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Insulating a basement that HAD a moisture problem

ARF9VudJeZ | Posted in Energy Efficiency and Durability on

Hello GBA,

1910’s Colonial in Climate Zone 5….

I am dealing with a basement that HAD a moisture problem. This past September, perimeter basement trench (French) drains and new sumps were installed. We have gone through several rounds of significant rain and snow melt, and the basement is dry as a bone. The moisture issue has been dealt with.

What, now, is the preferred method of insulating the basement walls? Closed cell spray foam?
XPS? Poly-iso boards? Do I want some sort of drainage mat against the wall behind the insulation?

It must be noted that we will also be insulating exterior balloon framed walls and properly air sealing/insulating the attic in the process of this home performance retrofit. We are expecting to significantly tighten the envelope through the course of this project.

My inclination is CCSF in the basement, but I wonder if it is too vapor impermeable and if that will too drastically inhibit drying to the inside.

Any thoughts?


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  1. GBA Editor
    Martin Holladay | | #1

    It's up to you to choose which insulation to install -- closed-cell spray foam, XPS, EPS, or polyiso. Obviously, you shouldn't use fiberglass batts or cellulose.

    Most green builders prefer to use EPS or polyiso, because they have a lower global warming potential than XPS or spray foam. However, if your basement walls are irregular (stone and mortar, for example), spray foam makes more sense than rigid foam.

    Don't skimp on the insulation thickness. The 2006 IRC requires a minimum of R-10 basement wall insulation in Climate Zone 5, and R-15 in Climate Zone 6. More insulation is always better than less.

  2. ARF9VudJeZ | | #2

    Thanks Martin. Are there any issues with specific regard to condensation on the masonry and/or the ability of the wall to dry that would factor into the decision? Are there any specific concerns about the necessity of a drainage plane that will usher any inward driving moisture downward to the French drains?

  3. GBA Editor
    Martin Holladay | | #3

    I assume your walls do not have water trickling down the interior. Even so, it would be good insurance (if the step is within your budget) to install a dimpled drainage mat between the wall and the foam insulation. Be sure the drainage mat extends to (and connects with) the perimeter French drain.

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