# Partial Basement Wall Insulation



## djbarber2 (Nov 4, 2017)

I am finishing my basement and need some advice on insulation. The back half of my basement is ~1/2 within the foundation and the other ~1/2 is above the foundation and is 2x6 framed wall. Picture attached. I attached Foamular insulation to the half foundation wall and then framed a full wall from floor to ceiling in front. I then removed the existing fiberglass batt insulation and plastic sheeting from the 2x6 wall to install 3/4" EPS + 1" Foamular insulation sheets and sealed around edges with goodstuff. I plan to re-install the batt insulation in the 2x6 cavity (stripping off ~2" to account for the EPS/Foamular thickness). On the bottom part of the full wall I plan to install unfaced fiberglass batts. My question is what to do about the upper half of the full wall. It is approximately 9.5" to from the front of the wall to the front of the 2x6 framing, so unfaced attic insulation would seemingly work well here. Just concerned about having "too much" insulation in the wall to where moisture could collect/grow mold. Any advice is welcome.


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## Bud9051 (Nov 11, 2015)

Hi dj and welcome to the forum.
There is one concern dealing with your climate region where the ratio of rigid insulation to batts is needed be sure the inside surface of the rigid remains above the dew point. I'll go find the link and add it but your 1 3/4" is probably good for a moderately cold climate with 5" of batt insulation. But in the upper section it might be a problem if you fill everything with fiberglass.
Here's the link and check your climate zone.
http://www.greenbuildingadvisor.com...rmal-mass&utm_campaign=green-building-advisor

I'll be back once you post your climate zone and approximate location.

Bud


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## djbarber2 (Nov 4, 2017)

Looks like zone 5 which appears to require 27% of the total R value be foam board insulation. So, would it be ok to add fiberglass behind the front wall stud bay to meet this requirement and leave the stud bays empty?


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## Bud9051 (Nov 11, 2015)

The objective is to keep the inside surface of the rigid above the dew point. That means more rigid or less fiberglass. Leaving the gap empty moves in that direction, but the gap is isolated from the heat by the empty stud wall. Not sure how to count that.

But, the article is referring to fully exposed exterior walls where some of your foundation is below grade. Plus, basements often don't get heated to the 70 degrees that he was probably using for his calculations. The good news is, most numbers in the energy field have wide tolerances, like those r-values and his calculations. It's not like 27% and everything will be fine where at 26% the house will be destroyed. I work in a shotgun world where close is plenty good. Monitor your basement humidity and keep it reasonable, below 45% RH and all should be fine with that gap empty.

In any case you would want to finish the wall behind the stud wall to give it a solid air barrier.

Bud


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