To Seal or Not to Seal...the vaper retarder


Old 01-05-11, 03:43 PM
Yukon Youngun's Avatar
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Join Date: Sep 2005
Location: Missouri
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To Seal or Not to Seal...the vaper retarder

Two projects, one common it helpful, important, or critical to seal "seams" or tears in the kraft facing, as in tape or stapling together?

First, I am repairing some places where the cats have damaged the exposed fiberglass batts in my unfinished basement. It has 9 ft. walls and the previous owner/builder used batts, so there are short pieces added at the bottoms to fill the gaps.

Second, I need to rewrap the attic ductwork where about 1/3 of the faced fg batts are missing. I've not seen this before and the batts are wrapped edge to edge rather like cardboard boxes stacked over a fence post.
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Old 01-05-11, 04:40 PM
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A layer of plastic with holes from staples and a few tears is still a vapor barrier. What it has lost is its performance as an air barrier. If it were to be covered with sheetrock, which is a good air barrier, then the sandwich is complete, an air barrier, a vapor diffusion retarder, and it is rigid. Leave out any one of the elements and you could have trouble.

As for your basement, even I am unable to find a consensus on how basement walls should be insulated. However, the one most often identified as not being good is fiberglass. But the problem actually stems from the differences from one basement to another. Different basements warrant different solutions, and what is best for yours will require a longer thread, much longer.

As for faced fiberglass against or near a basement wall, generally no vapor barrier is best. The idea is to allow some drying to the inside, since it can't dry to the outside. But several other factors need to be considered before I can suggest a best approach.

To give you a start, the wood rim joist at the top and the exposed foundation wall (plus one foot) are the highest priority. The foundation that is below grade will lose much less heat.

Now for those heat ducts, before you wrap them, they need to be air sealed. Something like 20% of your heat is lost through conditioned air leaking out or unconditioned air being pulled in through all of those leaks. Once sealed, wrap and tape a good layer of insulation over all ducts, especially any in unconditioned spaces. If the Kraft facing is missing, just replace with plastic over the insulation. with them sealed, there should be no air pressure to move air in or out through the insulation.

Old 01-05-11, 07:49 PM
Join Date: Mar 2009
Location: WA
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It is unsafe to leave the paper facing on the basement walls exposed. The cats could easily breathe the glass fibers to cut their lungs up from the inside, and yours.....The paper is also a fire hazard which REQUIRES covering per minimum Building Code. Especially if near the furnace which will spread the fibers throughout the house....

The duct work has special insulation (more dense) than standard fiberglass batts that will lose R-value when compressed, use it instead, and double it rather than the minimal R-7 per Code.

Old 01-06-11, 03:05 PM
Yukon Youngun's Avatar
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Join Date: Sep 2005
Location: Missouri
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Thanks for your comments. Air flow is the main reason I asked about sealing the facing since I discovered quite a bit of it while filling in the empty spaces. I started out planning to pull down the batts in each stud space as I went along and caulk any air leaks I found along the way. It seems to me that method could get pretty tedious, but it would pretty much eliminate any incoming air. Would it make much difference to seal on the back side of the sheathing vs. the inside facing?

As for the cats, they are relatively new and we put up an assortment of plywood panels once we saw what they were doing. No more problem there.

I checked the duct work this afternoon and found it wrapped with R-19 glass batts, although they are obviously compressed by about half. Basically, they are stapled to the floor on one side, draped over the duct and tucked slightly under, into the blown-in cellulose around it. There still are some pretty large gaps where the tucking is not so snug. I can't imagine why they left four feet of it exposed where it comes up from the furnace, but there is no evidence that any was ever in place there.

You mention higher density insulation for the ducts. Are you referring to another type of fiberglass or a more rigid board product? Would it make sense to replace what is there now vs. just finishing what they left off?

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