I am recieving some 3" Foamular 400 free. I am curionus if I can put the sheets in my attic as added insulation?
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Tough question...and probably depends upon your local.
If you're located where winter is sustained and very cold, then placing a vapor barrier over the top of the insulation in the attic wouldn't be advisable. (XPS foam insulation is a vapor barrier) If a vapor barrier was placed at all in a ceiling application, it would normally be placed under the insulation and not over it....in a "heating zone".
However....since you'd be placing 3" of foam insulation in addition to the VB effect, then the game changes. The added R-value changes the location of the potential dew point. IOW, depending upon the potential temperature differential (between the heated zone and the attic).....there might not be a dew point at all inside the existing insulation (whether cellulose or FG batts or whatever).
So.....the bottom line is......it depends. If you're in a "winter zone", but your lowest temps are 30F rather than -25F......the delta-t is very different and so is the potential location of a dew point inside the insulation.
If you're in the deep south or similar (primarily a cooling zone).........where VBs are placed on the opposite side (outside) of the insulation.........I'd say go for it. Should be just fine.
If you know that there is a VB in place under the existing insulation already.......then whether in a heating zone or cooling zone........I wouldn't advise adding the foam as the additional insulation. That would create two vapor barriers and any moisture that made its way between the two couldn't readily escape/dry-out to either side.
I am in Atlanta GA, and the only thing between the attic and living space is sheetrock. There is about 6" of blown in inbetween rafters. Traping moisture and fire hazard are my main concerns. I was just going to lay the sheets perpendicular to the rafters throughout. I hate to waste things, and if it helps control temp great.
Okay, sounds like you can do this.
However, I'll suggest that you don't just lay those sheets loosely across the joists. I'd recommend that you make sure each sheet is tightly sealed onto the top of its joists (maybe use a bead of caulk thats compatible with foam insulation and bed the sheet in that)... and that you tape the seams between the sheets with an appropriate tape made just for foam insulation sheeting.
You'll also need to make some type of provision at the perimeter of the attic or wherever becomes the perimeter of your insulation.... (don't know if you have enough to cover the whole attic).......so that air cannot enter freely between the joists (into the joist bays). You might end up nailing some blocking between the joists at the perimeter edges to accomplish this ...or use pieces of the foam itself, cut to wedge tightly between the joists. Or cut those pieces so the fit is loose and then use a bead of some canned foam (like Great Stuff) to create the actual seal.
If you don't do the careful install of the foam sheeting, I fear that much water vapor from your hot humid air down there in Atlanta will make it's way under the foam.(I presume your attic is vented to the out-of-doors) There the water vapor will contact the cooler temps that will be present from ACing the living space below. Condensation is bound to occur. The less condensation that forms in there, the better. Hence the recommendation to seal those sheets tightly to prevent the entrance of that hot humid air.
Thanks for info. I am going to do all I can reach, ranch house. I think I'll cut pieces of the foam and wedge and lay the foam on top and seal it all up with caulk and tape. Thanks again for advice.