Poly vapor barrier and venting
Jlooney
| Posted in General Questions on
Please see the following attachment.
I have a room that has what you would call I guess an “attic space”. I removed the bottom chords of the roof and moved them up higher to open up the ceiling.
The house was spray foamed on walls and roof deck. In order to make sure moisture stays within the house and doens’t go into the airspace between drywall and sprayfoam to condensate, should I poly sheet the entire room and would this be beneficial?
Also, in the space above the chords holding the ceiling rafters together, the “attic space” (see pic) I’m no so sure on the venting.
1. Should I allow the pre-existing gables vents to be opened (sprayfoamers covered them).
2. Should I allow for ridge vents to be open?
3. Should I poliy sheet the entire from inside to keep moisture from moving up and dripping down the sprayfoam and building up?
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Replies
If the attic space is now insulated at the roof deck and walls, you don't need or want the gable vents or ridge vents. The attic has become "conditioned space", that is, inside the pressure and thermal boundary of the house.
Polyethylene sheeting is true vapor barrier, and a double-edged sword. It prevents conditioned space moisture from migrating out to the roof deck (or foam), but prevents drying toward the interior. You may or may not need some sort of vapor RETARDER, but a vapor BARRIER could create problems.
To figure out the best approach going forward there needs to be more information, such as:
Where is this house located? (By US DOE climate zone.)
How much foam, and of what type? (Open cell? Closed cell?)
What type of siding?
It’s a stone house. It has about 3 inches sprayed. We are right on the 4-5 line zone. Probably zone 4 (since world temps increasing)
A simple rule is if you use AC you shouldn't use interior poly.
Measure and maintain your interior relative humidity no higher than 35% in the winter and not greater than 50% in the summer; that will help reduce vapor drive into your building assemblies.
And even though you have insulated with an air barrier material, get your building blower door-tested; way more air and moisture make their way into building assemblies by way of air leakage than vapor diffusion.
Peter