Adding foam insulation between two layers of drywall without tearing out the existing drywall can be a bit tricky, but it is possible using a few different methods. Keep in mind that these methods may not be as effective as traditional insulation between studs, but they can still provide some additional insulation value.
Inject Foam Insulation: One approach is to inject foam insulation into the wall cavity. There are expanding foam products designed specifically for retrofitting existing walls. To do this, you will need to create small holes in the existing drywall, either from the top or bottom of the wall, and inject the foam into the cavity. Once the foam expands and fills the space, you can patch up the holes in the drywall.
Foam Board Adhesive: Another method is to use foam board adhesive to attach rigid foam insulation boards to the existing drywall. This will create an extra insulating layer. You can cut the foam boards to the appropriate size and adhere them directly to the drywall surface. Afterward, you can cover the foam boards with a new layer of drywall or simply repaint the surface.
Blow-In Insulation: Blow-in insulation can be used if you have access to a small gap between the two layers of drywall, such as around electrical outlets or switch boxes. Specialized blow-in insulation machines can fill the gap with loose insulation material like cellulose or fiberglass.
It's important to note that these methods may not be as effective as installing traditional insulation between studs, but they can still provide some level of additional insulation. Additionally, be cautious about potential fire hazards and ensure that the insulation materials you use are safe for this application.
Before attempting any of these methods, consider consulting with a professional to assess your specific situation and ensure that the chosen insulation materials and techniques are appropriate for your needs.