Foam Those Walls!
It is time to replace fiberglass insulation in wall cavities. We have known for years that if you put fiberglass in a stud wall and let air blow on it, the effectiveness of the insulation value will evaporate. The only way it works is to trap air because the trapped air is the real insulator.
Research now confirms that even inside walls closed and not subject to direct air current, there is air movement within the wall. A convective air current is set up when there is a cold outer wall and warm inner one (sort of like your house in the winter). The air against the outer wall cools and cool air drops. The air against the inner warm wall rises. The more tightly the insulation in the wall is, the slower the movement, but move it does. This process cools your living space. You make up this loss by burning stuff to make it warm again.
It’s time to stop the chilling of our indoor space. OK for you purists, we know the warm inner temps will eventually migrate to the cold (you know physics and all) but the goal is to slow the process to a crawl. Two types of products come to mind: cellulose blow-in systems and foams. I can’t comment on the cellulose systems but I have definite opinions on the foams.
Don’s Two Step process. One: put as much foam in the wall until you reach the limit of diminishing returns. Then cover the framing members because the foam only goes into the spaces between the framing. Two types of foams: closed cell and open cell. If you want to get technical, Google the two terms but here is the short version. Use closed cell foam in walls and basements and open cell foams to insulated roof assemblies. The open cell foam (Icynene and cousins) lets water and water vapor pass through so in a wall, it requires a vapor barrier. It also is very wasteful. It expands so much the installer has to cut off half of it and toss the material in the dumpster. Bad Idea.
The closed cell foams repel moisture and won’t let it through. This is good for walls and basements. The closed cell product will stick to the concrete wall and insulate and moisture proof the area. It also adds strength to the structure since it adheres to the framing members. Don’t use it for roof assemblies since if you have a water leak, you will never see it until the roof starts rotting (bad karma). I have found a soy based closed cell foam with R values of a little less than 7 per inch. 3 inches = R19 or so. 4”=R26. This stuff is not cheap. For 3” it can be 6 times the price of fiberglass but it is worth it because the R19 fiberglass only delivers this R value in the laboratory. They do make a ½” foam that they line the cavity with and then install fiberglass which is OK if you just can’t afford more. For my money, I’d skip the hot tub and go with the 3” of foam.
The next step in my ideal wall is the framing. If you build a 2”x6” stud wall, the insulation in the cavity is R19 but the stud is R6.9. It’s easy to see where the weak spots are. You need to do more (unless you own Exxon/Mobil). My choice is 1” of rigid extruded foam over the entire exterior of the building. This is like putting a coat on your home. It adds about R5. Now we have a R25 wall assembly. There is no rocket science here. You have to make the up-front money investment. You will actually get this money back and then go on saving for the life of the home.
Even with an existing home, don’t even consider new siding or major remodeling without addressing the building envelope. I recently visited a potential customer who wanted an addition. They had a beautiful 1939 vintage home. Some contractor sold them a siding job and did a nice job. The problem was, the home had zero insulation in the walls. Covering those walls without insulating them seems just criminal. More on retro fits next time. If anyone has experience with blown cellulose, let’s hear from you.
Tags: Foam Insulation