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.

Saturday, February 25, 2017

Design - Maximizing Passive Solar Gain (Cont'd some more) - Windows, Thermal Mass and Surface Colors

This is the third of four posts on passive solar gain.  The first post was an overview of the subject and the second post zeroed in on the shape and orientation of the building, room arrangement within the building and a protected entry for the building. Here we are continuing to use Mazria's book on passive solar and our project as a bases for a look at windows, heat storage and surface colors. And it goes without saying that we are talking about passive solar dwellings that are well-insulated, if not super-insulated.

A previous post several months ago explored the ins and outs (no pun intended)
of energy efficient windows and doors.  In the process, I discussed the advantages of......
  • Casement, hopper or awning, whereby closure is against an airtight seal -- as opposed to sliding windows
  • Fiberglass frames
  • Double glass
  • Low-E coating
  • Argon filled
However, our discussion of windows here goes beyond energy efficient glass, frames and ventilating style.

One of the recommendations in the second post on passive solar was for an east-west orientation for the building so as to maximize the amount of south-facing glass. But how much glass? Mazria offers ranges for the ratio of glazing to floor area (assuming that the floor is insulated thermal mass, i.e., capable of storing and radiating heat) that is needed to maintain average indoor temperatures in the upper 60 degree range in various climates.  For example, his chart shows 0.19-0.29 square feet of glass for each square feet of floor in the not-so-cold north to 0.27-0.42 sq ft in the cold-cold north.  For temperate climates, the glazing should range from 0.11-0.17 sq ft per square foot of floor in warmer climes to 0.16-0.25 in cooler climes.  Any fall-off from the recommended ratios means less solar gain and more supplemental heat.

Double and triple glazing and low-E coatings reduce solar gain to some extent but the loss is more than offset by a reduction in heat loss back out through the glass at night and on cloudy days. Gain can also be diminished by shade on the glass from the surrounding wall when windows are recessed, as is common with many energy-efficient structures.  There are three options for the problem: accept the loss of radiation as the lesser to two evils over thinner walls, move the glass closer to the outside plane of the wall and suffer more heat loss from wind washing or bevel the outside wall away from the glass to let the sun in.

Translucent glass is especially useful for direct gain passive solar.  It diffuses solar energy over a wide area, which helps when the energy would otherwise overheat low-thermal-mass structures like framed walls instead of finding its way to structures like masonry and soil that can absorb it.  And, compared to raw sunshine, the diffusion makes for a much brighter environment without unpleasant glare and helps to prevent color changes in furniture and fabrics.  These characteristics make translucent glass especially suitable for clerestory windows where a view through clear glass may not be critical.

In order to preserve heat gained during sunny days, it may be necessary to use thermal shades to cover the windows at night and on cloudy days.  The literature is replete with designs for thermal shades -- homemade and store-bought.  The best designs cover the inside of the window and seal against the sash on all four sides so as to prevent convective heat loss.

Heat Storage
Except for the work of Hiat and Stephens (for details, click on the "Featured Post" in the left
column), references to thermal mass for storing heat are typically focused on mass inside passive solar buildings.  For direct solar gain systems, concrete is king -- floor, walls and sometimes roofs. When it is in the envelope of the building, it is insulated on the outside surface, including under the floor. Research has shown that the concrete need not be thicker than 4" because daytime solar heat only penetrates to this depth before heat is withdrawn by falling nighttime temperatures.  

For indirect solar gain systems using thermal mass between the windows and the living space, the most common material is again concrete but thickness matters.  The thicker the wall the less temperature fluctuation within the living space.  The other, less popular, option is the "water wall", typically steel drums or other containers filled with water. For all practical purposes, the thermal performance for a given thickness is the same for concrete and water even though each behaves differently with regard to absorption and radiation of heat.
Hiat's umbrella in conjunction with maximum earth contact

With regard to thermal mass, the AGS system Hiat and Stephens co-fathered combines uninsulated earth contact with internal thermal mass like concrete.  They recommend a below-grade "umbrella" extending outward from the building to waterproof and insulate a volume of thermal mass (earth) much larger than the interior mass of traditional passive solar installations. The large thermal mass means that temperature swings in the living space are modulated to the extent that, instead of being measured in hours, remain relatively constant year-round.

Surface Colors
Our discussion of surface colors here is limited to direct solar gain systems since they predominate.  In general, dark colors are okay wherever the sun doesn't shine, which may be counter-intuitive because they would absorb more radiation -- a good thing.  However, they often overheat because absorption is faster than penetration and storage.  This is absolutely true for surfaces containing minimal thermal mass, like frame-and-gypsum-board surfaces, but it is true for concrete as well. Therefore, all surfaces receiving direct sunlight through transparent glass should be light in color so as to diffuse and scatter solar energy for distributed absorption by mass throughout the structure.  The possible exception might be a medium-shade for a masonry floor.

Surface colors become less critical when sunlight enters through translucent glazing. The glass itself diffuses the energy making overheating difficult even for dark colors.

Our Project 
In our warm temperate climate, the ratio of glass to floor area to maximize passive solar is rather modest. Mazria's chart seems to indicate that our 400 heating degree-days/mo calls for 390 sq ft of glass.  We will actually have 420 sq ft which was determined more by meeting the glass-to-floor-area code requirements than any intentionality about passive solar requirements.  

Our walls will be right at 17" thick and, since the glass will be recessed 10- 11" in from the exterior plane of the wall, the wall will shade the periphery of the glass.  But the deep-set windows will be ideal for minimizing wind washing.  Since we will depend on the AGS system as our primary heat source, we can easily tolerate a minor loss of wintertime solar gain. (For an explanation of "wind washing", go to the previous post on windows.)

Heat storage?  Thermal mass?  Our design is all about heat storage in concrete but more so in soil:
  • 900 sq ft of earth sheltered west and north concrete walls, largely uninsulated, which means that the contiguous earth is part of the mass
  • 2800 sq ft of concrete floor having no insulation under it, which means again that the earth is part of the mass 
  • 4,500 sq ft of insulated earth under the umbrella extending 16-20' outward from the living space of the house in all directions
So much thermal mass is essential to the AGS system.  It will store heat from passive solar gain in winter only secondarily.  But each BTU stored and radiated from solar gain means one less BTU from the AGS system. 

The long and tall 2 x 4 and gypsum board wall between the living space and the north earth contact concrete wall would seem at first glance to isolate the concrete from solar gain through the windows.  However, for its entire length, the stick-built wall will have continuous openings at its top and bottom to allow air to reach the wall via natural convection. Having said that, it is important to reiterate that the heat from the windows that does reach the concrete wall is welcome but less consequential than the heat emanating from the soil behind and below the wall that was deposited there by the AGS system during the summer.  Consequently, the openings in the framed wall are there more to move cool room air to the concrete wall for warming than to move warm air from the windows to the wall.

The solar gain through the transparent windows on the first story will find its way directly into the floor as is typical with most passive solar systems. Some of it will be reflected/diffused and find its way through the high-low wall openings in the framed wall to reach the concrete wall.  

Second floor layout showing long stick-built wall
between the clerestories and the concrete wall
(click on the image to enlarge it)
The clerestories will comprise more than half of our south-facing glass.  Not only will they be facing the long 2 x 4 wall but they will be 15' above the concrete floor or backed by the second story wood floor -- all low mass scenarios.  For the sun's rays through them to reach the thermal mass in the floor and the back wall, they will have to be diffused by a preponderance of translucent glass and light colors on low-mass surfaces.