I'm planning on replacing the 1" seaweed insulation in my 1950's home with a more modern alternative. The most common (and least expensive?) option seems to be using some R12 pink fibreglass insulation. But I notice that it indicates that it is for 3.5" spaces. I believe that because my house is from the 50's it is using actual 2x4's, so does this mean that the insulation will actually be slightly to small for a retrofit application?
(I realize that it will fit in just fine, but I've been told that the slightest air gap with insulation will destroy the R Value...)