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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?

BWDesign
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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.

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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.

Ed Beal
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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.

Tyson Hilmer
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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.

user19474
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