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Old 08-25-2013, 01:21 PM   #1
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Basement Vapor Barrier or no?


Hey, I'm posting this to you guys because I am lost. I've been reading conflicting theories all over the internet on this subject for the last few days.



Background- Working on refinishing my basement. I tore everything out to the perimeter stud walls. I was left with stud walls with unfaced fiberglass insulation covered by a 4 or 6 mil poly vapor barrier.


Since then I've framed up a bedroom and had to remove the vapor barrier to run electrical. I'm ready to drywall now. I was able to keep the insulation as most of it appears in good shape still.



Now here's my question. Do I need that vapor barrier? Or will it just cause problems below grade? I've read over at building science that it could cause mold growth with a vapor barrier below grade. There is conflicting information everywhere.


I know the "Right" way to do it would be to rip out the fiberglass and put in XPS rigid foam. But I wouldn't be able to get it in behind the studs properly.



So what is the right course of action? Leave the fiberglass unfaced and uncovered with poly in order to "breathe". Or cover it with a 4 mil poly vapor barrier.


Thanks, sorry for the long story!
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Old 08-26-2013, 06:44 AM   #2
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(a) No, no the vapour barrier goes next to the outside wall
(b) No, no the vapour barrier goes on the inside wall
(c) You don't need any vapour barrier
(d) No, you need a vapour barrier

Get it?

Depends - partly- on where you live
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Old 08-28-2013, 11:03 PM   #3
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I understand it depends partly on where you live. But isn't that for above ground? Aren't basement's everywhere typically the same with the earth being a constant temperature?

I'd hoped to get some more feedback.
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Old 08-29-2013, 11:46 AM   #4
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"I'd hoped to get some more feedback."

Why? when you won't tell anyone even where you live...?

OR at least in exactly what climactic region do you live? - or indeed even if in the US...get real.

Look, there are piles and piles of info on each different climactic region of the US - and there are about 5 that I know of. Each region has different building codes as well, so you expect some expert here to expound on all 5 not knowing which one pertains to you?

I told you it depended on what region you live in...now you're beefing because you get no feedback?

I guess you're in the top half of the US....am I even close?

I'm losing interest in this dialogue.

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Old 08-29-2013, 10:20 PM   #5
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illinois.... After reading building science and some other material I thought it didn't matter where you lived if it's below grade. sorry, i posted this on a few other forums and was getting frustrated because there is so much conflicting information.
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Old 08-30-2013, 09:00 AM   #6
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Ah, OK then now we're getting somewhere.

The folllowing link explains a lot about vapour barriers, vapour retarders and air barriers and the effects of climate on your choice of which one(s) you should have:

http://energy.gov/energysaver/articl...sion-retarders

Now as for your particular circumstances, it depends on what your budget is; remember that your money is better spent on air sealing (which your polyethylene plastic is already doing) than on anything else, seeing as how this is a retrofit.

Hope this helps a confusing issue.
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Old 08-30-2013, 10:24 AM   #7
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Quote:
Originally Posted by daphillenium View Post
illinois.... After reading building science and some other material I thought it didn't matter where you lived if it's below grade. sorry, i posted this on a few other forums and was getting frustrated because there is so much conflicting information.
Yes there is a lot of conflicting information. When I finished the basement in my previous house 5 years ago, I put the plastic sheet up against the block wall and framed in insulated in front of it...under the watchful eye of my city building inspector.

For this latest basement buildout i properly put the XPS to the wall with the framing snug up against it and insulated.

The codes are lagging.

B
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Old 08-30-2013, 11:00 AM   #8
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"under the watchful eye of my city building inspector."

Yeah, I almost forgot that under the various building codes and jurisdictions, there are x number of inspectors each with their own particular version of what is a complicted subject...I hear ya'!

Usually, the state-of-the-art precedes the building code changes by some 5-10 years, on top of everything else we know.
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Old 08-31-2013, 07:32 PM   #9
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Interesting. Thanks for the link. I went ahead and built up a new top plate so the perimeter walls go all the way up to the floor joists above. I put in cripple studs on 16's between the old and new top plate and put insulation in there.

I just finished putting the poly sheeting in and now have a true air barrier.

We do plan on selling and moving out of this house in a few years. Our plan is to build.

I can't wait to have an empty slate and do things the RIGHT way!

Thanks for the help guys. I'll post a few pics of the finished basement when I'm done!
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Old 08-31-2013, 10:28 PM   #10
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IMO, no poly for your location, below grade- (Start reading below photo 6); http://www.buildingscience.com/docum...commendations/
This is the safest route; with foamboard. It can be done with faced batts or poly but summertime condensation can be a problem in Zone 5.

The poly is a good vapor barrier, not so much an air barrier with all the drywall/trim fastener holes; pp.5-6; http://www.buildingscience.com/docum...researchreport

Need to foam board/canned foam the rims, ADA the drywall (good air barrier); http://www.buildingscience.com/docum...wall-approach/

http://www.buildingscience.com/docum...l_seal_rev.pdf

Gary
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Old 09-01-2013, 07:46 AM   #11
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Ah-ha; see, that's why I brought up the budget idea, in that it depends on what your budget really is in order to do all the things that are theoretically 'right'; now we find you're moving in a few years, so this reinforces the idea of just putting things back the way you found them and let someone else worry about the different codes and the recommendation of the industry via the BSC. In other words, before the effects of the placement - or not - of a Class I vapour retarder (the poly sheet) - or anything else - comes into the picture, you'll be long gone.

And even then, the effects may be marginal at best, because the most important heat loss effect comes in the form of having insulation there in the first place, let alone what type or what placement it has. After the first few inches of insulation, the difference between all the choices falls under the laws of diminishing returns...in other words, the payback becomes longer and longer.

Again, the best value for (little) money is air-sealing which you will be doing as soon as you drywall it all up; by doing a good sealing job here, you'll take care of the major source of moisture damage: condensation.

See, your location was important info for us to have; now you have the proper info - thanks to Gary - that you need for the new house.
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Old 09-01-2013, 10:44 AM   #12
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Interesting. Just for curiosity's sake....Is there any way to do airtight drywall with a drop ceiling?
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Old 09-01-2013, 01:28 PM   #13
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Sure, just air seal the rim joist area(s) right up to the floor above; use XPS, or spray foam - or even drywall properly caulked.
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