Insulation question

[jirocpa]

Hey all,
I am re-insulating the space between a garage and second floor bedroom. The space is 7" high, so I was going to install r-19 paper-faced fg. As the r-19 is only 5.5" high, I was thinking of first putting a 1" layer of Foamular rigid board against the flooring, then install the fg insulation, then closing it up with 5/8" sheetrock.
My question is whether the foam board will cause vapor issues with the paper backed fiberglass insulation or not. With the foam in place, should I use unfaced insulation?
Thanks for the advice, Jim

[DIY Guy]

Moisture issues are always a serious concern when installing insulation. You are wise to be considering them.

Experts differ in their opinions, and opinions will differ depending on where you live too. I know many pros who would say never use faced paper. Others will tell you it is a must. I heard of an architect who caused over $100k damage to a house by his "expert opinion" on insulating a south facing wall.

If I understand you, you plan to put solid foam panels (unfaced) between the joists in the ceiling between the garage and an upstairs bedroom. After that you will install fiberglass batting.

I am not an expert, so take that into consideration when evaluating my opinion.

The foam insulation will, more or less, act as a vapor barrier. That means that if warm moist air gets under the floor from the upstairs room, it could be trapped between the foam and the floor. On the other hand, the floor is pretty well insulated by the flooring material and so may not be a serious concern.

You could also consider installing two layers of 3.5 inch insulation, one faced and one unfaced. The stacked layers should get you to an R-value around R-26.

Other opinions? Please let us know, we could use the expertise.

[jirocpa]

Thanks for the advice. Just fyi, the floor of the bedroom above is 3/4" t&g subfloor and on top of that is 3/4" oak.
Yes, I was going to seat the foam unfaced board against the subfloor, held in place with a little adhesive.
But I like your idea of dual fg layers - it takes the uncertainty out of it. And gets me to 7" of insulation. Considering I have some old thin insulation there now, R-26 is a huge upgrade.
Thanks again!