One of the cornerstone requirements in the LEED for Homes system is quality insulation. Of all the credits and prerequisites, this one and the Good Windows credit are the ones that Dan and I have wrestled with most.
In our region ("Northern"), LEED requires a minimum R-value of 19 for exterior walls. R-value is basically a way to rate how well a wall is insulated. The higher the number the better. The more space you have in your exterior walls, the more insulation you can fit inside. My friend Kevin recently constructed a new LEED house in Shaker Heights where he was able to achive R-40 walls because he built them to be 8 inches thick.
The challenge in existing homes is that you're working, of course, with existing walls. So you're limited in the amount of insulation you can install. In our case, our exterior walls are constructed either of 2x6 studs or true 2x4 studs (we're still trying to confirm which). So at a maximum we have 6 inches of spaces to work with; at a minimum we have 4 inches.
It would be possible to make our walls thicker in one of two ways -- either on the inside or outside of the exterior walls. Today I talked to Brian Schultz of Grand Rapids Community College. Brian has worked on several LEED Homes renovations in Grand Rapids for the local chapter of Habitat for Humanity. On their projects, they work with houses where the existing siding is in very bad shape. So they simply rip it off and install rigid polystyrene panels on the outside of the studs, as is done in conventional new-construction house projects. This gives them a slightly larger exterior area (or building envelope) than they had before, but it's not a problem because they end up having to install new vinyl siding anyway. On the inside of the house, they blow fiberglass insulation into the wall cavities. These two measures combined get them to R-19.
Another approach is to expand the walls from the inside. This allows you to install a greater thickness of insulation. But, of course, you're sacrificing some area from the interior of your house.
Dan and I don't want to take either of these approaches. We have original wood siding in great condition that we want to keep. It looks better than vinyl, and it's much more environmentally friendly (both because we're reusing what's already there and because it's not vinyl, which is pretty much the most evil building product around). We also don't want to give up any interior floor space, because we're already working with small, Victorian-sized rooms.
So we've got to work with what we've already got. Can we get to R-19 in our existing walls?
We think so. After much research, we know we want to use a spray foam insulation. Spray foams are malleable and can get into all the little nooks and crannies in a wall. So they generally provide better R-value than batts or blow-in insulation because they can get into more spaces. They're generally also more air-tight, which is another requirement of LEED.
Different types of spray-in insulation have different R-values. The ones we've been looking at are BioBased; Icynene; polyurethane; and tripolymer. Of these, polyurethane provides the highest R-value per inch (around R-7). But it's also very environmentally unfriendly and off-gasses all kinds of harmful chemicals. BioBased is made out of soy, which sounds appealing, but it's got a lower R-value and I've heard it's appealing to insects, who like to munch on the soy. Also, the closest installer is in Columbus, two hours away. Icynene is quite widely used and is also green -- it's made out of castor oil instead of petroleum.
But Tripolymer may be our best choice. It seems to be relatively green; it's made out of phenolic resin and doesn't off-gas. It also doesn't expand when it's installed, like the other spray foams do. This is good for us because we've got relatively fragile plaster walls that could crack under the pressure of expanding foam. It's also got a higher R-value per inch (5.1) than soy (3.7) or icynene (3.6). We're having an installer come by to give us an estimate next week.