Wednesday, July 5, 2017

Construction - Ventilated Cathedral Roof

The design of our cathedral roofs incorporates a "mini-attic" between two layers of sheathing.  The space between layers offers several advantages:  (a) primary air sealing at the sheathing level (secondary air sealing will be at the drywall level), (b) ventilation for a cooler roof in summer and to dry any moisture making its way into the ceiling/roof cavity and (c) a much more interesting interior space than would be the case with a typical horizontal 8' ceiling.

This post describes the construction of the south-facing cathedral roof over the single story portion of the house.  A prior post described the first cathedral ceiling/roof but left the story untold beyond the first layer of sheathing.  Here the entire procedure is described except for installation of the final metal cladding. Reminder:  click on any photo to enlarge it for detail.

Roof Trusses
The actual R-value of a roof or wall is not only about the R-value of the insulation but also the amount of thermal bridging through the structural members, especially through solid 2x lumber.  Because trusses minimize through-and-through structural members, they have less thermal bridging than solid rafters like 2 x 12s (manufactured I-beam rafters have even less).  The roof trusses that we used for the second story cathedral ceiling were 16" tall, i.e., they accommodated 16" of insulation that would provide R-50+ before thermal bridging.  

After the trusses were installed, I wondered how expensive it would have been to have increased the height of the trusses to 18" as a way of compensating for at least some of the thermal bridging.  When ordering the new set of trusses for the first story roof, I had them quoted both ways -- 16" and 18".  I was pleasantly surprised that the difference in price was less than $4 per truss so, naturally, I bought the taller trusses. Of course, the additional 2" will increase the cost of insulation by 10% but we will be using inexpensive rice hulls and, after buying a trailer-load, will have a surplus anyway.

Roof Slope Issues
The plans called for a roof pitch of 12:2.5 which would have more than satisfied the code
The left ends of the trusses were designed to rest equally
on both sets of double top plates; the right ends rest on

a double 2 x 6 ledge board; at a little more than 12:2
 barely meets code for standing seam metal roof
minimum of 12:2 for standing seam metal roofing.  However, the plans show 2 x 12s instead of 18" trusses and the truss manufacturer dropped the pitch by 3" inches for unexplained reasons. The two together meant that the slope was not as steep as planned. While it meets code, I am faced with the same issues that I discussed in detail in the most recent post that lead to the adoption of a wet prevention strategy under the metal roof as opposed to the more ideal drying strategy.  

Truss Installation
The trusses were supported on one end by the top of the exterior wall and on the other end by a ledger comprising two back-to-back 2 x 6s fastened to the second story wall with nails and construction screws.  After the layout lines were drawn on the ledger and top plate of the wall, installing the trusses becomes realistically a two-person operation.  One person lines one end of a truss with the layout line and with the outer edge of the top plate of the wall and the other person lines the other end with the layout line on the ledger.  We used both nails and construction screws to fasten the trusses.  The latter, angled up through the top plate and into the bottom chord of the truss, is a code-compliant alternative to steel rafter ties. In addition to nailing, the other end was secured with one set of construction screws up through the bottom of the ledger and another set near the top of the truss through blocking in the wall.


First layer of sheathing.  All
cracks are air-sealed with
 flashing tape
The function of the first layer of sheathing is twofold:  to provide easy air-sealing at the sheathing level and to serve as the floor for the "mini-attic".  The intentional gaps between sheets that would be hard to caulk or foam from below but are easily handled from above with flashing tape. Even the junction between the sidewall sheathing and the roof sheathing can easily be sealed with tape.  But before doing so at the eave side of the roof, we backed up the junction with 2 x 4 blocking internally.  We used 1/2" plywood for the first layer of sheathing because is "breathes" better than OSB should any moisture find its way into the ceiling cavity. 

Traditionally, a leaky area for air infiltration/exfiltration is the space between the double top plates which, if it is addressed at all, is typically caulked interiorly.  I as the nearby photo shows, I used tape on it, albeit duct tape instead of flashing tape because it is cheaper and fit better.  Duct tape is not nearly as sticky as flashing tape but will be held in place by the sheathing should the stickiness wane.  And, with my being the belt and suspender type, it will get caulked on the inside as well.
Duct tape is used to close the gap between the two top
plates before the sheathing goes on; the junction
between the wall and roof sheathing is not well supported
 until blocking is added between trusses for support from
the inside;  taping the junction had to be postponed until
it was supported

The next job was to add another layer of sheathing to create
The 2 x 4 supports for the second layer of sheathing
fastened directly on top of the trusses using construction
screws; notice the space at the left next to the second
story wall that will allow a free flow of air between all of
the bays; eventually roof vents will be installed over it to
allow hot air to escape
an air space for ventilation which I am calling a "mini-attic".  In order to create space and support the second layer of sheathing  we used construction screws to fasten 2 x 4s on edge over the trusses.  The first layer of sheathing was 16' wide, i.e., 16' from the second story wall to the eave. We held the 16' 2 x 4s away from the second story wall enough to accomplish two things (1) to extend past the lower story wall enough to serve as rafter tails for a 24" eave overhang and (2) to create space next to the second story wall that would be continuous from east to west such that air convected up the roof from eave vents could co-mingle with air from the other bays.  In this way only a four or five roof vents will suffice for 24 bays. 

Extensions of the lookouts (left) and rafters (right); notice
 the flashing tape at the junction of the rake wall and the
 roof but the absence of it on the eave side -- it had to be
 postponed until the junction was supported by blocking

We used OSB for the second layer of sheathing over the 2 x 4s for a couple of reasons -- in order to save cost and because, with the mini-attic below, trapping moisture will not be an issue.  If I had it to do over again, however, I would have installed 2 x 4 blocks in the gap between the long 2 x 4s and the second story wall.  I would have aligned them with the long 2 x 4s and made them short enough not to interfere with air movement but long enough not to split when the OSB was nailed to them.  Without them, the OSB was too springy as it rested on the ends of the long 2 x 4s then bridged the 21" gap before resting on the 2 x 4 next to the wall.  As it was, I had to use metal strapping to stiffen the junction between OSB sheets but could do nothing about supporting the sheet between edges.

Temporary Protection
It will be several months before all of the roofs are ready for installation of the metal roof so temporary protection for the sheathing was mandatory.  I used battened-
The mini-attic completed:  the subfacia and the second layer of
sheathing are installed and protected by 30# felt paper
down 6 mil plastic to protect the sheathing on the two roofs that were installed earlier -- the north facing roof over the second story and the west facing first story roof. After much waffling about what kind of underlayment to use under the eventual standing seam metal roofing, I settled on, not just one layer, but two layers of 30# felt.  The reasoning was that a second layer might add a measure of insurance for the lower than expected roof pitch.  My plan was
Masonite protects the felt while working on the wall
to use the first layer for temporary protection and lay down the second at the time the metal was installed.

As will be explained in the next post, using 30# felt as a temporary covering proved to be a bad idea.  Despite conscientious fastening with roofing nails, several areas peeled away with the first heavy wind. Augmenting with roofing cement was only marginally better -- several pieces even then lifted off as if the cement did not adhere well to the felt at least at high summer temperatures.  Also the felt that stayed put badly wrinkled but I am not sure why -- whether it was from getting wet or from thermal expansion or both.  Some of the wrinkles were so severe that I was afraid they might keep the metal roofing from laying flat as it should for good appearance. So I punted and added 6 mil plastic over the felt until it could be removed and replaced in conjunction with installing the metal roof.

I had a surplus of Craigslist Masonite so I screwed down a few sheets next to the second story wall so the felt will be protected while we build the overhang for the second story windows and install the windows.

Thursday, June 15, 2017

Construction - First Rake Walls; First and Only Conventional (Non-Cathedral) Roof

This post discusses three diverse topics.  First, it describes built-in-place wall trusses for the rake walls (as opposed to the pre-made trusses used for the eight foot walls).  Second, it explains how our low pitched roofs evolved.  And, third, it shares a bad design for a roof assembly that necessarily extends the conversation on vapor barriers that was started in the first of two posts on air and vapor barriers and a conversation that will undoubtedly come up often in future posts until the building envelope is fully completed inside and out.  When I set out to build a house, I understood the importance of air sealing but lacked an appreciation for the importance of moisture control and how controversial and misunderstood it is for contractors and permitting authorities.

Rake Walls
We stick-built the east rake wall for the second story on top of the first story pre-made wall
The east rake wall on top of the first story wall trusses
trusses that separate the living quarters from the garage. The west second story rake wall rested on top of the beam over part of the master bedroom. Two-by-sixes were used for both walls instead of 2 x 4s only because I had exhausted my supply of salvaged 2 x 4s but had plenty of salvaged 2 x 6s left.  In order to provide a 15" wall cavity for insulation like the rest of the stick-built exterior walls, the rake walls were essentially "double walls" patterned after the pre-made wall trusses, even to the extent of turning the 2 x 6s 90 degrees like the 2 x 4s in the trusses and stabilizing them with gussets like those in the trusses.  

The west rake wall on top of the LVL beam

Sheathing the rake walls had to be postponed until the shed roofs extending from them had been completed. So we covered them with battened-down sheet plastic in order to protect them, especially their plywood gussets, from the elements.

A Word About Temporary Coverings
The blue tarp covering the roof in the second photo had enough UV deterioration from a prior use that did not protect the sheathing and had to be replaced with battened-down 6 mil plastic sheeting. Black plastic resists deterioration longer than garden variety tarps and much longer than clear plastic.  The reason for its longer life is that UV rays cause minor damage only to the surface rather than penetrating through and damaging the full thickness of the material as with clear plastic.  Staples alone do a pretty good job of securing tarps in the wind while plastic easily tears loose from staples.  It takes screw-retained batten boards to hold it. Nailing the boards to 1/2" thick sheathing, particularly with bright nails rather than serrated nails, does not work either.  In a stiff wind, the boards end up on the ground or, worse yet, flailing around on the roof with nails protruding, tearing holes in the plastic.

Conventional Roof
The living space between the LVL beam and the west concrete wall as seen in the second photo needed to be covered by a shed roof attached to the rake wall and resting on the concrete wall.  I wavered between the cathedral type trusses we used for the second story roof or more typical trusses.  I made the mistake of choosing the latter.
OSB blocking attached to the top chord of the trusses; they
extend from the outside wall to  a height on the chord that
 will allow 18" of  insulation without any of it blocking the
ventilation pathway between the attic and the eaves  

The cathedral approach would be easier to insulate (by a technique I have planned for the other cathedral ceilings and will be detailing in future posts). The conventional attic created by the low roof pitch is so confining that crawling around in it to blow in the insulation won't be fun. Extra work was required for installing blocking between the rafters to hold the future insulation at bay and maintain a patent airway to the eaves for proper ventilation.  And, to make matters worse, I was unaware that I needed specifically to request that the truss company make the height of the two end trusses 3 1/2" shorter to allow for stick-built lookouts to support the fly (facia) rafters. As a result, I will have to "Jerry-rig" (pun intended) something to make the lookouts strong enough to support a two-foot overhang.

Why Such Low-Pitched Roofs?
The building code specifies that the window area be 8% of inhabitable space.  All of our windows but one are confined to the south facade so, in order to meet code, the clerestory windows on the second story had to be much larger than is typical for clerestories.  The height of the second story wall was increased by 30" over a standard 8' wall in order to create enough space between the bottom of the windows and the shed roof to allow for the pitch of the roof.  The 30" figure was purely arbitrary as I was trying to strike a balance between making the wall inordinately tall and providing for adequate roof pitch.

The plans called for a 2.5 - 12 roof pitch for all of the roofs but envisioned 2 x 12s as rafters. The trusses that I chose instead of 2 x 12s were 16" (north slope) and 18" (south slope) deep to give more room for insulation. The added height caused both roofs to drop below the target roof pitch.  Knowing what I do now, I would have raised the second story wall by at least 6" and lowered the exterior walls by at least 6" to give a steeper roof pitch.  Even then, since there were no windows involved, I could have pitched the west-facing conventional roof higher but, for aesthetic reasons wanted to hold it to slightly below the height of the south-facing shed roof.

Foil-backed Sheathing
I had already purchased and installed OSB sheathing with foil backing over the conventional trusses by the time I had finished the research for the recent post on barriers. The research convinced me that OSB without foil or plywood would have been a better choices.

(In order to make more sense of the next couple of paragraphs, I would recommend recent post on barriers.)

The reason why foil-backed OSB was not a wise choice is that it is a vapor impermeable on the foil side, meaning that any moisture that breaches the roof cladding and the fabric or felt paper under it will have to dry towards the attic side of the sheathing.  And it has to be assumed that some moisture will find its way under the metal roofing, especially at our low roof pitches.  As discussed at length in the most recent post on barriers, a vapor permeable barrier such as Slopeshield could be used to shield the OSB from moisture penetration but allow any moisture that does breach it to return back out through it for drying, but it is not recommended for roof slopes as low as ours.

In a recent post on barriers. I quoted Listiburek's as follows:

"Avoid using vapor barriers where vapor retarders will suffice; avoid vapor retarders where vapor permeable materials will work; thereby "encouraging drying mechanisms over wetting prevention mechanisms."  (Italics and underlines are mine)

Since our low pitchness for standing seam metal roofs increases the potential for moisture penetration, I am stuck with going against his recommendation and using a wetting prevention approach for all of the roofs, relying on the interior surfaces of the sheathing to be vapor permeable enough for drying .  Therefore, I intentionally scuffed up the (expensive!) foil so as to make the OSB at least somewhat vapor permeable interiorly without totally compromising the radiation reflectance of the foil.

Temporary Protection  
The black plastic temporarily protecting the rake wall and the roof was removed in conjunction with sheathing the rake wall above the new roof. It was reapplied over the wall sheathing but not the roof sheathing -- 30# felt was applied instead.  As will be fleshed out in subsequent posts, 30# felt was or will be used on all of the roofs as interim protection until the metal cladding is available.  Then a second layer of 30# felt will be applied in conjunction with installing the cladding. 

Friday, May 19, 2017

Design - Vapor Barriers and Air Barriers (Cont'd)

The previous post tried to make sense of vapor barriers.  This post tries to do the same for air barriers.  However, air barriers are hard to address without further discussion of vapor barriers since the two are so intertwined. 

My primary source of information for the last post was an online paper by Joseph Lstiburek on vapor retarders; my authority here is another Lstiburek paper, "Understanding Air Barriers".   Some of what appears below paraphrases his work.

What Is An Air Barrier? 
An air barrier controls airflow between conditioned space and unconditioned space. It is located in the assemblies that separate the building interior from the outside environment but can be in assemblies that separate living space from potentially dangerous space such as a garage.  The barrier can exist on the exterior of wall/ceiling assemblies, on the interior, or both.  In cold climates in the absence of air conditioning, they tend to be located interiorly to control exfiltration of moisture-laden air. In warmer climates and any climate with air conditioned space, they are located exteriorly where they not only control infiltration of exterior air, they also prevent air penetration into the insulation materials within the wall or ceiling (wind washing). Very often air barriers are both exterior and interior as will be the case with our project.

Merits of Exterior and Interior Air Barriers
The advantage of exterior air barriers, such as house wrap, is ease of installation and automatic control of wind washing.  The disadvantage of an exterior system is that it is on the wrong side of the wall to stop exfiltration of moisture-laden interior air that degrades cavity components.  

The advantage of interior air barriers, such as latex painted drywall, is they stop exfiltration of moisture-laden air. Their disadvantage is that they are on the wrong side of the wall to stop wind washing or infiltration of insulation-compromising moisture from the exterior.

However, installing both exterior and interior air barriers can compensate for the disadvantages of each.  Interior barriers were covered in a recent post. The take-away was that, in all but the coldest climates, drywall, carefully detailed and painted with latex paint, suffices as an interior vapor barrier.  Lstiburek also recommends drywall as an air barrier and uses the nearby illustrations to demonstrate the detailing that is necessary for it to be successful.  We plan to follow his recommendations.

Cathedral Ceilings
Tentatively, most of our ceilings will have tongue and groove pine boards instead of drywall because the exposed wood enhances our country decor and the 3/4" boards will be better able to support 16" of rice hull insulation. The boards will serve as a Class III semipermeable vapor retarder (same recent post ) to help protect the insulation from exfiltration of moist interior air. The boards will serve as an air barrier but not well enough but what the definitive air barrier will have to be at the top of the ceiling trusses.  Accordingly, I am in the process of covering the tops of the roof trusses with 1/2" plywood then sealing the cracks between sheets with flashing tape (bottom photo). Above the plywood will be ventilated space in the form of the "mini-attic".  All of this information appears in the post immediately preceding this post.

Blower Door Testing
Verification of air barrier efficacy is easily done with one or the other of two "blower door" tests that create an air pressure differential between the interior and the exterior of a building.  For small buildings, the fan is embedded in a temporary exterior door; for larger buildings, it is installed in the building's air handling system.  The amount of air leakage in the envelope of the building can be quantified and, with the help of a smoke gun, can be pinpointed for remediation

Choosing An Exterior Air Barrier
All cladding materials admit varying degrees of rainwater to underlying structures. Some, like brick veneers, fibercement siding. stucco and even wood siding, are worse than others, like vinyl or metal siding, in that they absorb and hold water that the sun can then drive through the wall (solar vapor drive).

While the primary selling point for exterior barriers (such as the ubiquitous "Tyvec") is to reduce air infiltration, their vapor barrier specs turn out to be the thoughtful basis for choosing between them.  The choices range from something like 30# felt paper as an attempt to keep water out entirely, thereby favoring wet-prevention mechanisms, or something like Class IV permeable house-wrap to keep most of it out then let that which does penetrate the opportunity to exit back out through the barrier, thereby favoring drying mechanisms.

The exterior walls and the floor of the mini-attic (last post) are being sheathed with plywood, rather than OSB.  (The rationale for using plywood is explained by Joe Listburek at Building Science Corp).  At the time of this writing, I was still undecided whether to use foil-backed OSB instead of plain OSB for the second layer of sheathing that forms the roof of the mini-attic and provides the decking for the metal roof.  The advantage would be that its radiant barrier would provide a cooler roof. Its disadvantage would be that the decking would be vapor impermeable on the inside so that, for proper drying, the underlayment for the metal roofing would have to be vapor permeable to allow drying outwards.  Our dilemma is that the slope of some of the roofs may not be steep enough for a vapor permeable underlayment for the metal roof, in which case, we may have no choice but to use a vapor impermeable underlayment and no foil backing so the roof could dry inwards.  It will take more research before making a final commitment but I am inclined to forego the foil backing and rely instead on a metal roof color that has a high solar reflectance.

With metal roof or metal wall cladding, we can expect some moisture to make its way into the underlying assemblies,
Plywood sheathing sealed with flashing tape; holes
that were created by first unsuccessful attempt at a
 temporary covering using a tarp and sheet plastic
 have been closed with spackling
especially during horizontal rains. So, assuming that any house wrap or roof underlayment is an adequate air barrier, we need to choose wraps that minimize water penetration but allows outward drying of the underlying wood when necessary.

With regard to the wall assembly, there will be five layers:
  • Metal cladding
  • Wrap
  • 1/2" plywood sheathing
  • 15" wall trusses filled with rice hulls
  • 1/2" drywall detailed per Lstiburek and sealed with latex paint

With regard to the roof-ceiling assembly, there will be seven layers:
  • Standing seam metal roof in a color that has a high solar reflectance
  • Air/vapor barrier (type to be determined after more research)
  • 1/2" OSB without foil backing
  • 3 1/2" air space (mini-attic)
  • 1/2" plywood nailed to the tops of the rafters (air barrier)
  • 16" tall trusses filled with rice hulls
  • 3/4" tongue and groove pine ceiling
In an ideal world, the best choice for our metal roofing and siding would be a vapor permeable wrap instead of a vapor impermeable wrap, thereby favoring drying mechanisms over wet-prevention mechanisms.  The barrier that we use for the walls will definitely follow this approach.  The barrier that we use for the roof will be dictated by the roof pitch, has yet to be determined and will be discussed in future posts on roofing.

Friday, April 28, 2017

Construction - First Cathedral Ceiling

The first cathedral ceiling presented an opportunity to apply the concepts detailed in the two posts on vapor and air barriers.  And it is interesting how much the final design of the ceilings deviates so much from the original design that I so naively and confidently detailed in a prior post before fully understanding vapor and air barriers.

I am deliberately inserting this post between the two posts on vapor/air barriers in order to reference it while discussing air barriers in the second of the two posts.

Original Design
My original design called for a 3 1/2" tall "mini-attic" between a fabric stapled to the tops of 2 x 12 rafters (with which to confine the blown-in rice hull insulation) and the roof sheathing. After more insight into moisture control and air infiltration/exfiltration in wall and ceiling assemblies, and, after a meeting with the consultant who will certify our project for energy efficiency, a different design for the mini-attic emerged. Also, instead of using 2 x 12s, I opted for trusses but only after thoroughly parsing I-joists as well. 

Perhaps the most succinct article on the subject of cathedral ceilings that I have seen is How to Build An Insulated Cathedral Ceiling on the Green Building Advisor site.  It clearly informed me that my original design would have been a disaster, that air sealing the ceiling/roof assembly is much more important than ventilation between the insulation and the sheathing, but that a dedicated air space sandwiched between a double layer of sheathing as described below for our project would be an advanced design worth the additional time and expense.

Roof Trusses
Two-by-twelves seemed a bad choice for three reasons.  First, 11 1/2" of insulation would yield an R-factor of only 35 when our target was at least 45.  Second, the thick 2 x 12s would allow considerably more energy-robbing thermal bridging than either trusses or I-joists. (Ever notice how easy it is to identify cathedral ceilings vs. conventional attics by the snow melt pattern on the roof?  Striations appear over cathedral ceilings because melting is faster over the 2-bys than over the insulation between them, whereas melting over conventional roofs is more uniform.) The third reason for avoiding long 2 x 12s is that their length and girth make it more likely that they come from old growth trees while I-joists and trusses have certifiably sustainable sourcing.  

I-joists 16" tall would have been a little cheaper than 16" trusses but would have required considerable job-site customization.  They would have
Roof trusses resting on truss walls (click on the picture to enlarge)
had to have been plumb-cut and reinforced on both ends and, since bird-mouths cannot be cut into the lower chords, the top plates of the walls would have had to have been fitted with wedge-shaped support boards.  And, while the trusses could be customized at the factory to fit flat on both 15" wide walls, the I-joists would have rested on the inside top plate on one wall and the outside top plate of the other. These considerations made the minor 
up-charge for the trusses a good trade-off.

The one downside to the decision, though, is that, while the trusses are significantly less thermal bridging than 2 x 12s,  I-joists would have been even better.  In hindsight, I would have used trusses 18" tall instead of 16".  The difference in cost would have been manageable and the extra two inches would have boosted the R-value by at least 7 points which would presumably off-set the loss from thermal bridging.

Mini-Attic Design
The air barrier for a conventional attic must be done at the level of the drywall as discussed in the recent post on vapor/air barriers.  In our case, however, the barrier will move to the level of the tops of the trusses due to our choice of tongue and groove pine ceilings that will be more permeable than drywall.  Instead of mesh on top of 2 x 12s as originally envisioned, I installed 1/2" plywood sheathing as the first of two layers of sheathing. The first layer will serve as the "floor" for the mini-attic; the second layer will double as its "ceiling" and as decking for the roof.  Since vapor passing through a wall or ceiling largely depends upon moving air, air sealing the floor of the mini-attic as described below will virtually eliminate vapor penetration through the pine ceilings.

At the time of this writing, I had covered the plywood with 6 mil sheet plastic anchored with batten boards to protect it for a few months until the mini-attic could be completed in conjunction with roofing the rest of the house.  And, for whatever it is worth, the first attempt to protect the plywood was a failure.  I conscientiously anchored the plastic with batten boards fastened with nails.  However, it took only a short time for wind blowing across the surface and coming up through the spaces between the sheets of plywood to heave the plastic enough to work the nails loose from the relatively thin (1/2") plywood.  The battens either blew off the roof or clung loosely to the plastic.  In either case, the nails protruding from them ripped holes in the plastic to the extent that I had to recover the roof with new plastic after taping the seams between the plywood sheets and filling the nail holes (last photo below).  This time I screwed the batten boards to place.  The moral is "use screws"; do not depend on nailed battens and don't even think that staples alone will work.

Just before the final roofing goes on, I will use construction screws to fasten 2 x 4s on edge on top of and fastened to the roof trusses through the first sheathing. I will then nail the second layer of sheathing to them.  The result will be a 3 1/2" ventilation space -- mini-attic -- that communicates with the outdoor air via continuous soffit vents in the north eave and a continuous ridge vent between the south edge of the roof and the overhangs for the second story windows.  

According to Joe Lstiburek at Building Science Corp., plywood for the first layer of sheathing is a better choice than OSB because it will allow water vapor to pass through it should vapor escape the living space, negotiate the less-than-impermeable wood ceiling and rise through the insulation. By contrast, the impermeability of OSB would block vapor which then could harbor mold, rot the sheathing, if not the trusses, and degrade the R-value of the insulation. OSB for the second layer of sheathing is acceptable however because any vapor from below will be vented from the mini-attic through the soffit vents and does not have to find its way through the second layer of sheathing.

The code calls for 1" minimum ventilation space between the insulation and the sheathing of a conventional cathedral ceiling.  Lstiburek suggests at least 2" for the air space while questioning the efficacy of any air space directly in contact with the insulation.  Our mini-attic will not only provide 3 1/2" instead of an inch or two but will also have sheathing separating the air space from the insulation.

The roof will overhang the walls 24".  I plan to extend the edgewise 2 x 4s that carry the second sheathing outward as support for the overhangs.  As discussed below, the 2 x 4s will not complicate air-sealing as would rafter tails extending from the roof trusses.

Sheathing the Short Truss Wall
The trusses are plumb cut flush with the short wall, i.e., there
 are no rafter tails extending from the trusses to interfere with
sealing the junction between the wall and the roof with a
continuous run of flashing tape
For the same reason I used plywood instead of OSB under the mini-attic, I used it for sheathing the short truss wall (and plan to use it for all of the exterior walls). It is important to note that, by plumb-cutting the roof trusses and leaving off the rafter tails, all of the wall sheathing could be abutted against the roof sheathing in order to simplify air sealing at the junction between the two. If rafter tails had been present, the sheathing would have had to have been cut and fitted around them -- a tedious job with a less-than-ideal outcome when it comes to air-sealing.  I was able to use a continuous run of flashing tape to seal the junction between roof and wall whereas, with rafter tails, tape, caulk and spray foam on the
 inside would also have been necessary for the inevitable gaps between the  tails and the wall sheathing.  

Air Sealing the Roof Sheathing
Blocking between trusses to stiffen the junction of the roof
sheathing and the wall sheathing and to facilitate caulking it
 from the interior, in addition to having taped the junction on
 the exterior (click on photo to see detail)
The clips used between sheets of plywoodsheathing are spacers to allow for expansion without buckling.  However, the space also would allow air infiltration and exfiltration that would be totally unacceptable.  Using caulk in the cracks would be counter-productive eventually since it loses its flexibility with age.  Spray foam would be rigid from the git-go.  So thank god for flashing tape. I used it not only to close the gaps left by the clips but also where the sheets of plywood met over the trusses. The nice thing is the tape will remain flexible indefinitely.

After the front wall and the rake walls for the second story have been sheathed withplywood, the junction between them and the roof sheathing will be handled in the same manner as the junction between the short wall and the roof. Then, considering that (1) the roof-wall junction is sealed with tape on all four sides, (2) the cracks between sheathing panels of both the roof and the walls are sealed with tape and (3) proper air-sealing is done around the windows when they are installed, the second story would theoretically be ready for a blower door test well in advance of the drywall stage. 

Sunday, April 23, 2017

Design - Vapor Barriers and air barriers

There are myriad materials marketed for controlling the flow of moisture and air through wall and ceiling assemblies.  However, it doesn't take much research to become confused about which to use where.  For example, the nearby map shows that, for our lower Midwest climate zone, we need an interior vapor barrier.  As we will see, this would be an unwise choice. When step-son, Keith, and I were building his house in a nearby county, even the building inspector was sufficiently ambivalent to accept the wall construction with or without a polyethylene sheet plastic vapor barrier.

This post is limited to those materials used on the inside of assemblies. The next post will tackle those used exteriorly. Furthermore, this post emphasizes vapor barriers for wall and ceiling assemblies while the next post completes the story by discussing air barriers.

The Problem
Anyone who is involved with building a house would do well to read "Understanding Vapor Barriers" by Joseph Lstiburek on which the following discussion is based.  He says that "Vapor barriers are...a cold climate artifact that have diffused into other climates more from ignorance than need. The history of barriers itself is a story based more on personalities than physics.....It is frightening indeed that construction practices can be so dramatically influenced by so little research...  Incorrect use of vapor barriers is leading to an increase in moisture related problems. Vapor barriers were originally intended to prevent assemblies from getting wet. However, they often prevent assemblies from drying. Vapor barriers installed on the interior of (wall or ceiling) assemblies prevent assemblies from drying inward.  This can be a problem in any air-conditioned enclosure. This can be problem in any below grade space.  This can be a problem when there is also a vapor barrier on the exterior.  This can be a problem where brick is installed over building paper and vapor permeable sheathing."

Simplified Terminology
Lstiburek proposes simplifying terminology.  He suggests that all of the materials used in wall and ceiling cavities that are capable of influencing the behavior of moisture vapor, such as house wraps, polyethylene sheeting, felt paper, OSB, plywood, foam insulation board with or without foil backing, drywall, latex paint, vinyl wallpaper and all cladding materials should be called "vapor retarders" because they all have the capacity of retarding the movement of water by vapor diffusion.  Vapor retarders should then be sub-classified into four groups according to the rate at which vapor diffuses through them as measured by their vapor permanence or "perm" as follows:

  • Class I Vapor Retarder:        0.1 perm or less
  • Class II Vapor Retarder:       Between 0.1 and 1.0 perms
  • Class III Vapor Retarder:      Between  1.0 and 10 perms

Then Lstiburek goes on to categorize materials generically into four groups based upon the above three classes as follows:

  • Vapor impermeable               Class I Vapor Retarder       (vapor barrier)
  • Vapor semi-impermeable       Class II Vapor Retarder      (vapor retarder)
  • Vapor semi-permeable           Class III Vapor Retarder    (vapor retarder)
  • Vapor permeable:                    Greater than 10 perms
Air moves through wall and ceiling assemblies due to differences in air pressure and contains varying amounts of water in the form of vapor.  All of the materials listed above as vapor retarders have some capacity for blocking air movement and, in so doing, might be called "air barriers".  

Examples of Vapor Retarders
The following list comes from
  • Class I:  Glass, metal, polyethylene sheeting, rubber membrane
  • Class II:  Unfaced extruded (XPS) or expanded polystyrene (EPS), 30# felt (asphalt coated paper), plywood, bitumen coated kraft paper
  • Class III:  Gypsum board, unfaced fiberglass insulation, board lumber, concrete block, brick, 15# felt (asphalt coated paper), house wrap
Choosing a Vapor Barrier
Lstiburek is quite clear as to best practices for choosing vapor barriers  Paraphrased from his work, they are as follows:
  • Avoid using vapor barriers where vapor retarders will suffice; avoid vapor retarders where vapor permeable materials will work; thereby "encouraging drying mechanisms over wetting prevention mechanisms
  • Avoid vapor barriers on both sides of an assembly so as not to block drying in at least one direction
  • Avoid installing on the interior of air conditioned space such vapor barriers as polyethylene sheeting, foil faced batt insulation and reflective radiant barrier foil
  • Avoid vinyl wallpaper on the interior of air conditioned spaces
Interior Vapor Barriers
A reasoned summary for when to use an internal vapor barrier like polyethylene sheeting is found in Green Builder and reproduced verbatim below:
  • Most buildings don't need polyethylene anywhere, except directly under the concrete slab or on a crawl space floor.
  • The main reason to install an interior vapor retarder is to keep a building inspector happy.
  • If a building inspector wants you to install a layer of interior polyethylene on a wall or ceiling, see if you can convince the inspector to accept a layer of vapor retarder paint or a "smart" retarder (for example, MemBrain or Intello-Plus) instead.
  • Although most walls and ceilings don't need an interior vapor barrier, it's always a good idea to include an interior air barrier.  Air leakage is far more likely to lead to problems than vapor diffusion (the italics are mine).
Both Lstiburek and say that drywall with latex paint on it, when installed
correctly, forms an adequate semipermeable vapor retarder for most of the US. As we sit at the junction of the mixed-humid and hot-humid zones depicted on the above map, this is obviously the best option for our project as well.  The only caveats that make our project different is that (a) we will not have conventional air conditioning for cooling and humidity control as would be expected for our area and (b) our earth sheltered design means our living space is partially below grade.  However, the energy recovery ventilator that we have planned should adequately replace air conditioning for humidity control and the earth contact walls will be not be in contact with moisture because of the French drains and the insulation/watershed umbrella. And the earth contact walls will not be subject to sweating because the earth behind them will be warmed by the AGS system (for info on AGS, click on "Featured Post" in the column to the left).

Vapor Control Varies According to Climate and Assembly Components
Despite the above advice for avoiding interior vapor barriers, there is no universal solution to vapor control for all situations.  Lstiburek's paper discusses various scenarios for exterior wall assemblies and specifies the best climate zone(s) for each (not only for zones depicted by the map above, but for severe cold climates further north as well).  In northern climates, for instance, the best practice is in fact to use an interior vapor barrier.  But, in another paper on air barriers, he warns against using them even in cold climates for air conditioned spaces.

Even though the primary function of air barriers is to limit air infiltration and exfiltration, they are also vapor retarders in that they control the movement of moisture-laden air through an assembly -- as we shall see in a subsequent post.

In the meanwhile, I will devote the next post to the newly-built cathedral ceiling so as to be able to reference it later for the follow-up post on air barriers.

Thursday, March 9, 2017

Design - Maximizing Passive Solar Gain (Cont'd yet some more) - Supplemental Heat, Thermal Environment and Exterior Colors

This is the fourth and last post on passive solar design.  The first post was an overview and ended with a list of design considerations.  The second post  discussed three of the considerations:  the location of the building, the room arrangement within the building and a protected entry to the building.  The third post delt with windows, thermal mass and surface colors.  Here we wrap up the series with supplemental heat for passive solar structures and the thermal environment.  Again I am relying heavily upon Mazria's definitive text as background and our project as an example.

Supplemental Heat
Wood burning stove
Only in places like southern California with mild winter temperatures and lots of sunshine is it possible to forego supplemental heating. According to one study, the annul percentage of heating provided by passive solar is closely associated with latitude and somewhat less with heating degree days. The percentage of heating that can be expected from passive solar ranges from 31.9% in Ottawa, Canada (45.3 degrees latitude and 8,838 heating degree days) to 60.2% for NYC (40.6 and 5,254) to 80.8% for Ft  Worth, TX (32.8 and 2,467).

The literature suggests that, for passive solar purists, wood-, corn- or wood chip-burning stoves and masonry heaters are commonly used to raise the ambient temperature a few degrees to a comfortable level, particularly in earth sheltered structures.  My guess is that most conventional homes embracing passive solar have conventional heating and air conditioning but down-sized to fit their passive solar capabilities.
Masonry heater

What makes Annualized GeoSolar conditioning (AGS) a significant upgrade from classic passive solar is that it maintains the same comfortable year- round temperatures that conventional HVAC systems provide.  (For details on AGS, click on "Featured Post" in the left column.)  And it does so by first increasing the size of the thermal mass then using the summer sun to heat it.  According to Hiat and Stephens, it takes a couple of years of solar input to reach the desired room temperature, during which auxiliary heat will probably be necessary.  Accordingly, we plan to use wall- or ceiling-mounted infrared heaters in tandem with wintertime passive solar as our secondary heat sources for the first couple of winters while the AGS system is heating the thermal mass. 

Eventually, passive solar alone should be sufficient to supplement the AGS system except possibly on below-zero cloudy days.  Then we will resort to infrared heaters.  Also not to be forgotten is the heat generated by people living in a structure.  The amount of waste heat from cooking, lighting, water heating and from human bodies is not inconsequential.  In fact, there are case studies in the literature in which waste heat provides half of the necessary supplemental heat for well-insulated passive solar installations.

Thermal Environment
Mazria discusses something that I have not seen in the other sources with which I am familiar -- what he calls the "thermal environment".  The topic is somewhat hard to grasp at first, much less explain, but here's a go at it.

There is a relationship between the air temperature and something called the mean radiant temperature (mrt) which is the average temperature of all of the surrounding surfaces.  Both mrt and air temperature influence the feeling of comfort but not equally.  Mrt has a 40% greater impact on comfort than air temperature which means that, for the same feeling of comfort, the air temperature can be reduced by 1.4 degrees for each degree mrt is raised.  The following examples come from a chart in Mazria (p. 64)

  • Mrt of 65 degrees means the air temp has to be 77 degrees for a comfortable feeling of 70 degrees
  • Mrt of 70 degrees means the air temp can be 70 degrees for a 70 degree comfort level
  • Mrt of 75 degrees means the air temp can be 63 degrees for a 70 degree comfort level
  • Mrt of 80 degrees means the air temp can be 56 degrees for a 70 degree comfort level
But how does all of this relate to passive solar?  Many things contribute to the mrt, or average temperature of all of the surrounding surfaces, but thermal mass -- the concrete floor and walls and, for the AGS system, the soil -- is by far the most important contributor. Once the mass reaches and maintains a constant temperature of say 75 degrees, the air temperature at night or on a cloudy day can drop to 63 degrees but it still feels like 70 degrees in the space, whereas in structures without thermal mass, a 63 degree air temperature feels like 63 degrees (or chillier due to air currents). However, if the thermal mass in contact with earth or exterior environment is not sufficiently insulated, the mrt might be so low that an inordinate amount of sunshine and supplemental heat would be needed for comfort.  This was one of the problems with older earth sheltered homes.

Ideally, the beginning of the heating season finds the temperature of the thermal mass already at a comfortable level naturally or due to air conditioning.  Then, the combination of solar gain, supplemental heat and waste heat maintains or increases the temperature such that swings in room air temperature are modulated within an acceptable comfort range.  Studies have shown that lower air temperatures are more invigorating and that one's ability to think and work improves when one feels warm in air temperatures below 70 degrees (Mazria) therefore the mrt of the space needs to be higher than 70 degrees.  A feeling of comfort is also enhanced by warm floors and the lack of air movement that occurs in most homes when a difference in floor and ceiling temperatures causes air currents -- from ceiling to floor and back.

Exterior Colors
Dark colors absorb more sun energy than light colors so the color selection for exterior surfaces of a passive solar home might vary with climate.  Up north, darker colors could be a good balance between heating assistance in winter without impacting cooling in summer.  In temperate and warm climates, light colors can be used to reflect solar energy as part of the thermal barrier for the envelope.  Radiant barriers can also be used in the attic to intercept solar energy before it has a chance to challenge the insulation. 

Our Project 
At a latitude of 39 degrees and heating degree days of just south of 5,000, we have the potential for getting around 65% of our heating from passive solar during the winter. However, we consider this option a bonus.  The AGS system will meet all of our conditioning needs because of the design of the thermal mass :  (a) it is considerably enlarged, (b) it is well insulated and (c) most importantly, it is heated from the earth side by the heat from the summer sun instead of from the house side from solar gain through the windows.  Our goal is a mrt that gives an average comfort level of 74 degrees year-round with the expectation that the mrt might drop a couple of degrees by the end of the winter and rise by a couple of degrees by the end of summer. 

Once we get past needing supplemental heat (2-3 years), it remains to be seen whether the combination of AGS, wintertime passive solar and waste heat produces more heat than we need. Already, we are optimistic enough to postpone thermal shades until the need for them is apparent. And we are prepared to mothball some of the AGS conduits should there be overheating from the summer sun.  Too much passive solar heat?  What wonderful problem to ponder.

As to exterior colors, our plan is to use white cladding and light colored roofing.  As explained in a prior post, we will have a well-ventilated "mini-attic" between the cathedral ceilings and the roof itself.  The sheathing to support the roofing will be OSB with foil backing as a radiant barrier to keep the roof cooler in summer.