# vapor barrier behind green board?



## BRO931 (Jul 12, 2010)

I had to tear out some cheap paneling and the frame walls behind it to remedy a major mold problem in my basement. The walls were insulated with R-13 fiberglass which appeared to have wicked up moisture from the unsealed concrete blocks. What a mess!

The concrete block walls are now well sealed and I installed 2" extruded polystyrene panels (R-10) against them before putting up the new frame walls. The frame walls have new R-13 fiberglass and I've got mold-resistant green board ready to hang, but I wonder if a 4-mil plastic vapor barrier on the interior side of the frame walls would be a good idea.

I've heard pros and cons about vapor barriers with green board. The XPS panels should already be acting as a vapor barrier. I'm beginning to wonder if going with green board here might be a mistake.

Anybody care to offer advice here? Thanks in advance.


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## algored2deth (Jan 7, 2011)

Provided you have 2" XPS installed, I think you can forgo the vapor barrier altogether. The XPS will act as the vapor barrier.

The green board is just supposed to be moisture resistant. It is a better choice for a basement than standard drywall. There are other choices like fiberglass faced drywall but if you have already bought the green stuff, party on. 

When you say the walls are well sealed, what exactly do you mean? Are you talking about drylok stuff on the inside? If so, that is a temp measure of remediation . All moisture problems that come into a basement need to be dealt with from the outside. Drylok may hold for a bit but it does have to be reapplied.


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## cocobolo (Dec 16, 2008)

Bro...
For reasons that I do not understand, it seems that many of the local building codes in the US do not seem to require a vapour barrier.

Any time you have a break in the insulation on an exterior wall, air is free to pass back and forth at will. It is unlikely that you can physically install your panels without some air gaps.

Therefore, the air movement, which will carry moisture with it, will always be present. I would not hesitate to be adding 6 mil poly, rather than 4 mil, as it is extremely cheap insurance. Up where I am, 6 mil is the standard requirement. Joins in the poly need to be taped and all penetrations sealed. It is a step well worth taking. Your pocketbook will be thanking you indefinitely after you do this.


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## algored2deth (Jan 7, 2011)

cocobolo said:


> Bro...
> For reasons that I do not understand, it seems that many of the local building codes in the US do not seem to require a vapour barrier.
> 
> Any time you have a break in the insulation on an exterior wall, air is free to pass back and forth at will. It is unlikely that you can physically install your panels without some air gaps.
> ...


In general, I agree with you but I left off some implications with my original message. If you do put up 2" XPS and glue it to the walls, the assumption that I make is that the between panel joints are at least tongue/grooved or lap jointed together WITH caulk or the appropriate tape. Any other "exposed" joints will then have spray foam applied. You are looking to make it air tight as possible. It is when you do this much effort, you can get away w/o using a vapor barrier. I apologize for my lack of details previously.

Building codes in the US are just not well known. A lot of this resorts back to that only certain people need to know these things and we generally have the attitude that is the way it is. I am not saying this is right but many people pay others to do the job. 

Because of the weather differences throughout the US, what works in the northeast may not work in the desert by Arizona. It is hard to generalize. And then you get into a whole other realm of who is technically competent. That is another matter.

dennis


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## Gary in WA (Mar 11, 2009)

“In climates with less than 4,000 Heating Degree Days, materials like painted gypsum wallboard and plaster wall coatings impede moisture diffusion to acceptable levels. Usually, no further vapor diffusion retarder is needed.” Your State has 3500 HDD. From; http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11810

The foam board assembly has to dry to the inside below grade, no vapor barrier; http://www.buildingscience.com/documents/digests/bsd-103-understanding-basements?full_view=1

I don’t have it front of me but greenboard should never be used with a vapor barrier. Also, it is not allowed in bathroom “wet” areas; tub, shower, as per Code.

Gary


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## cocobolo (Dec 16, 2008)

algored2deth said:


> In general, I agree with you but I left off some implications with my original message. If you do put up 2" XPS and glue it to the walls, the assumption that I make is that the between panel joints are at least tongue/grooved or lap jointed together WITH caulk or the appropriate tape. Any other "exposed" joints will then have spray foam applied. You are looking to make it air tight as possible. It is when you do this much effort, you can get away w/o using a vapor barrier. I apologize for my lack of details previously.
> 
> Building codes in the US are just not well known. A lot of this resorts back to that only certain people need to know these things and we generally have the attitude that is the way it is. I am not saying this is right but many people pay others to do the job.
> 
> ...


Dennis...thanks for the clarification. It would seem that you have made yourself a pretty good v/b.

Your other comments are completely understood, as are Gary's.


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## BRO931 (Jul 12, 2010)

Thanks for all the advice folks. I did apply Tyvek tape to the joints between the XPS panels. The block walls were previously sealed (NOT!) with one coat of DryLoc. I cleaned the block with a wire brush and sealed it with two heavy coats of Ames' Block and Wall Liquid Rubber. I know that any waterproofing on the inside of a wall is like patching a tire from the outside, but I didn't see any signs of actual seepage. The basement has a footer drain on three sides and the ground slopes steeply to a walk-out side. I suspect the moisture may have been getting through the wall from the inside during the spring and summer when the ground is colder than the typical dew point in the mountains of N.C. I'm at about 3500 feet elevation and I'm pretty sure we are in the 4000+ heating degree day category.

Gary, you said green board should not be used with a vapor barrier. Why so? I've seen and heard that before, but I fail to see the logic.


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## oldrivers (May 2, 2009)

because you get moisture trapped inbetween the two vapor barriers.


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