Insulation, Radiant and Vapor Barriers - Insulation behind backerboard/Durock in a bathroom...

Hi,

I just finished ripping out my bathroom completely, I went to the studs. All interior walls are traditional 2x4 framed, however on a 5 foot run on the exterior wall (where the tub runs) they had used 2x4s running the opposite way - so what is jutting out into the room is 1 1/2". This was done to conserve space I'm sure, I'd like to keep it as losing any more space would mean some major plumbing work.

The area is going to be tiled, behind the tile (on the 2x4s) will be backerboard, Durock or equivelant. The rest of the room will be insulated with R-13, but that won't work here unless I compress the insulation which I understand is never a good idea. I was thinking of using the rigid foam type of insulation - cutting it to fit. The home is brick on cinder block, I imagine that provides some decent R value but while it's open, I'd like to do more.

So does this sound like the right approach? I can get this rigid foam in different depths, would you recommend going to the full 1 1/2 inches or leave a bit of a gap between the insulation & backerboard?

Thanks!
Bob

You do not have to leave the air gap between the masonry wall and the insulation on an above grade masonry wall. If you can get rigid board to fit this area, then go ahead and fill it. Because it is a bathroom, I would strongly recommend covering the insulation and studs with at least 4 mil plastic, then installing the Durarock.

Great! I'll fill it in with rigid foam and then cover w/ plastic. Thanks for the suggestion!

Bob

By the way, I was thinking I'd wrap the tub area with plastic around the tub and staple the plastic to the studs. Is this OIK? Also, could I use duct tape for overlapping seams? And finally (as if), should the plastic be behind the tub or should I try to have a little over the lip or flange of the tub?

Thanks!
Bob

Vapor barriers only get applied to insulation. Installing plastic in areas where there is no insulation in a high humidity area will cause a moisture problem. The reason for this is fairly simple. Insulation slows down heat flow but does not slow down moisture flow. Vapor barriers slows down moisture flow but does not slow down heat flow. This explicitly implies there is a constant flow of heat and moisture through a thermal boundary, regardless of the material used in the thermal boundary. The insulation with the vapor barrier induces a balance, which is the best way to describe nature. It is when we do things that induce inbalances is when we run into problems.

If you are using the cement backer board then using plastic between it and the interior wall studs is overkill for protection. I presume its just in case there is a leak, in which case the water will travel down the plastic and collect on the floor under the tub

If you are going to do this then you might as well use the red Tuck tape (or equiv) where the plastic overlaps.

On the exterior wall (where the tub runs) is a somewhat different story. In most cases code calls for an interior vapor barrier (6 mil poly) in which case this would run from the floor to the ceiling.