Rule #1: you can’t stop heat, but you can slow it down. Heat always moves from hot areas to cold areas. In summer, exterior heat will flow toward the cooler interior of a home. In winter, interior heat will flow toward the exterior. The role of insulation is to slow this heat flow. In general, thicker insulation is more effective than thinner insulation. Many energy consultants have proposed the following rule of thumb: The R-value of insulation installed in a green building should be about twice the code minimum. This is, of course, a guide to planning rather than a hard-and-fast rule.
More insulation is better, to a point = Doubling the thickness of a layer of insulation will double the insulation’s R-value, cutting heat loss in half. Each time that the insulation layer is doubled in thickness, this rule applies. But the energy saved per year by doubling insulation from R-10 to R-20 will be considerably more than the energy saved by doubling insulation from R-20 to R-40 because of the law of diminishing returns.
It’s best to insulate outside the box = The most common types of insulation used in residential construction are fiberglass batts, cellulose, spray polyurethane foam, and rigid insulation. Insulation materials are usually sold with a label indicating their R-value. The higher the R-value, the more effective the insulation. To achieve the R-value on the label, however, the insulation must be installed without compression or voids. Although residential wall insulation is traditionally installed in stud cavities, the best place to locate wall insulation is outside of the frame. This reduces the thermal bridging effect that studs have in a wall — each piece of framing is a thermal bridge through the insulation. These thermal bridges seriously degrade the performance of the wall. The thermal bridge effect can be partially addressed by using rigid foam sheathing — usually 1 or 2 inches of XPS or polyisocyanurate. Even better are wall designs that place all of the insulation — 6 to 10 inches of rigid foam — outside of the framing. When insulation is outside of the frame, framing materials stay warm and dry. When stud bays are not filled with insulation, the work of electricians and plumbers is greatly simplified. Houses that have foam sheathing should not include an interior polyethylene vapor retarder.
For more insulation information, see the “Insulation Overview” section of the GBA Encyclopedia, or give Westhill Construction a call today!