Interior ‘vapor barrier’ in Eastern NC
I’d appreciate any help I can get on this one. Just purchased a home in coastal North Carolina. It was built in 1985, when for some strange reason, it was code here to apply a layer of poly to the inside of the stud wall before drywall. The smarter contractors waited until rough inspection was completed and then cut out the poly before drywall was applied. Unfortunately, that didn’t happen often enough. I’ve heard horror story after horror story about 12-30 year old homes rotting from inside out in this area. I was hopeful that my newly purchased house would fall into the ‘cut the poly out’ category, but alas.
I’ll cut to the chase. I’ve pulled baseboard and inspected for water/mold/rot in the wall cavities and so far have found none. It is a very wooded lot, with the house shaded on all sides almost 100 percent of the time. Cedar ship lap siding with true plywood sheathing and roof decking. Previous homeowner ‘sealed’ the crawlspace and installed a fan in the floor of the house to keep crawlspace pressurized (they kept cardboard boxes of books and papers down there with no damage whatsoever. I witnessed this with my own eyes.)
I can’t afford to remove drywall and cut out the poly. However, as a former contractor, I can do any other work necessary in the form of some elbow grease. Should I drill holes in every studbay to puncture the poly and then patch them? Could that do the trick? If so, how many holes and how big? Please advise. Thanks in advance!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Colin,
Did the previous owners have air conditioning?
If the house has been operated for years without any apparent damage to the walls, and if the previous owners used air conditioning, then I see no reason to worry. Life is too short to waste hours and hours solving a problem that hasn't occurred yet.
However, if you are installing air conditioning for the first time, you will be changing the conditions under which the house has been operated, and all bets are off.
Martin,
Yes, there is central air conditioning. I can't be certain of how they used it, but being that this area is designated humid sub-tropical and we are in the mid 90's most of the summer, you'd be hard pressed to find someone who didn't use their air regularly. I too considered the "If it ain't broke, don't fix it" scenario, but figured it would make sense to do as much research and garner as many opinions as I could.
With that said, if I did decide to go ahead and try and get rid of the poly by rigging a way to remove it from the bottom of the wall behind the baseboard or by drilling holes, do you think I could potentially cause other problems? Would drilling say 10 1/4" holes in each studbay effectively negate the affect of the poly or would that be an exercise in futility? Thanks for your response Martin!
Colin,
Q. "If I did decide to go ahead and try and get rid of the poly by rigging a way to remove it from the bottom of the wall behind the baseboard or by drilling holes, do you think I could potentially cause other problems?"
A. Yes, of course you could cause other problems. The main way you could cause other problems would be by adding air leakage holes that didn't formerly exist. If you are conscientious about air sealing, however, it's possible to avoid this danger.
Q. "Would drilling say 10 1/4" holes in each studbay effectively negate the affect of the poly or would that be an exercise in futility?"
A. I would say the latter. Vapor diffusion is directly proportional to the area of the wall that is covered by polyethylene. You are suggesting that you would drill 10 holes, each of which has an area of 0.196 square inch. Each stud bay would end up with holes in the poly measuring 1.96 square inch (if your drill bit managed to pulverize the polyethylene and not just push the poly aside). Since each stud bay measures about 1,348 square inches, you will remove about 0.14% of the polyethylene, leaving 99.86% of the polyethylene intact. That means that the vapor permeance of the poly will be 99.86% of what it used to be before you drilled the holes -- i.e., unchanged.