Heat travels in three main ways: it is transported via movement of air or water (convection), it radiates from a hotter surface to an unconnected cooler surface (radiation), and it is directly transferred through solid materials (conduction). In homes, you reduce unwanted convection by controlling air leakage. You reduce unwanted solar radiation by shading, window selection, and using light or reflective colors for your roof and walls. You reduce conduction by installing insulation (which also reduces convection and radiation in wall cavities).
Heat moves by conduction through walls, floors, and ceilings, from the warmer side to the cooler side. In winter, heat moves from conditioned living spaces into neighboring unheated spaces—attics, garages, and basements—and outdoors. In summer, heat moves from outdoors into cooler indoor spaces.
The ability of a material to slow heat transfer is called its thermal resistance, indicated by its R-value. The higher the R-value, the more effective the insulation.
Insulations like fiberglass, cellulose, and rock wool gain most of their thermal resistance by creating small air pockets that make it difficult for air to circulate and move heat by convection. To perform effectively, these air-permeable insulation materials need to be enclosed with a tight air barrier on as many sides as possible to keep air still inside the insulation, and to inhibit air from passing through the insulation. Wall cavities that are enclosed on the top, bottom, and all four sides (as in standard wood-frame construction) can work well with air-permeable insulations. Any air-permeable insulation will be less effective in floors, ceilings, and open wall cavities, where it is not thoroughly enclosed and may be exposed to air movement.
Sprayed-foam or rigid foam insulation gains its thermal resistance from gases trapped within the insulating material; these types of insulation are not permeable to air.
People often confuse insulating a house with air sealing it, but these are not the same thing; you will usually need to do both. Use the air-sealing techniques described in Chapter 8 to stop air leakage. Then use insulation to reduce heat conduction. Only foam-in-place insulation, rigid insulation caulked in place, and dense-pack cellulose insulation allow you to seal and insulate at the same time.
Know Your Thermal Boundary
The walls, ceilings, and floors that enclose your living space make up your home’s thermal boundary—the separation between conditioned (heated and/or cooled) and unconditioned space. Think of this as the boundary within which your conditioned air should be contained. This is where your insulation should be.
Before proceeding, it’s important that you understand where this boundary is in your house. It should be clearly defined, well insulated, and aligned with a continuous air barrier. If the air barrier and thermal boundary are not properly aligned, warm moist air—from inside or outside, depending on the climate and time of year—may condense, leaving water within your roof, walls, or floor. This can produce cosmetic or structural damage and can encourage the growth of mold. Sometimes there is a question about where the thermal boundary should be—for example, if you’re enclosing a porch or considering heating your attic or crawl space. Your home performance assessor or architect should be able to help with this.