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Basement block foundation wall insulation

user-270695 | Posted in Energy Efficiency and Durability on

I have been auditing & retrofitting homes as an additional service to my remodeling company for 3 years now. My all-star insulator sadly passed away recently and I am just getting started with a new provider.

This new provider has specified Polypropylene Scrim Kraft to insulate unfinished basement block walls:

Thus far, I have used Thermax on my projects: 2″ foil faced poly-iso. He indicated that the PSK (Polypropylene Scrim Kraft) is cheaper, also satisfies the code for fire, and has a white finish which looks better. The JM product is formaldehyde free.

The 2 products have very close thermal values and I suspect will perform similarly when taped at seams and installed continuously.

My concern is moisture and durability.

I look forward to your input.

Thanks in advance.

Rob P.

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  1. Foamer | | #1


    No doubt the JM product is cheaper to purchase and faster to install but I share your concerns about moisture. Over the years we have removed quite a few such blanket systems and found moisture behind them. I prefer two inches of closed cell spray foam but Thermax will work well provided that the installation is detailed properly. Check out Building Science Digest # 103 "Understanding Basements" at Thermax is available with a white foil finish if you prefer.

  2. user-270695 | | #2

    How do you attach thermax? We have used foam compatable construction adhesive, but there must be a better way. The panels also want to stay in place due to friction. We then tape all joints with UL foil tape. It is a costly installation. How do you deal with taping? Does the white thermal cost about 30$/ sheet?


  3. pSpoW2tSEw | | #3


    Good question about the JM Basement Wall Insulation. As an engineer for Dow, I admit that I am biased. But, there is no doubt that fibrous insulations have had a history of wicking moisture in applications where they come in direct contact with wet surfaces. Even the JM literature you provided a link to cautions "Walls that leak water must be repaired before installing." Any warm, humid air that reaches the cool surface of the block or concrete is at risk of condensing at the intersection between the insulation and the wall. I would feel much less comfrotable with a fibrous insulation than with a rigid foam board like Thermax that has proven itself over and over to resist moisture uptake, maintain its R-value, and perform as a 15-minute ignition barrier when left exposed (it has been certified to FM-4880 - see attached data sheet)

    As for the attachment method, Hilti markets an excellent system for attaching Thermax to block or concrete. It is the X-SW series of fasteners you can find on and go to: Find Products by Trade -> Building & Construction -> Waterproofing and Insulation -> Waterproofing

    Thanks and feel free to shoot me an E-mail if you would like any further information on Thermax and how to use it in a basement applicaiton

    Scott Cummings
    Sr. Technical Service Engineer
    BPI Building Analyst and Envelope Professional - Certified
    [email protected]

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