Saturday, November 5, 2016

Design - Thermal Bridging and Air Infiltration (Cont'd)

This is the second of two posts on thermal bridging and air infiltration. The first post defined the three ways heat gets transferred -- by conduction, by convection and by radiation -- and what is meant by the building envelope.  Then the post focused on the transfer of heat by conduction through the building envelope. This post covers the other two heat transfer modes -- convection and radiation.

Convective Heat Loss
Air infiltration and exfiltration refer to the heat transferred in and out through the building envelope by air in motion -- convection.  Air infiltration is at it worst when winter winds push air through holes in the envelope, especially on windy days when there is a air pressure differential between the side of the building against which the wind is blowing and the leeward side of the building.  Heat is lost even in the absence of wind, however, as interior air exfiltrates, not only because heat seeks cold, but due to a pressure differential.  In summer, air infiltrates for the same reasons.  So here is my understanding of the best practices for controlling convective heat loss via what has come to be known as air sealing:

  • In the first place, design intentionally and build with precision in order to minimize potential holes in the envelope
  • Then seal all unavoidable holes with gaskets, caulk, tape, spray foam and drywall mud
    • Between parallel and touching structural members, such as double top plates
    • Between mud sills and concrete foundations
    • Between structural members and the sheathing
    • Between windows and doors and their rough openings
    • Between drywall intersections, especially walls meeting ceilings
    • Around penetrations in the envelope for such things as vents, furnace pipes, wiring, coaxial cables, electrical boxes and can lights
    • Within window frames; choose those that close against a sealing gasket instead of sliding -- such as casement, awning or hopper instead of horizontal sliders or single or double hung
    • Within double-glazed window panes by the addition of argon gas which, by being heavier than air, impedes heat-conveying convective currents between the layers of glass
    • Through fireplace doors and dampers
  • Use an airlock between outdoors and living spaces
  • Orient exterior doors away from the prevailing winter winds (in our locale, that would be south and east sides of the building)
  • Use air (and moisture) barriers such as latex-painted drywall interiorly and house wrap exteriorly*
  • Then, use a blower door test to measure the integrity of air sealing and identify leaks to be corrected before insulating and drywalling
Convection and Our Project
Super-insulating the walls and ceilings and going all out to eliminate thermal bridging
would not produce a zero energy, or energy neutral, home unless air infiltration is eliminated as well.  Johnston and Gibson in their book say, "A typical house has 2,000 liner ft. of cracks and gaps that allow air in and out, which can represent up to 50% of the heat loss in a building".

One of the advantages of being a task oriented DIYer instead of time oriented contractor, is that there is no reason not to be precise with construction first then compulsive about hole plugging.  I intend to buy a pneumatic caulk gun because I know how tedious, tiresome and time-consuming the caulking will be.

For at least the first story and as much of the second story as my stash of 3/4" salvaged lumber will allow, I intend to use the 3/4" individual boards for sheathing.  I will install them at a 45 degree angle as was commonly done before sheet goods were available.  The 45 degree orientation has two benefits.  When the cladding is attached over it, a row of fasteners will be spread over multiple boards so as not to cause splitting of a given board. And the diagionalization provides shear strength to the wall. The disadvantage of using 1x lumber is that it is impossible to seal all of the spaces between so many boards.  Consequently, I will use recycled 4 x 8 sheets of Masonite on the wall trusses before nailing on the sheathing boards.  I can then caulk from the inside just as if the wall was sheathed with OSB board. The Masonite will also add a quarter inch of thickness for fastening of the metal cladding. Unlike most of the contemporary man-made sheet goods, Masonite is manufactured with natural binders so that there is no worry about VOCs.

As far as air coming in through exterior doors, we will have a large 8' x 14' airlock so that the semi-conditioned air in the lock will attenuate incoming outside air before an interior door is opened.  The door between the kitchen and the screened patio will not be protected by an airlock so it will see little use during the winter.

And needless to say, the earth sheltered parts of the envelope totally eliminate any chance of convective heat loss -- air doesn't pass through dirt and concrete very well!

We are seeking green building certification by either HERS or NABH Green Building Standards (Timeline - Alternative Certifications to LEED).  Part of the certification process is blower door testing to measure air infiltration.  I think we will be ready.

Aside from stoves and fireplaces in the living space, the principal source of radiant heat is the sun.  The game here is to admit solar gain when you want it and and exclude it when you don't through the following measures for the northern hemisphere:
  • Orient the building for major solar gain through south-facing windows in winter but block the gain during summer with overhangs, deciduous trees and trellises
  • Incorporate well insulated thermal mass into the structure so as to trap and hold any intentional gain
  • Minimize the amount of glazing on the north and west and, to a lesser extent, the east
  • Use low-E glazing so as to slow the loss of solar heat back through the glass
  • Distribute (diffuse) incoming solar radiation with by either using translucent glass, rather than transparent, or light colors where the sun shines (ceilings, floors and walls) unless there is enough thermal mass to absorb the energy without overheating
Radiation and Our Project
We are intentionally avoiding stoves and fireplaces for the sake of better indoor air quality and to eliminate air infiltration/exfiltration via envelope-piercing chimneys.  Our heating (and air conditioning) system will be passive solar by virtue of the AGS system. But, rather than depend on solar gain in the winter, the gain will come from the summer sun.  (For basic information on the AGS system, click on "Timeline - Annualized GeoSolar" under "Featured Post in the column to the left and follow the trail of posts.  Or go to Wikipedia for a more succinct explanation.)  

Our design calls for south-facing glass with overhangs except possibly one east-facing window in the laundry room.  The overhangs will restrict direct solar gain to the cool/cold months (but with too much in the early fall, which will be grist for a future post on the design of the overhangs). We intend to use translucent glass in most of the second story clerestory windows backed up with light colored ceilings and walls so as to diffuse incoming sunlight instead of creating hot spots on the walls and furnishings. Because of
the AGS system, the concrete floor will remain essentially the same temperature year-round and will absorb the diffused energy gradually. Sunshine falling directly on it will be absorbed without hot spots by coloring it with medium, rather than dark, tones.  (My information on color selection and translucent glass comes from "The Passive Solar Energy Book:  A Complete Guide to Passive Solar Home, Greenhouse and Building Design" by Edward Mazria -- a good read for anyone contemplating passive solar construction.)
*As I will discuss in detail in upcoming posts on air barriers and vapor barriers, air control and moisture control are closely related to the extent that to control air is to control moisture as well.