Today I’ll talk about an inexpensive way to buffer your attic insulation from the elements and help keep your home comfortable, cool and efficient. This method is a cost-effective alternative if you are close to the point of diminishing returns for attic insulation, where it is not longer cost-effective to add additional insulation. Keep in mind that in the Phoenix area windows are our greatest source of heat gain (~50%), not from the ceiling (~6% and by the way the smallest source of heat gain in homes).
A problem arises when you only need a small amount of insulation to reach your point of diminishing returns, say, 2-inches or R-7 value, and insulation contractors charge by the square foot for 4-inch, 6-inch or 8-inches of additional insulation. Now, it may be a smart idea to put that extra insulation in your attic if your comfort is more important than your ROI, but each case is different. The solution I’d like to share today is insulating with foam boards along the roof truss, or roof ceiling. Next time you visit any of your major home stores, take a walk down their lumber isle and look for large 4’x8’ sheets of foam board. I found foam boards ranging from $13 to $20 in R-values of R=5 to R=6.4. The idea is to use a utility knife and straight edge to cut the foam board to fit between your roof truss (either 16-inch or 24-inch oc). You will also need wire fabricators, or wire insulating holders which are simply small metal rods designed to hold the foam board in-between your roof studs via tension (no tools required).
These boards run from $0.44 to $0.63/ ft2 but have several features that make them cost effective versus the $0.36/ ft2 cost of an insulation contractor or $0.80 – 1.00/ft2 for a radiant barrier. One is their ability to be added to your roof truss rather than the ceiling floor, which is a much more effective way to keep summer’s heat out of your home. The second feature is that these foam boards have a higher insulation value per inch than fiberglass batts or blown-in cellulose. The third reason is that the foam board comes with a reflective backing to act as a radiant barrier for additional heat protection (without the extra costs of a reflective barrier). The last attractive feature of the foam board is for homes with cathedral ceiling and second floor hot rooms -- a DIY homeowner could target specific hot rooms by adding foam board to only certain areas of their roof for a small initial cost. For these reasons I would make the argument that foam board insulation is more cost effective and the in certain cases a recommended way to go green. Feel free to leave comments and questions and expect more from your energy audit!