Hello, I am in the process of building a 2500 sqft two store home in Maine (climate zone 6). The current code at a minimum is R 20 cavity, and R 5 continuous on either the exterior or interior. In Maine it is common practice to have the vapor barrier on the interior side of the wall in order to dry to the outside. As im sure most of you know Maine is a heating dominated state. We do have hot summers at times, but we also get bitterly cold winters. Example, the design temperature for heating equipment is 1 degree F in my area. Here is my issue, I have looked at the dew point calculator on CI's website, and found that the R 5 (even though allowable by code) is not sufficient to stop the exterior sheathing from having condensation potential in the winter months. i.e it is not enough to keep it above the dew point. I feel as though there are a couple solutions to this, but I just wanted to hear opinions on best practices on a budget. With the above situation in mind, if the vapor barrier is installed perfectly/continuous, can that mitigate the migration of moisture into the wall cavity so there would not be an issue all together? Or should I just opt to go with a greater R value on the outside of the building, to keep the interior side of the sheathing warm enough in the winter months. Also on a side note...am I causing problems by having an interior vapor barrier (where it should be in our climate), and foam (also a vapor barrier) on the outside? Just doesnt seem like there is any way for drying potential. Here is the wall outside to inside: Foam (R-5 or R10), Tyvek, OSB, 2x6 w/ R-23 Rockwool, 6 mil poly, 1/2 Drywall.

Have you checked out this article? https://www.greenbuildingadvisor.com/question/how-to-deal-with-an-interior-vapor-barrier-on-a-rigid-foam-retrofit

Yes, you always need to retain a drying path as moisture will always get into a cavity. So don't use the 6 mil poly on interior side and have your drying path be to the inside. Increase the foam R-value as needed to avoid creating moisture in the buildup. You can use our calculator to confirm this:

https://www.borstengineeringconstruction.com/Building_Assembly_Moisture_Analysis_Calculator.html

Or better yet, consider using ICF. Honestly, I don't no why people still do 2x construction anymore.

Gayle

Posted By sailawayrb on 24 Apr 2024 02:36 PM
Yes, you always need to retain a drying path as moisture will always get into a cavity. So don't use the 6 mil poly on interior side and have your drying path be to the inside. Increase the foam R-value as needed to avoid creating moisture in the buildup. You can use our calculator to confirm this:

https://www.borstengineeringconstruction.com/Building_Assembly_Moisture_Analysis_Calculator.html

Or better yet, consider using ICF. Honestly, I don't no why people still do 2x construction anymore.

Gayle

They don't because stick construction is a mature (OLD), fairly well understood technology.
Also, stick construction has something concrete doesn't, it's a natural hygric buffer.

2x construction is certainly OLD. I suspect vast number of contractors have only done 2x construction and that's what they push. ICF can get wet and stay wet forever and it won't grow mold and won't rot either. But ICF can be sealed better and easier than 2x construction and can be DIY constructed easier and for less cost too. Yes, contractor prices may disagree with this...but ICF material cost and labor hours are way less than 2x construction material costs and labor hours these days.

Gayle

Hi there,

For your Maine home, consider increasing the exterior insulation to R10 to better manage condensation and keep your sheathing warmer in winter. Regarding vapor barriers, using R10 foam outside and possibly a smart vapor retarder inside could provide the best balance for drying potential. Consulting a local expert might help refine your approach.

Good luck with your build!