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    logan176's Avatar
    logan176 Posts: 341, Reputation: 6
    Full Member

    Feb 18, 2016, 07:43 AM
    Insulating Basement with XPS Rigid Foam
    I originally asked this question within another thread titled "Basement Heating Options." I noticed that the thread was quickly becoming more about insulation rather than heating, I decided to post my situation with updated questions in this new thread with an appropriate title.

    I'd also like to apologize in advance if it seems like I am asking the same question different ways or if I keep changing my mind. The more I read about basement insulation, the more my head spins.

    I have a Cape Cod style home built in 1951. I live in southern New York, climate zone 5. The foundation is made from concrete block. The house had water infiltration problems when I first moved in 8 years ago. Since then, I have installed an interior perimeter drainage system below the slab and a sump pump.

    I currently have a moisture issue in one corner of the basement after several days of heavy rain. I plan on digging down and sealing the exterior foundation wall with tar extending out about 5-6 feet from the corner. I will also regrade and extend the downspout 10 feet from the house.

    What combination of insulation should I use for my basement?

    I started by insulating my rim joists with 2” XPS as described in this article from Family Handyman. Based on the reading that I’ve done and the recommendations in the previous thread (thank you Ballenger), I want to go with XPS rigid foam boards glued directly to the foundation. My building inspector said that I need to have at least R13 insulation in the walls, so the 2” XPS alone (R10) is not enough.

    This second article from Family Handyman says to use ¾” XPS on the walls, frame, and later fill with fiberglass.

    Then I came across two articles from Building Science Corporation. The first BSC article says using 2” XPS along with R13 fiberglass works very well at controlling moisture when used with spray foam on the rim joists and another 2” of XPS below the slab. But I don’t have anything below the slab and used XPS on the joists instead of spray foam.

    If I’m reading the second BSC article correctly, it says that using 1” XPS with R13 fiberglass can also be effective at controlling moisture as long as the sheet rock is covered with vapor barrier paint and a dehumidifier is located in the basement. But… the article states that “some variations not recommended” and the article does not explain what that means.

    So, will 1” XPS (with fiberglass) be thick enough for moisture control or will I need to go with the 2” XPS?
    ballengerb1's Avatar
    ballengerb1 Posts: 27,379, Reputation: 2280
    Home Repair & Remodeling Expert

    Feb 18, 2016, 07:18 PM
    Before you do any insulation you must fix that moisture issue and allow enough time to see if it stays fixed. Block is not a good wall material if you want to finish the basement. You could consider something like DryTrak® Basement Waterproofing for Monolythic Foundations Once you do not have any water coming in then cover all the block walls with 6 mil visqueen before studding and insulation. With visqueen its safe to use fiberglass insulation. I would also consider using greenboard.
    joypulv's Avatar
    joypulv Posts: 21,593, Reputation: 2941
    current pert

    Feb 19, 2016, 03:30 AM
    I'm not a building expert. All I know is what I read and see. My parents finished basement 1970 poured concrete foundation was an absolute mess by the time I ripped the interior out in 2007. Not just dampness, but millions of bugs (and one area of termites in a few interior walls), who love moisture.

    My basement, also 1970 poured concrete, gets wet every few years during very rainy springs, no matter how good my gutters and downspouts are. Ground water weeps in through rebar ties (WHY do so many builders do that?)!! I know there is nothing at all that will solve that from the inside. Pressure from the outside is too great. There are many new materials for outside that waterproof, insulate, AND keep bugs from chewing channels all through insulation panels that people for years were gluing right to the foundation, with lousy results.

    You have no vapor barrier under your slab. That to me is an indication that you should go slow even thinking about all of this. Even with a dehumidifier. You can buy rolls of 3' wide plastic that has a thin layer of foam attached for evening out bumps in the concrete, and glue it down super carefully with not one single gap, and put a non-wood flooring over it, but I don't know if the jury is back yet on that.

    This Old House won't make basement walls that are attached to the concrete. They build them out slightly.

    NOTHING can be a real vapor barrier from the inside.

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