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how to install vapor barrier???

Posted by jaansu (My Page) on
Wed, Mar 17, 10 at 13:02

I will be installing a 6mil poly vapor barrier on my kitchen wall after I put in the insulation. I assume this is best installed by stapling into the 2X4s? How close should the staples be? For a good seal, should the stapled points be caulked as well?
I see that it is recommended to use something called acoustical sealant to seal the poly seams. Never heard of this. Is this and sheathing tape at big box stores? Can someone explain how the sealant is supposed to be used in order to seal at the outlet boxes?
thanks!


Follow-Up Postings:

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RE: how to install vapor barrier???

I've been told Tyvek tape works for sealing polyethylene sheets together but no real luck in finding acoustical sealant. Has anyone had luck with acrylic or other caulks that stay flexible?


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RE: how to install vapor barrier???

I use polyurethane sealant and have the seame fall over a framing member; a stud, a cap or sole plate, for example.

I don't seal staple holes. Your larger concern will be detailing penetrations like junction/outlet boxes.

Only use enough staples to hold the poly in place.


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RE: how to install vapor barrier???

geff is a spammer paid to link to a commercial site.

There is not just one way to install a vapor retarder in a wall assembly. The installation details depend on the location of the project and the design of the rest of the wall assembly. Since we have none of that information there's not much anyone can tell you with certainty.

In general the advice already given here assumes a very northern climate and that the interior vapor retarder is also an air barrier (instead of relying on an exterior air barrier like a plastic housewrap).

In a more moderate climate with an exterior air barrier, such careful sealing the joints and penetrations of the vapor retarder is probably not necessary.

You should design the wall system as a whole rather than designing each part of it and then putting them all together. The latter approach can often lead to poor performance and unnecessary effort and expense.


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conclusion

I found that I could not access the acoustical sealant that was recommended so I decided to forego the polyethylene and just use faced R-21. I sealed the tabs to the studs with many staples, and penetrations as carefully as I could using foilbacked tape, which seemed to be very sticky. I decided it didn't matter too much if the kitchen wall wasn't done perfectly as the entire rest of the house was only poorly installed R-19.


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RE: how to install vapor barrier???

Sheet poly can be bought in a roll roughly the height of your wall and far longer than the dimension of an average room. therefore there shouldn't be many joints in the poly. Such joints, when they occur, are no worse than the top and bottom edges of the sheet so I don't know why anyone would think sealant is necessary at joints and not at the perimeter ... and insisting that it be acoustic sealant borders on being silly; the stuff will be trapped behind the drywall against the stud so anything will do.

Perhaps someone mistook a specification for an under slab vapor retarder for use in a wall and spread it all over the internet. I find that kind of mistake frequently on how-to sites including Vob Billa's shrine to himself.

The disadvantage of faced fiberglass insulation is that it creates a joint in the vapor retarder not only at the perimeter of the wall but full height of the wall every 16" along the wall. Stapling and taping so many joints is a waste of time and money. I have no idea why faced fiberglass insulation is sold in a cold climate but it would be appropriate in a moderate climate. But we don't know where the project is located.

And the fact that the other rooms were done poorly is no reason to do a poor job in the kitchen. Incidentally, there is usually not enough moisture in a house in the winter to require a vapor retarder except in a bathroom or a kitchen so the wall behind the kitchen sink is not only the most important wall to protect from vapor migration it is probably the only wall where that is necessary in any climate.


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RE: how to install vapor barrier???

Alas, the drywallers put up the wall so I will have to live with it now. It's usually my nature to overbuild anyway so I would have preferred the poly barrier. My mistake was not starting my research earlier. The sites I visited seemed to indicate that not any sealer would do. What do I know. And I phoned all over NJ trying to find sheathing tape. Tyvek tape would have done.

Sorry, I should have mentioned before. I live just north of Philly. There are some cold days but rarely below 0 F.

You make a good point about higher humidity in kitchens. But considering the rapid diffusion of air, wouldn't it be more likely any water vapor generated in the kitchen will disipate to the adjoining rooms before it would penetrate the cabinets/tile/drywall?


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