Plastic over Insulation?

[msmity29]

Thanks for everyone's help so far! This site and its member have been great! I have another question....our new home is being insulated today and I am wondering if I should add plastic over the top of it. As it now stands, from exterior to interior, the walls consist of: vinyl siding or stone, 7/16"OSB, 2x6s filled with R19 insulation with a kraft backing, drywall. Would it be beneficial to cover the insulation-filled 2x6s with sheets of plastic? If so, should I just cut out windows, outlets etc? Any other techniques etc I need to know about? Thanks in advance?

[CThomp]

This is a very bad idea. Plastic will hold moisture in your walls and then you will mold.

Why are you wanting to do this?

If your looking for even more of a moisture barrier you could run tyvek over your insulation on the inside of your walls.

This is unnessecary though because the kraft face on your r-19 already acts as a vapor barrier not to mention the house already being rapped.

[msmity29]

I thought I read it somewhere but I must have been mistaken.

[CThomp]

Don't sweat it. What you have now is adequate.

[David]

Here is a valuable source of info:
http://www.buildingscience.com/designsthatwork/default.htm

Here is the pdf for your climate:
http://www.buildingscience.com/designsthatwork/cold/DTW_Cold.pdf

As for plastic, I am not sure which kind to use. I have read the same thing from Joe Libstrek (sp?). A plastic membrane will not condense in winter because it's on the cold side. In summer, though, I think there is a chance, but it's minimal.

However, my suggestion would be to forget the plastic and learn about airtight drywall approach (ADA). The most basic method is to caulk the seams of exterrior walls (top, bottom, and sides) underneath the drywall.

[CThomp]

Yeah the ADA is the way to go. Just seal the inside shell and you're rockin and rollin.

Almost all of my seams are sealed with some variety of trim and the gaps smoothed over with caulking and then painted. It makes for a good continuous look.

[David]

You'd best caulk the drywall if you have stain-grade trim. On a production home, I don't think you'll get stain-grade.

CThomp - Agreed. The continuous look can look very nice. But if not done right, it looks very sloppy.

And don't forget to practice your smilies ;) .

[stuccoman]

Moisture barrier on the out side and plastic on the inside spells terrarium! If thats how it is spelled. Rot city. In my neck of the woods condensation will ruin your house.

[David]

Smity - I looked in my Builders Guide book by Lstiburek. Polyethylene air barrier is for very cold climates in non-air conditioned buildings. Hence, don't use the plastic!

[CThomp]

It can definitley get sloppy. Don't ever just shoot the caulking out of the tube. Thats where it gets sloppy. Put it on your finger and smear it in. Lots of wiping and smearing. Tedious stuff but it pays off in the end :) :) :)

[msmity29]

The insulators came yesterday. On some places they installed insualtion with kraft paper. In other places they installed unbacked insulation with plastic stretched over the top. Does it really make a difference?

[Cole]

No, both act as a vapor barrier.

[msmity29]

Some of the kraft paper areas are ripped and don't seem to be sealed very well. It would not be good to stretch plastic over the insualtion (plastic against kraft paper), correct?

[David]

Hmm. What are these "other places?"

They do both act as a vapor barrier, however the plastic acts as an air barrier, too, while the kraft paper does not. Lstiburek was very clear in his book not to use plastic unless in non-airconditioned structures.

[stuccoman]

No plastic!

[David]

Some of the kraft paper areas are ripped and don't seem to be sealed very well. It would not be good to stretch plastic over the insualtion (plastic against kraft paper), correct?

I like the attention you are paying to details. If only the subs were as diligent.

In small areas, a little plastic is not going to hurt. However, if it is just ripped, you may be able to tape it, no?

[CThomp]

Or just cut a piece of tyvek and staple it to the studs over that area. Staple is from floor to sealing on the studs to seal in that area. Thats what i'm going to do

[Cole]

Guys the plastic he is probably talking about is probably VisQueen which has been used forever.

[stuccoman]

Yes and it is not good. In a heating and cooling climate. It traps moisture in the walls.

[CThomp]

But wouldn't visqueen hold moisture in the walls? That stuff doesn't exactly breathe does it?

[msmity29]

The "other places" are various. It seems like some rooms they used kraft paper and others plastic. It's very odd. In areas where the paper doesn't reach the bottom or top of the wall 2x6 and in areas where you can see gaps between the wall and the kraft paper, what should I do? And what kind of tape can I use to repair rips?

[CThomp]

I'm just used to Florida where everything molds and rots.

[David]

Small rips are acceptable, I guess. But for large areas that don't have kraft paper...unacceptable. Go in at night and rip the $hit out that they used. Do the same where they used plastic. This is bullcrap.

Tape? Well, I am not sure. I would use duct tape. A 2" strip of tape is not gonna effect the performance of your wall! It won't cause moisture and mold problems, nor will it hamper the insualtion.

[msmity29]

So I should take down the plastic and replace with kraft backed insulation? That's approximately half the house and would cost 100s of dollars!

[stuccoman]

Spend it now or more later!

[David]

Half the house!!?? :wow:

I told you to tear it down, not replace it. :whistling: The builder should replace it, not you.

[CThomp]

Why not just take the plastic and tyvek over it? Wouldn't that solve the issue?

Dave is right...the builder should be replacing this not you.

[woodall83]

First ask him WHY it was done this way then ask him to replace it.

[woodall83]

Also, the insulaters should take care of your firestopping. Make sure all holes are sealed from floor to floor and to the outside.

[wmax]

I just insulated my garage with Owens Corning R13 faced batt, I'm drywalling the bottom half of the garage and ceiling and then I will cover the attic Insulation with Film-Gardn 8' 4" x 100' Flame Retardant Clear 4 Mil Plastic

I live in Minnesota (cold & hot) should I even bother covering the attic Insulation with plastic sheeting?

What the PRO's and Cons?

[roklin]

Plastic over insulation that require building codes permit the direct application of foam plastic insulations (polyisocyanurate, expanded polystyrene, and extruded polystyrene) over steel roof decks, provided that the insulation manufacturer has tested and passed.