Image credit Lloyd Alter, after John Straube
TreeHugger founder Graham Hill is trying to radically reduce his footprint and live happily with less space, less stuff and less waste on less money, but with more design. He calls it "LifeEdited." You can help: Enter the LifeEdited design competition and win up to $70,000 in prizes and the opportunity to design the apartment!
Graham wants to build the greenest apartment in Manhattan, and has said that he wants to "super-insulate" the walls. But can you put in too much insulation? If you have a thick brick wall, the answer is yes. Brick is permeable to moisture, and if you insulate too much, the moisture will freeze in it, destroying the wall. So how much insulation can you put in?
At Green Building Advisor, Dr. Joe Lstiburek says: "it depends."
This is more of a Canadian issue, and a Northern New England issue, and a Minnesota issue than it is a Chicago issue, a Boston issue, a New York City issue, a Philadelphia issue. OK, can I insulate a wall on the inside in Boston that's a mass assembly? I'm going to say, pretty much yes, with a restriction in terms of R-value. Should I have a vapor barrier on the inside? No, you shouldn't. And it would be nice to keep water from being concentrated on the surface of the building. When we start getting into, say, Portland, Maine, I'm going to say you probably shouldn't insulate more than R-10. What's that based on? A gut feeling. I have a really big gut, I've been around a long time, and it's an intuitive, experience-based thing. I've looked a buildings a long time, I know what works and what doesn't based on what I've seen out there.
His partner at Building Science, John Straube, is a bit more specific, suggesting a vapour permeable spray foam and no vapour barrier; moisture has to be able to get out of the wall and into the unit.
The interior finishes must all have high vapor permeance or be back-vented. This retrofit has the advantage that all air leakage condensation is strictly controlled, and it is the most practical approach to achieving high levels of airtightness in existing buildings. The use of spray foam also acts as a moisture barrier, and any small amount of incidental rain penetration will be localized and controlled. Hence, interior finishes will be protected, water will not run down and collect at floor penetrations. Water that is absorbed into the masonry can wick to the outside (where is will evaporate and diffuse into the exterior air) or wick to the inside, where it will diffuse through the semi-permeable spray foam and interior finishes.
Superinsulated wall: 7" closed cell polyurethane foam on 8" masonry. Image credit Lloyd Alter
All of this goes absolutely contrary to what I was taught in architecture school, which is that you want the best possible vapour barrier on the warm side of the wall. It also goes against what a lot of green renovators are doing, which is pack as much insulation as possible into the building. (Have a look at the discussion of the REEP house in Kitchener, Ontario)
Does this mean that an old building can't really go green? No. Last word goes to Joe Lstiburek:
I wouldn't let them touch the walls at all --until after they did the roof, until after they did the foundation, until after they did the windows, until after they did lighting, and the heating, and mechanical systems. And over my dead body, then, maybe then, would I let them do something with the walls.
There is an order to these things, and if you have a masonry building, insulating the masonry walls is pretty far down the list.