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Surlycraftsman 11-27-2009 12:31 AM

Multiple vapor retarders
 
Hi all-

Question: Should I remove the interior layer of tar paper on my house. I'm concerned with trapping moisture inside the walls. Given the house 'breathed' fairly well it didn't seem to present any previous issues, but the goal is to stop allot of the air infiltration and removing the interior layer seems like common sense. I did notice that above most of the windows the paper was de-laminating from the plaster brown coat and that there was no interior tar paper above the windows.Thoughts.

Details and Existing Construction.
1907 home in climate zone 7 (Tacoma, WA). 900sqft. Stick built with dimensional and rough cut cedar throughout, with the exception of two room additions. Lath and Plaster in orignal part of the house. CMU crawlspace foundation. Exterior finish consists of 3/4 ship lap with a layer of asphalt impregnated paper on the interior side and 30lb paper on the exterior side covered with a single course of raked ceder shingles set at the max 12" drop spacing.

Proposed:
Use R-15 non craft faced batts with a 4 or 6mil vapor barrier on the walls and coat wallboard with non PVA primer. Use blow in in the attic with a course of fiberglass batt over that and cover the ceiling wall board with a PVA primer.

Heat will be via electric hydronic baseboard heaters. I had a new roof installed last year with ridge vents and have since drilled out the bird blocks to add roughly 9sqft of free air space of low end venting. I may use an existing gas vent stack that is being eliminated to run a small exhaust fan to ensure enough air removal from attic space.

Thanks!

Scuba_Dave 11-27-2009 09:37 AM

You are only supposed to have one vapor barrier/retarder
Otherwise moisture can be trapped between 2 layers

MJW 11-27-2009 03:57 PM

Tar paper is water repellant, not to be confused with waterproof or a moisture barrier or a vapor barrier.

Do a google search for 15lb felt or 30 lb felt. You will find out what it really does and it's uses.

Poly should be on the inside and felt or house wrap on the outside.

Gary in WA 11-27-2009 07:31 PM

For our Marine climate, don't use poly inside. http://www.google.com/imgres?imgurl=...ed=0CAoQ9QEwAA

Felt paper has a permeability rating of 5 when dry and 60, wet. Asphalt coated faced insulation has a rating of less than 1. You shouldn't use either on the inside of the wall construction.

"I did notice that above most of the windows the paper was de-laminating from the plaster brown coat and that there was no interior tar paper above the windows." ------ please explain.......

Be safe, Gary in Tacoma


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