My new home has 2 bedrooms and a bathroom over the garage. Per the builder, there is a R30 insulation (so I guess about 9" batt, but haven't drilled in yet to check). This is clearly not working as the temp diff can get up to 10 F from the rest of the house if the heat doesn't need to blow in the rest of the upstairs.
I called a company that does both injection and spray foam. Since the walls are likely only insulated at R13 (under code) in those room, I thought the bigger bang would be adding injection foam in the walls. They suggested I would do better with spray foam on the ceiling of the garage. My garage has finished walls and a finished ceiling. They said that I would need to have the drywall ripped down, then they would spray foam, then I would need to put up new drywall.
One company on the net who provides similar service said this on their blog: "If there is drywall on the ceiling, then injection foam would be used. "
I am wondering why the company I am talking to would want me to rip everything down rather then inject? Curious to know if this would be better or cheaper or other pro/cons? Is this because its likely a 9" gap and the injection foam would sit on the garage ceiling and not on the subfloor of the rooms above? I imagine 9" of injection foam would be overkill and waste of money... Could they spray foam by just making a bunch of holes - or would that just be ridiculous?
They said I would see a difference with just 2" of closed cell spray foam versus the R30 batt. 2" spray foam is about R-12. From what they said and I researched, the R-value is lower but its a full air barrier which compensates. They said 2" would be 94% efficient, going to 3" would be 96% - not sure what they mean by efficient in this case)
At the end of the day, is 2" or even 3" of closed cell the way to go? Would you still then fill the rest of the void with the left over batt?
I've read lots of good things about spray foam for the garage ceiling with habitable rooms above, but I want to make sure I am going to get better results with ROI /payback period being secondary concern. I don't want to find out that it won't make a big difference or that I should have focused on the walls or something else?
Other thoughts to what I should be considering while making the decision on how to proceed?