Let start with a discussion of how foam reduces the overall heat load of your house and saves you money. Please be aware that parts of our discussion only apply to hot, humid Southern climates, specifically Houston, Texas.
Your attic absorbs heat from the sun, which passes through the roofline and accumulates in the attic. This heat transfers into the living space below and offsets a lot of what your AC accomplishes when it cools the house. Blown fiberglass is very sexy in that you think you're getting a ton of insulation for your money. You're not. Fiberglass simply doesn't do enough to keep that heat from transferring into the living space below. Foam insulation keeps the heat from entering your attic in the first place.
Now, instead of putting fiberglass insulation on the floor of your attic, you spray foam insulation against the underside of the roof decking. It adheres to the roof deck and will never come off on it's own. It takes about 36 hours of direct sunlight for heat to pass through 8 inches of foam. Because we only have about 12 hours of sunlight each day, it's virtually impossible for any significant amount of heat to pass through foam. This means that all of that heat that you usually accumulate in your attic is no longer entering your attic. That heat is no longer transfering down into your living space. The heat penetrates the foam to a certain depth and the rest is just rejected back out of the attic. No this won't void the warranty on any of your roofing materials. Yes, this means you can skip paying extra for roof decking with radiant barrier.
The same concept applies to the walls. Foam insulation resists the heat transfer better than fiberglass or cellulose insulation. This keeps the heat from the sun beating on the walls of the home from penetrating the wall assembly and making the house hotter than it should be. Thus, when you cool your house, it stays cool longer.
Because you're not gaining nearly as much heat through the roofline or the walls as you were, you're reducing the overall heat load of your house. This means it's easier to cool your house. It also means that what your AC cools stays cool longer. This all translates into less wear and tear on your AC system and thus, lower energy bills. Have you ever asked yourself why they insulate igloos and refrigerators with foam and not fiberglass?
This all means you can also reduce the size of your AC system but we'll talk about that some other time.
Have a great day.
-James
March 24, 2009 2:17 PM
Anonymous said...
I am trying to determine if I should install tradional insulation or foam insulation. Does anyone know how much a bungalow ceiling would cost that is approx 500 sq feet? I am in Michigan. What specificly should I get depth wise?
April 8, 2010 5:45 PM
James Davis said...
Anonymous in Michigan,
I recommend foam insulation over traditional (fiberglass?) all day long.
I can only speak to installation costs in the Houston area. 500 SF of roofline/ceiling should cost about $1.75/SF for 5" of open-cell foam, which comes out to $875. I recommend at least 4 or 5 inches in order to keep the heat out of the conditioned space. To be clear, if you're going to insulate the CEILING (attic floor above a room), you can get away with 3 or 4 inches of foam. If you're insulating the underside of the roof deck, you want a little more so the radiant heat from the sun that soaks through the roof deck doesn't penetrate the foam and heat up the attic (thus transferring into the conditioned space).
Hope this helps.
April 13, 2010 9:41 AM
Anonymous said...
Hello do you still do this work? I'm south of Houston. What is the cost per square foot for spray foam on interior wall space? Thanks
September 7, 2010 9:32 AM
James Davis said...
Hello, Anonymous, I don't still do this work, which makes me super-objective. I do have a contractor that I recommend, but I will not post their name on this site. Please email me directly for more information.
December 7, 2010 2:49 PM
