Wall Insulation and AC Performance — What Homeowners Often Miss
While the attic gets most of the attention, your home’s exterior walls represent a substantial surface area through which heat enters during summer. Properly insulated walls work in concert with your AC to maintain stable indoor temperatures and reduce cooling loads throughout the day.
How Much Heat Enters Through Walls?
In a typical home, walls account for roughly 15–20% of total heat gain in summer. While that’s less than the attic, it’s significant — especially in homes with large exterior wall areas, minimal window shading, or walls that face west and receive intense afternoon sun.
Types of Wall Insulation
Batt insulation (fiberglass or mineral wool): The most common type, installed between studs in new construction or during major renovations. R-13 to R-21 depending on stud depth.
Blown-in insulation: Can be added to existing walls through small holes drilled in the exterior or interior, then patched. Ideal for retrofitting older homes.
Spray foam: Provides both insulation and air sealing in a single application. More expensive but highly effective.
Rigid foam board: Often added to the exterior of homes during re-siding projects, providing continuous insulation that eliminates thermal bridging through studs.
Thermal Bridging — The Hidden Weak Spot
Even perfectly insulated stud bays have a thermal bridging problem: the wood studs themselves conduct heat far better than the insulation around them. In a standard 2×4 wall with R-13 insulation, the studs reduce the whole-wall R-value to closer to R-10. Adding a layer of exterior rigid foam board breaks this thermal bridge and significantly improves real-world performance.
When Wall Insulation Makes Sense
Retrofitting wall insulation in an existing home is most cost-effective when the exterior is already being replaced (new siding) or during a major interior renovation. If walls are otherwise in good condition, the cost of opening them up just for insulation may not deliver a quick payback — though it does improve comfort noticeably.