monticellohomes

I am wondering the best way to insulate this vault. I am working on a home with a vault in the living room. To make the span and carry the load we are having to use 2x10 that will double as the rafter and ceiling joist. On the rest of the home (flat ceilings) we are going to use 1" of open cell foam and then place cellulose on top of that to the thickness of R50. I am having a time to please the HO with the process on the vault. Due to the small cavity I don't feel the foam installers will be able to get the "skim coat" on good so I had thought of building down a false cavity or use air baffles and fill the cavity....but the HO is worried the R factor will not be the same and in his defense per the manf. specs it would be less R factor.

Does anyone have any ideas or suggestions?

Jason

Kgmz

What is the code there for roof/ceiling insulation? R38 or R30

Around here it is R38, and if you use 2x12 rafters you can get R38 fiberglass batts that will leave one inch of air space.

If your code/specs are R30 then they also make R30 batts for 2x10's that will leave a one inch air space.

But you are required to have R38, you should have used 2x12's.

Other things you can do is go to a closed roof sytem and have them spray it with enough foam to get whatever R rating that is required. Or build out a 1" air space with 3/8" plywood and then have it filled with spray foam if you want the air space.

monticellohomes

Code around here would call for R30, this one is in a county where they casually check setbacks and then you never see them again. But what you suggested with the air space is what I have suggested to the HO and they seem worried about how much R factor we will have and will be be equivalent to the R factor on the remainder of the ceiling. I believe it will, I think the actual R factor of the foam is misleading so it opens room for doubt. And when we were meeting with the HEERS rater and we mentioned we'd have a full foam deck they said the R factor wasn't high enough but the R equivilant was near or over R50. So I kind of have a catch 22 where even if it does perform at or higher I've got the be able to show that to the rater so the home still meets EnergyStar.

sawyerEd

I have to respectfully disagree with Kgmz. Ceiling joists should be sized according to code requirements for the expected live and dead loads as you have done. If you need to improve thermal efficiency your idea of providing another layer is much better. Back side of glass batts need to be allowed to breath so air space, reflective layers, and capillary breaks correctly placed into the envelope will produce much better results than simply loading in more glass fiber. My humble opinion.

Kgmz

I was comparing his installed 2x10's to using a 2x12's to get the required insulation space.

So I think the 2x12's would exceed his design with 2x10's.

I see this problem a lot around here. 2x10's will meet the engineering, but then no one thinks about the insulation and air space. So it is a lot cheaper and easier to use 2x12's in the first place, than trying to retrofit something.

joasis

And the next item to get rid of is fiberglass anything and call it insulation.

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