The answer is yes, but drywall needs to be removed before you can install the foam. Spray foam expands rapidly when it is applied to the inside of walls. If you try to add the insulation without removing drywall, the expanding foam may overfill the panel and exert pressure against the inside of the plasterboard.

That being said, if you are interested in insulating your attic with spray foam roofing, you must first remove old insulation solution. You might think that leaving it there is a good idea, that it will pair well with the spray foam solution. However, this isn't the case.

The answer is that it depends on the style you use. For closed cell spray foam, you'll want to make sure that your foam is 2-3 inches thick on the walls and 4-5 inches thick on the roof deck. For open-cell foam, the same thickness is needed for the walls, but the roof deck should be sprayed to be 6-10 inches thick.