The Envelope and Green Building
The skin of a building
comprises the walls, roof or ceiling, floor, windows and doors collectively
known as the "envelope". Green
building is largely a matter of keeping heat from entering or exiting the building through its envelope and doing so with minimal impact on finite resources.
Heat Transfer
Heat is transferred in three ways:
- Conduction
- through solid objects, called "thermal bridging" when it is
applied to green building
- Convection
- through fluid motion (air is a fluid), called "air infiltration"
in green building parlance
- Radiation
- heat transfer in a straight line through space such as sunlight passing through a window and warming
a floor
R-factor
Insulation is just another solid object through which heat
passes. The better the insulation,
the slower
heat passes through it. In fact, the
"R" in "R-factor" means "resistance" to the
passage of conductive heat and is usually expressed either in terms of R-factor per
inch or R-factor for the entire thickness.
For example, 2" thick solid foam is labeled R-10 which makes it is R-5
per inch while batt fiberglass insulation for 2 x 6 walls is R-19 or R-3.5 per
inch.
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| Comparison of fiberglass batts vs. blown-in cellulose |
Air Infiltration
Even though it is not its primary function, insulation can also be a barrier for air infiltration. Blown-in foam insulation totally stops air leakage. Cellulose can be packed so densely that little or no air passes through it. Loose fiberglass (not batts) can be similarly densely-packed. The low-tech rice hulls that we will be using for insulation in a 15" wall also makes air leakage relatively moot, although we still plan to do all we can with caulk and spray foam to stop leakage. Fiberglass batts, on the other hand, are difficult to fit into the space between studs precisely enough to stop air. And it is even more difficult to do so around such obstacles as electrical and plumbing components. For dense-packed fiberglass and cellulose, for spray foam and for rice hulls, it is inappropriate to base the R-factor on resistance to conductive heat loss alone. All of these insulations, ramp up whole wall R-value by eliminating air infiltration either nearly or totally.
Even though it is not its primary function, insulation can also be a barrier for air infiltration. Blown-in foam insulation totally stops air leakage. Cellulose can be packed so densely that little or no air passes through it. Loose fiberglass (not batts) can be similarly densely-packed. The low-tech rice hulls that we will be using for insulation in a 15" wall also makes air leakage relatively moot, although we still plan to do all we can with caulk and spray foam to stop leakage. Fiberglass batts, on the other hand, are difficult to fit into the space between studs precisely enough to stop air. And it is even more difficult to do so around such obstacles as electrical and plumbing components. For dense-packed fiberglass and cellulose, for spray foam and for rice hulls, it is inappropriate to base the R-factor on resistance to conductive heat loss alone. All of these insulations, ramp up whole wall R-value by eliminating air infiltration either nearly or totally.
Whole Wall R-values
The effectiveness of an insulated wall is compromised when
structural members, like studs and headers, penetrate completely through the
wall. They serve as bridges for heat transfer (hence "thermal bridging") that degrades the R-factor for the whole wall. (Have you ever noticed how snow melts faster over cathedral ceiling rafters than over the insulated spaces between them?) Therefore, the whole wall R-value that takes into consideration thermal bridging is an important concept. A 2 x
6 wall on 24" centers, for example, goes from the R-19 stamped on fiberglass batts to
R-13.69 due to heat loss through the studs, a 27% drop. A 2 x 4 wall on 16" centers experiences the same 21% drop, going from R-13 to R-10.* To brag a little, the super-insulated home we are building not only widens the space for insulation to 15", but it also uses truss walls instead of stud walls to minimize thermal bridging. Altogether we expect a whole wall R-value of over 50 which will be necessary for sole dependence on passive solar heating and air conditioning.
Air Sealing is the Secret
The recommended minimum for wall insulation in our climate
zone is R-18. Not even 2 x 6 walls
insulated with fiberglass batts with a whole house value of R-13.69 meets the
minimum, much less the ubiquitous 2 x 4 walls. However,
if the envelope is thoroughly sealed against air infiltration, the house is significantly more comfortable than a drafty house with the same amount of
insulation. Conversely, a house with serious air infiltration might be impossible to make comfortable
with any amount of insulation.
Air sealing is finally getting its due. Building codes for new construction now require that all potential air leaks be caulked or spray-foamed before closing up the walls and ceilings. And blower-door testing can be added as a way of measuring the thoroughness of air sealing. The reward for doing the right things for sustainability -- sealing and testing -- nets a speedy return on investment through lower energy costs.
Replacement Windows
The energy benefit, from replacement windows derives not from upgrading the glass itself from double pane to single pane, considering that single pane is R-0.85 and double pane is R-1.5 - 2.0 -- nothing to write home about. The benefit from window replacement is the opportunity it provides for sealing air leakage through and around the window openings, especially the leaky compartments on either side of a window that previously housed the window weights. Replacement of metal framed windows has the added benefit of significantly reducing conductive heat loss.
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* These whole wall figures were used on page 12 by Michael Morley in his book,
"Building with Structural Insulated Panel (SIPS)" while comparing SIPs, with their minimal thermal bridging, with conventional stud wall construction.Air sealing is finally getting its due. Building codes for new construction now require that all potential air leaks be caulked or spray-foamed before closing up the walls and ceilings. And blower-door testing can be added as a way of measuring the thoroughness of air sealing. The reward for doing the right things for sustainability -- sealing and testing -- nets a speedy return on investment through lower energy costs.
Replacement Windows
The energy benefit, from replacement windows derives not from upgrading the glass itself from double pane to single pane, considering that single pane is R-0.85 and double pane is R-1.5 - 2.0 -- nothing to write home about. The benefit from window replacement is the opportunity it provides for sealing air leakage through and around the window openings, especially the leaky compartments on either side of a window that previously housed the window weights. Replacement of metal framed windows has the added benefit of significantly reducing conductive heat loss.
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