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Vapor Barrier

1016 Views 7 Replies 8 Participants Last post by  ron45
Hi... I am construction a 24X20 two story house.

I want to install a cheap vapor barrier. Can I use cheap 4 mil plastic on the outside over my sheeting and on the inside between the insulation and the sheetrock?

I am in Montana. Cold and dry in the winter. Warm and dry in the summer.


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Why "cheap"??? Why not do it right and use Tyvek on the exterior sheathing as a weather resistive barrier and 4 mil over your interior insulation.
VB is 6 mil and house wrap on the outside.
To wrap a house in my city you can use:
- house wrap like Tyvek or similar,
- 6o minute tar paper, 2 layers.
No plastic.
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4 mil plastic on the outside over my sheeting and on the inside
Never - that's a double vapor barrier and is to be avoided. Housewrap on the outside. Vapor barrier, if used, goes on the warm side of the insulation.
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If you want cheap, use 15# roofing felt on the outside (a lot of us think roofing felt is a better siding underlayment than Tyveck). 4 mil poly will be fine on the inside. DO NOT use poly on the outside...that will just trap moisture and lead to rot.
Save the 4 mil for the dropcloths. If you use a vapor barrier on the inside, use the minimum 6 mil, it costs a whopping $11 more on a 1200sf roll.
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Just my opinion.

I would never put plastic in my walls unless it's smart plastic and knows the difference between heat and air conditioner.

Not to mention if you have a leak.
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