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We are building a new house and are getting ready to install the drywall on the ceilings. We have installed a 6 mil plastic barrier on the underside of the rafters to act as an air & moisture barrier. The barrier will be sandwiched between the drywall and the rafters and under the blow-in insulation. I talked with another insulation guy that said he thinks it might trap moisture in the house and cause problems with the ceilings. He thinks we should tear out the plastic before we install the drywall ceilings.

Does anyone know if the plastic barrier will help or hurt us?

  • There may be some truth to that. There's a reason people use Tyvek v. plastic for a housewrap. I suspect worse than trapping moisture in the house, it could trap moisture against the backing of your drywall, which is a prime way to start mold. I've really only seen plastic used against earth/concrete. – BrownRedHawk Oct 14 '15 at 14:51
  • Related if not a duplicate: diy.stackexchange.com/questions/6011/… – JPhi1618 Oct 14 '15 at 15:01
  • Your guy is right. Remove it. – iLikeDirt Oct 14 '15 at 15:54
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It depends on your local climate.

For climates that are higher humidity inside than outside in summer (a dry climate) plastic on the interior is OK. For climates that are higher humidity outside during the summer (a humid climate) plastic on the interior could be a serious mistake.

Here is a link to a good brief discussion titled, "What happens when you put a plastic vapor barrier in your wall?":

http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/75130/What-Happens-When-You-Put-a-Plastic-Vapor-Barrier-in-Your-Wall

If you have very cold winters in your area without very high summer humidity, then a vapor barrier near the inside will help keep your insulation dry when it is cold outside and your house is relatively warm and humid, otherwise take the plastic out.

  • What about Ohio where it is hot, cold, wet and dry? I guess our only option is to move to Florida :D – BWDesign Oct 14 '15 at 19:25
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the plastic will trap moisture in your sheetrock and can cause black mold some areas require hazmat teams for black mold removal, Ran into this a few years ago and it was quite expensive for the home owner.

  • Thanks for the reply. I'm just wondering if you have any hard evidence to back up your statement. The answer to the related question pointed out above is that the plastic is a good idea (we live in Ohio where it gets very cold for several months). It seems like every other person I talk to has a different opinion about it. – BWDesign Oct 14 '15 at 16:05
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    I understand your conditions I was stationed at wrightpatterson in dayton,,, the Black mold issue was in a 1980 era home in albany Oregon the contractor I was working for was hired to do a large remodel and remove the moldy Sheetrock, several contractors were asked to bid on the job and one reported it , the county inspector posted the house as unsafe until a team came in and removed the sheetrock with black mold, it had plastic above the sheetrockand the areas that were ok we just cut the plastic out this was in ~1999 – Ed Beal Oct 14 '15 at 16:33
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Where I live, Canada, the building code requires a vapour barrier like you have installed. It doesn't lead to mould here, because there is always insulation between the vapour barrier and the outside of the house. Therefore, the barrier is always warm, and there isn't any condensation.

If you don't have a vapour barrier, moisture from your house will condense on the outside of your insulation, reducing its effectiveness, and possibly creating a mould problem in your attic.

I can see that if you live in a very hot, humid climate, and your house is air conditioned, the vapour barrier could create a problem. However, I believe the climate in Ohio is not too much different from the climate in southern Ontario, where these building code requirements apply.

  • Thanks for the advice. My concern is during the summer. In Ohio, it can get up with 100 degrees (Fahrenheit) with 90+% humidity. I talked with a local builder who said in 30 years of construction he has only seen plastic under the attic insulation in one house. I think I will tear it out just to be safe. – BWDesign Oct 15 '15 at 0:42
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There are construction products specifically for this purpose. Unlike the moisture-vapor impermeable polyethylene foil you have installed, they are designed to allow some moisture vapor to diffuse. The diffusion rate is sometimes a function of the temperature.

There are many anecdotal stories of moisture puddles building up on top of PE foil. Some amount of diffusion is desired; you don't want to live in a plastic bubble.

Google vapor barrier vs vapor diffusion. Lots of good building advice.

Looks at products like Isover diffusion foils. I used ProClima Intello Plus in my home renovation. The only place I used a true barrier (PE) was in the bathrooms.

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