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Community and Q&A

Foundation insulation transition to SIPs

user-1142739 | Posted in Green Building Techniques on

Hi All-

I’m getting ready to build a couple of single family urban infill homes in Denver, Colorado (climate zone 5b). Local zoning creates a very narrow home (19′ wide from exterior finishes) which has me leaning towards using PUR SIPs (4.5″ or 6.5″) for the exterior walls. The exterior finish will be limestone panels, making using exterior foam and strapping challenging due to the 14lbs per sq weight of the panels.

Zoning also limits the home’s height, forcing us to hang the first floor’s joists off of the foundation to gain the needed interior headroom. The homes will have full height basements and normally I would insulate the interior of the foundation walls, but since the joists are hanging into the basement I haven’t found a clean way to detail the insulation. For this reason I’m thinking about insulating the exterior of the foundation wall with 4″ of mineral wool, but I’m concerned about the potential thermal bridge at the base of the SIPs.

Attached is a PDF drawing of the way I am planning on detailing the transition. I would appreciate any insight or feedback. Alternatively, has anyone found a good way to insulate the interior of the foundation wall when hanging the floor joists?

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  1. GBA Editor
    Martin Holladay | | #1

    C. Butler,
    1. I think that your proposed detail for foundation insulation would work.

    2. If you decide instead to insulate the foundation wall on the interior, you would need to install an adequate amount of closed-cell spray foam insulation between the floor joists. If you're doing that, you might want to use spray foam insulation for the entire wall, from the footing to the top of the wall. For more information, see How to Insulate a Basement Wall.

    3. Your plan to install limestone panels as cladding on SIPs sends shivers down my spine. That's risky, for many reasons: for one thing, you will be fastening heavy cladding to OSB rather than framing; and for another, you should be worried about the fact that some SIPs have developed exterior OSB rot.

  2. iLikeDirt | | #2

    With a ventilated rainscreen gap and decent roof overhangs, your stone veneer detail probably keeps the OSB safe enough from water damage, especially considering the dry and low-precipitation Denver climate (about 16 inches a year).

  3. GBA Editor
    Martin Holladay | | #3

    C. Butler,
    I agree with Nathaniel that your ventilated gap is good, and that there is no reason to worry about OSB rot. (Still, take care when you tape the seams on the interior, to make sure that you don't have any exfiltration at the SIP seams.)

    I'll leave it to the engineers and stone distributors to give you advice on how to hang stone off of OSB. Good luck.

    I wouldn't worry too much about your "diagonal thermal bridge," but if this detail worries you, you can proceed with your plan for exterior foundation insulation.

  4. user-1142739 | | #4


    Thank you for the timely and thoughtful response.

    I read your article on How to Insulate a Basement Wall, but I'm still left with a concern changing to interior insulation. Since the foundation wall is 8" thick and the SIP is 4.5" or 6.5" think, there would be a potential thermal bridge diagonally through the foundation wall between the SIP base and the floor joist foam. Is this worth being concerned about?

    On your third point, I can understand both of your concerns. The limestone is installed with continuous horizontal supports at the top and bottom of each row. While this doesn't eliminate your concern, it does help the panels and the OSB to work together as a system, decreasing the chance individual panels would have issue. Additionally, the limestone manufacturer is willing to guarantee the system.

    As to the rot issue, the panels will be installed over a high quality WRB (eg. siga, proclima). The panels are installed as a rain screen, with a 1/2" air gap between them and the WRB. Additionally, there will be a ERV on the interior of the house to help maintain reasonable interior humidity levels. Would this reduce your concern about rot?

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