# Dewpoint in R-57 wall



## wwsteeel (Nov 5, 2014)

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?


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## Windows on Wash (Aug 30, 2011)

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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## wwsteeel (Nov 5, 2014)

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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## SeniorSitizen (Sep 10, 2012)

wwsteeel said:


> 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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## ddsrph (Nov 23, 2013)

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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## Jnaas2 (Mar 29, 2014)

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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## wwsteeel (Nov 5, 2014)

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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## Windows on Wash (Aug 30, 2011)

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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## wwsteeel (Nov 5, 2014)

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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## Gary in WA (Mar 11, 2009)

Roxul is not air-stopping because it is a fibrous insulation. It does absorb some moisture; http://www.google.com/url?sa=t&sour...EN_j-wXM2AA2UNU07zZTmsW6TAjQ&cad=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/buildi...merica/high-r_value_walls_case_study_2011.pdf 

Gary


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## wwsteeel (Nov 5, 2014)

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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## Gary in WA (Mar 11, 2009)

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/docu...por-retarders/?topic=/doctypes/researchreport

ADA the drywall; http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/

Gary


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## SeniorSitizen (Sep 10, 2012)

wwsteeel said:


> 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 59°F which is dew point temp. at 58 percent RH and the air temp. at 75°.


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## Gary in WA (Mar 11, 2009)

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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## SeniorSitizen (Sep 10, 2012)

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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## ddsrph (Nov 23, 2013)

Gary in WA said:


> Actually, at 68*F (as stated) the exact dew point is 55% RH and 51*F temp of the poly. I stand corrected, Gary


Gary
Could you give me your opinion/advice on the wall construction in the house I am building? I will attach a photo. The house is concrete block, with all cavities filled with concrete. Brick covers block on exterior. The front wall is two by six framing. The house is cut into hillside with rear corners five foot and the other three foot below grade. The two end walls have 4 1/2 inches continuous insulation (from block inward 1/2 inch XPS, 4 inches polyiso) the studs then turned sideways and filled between with an additional 1 1/2 inch poly ISO. The back wall has 1/2 inch XPS, 1 1/2 inch polyiso, then the studs turned normal and the 3 1/2 inch stud cavity filled with polyiso. The front 2 by 6 wall will be filled with 5 1/2 inch polyiso. The ceiling will have average of nine inches of polyiso.

Thanks
JM


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## Gary in WA (Mar 11, 2009)

Below grade per code is only R-10, above grade is the same for your "mass wall", footnote "c" and "i"; http://energycode.pnl.gov/EnergyCodeReqs/index.jsp?state=Tennessee

Any of the ISO faced? Do you work at the factory...

Gary


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## ddsrph (Nov 23, 2013)

Gary
None of the polyiso is faced other than the black paper you see. I put a 1/2 inch thick XPS blue board against block in all areas. I don't work for a polyiso maker (retired Dentist and Pharmacist). I found the stuff on Craigslist being sold by a commercial roofing company . It came off a large commercial building but was in great shape and had never gotten wet. I hope it all works out. I took great care in water proofing the below grade wall. I coated the wall then also put 24 mil platon membrane and back filled with gravel. For the slab I used a 15 mil commercial vapor barrier.

Thanks
JM


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## Gary in WA (Mar 11, 2009)

Wow; with R-38.25 in end walls, R-35 in back wall and R-35.75 in front wall and no thermal bridging through the framing... R-58 in ceiling; be sure to add an ERV or HRV for air quality. Thanks for sharing, next time start your own thread with questions to be fair to the OP. 

Gary


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