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Insulating a stone house is an expensive and involved process because it requires building a double-walled house. The walls don’t need to be the same thickness, but they both need to be thick enough to handle any structural demands and meet fire and safety standards. The insulation (usually foam or fiberglass) is then installed between the two stone walls.

You don’t need to remove old insulation before adding new insulation. As long as the old insulation is still in good shape—not wet, physically damaged, or moldy—then you can stack new insulation on top of old insulation to get a compounding effect. You can also mix different types of insulation, like adding blown-in insulation around existing fiberglass batt insulation or installing rigid insulation boards over rafters in your attic that already have batt insulation installed. If your existing insulation is very old or damaged, though, you’ll likely see more benefit from removing it and replacing it instead.

Avoid sleeping in a house immediately after spray foam insulation is applied due to the off-gassing of volatile organic compounds (VOCs). These fumes can cause respiratory irritation, headaches, and other health issues. It’s recommended to wait 24 to 72 hours after installation to allow the foam to fully cure and the fumes to dissipate. The exact time frame depends on the type of spray foam used and the ventilation during the application, but your installation pro can advise you on the best timeline. 

Polyisocyanurate foam, or polyiso for short, has the highest R-value per square inch: 6.5 to 6.8. Polyiso is a rigid foam that comes with a reflective foil on both sides to serve as a radiant barrier. Given its high R-value, rigid foam is usually meant for foundations and basement walls.