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Rigid Foam Sandwich ABOVE Subfloor Zone 2

Smipau29 | Posted in General Questions on

I have built a small 200sqft office that I’m testing some different materials and air barrier assemblies on. The structure has 2” rigidfoam exterior with air gap with an air barrier behind this to the interior wall. There is 5” rigid foam insulation under a metal roof that has a 3/4” gap and an air barrier as well as some high temp roofing membrane. So far I have been able to seal everything very tightly but failed to pay attention to foundation/ subfloor detail. From some previous post & different vendors that provide radiant floor/rigid foam type heating, I believe the below install could work making for a better airtight assembly and still have air movement at the subfloor to dry during humid days. Basically I’m making a SIP, but instead of decking/rigid foam/decking it’s decking/rigid foam/membrane/high strength self leveling concrete. Here we go, let me know your thoughts on this!

Climate Zone: 2 
Interior F°/RH%: 68-75/40-55
Foundation: Pier and Beam (1-2’ clearance)
Floor Assembly Detail:
– subfloor decking
– 3/4” foil faced rigid foam polyiso
– Ditra Uncoupling membrane
– self leveling concrete 
– Resilient Tile flooring (LVT) 

To get this out of the way on why Ditra, I wanted to test an uncoupling membrane to see how it works with LVT especially for any type of movement. Schluter confirmed that this is the approach used on their Ditra Heat membrane and “should” work the same. Ok now back to the detail…

—The rigid foam would be adhered down to the subfloor with the foil facing the interior.
—Next would be the ditra uncoupling membrane, this would be adhered to the rigid foam and sealed to the inside of the structure making an air tight assembly.
— Self leveling concrete is laid
— Adhesive and LVT is installed. 

The Ditra membrane provides a slight gap between the foil face rigid foam due to the uncoupling portion of the membrane. Also, my Initial thoughts would be to not tape and air seal the rigid foam so that if any moisture collects above (interior side) it could escape. However now thinking this should be airsealed. But would you seal up both the rigid foam and then the uncoupling substrate as well?

So do you think the decking will rot In 2-5 years or longer or will the 3/4” rigid foam not provide any extra value in the end? I am documenting this install for testing purposes and Have taken base readings to date and will be combine all data throughout the next few years to hopefully publish something useful with my findings! Just need help with this final detail/experiment and get some thoughts on it. Also, I understand for my climate I am not meeting the code min for floor R-13, but hey that’s why I’m testing for my self!


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  1. Smipau29 | | #1


    I. Contacted the rigid foam manufacturer to see what their opinion was and suggested for the polyiso to be above the Ditra membrane, then to sandwich with Ply. This is as a recommendation and in no way a “definitely this will work if installed this way for your zone.”

    II. See the below link, I need to finish this up and make my bed and lay with it at this point. Would still love to hear opinions and thoughts on this!
    (Minus the heat strip and see line “wood floors”

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