Answer
Aug 04, 2020 - 08:04 AM
When your siding is off, it is a great time to insulate your walls. You can add cellulose insulation from the outside into your wall cavities through your old siding (or while it's off), and then your new siding won't have any visible holes from the insulation work.
Even if your walls are insulated, you can add better insulation and get better air sealing by adding a layer of rigid foam insulation under your siding. Make sure that you add at least R-5 insulation if your walls are 2x4s and R-7.5 if your walls are 2x6, however, so that you don't create moisture issues in your walls. (R-5 is 1 inch of Polyiso (polyisocyanurate) rigid foam insulation or 1.4 inches of EPS (expanded polystyrene, also marketed as Styrofoam); R-7.5 is 1.5 inches of Polyiso or 2 inches of EPS.) Your wall’s exterior side will match the outside air temperature. The wall’s interior side will be (hopefully) close to your thermostat setting. If the dew point temperature (which reflects the humidity in the air) occurs inside your wall (rather than in the foam insulation), then water will condense out of the air at that point and could create moisture inside the wall. That spells trouble in the long run. If the dew point occurs in the foam, then you're safe from condensation because the moisture can't get into the foam
When your siding contractor installs the new foam layer, make sure he or she tapes all the seams, and you'll increase the insulation of your walls by more than a third while increasing your airtightness significantly
