We are installing in the attic above the bathroom fiberglass insulation that has the paper on one side.
We installed the plastic moisture barrier then we installed the fiberglass insulation paper down.
Now our questions are, first did we install the insulation correctly. And, should we install another plastic moisture barrier on top of the insulation of the part of the fiberglass that is exposed. On top of the insulation we are placing plywood.
In other words, should the moisture barrier be on the top of the fiberglass insulation.
Replies
You did it right. Do not add another VB! That will trap moisture between the two.
The idea of the VB is to prevent moisture from moving from the warm humid bathroom into the insulation level.
But it might only stop 90-99.9% of it, depending how well you did and what other penetrations there are. Laying on another VB will trap that leakage ion the insulation where it will ruin it and promote mold growth.
My youngest brother bought a house where the attic had been insulated with a double VB that way. He found out too late. He, his wife, and his child are all sensitive asthmatics and the mold was a terrible problem. They ended up losing money on the house. I visited them shortly after he discovered the problem and they asked advice what to do. I could see the reality while they were into the dream of their first home together. So they were not too happy when I asked if they had five gallons and a match. Five years later, they realized I was right.
So don't double-bag it. Let it breathe to the top
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So, the best way is to the VB on the top of the drywall then put the insulation on top with the paper side down then the plywood.
Thank you so much for your speedy reply.
In Maryland (and especially for a bathroom) you should install the fiberglass with the paper facing inside. If you will be tightly covering the insulation with plywood you don't need any other covering on the outside. If you do use a covering it should be housewrap, not plastic.
So, before I put the plywood I can put the paper barrier. Where can I get the proper paper barrier. Will the insulation still be able to breathe.
Does Lowes or Home Depot sell it? What is the name of the brand to buy?
Thank you for your quick reply.
The "paper" would be Tyvek, Typar, etc. These allow moisture to escape but prevent drafts. Unfortunately you'd have to buy at least a half roll of the stuff for $50 or $75. You can do about as well in this case using landscape cloth for an investment of $10 or so.But if you seal the plywood tightly enough (plug any gaps somehow) then the "paper" is unnecessary.
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Plywood is a vapor barrier ... you wouldn't want to seal it, I don't think.
You have evidence of that?
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If plywood is a vapor barrier then millions of homes with plywood sheathing are in trouble.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
From my readings over the last year or so, the permeability of very dry plywood and OSB are very low, constituting a vapor retarder. As they absorb moisture, up to the safe moisture content, their permeability increases substantially. If the interior wall surface has been detailed well (no convective air flow into cavity), then the relatively slow diffusion of water vapor into the wall cavity is at such a low rate, in absolute terms, that the wood materials can safely absorb what diffuses through over a couple of months, then slowly dry out later.This was the conclusion of a paper by John Straube, a reference to which I posted somewhere here or on JLC/Building Science.Maybe someone can post a nice link to a reference on the permeability of plywood and OSB as a function of moisture content. Martin Holladay, got your ears on?
Edited 6/2/2008 8:10 am ET by DickRussell
I'll go for vapor retarder. VB was a bit over the top though.It is the resins and glues in it that contribute much of the good perm ratings.Note that Huber's Advantec line has ( I forget the exact name) the new one that is rated as a barrier with the seal strips at joints.
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Thanks for the clarification ... I couldn't have done that. All I know is that if you look up the tested 'official' perm rating of various materials, plywood tends to be low. This is part of the reason tight houses and/or houses w/ poor VBs have had some problems. Tightness raises RH which moves into the wall if the VB isn't good, then reaches the plywood and may not go through as readily ... trapping and increasing moisture levels inside. The exterior washes a lot more w/ moving air ... so I think there is probably a MUCH higher propensity to dissipate moisture on the outside ... through the plywood even though the plywood has low perm.
Moisture is a bit of a tricky science w/ simple principles, but complex conditions that can often make us scratch our heads. I'm not an expert like Listbruk, but I've seen a lot of strange situations that have taught me a lot.
You should have put unfaced batts over the poly vap barrier. Your approach is overkill and may provide a way for moisture to condense ... Don't add anything on top before the plywood! BTW ... plywood in itself is usually a vapor barrier (i.e. < 1.0 perm rating, I think. I'm not saying you have a problem ... since most plywood isn't sealed at seams and tends to breath better than poly.
This is ceiling to attic, Dan. Are you thinking walls?I've never seen tyvek go over insulation in an attic, though it might prevent some thermocycling in loose fill
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I know it's an attic. But covering FG batts will, as you suggest, prevent convection currents in the batts.
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