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Originally posted by LS1 GTO:

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Originally posted by Skins2881:

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Originally posted by arcwelder:
Whats wrong with that? You can blow cellulose over batts. No reason to take them out.

Every insulation company around here wants to remove the old before adding new. I assumed that meant it was the right way to do it? Maybe they are just trying to pad their pockets?

I found that here in San Diego. From what I gather, and maybe it's a money grubbing thing, by removing the old stuff the company can certify the R-rating they're selling. Otherwise they're just spraying a bunch of insulating atop an unknown rated insulation.

For spray foam, you want it to attach to the wood and such and it seals the air holes. If you spray it on top of loose insulation (cellulose, fiberglass), the other insulation will compress, compromising the firm spray foam. Either there will be voids between it and the old loose or fiberglass batt stuff or it will crack in places. I think spray foam is great on new builds, old houses that already have some insulation, it's best to just blow more fiberglass insulation on top of it IMO.

Here, they spray foam the underneath of the roof sheathing. On older houses one would then have to seal all of the roof vents, and all of the soffitt vents (there are many of these) for it to be most effective. I know someone who did this on a total remodel of a 1960's CBS house, where they kept the original roof/trusses/sheathing, it's a lot of work but they were re-stucco'ing the exterior anyways.