ceiling insulation

[aqua sport]

First off i just finished my roofover like in marks book i have 6 4x8 sheets of 3/4 tuff r foam board left over.Can i use this in my ceiling instead of the batten fibberglass stuff?im going to cover with drywall.if so do i put the 4mil barrier first or the foam then the 4mil barrier?i live in south florida. ur input will be much appreciated.

[Maureen]

Hello, welcome to the forum!

I can't give you a lot of help with your roof because I'm not expert in roof repair at all! I do have a feeling that the tuff r foam may add to condensation to the roof. I'd use the bat insulation instead.

I'm sure someone else will come along with more information soon!

Maureen

[JD]

I agree with Maureen. Personally, I would not use a foam board for ceiling insulation or anywhere where I would be filling cavities. Foam board is great as a layered insulation on flat surfaces. In cavities, it would be very difficult to get a complete seal of coverage of insulation. Small gaps on the edges of the board would funnel air of different temperatures. Air moves like water and will continue to move until there is a balance of mass or temperature. So what might happen is, cold dry air from inside your home would seek out these small gaps in the insulation, concentrating it's effect in that spot. This would promote condensation. This would be the same principle as using the foam insulation gasket on wall outlets and switches.

The vapor barrier is a trickier question because it depends on the area in which you live. I believe in the past, vapor barrier was always installed between the trusses and ceiling panels, at least that is where it is on homes in my area. But I have read studies that say that it should be on the warm side of the wall of roof cavity. Below the roof sheathing in warm climates and against the ceiling in cold climates. Then in the same study, they showed where there are exceptions because of high humidity areas.

I would install the vapor barrier where the factory installed it. Some homes are designed to allow a certain amount of home heat to flow through the "attic" area. Sometimes the vapor barrier could be engineered to work in conjunction with roof/gable vent systems. This is particularly true of the newer cathedral ceiling homes where they are starting to use trusses instead of 2x6 and 2x8's. It might be a good idea to ask you local building inspector. I have found them to be pretty accessible, and return phone calls pretty quickly. Your mileage may vary.

Hope this helps.

JD