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Old 05-14-2015, 12:46 AM   #1
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Dewpoint in R-57 wall


I wonder if anyone can tell me the dewpoint of my wall, assuming 20c interior and -20c exterior. Of course the outside temperature will vary, but the interior temperature will remain fairly constant between 20-22c.

My walls are 2x6 exterior wall with 2 batts of roxul R14 (R 28 total and yeah the batts protrude 1.5 inches from the studs), then followed by a third layer of roxul layed down, followed by a 2x4 wall (with poly on the outside edge of the interior 2x4 wall) and 4th layer of roxul R14 inside that 2x4 wall. That wall is then sheathed with 3/8 plywood on the interior followed by 1/2 inch drywall.

R-57 wall including the plywood and drywall.

Where/when is the dewpoint?

Will such a wall assembly never experience condensation or mold, and be healthy and strong forever?

Last edited by wwsteeel; 05-14-2015 at 12:49 AM.
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Old 05-14-2015, 07:47 AM   #2
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Whats the exterior sheathing?

That 2x6 is going to be the weak point in the exterior wall of that staggered stud wall.

The lack of moisture movement through the interior wall (via air tight drywall and vapor retarder) should keep the moisture to a minimum.

Are you designing this now or is it built.

Potential sounds like it is very low right now.
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Old 05-14-2015, 09:56 AM   #3
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It is already built, with vinyl siding.

My second perimeter wall, 2x4 filled with the 4th layer of roxul, is on 24" centers. The 2x6 wall is on 16" centers.

The 3rd layer of roxul, layed down and pinned to the 2x6 wall, completely covers all framing members of the 2x6 wall.

So basically my wall assembly is: vinyl siding, housewrap, 3/8 OSB, 2x6 cavities filled with 2 batts of R14 roxul (the roxul protrudes beyond the 2x6 by 1 1/2 inches), 3rd layer of roxul layed down from floor to ceiling and pinned to the 2x6 wall, 6mm poly (stapled to the exterior side of the interior 2x4 wall), 4th layer roxul inside 2x4 wall, 3/8 plywood attached to inside of 2x4 wall, and finally drywall attached to the 3/8 plywood.

I have tied/sealed the wall poly (on the exterior side of the 2x4 wall) into the ceiling poly and floor poly ensuring as close to perfect poly job as I can. The floor poly--I installed on the floor between the two walls a 16" piece of poly that cradles the 3rd layer of roxul laying on top of it, and let that floor poly come beneath the 2x4 wall so it can be sealed to the wall poly with caulking sealant.

I will be installing the Lifebreath HRV shortly.

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Old 05-14-2015, 01:10 PM   #4
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Quote:
Originally Posted by wwsteeel View Post

Where/when is the dewpoint?

Will such a wall assembly never experience condensation or mold, and be healthy and strong forever?
Plug in your numbers to determine when DP will occur.

With the word poly in the discussion, where condensation might occur is anyone's guessing game and in my opinion the wall will never be healthy anywhere except possibly where the relative humidity never exceeds 30 percent.

http://dpcalc.org/
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Old 05-14-2015, 02:59 PM   #5
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The possible weak point is having the six mil poly inside the wall. If used at all should it not be directly behind the Sheetrock? From everything I have read in preparation for the house I am building absolute vapor barriers like poly are no longer recommended.
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Old 05-14-2015, 04:13 PM   #6
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We use to use poly years ago but they found out that it traps the moisture in the wall instead of letting it escape to the interior or exterior. Google it and you will see pictures of the damage to the wall from the trapped moisture
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Old 05-14-2015, 05:37 PM   #7
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But code here (Saskatchewan) calls for 6mm poly in wall.
I placed it on the exterior of the 2x4 wall so as to preserve the integrity of the poly.

I was told so long as minimum of 70% of insulation is on the outside of the poly youre ok.
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Old 05-14-2015, 07:30 PM   #8
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The staggered stud wall make the poly placement less problematic in this case.

Use the calculator that he gave you but I don't see it being an issue as long as the drywall is night and tight.

The 2x6's shouldn't get that cold on the outside wall and it should be able to dry to the outside through the sheathing and housewrap.
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Old 05-14-2015, 11:04 PM   #9
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thanks for all your inputs.

I was told by an oldtimer that having 16 inch walls with 75% of the insulation on the cold side of the poly ensures no moisture condensation on the framing members or drywall regardless of usual relative humidity.

The type of insulation though must be Roxul, as it is effective at stopping air flow whereas fiberglass batt does not. And, Roxul doesnt settle or shrink.

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Old 05-16-2015, 12:32 AM   #10
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Roxul is not air-stopping because it is a fibrous insulation. It does absorb some moisture; http://www.google.com/url?sa=t&sourc...d=rjaCellulose is better at air-stopping, or SPF is optimum. Your 2x6 studs would be running about 35*F at inside face, because of that insulation thickness. But factor in the R-1.25 for solid wood per inch of thickness; studs will be at 4*f if -4*F outside (-20C)----- plus the insulation around them warming them to maybe 20*F- IMO.

The poly will condense vapor at 58%RH interior air as it is running about 53*F there. The studs are much more at risk because they will be colder longer. Page 42/70 and Fig. 13 on another page... here for wall #4- similar to yours but less insulation; http://apps1.eere.energy.gov/buildin...study_2011.pdf

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Old 05-17-2015, 06:16 PM   #11
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So, youre saying if I can keep my interior relative humidity below 58% at all times, I can guarantee no moisture on structure?

sorry, I am new at this
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Old 05-20-2015, 08:39 PM   #12
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Yes. It is extremely important to air seal the poly/drywall to prevent air leakage from the interior. This about 100 times more times volume condensation on the poly than diffusion alone; pp. 10/15; http://www.buildingscience.com/docum...researchreport

ADA the drywall; http://www.buildingscience.com/docum...wall-approach/

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Old 05-20-2015, 08:53 PM   #13
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Quote:
Originally Posted by wwsteeel View Post
So, youre saying if I can keep my interior relative humidity below 58% at all times, I can guarantee no moisture on structure?

sorry, I am new at this
If you can maintain a surface temperature above 59F which is dew point temp. at 58 percent RH and the air temp. at 75.
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Old 05-20-2015, 09:01 PM   #14
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Actually, at 68*F (as stated) the exact dew point is 55% RH and 51*F temp of the poly. I stand corrected, Gary
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Old 05-20-2015, 09:20 PM   #15
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Gary - I apologize for my reply seeming like a correction. I hadn't read all the details and was just responding to Mr. wwwsteeel's post.
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