A-frame insulation

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  #1  
Old 07-15-10, 06:37 PM
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A-frame insulation

I am going to put sheets of Owens Corning Foamular 150 2" sheets of rigid foam insulation on the interior of my A frame house (the house currently has no insulation---just the 2x6" boards that form the walls/ceiling, a 1" thick compressed fiberboard of some type (not sure) on the outside of that, and then the shingles. I am going to put the foam sheets directly in contact with the underside of the 2x6 boards and then cover them on the inside with 5x8" thick T1-11 grooved plywood sheets that have been stained. I know this is not an ideal way to insulate a house---but its what I've got. (Everything about this A frame is non-standard.) My question: do I need to put a vapor barrier (i.e, thin plastic sheating) between the plywood and the foamboard --- or is the foamboard enough of a vapor barrier in itself?
 
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Old 07-15-10, 08:03 PM
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2" of pink stuff is considered a vapor diffusion retarder, but is more than adaquate for your purposes. The key is to be sure the installation is air tight, as air will carry a lot more moisture than what could possibly dissuse through the foam board.

If the 2x6 rafters are not spaced 16" or 24" on center, then installing 4x8 sheets of anything will have it's problems. Tape all seams very well to seal.

Also, if the A-frame overhangs the foundation or has a small dead space way down along the edge, air sealing from the outside or between the floor joists under the small knee wall are important.

Here is a link on Vapor Barriers: Energy Savers: Vapor Barriers or Vapor Diffusion Retarders

Bud
 
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Old 07-15-10, 09:13 PM
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Thanks Bud. Actually the 2x6s run horizontally as cladding to form the walls/ceiling. They are held up by a number of pairs of massive angled beams that are the skeleton of the house. I plan on nailing furring strips to the sides of each beam and to screw a 2x2 vertically in the center of each bay. Then the foamboard to fill each bay. Then I'll nail the plywood sheets running horizontally to the vertical strips, going from the floor up to ceiling. I'll put a quarter-round stip at the side edge of each bay's panel to make sure there are no gaps and to make it look neat and clean. There should be no air gaps.

So from what you say I don't need to put a plastic sheeting vapor barrier between the plywood and the foamboard---right?
 
  #4  
Old 07-16-10, 02:55 AM
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If you tape, foam, or air seal well. Quarter round will not air seal, just looks good.

Bud
 
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