Closed cell spray foam is the number one product you should be using when insulating the underside of your home. In the picture you can see the foam is sprayed directly into the floor joint cavities by a spray foam installer. Once the foam is sprayed in place it will expand, dry and finally harden to the underside of the home. Once the foam has cured, usually over the course of 24 hours it will be there permanently. Unlike older traditional insulation, spray foam will not shrink, sag, fall out or ever loose its r rating. It’s a permanent solution for insulating the underside of your home.