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Old 02-05-2011, 01:33 PM   #1
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To VB or not to VB???


Hey folks,
First post here so please be patient with me.

I've got a situation that I was not expecting with respect to insulating over rigid. Major condensation problem!

Here are the details:

-Poured Concrete basement wall. 70% below grade.
-1" Pink rigid glued to Cement
-2x4 wall framed 1" - 2" away from Pink rigid
-Roxul R22 insulation between studs


I left it like this for about 1 week and was getting ready to drywall when I pulled off one of the batts to find water drops on the Pink behind it. Not just moist... water droplets large enough to run down the pink when I removed the Roxul.

Pulled off all the Roxul and let it dry out and now I'm ready to redo it. The guy at HD recommended I just cover it all with Vapor Barrier to keep humidity from penetrating the insulation and condensing on the Rigid.

Is this the right thing to do or am I encountering more problems somehow?

Also, do you recommend that I keep the Roxul from contacting the Rigid? Is there any benefit to an air space between the Roxul and the Rigid?

I've searched high and low but can't find a straight answer.

Thanks

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Old 02-05-2011, 01:45 PM   #2
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You shouldn't, that will be just a vapor collector

I believe that gap is what was causing your vapor problem, more specifically convection loops caused by the Roxul not being tight against the XPS, you may not have enough XPS as well.

http://www.buildingscience.com/docum...ent-insulation

You start by checking this out, I am sure Gary will chime in here very soon, I would consider him a SME, "subject matter expert"

Mark

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Old 02-05-2011, 02:33 PM   #3
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Thanks Mark,
My setup is exactly like figure 3 on the site you pointed me to. The only diff is that I used 1 inch rather than 2 inch Rigid.

Here are a few other variables I didn't mention.

1. I live in Ottawa and during the installation of the Roxul the outside temperature was -25deg C.
2. The house is very tightly sealed and I am also installing a home air exchanger (not installed yet)
3. I have carried the 1" Rigid across the slab with a 3/4 t&g sub floor
4. All seams are taped and glued
5. Joist cavities have Rigid on interior of rim joists (as per fig 2 of the site you sent me) I have Rigid on the top of the foundation wall also.
6. Interior walls are framed on top of sub floor.
7. H/W exhaust vent was sweating when I pulled batt insulation off (again may have been due to extreme low temp outside when we insulated.

From what I can see in the diagram, I would assume that by putting up drywall with semi-permeable paint right away may have kept the walls from condensing.

Bottom line here is that I am completely framed and can't rip the pink back out. Will an additional 1" of Rigid cut to fit between the studs fix the problem? May not be "perfect" but may be a solution...

Hopefully Gary gets the post and can offer a suggestion also.

Thanks
Mitch
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Old 02-05-2011, 02:35 PM   #4
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You are correct, the fact the walls were open allowed too much flow of that nice warm air, condensing when it easily reached the cold surface.

Mark
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Old 02-05-2011, 02:42 PM   #5
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That was my gut feeling... but unless you fully understand how humidity will react with cool, cold and extreme cold it's hard to guess how much is enough.

I forgot one last point. I built the 2x4 stud wall 2" away from the Rigid and used 6" batt. The objective here was to gain maximum insulating factor while keeping the studs off the Rigid. I figured Warm/organic material (wood) against cool exterior wall wasn't a good thing.

The Rigid has a R5 and the Batt has an R22. This is going to be a main living space for my children to play in and so it was important to me to insulate the h out of it.

I think I got lost in the weeds and forgot that the Rigid would actually get "cold".

Last edited by Gary in WA; 02-05-2011 at 03:35 PM. Reason: keep our "G" rating
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Old 02-05-2011, 03:34 PM   #6
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Mark pretty well covered it….. About the humidity;
“Experience has shown that where interior moisture levels in cold climates during the heating season are limited to the 25 to 30% relative humidity range at 70F (21.1C), relative humidities adjacent to the interior surfaces of exterior walls (of typical thermal resistance) fall below 70% and mold growth is controlled. The colder the climate (for the thermal resistance of any given building enclosure), the lower the interior relative humidity necessary to prevent 70% relative humidities from occurring adjacent to interior surfaces of exterior walls. Building enclosures of similar thermal resistance (building code minimums) located in Minneapolis, Minnesota and Cincinnati, Ohio should be limited to different interior moisture levels during the heating season. A 25% interior relative humidity at 70F (21.1C) would be appropriate for Minneapolis. Interior relative humidities up to 30% at 70F (21.1C) would be appropriate for Cincinnati. Correspondingly, the higher the desired interior relative humidity, the higher the thermal resistance necessary to control relative humidities adjacent to interior surfaces.”
From; http://www.buildingscience.com/docum...gs?full_view=1

Notice in fig.21 of that site about no v.b. and rim joist insulation.

Always check with your local B.D. about details of your construction.

No air gap behind batts; http://joneakes.com/jons-fixit-database/743
http://www.diychatroom.com/f98/how-b...ulation-90438/

The foam needs to be thicker for your climate or the inside humidity reduced, page #10; http://www.buildingscience.com/docum...lation-systems
Minnesota has 7981 heating degree days and Ottawa has 6331.

Use ADA on the drywall, so no air gets to the concrete (tape/canned foam the joints and under at the slab); http://www.buildingscience.com/docum...wall-approach/

Did you try our “Search” box above?

Gary
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Old 02-05-2011, 06:02 PM   #7
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Gary,

I'm still reading... Thanks for linking me to all this valuable information. I don't think I was as far off the mark as I originally thought. I've done virtually everything correctly except for using 1" Rigid instead of 2".

I've concluded that I may have hit the worst possible scenario when building the wall assembly. 50+ relative humidity in the house, almost 0 air exchange, extreme cold exterior temperatures, high interior heat, cold Roxul insulation (installed from from outdoor cold storage which may have held additional moisture), no air barrier...

Well I still have much more research to do on the subject but I may still pick your brain during the next few days.

I have a fireplace insulation question also but maybe I should start a new post. I have a very significant draft of cold air beneath the gas fireplace on the main floor... I do not believe the fireplace is insulated at all...

In any event, thanks again for pointing me in the right direction

Mitch
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Old 02-05-2011, 08:55 PM   #8
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Gary,
This baffles me a little.
I was on the CMHC (Canada Mortgage and Housing Corporation) http://www.cmhc-schl.gc.ca/en/co/mah...efcosa_002.cfm site this afternoon and found that one of the diagrams (Figure 1) shows XPS on the exterior wall and Vapor Barrier on the warm side.

If there is 1.5" XPS on the exterior and vapor barrier on the inside... wouldn't that create the "double barrier" that we're trying to avoid?

Granted this is a studded wall and not a concrete wall so the physics may be different... but I am still wondering about the humidity trapped between vapor barriers?
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Old 02-05-2011, 11:49 PM   #9
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Above grade is a different ball-game. In one of those sites, a foil faced poly board is used in the basement--- above grade only; below grade the concrete has to dry to the inside. Read this on vapor control and barriers, notice in cold and very cold v.b. are used on the inside; http://www.buildingscience.com/docum...gs?full_view=1

In the U.S., v.barriers are not required in basements, period. Our Codes (building and energy) are following BSC on v.b. and non-vented roof assemblies.

Gary

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Last edited by Gary in WA; 02-05-2011 at 11:51 PM. Reason: typo
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