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2 Layers of foam

A B | Posted in Energy Efficiency and Durability on

long time reader, first time poster. 
I had a question regarding adding 2 layers of foam under a radiant slab.
I am in zone 6. I am planning on adding 1 layer of 2″ under slab 35 psi styrofoam r10 (recycled) and adding on top of that amvic radiant panel r10. This will bring the insulation value to r20. (I will also be having 2″ foam on the inside perimeter)
my question: should I be taping the 1st 2″ layer of recycled foam at the seams or should I leave it untaped so that if any moisture were to get trapped between the layers it could possibly escape between the seams? The top layer panel also acts as a vapour barrier.
I’m leaning towards leaving it untaped but just wanted to hear other opinions.
thank you in advance.

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  1. Expert Member
    Dana Dorsett | | #1

    Tape the seams of each layer. At 2" XPS is not going to be a major moisture trap. With a second layer of foam on top the water isn't going to flow from gravity anyway- it will be wicking in the micro-space between sheets.

    From a design point of view don't count on the 2" XPS for more than R9 (it's warranteed rating), and long term you're really looking at R8.4 at full depletion of the HFC blowing agents.

  2. GBA Editor
    Peter Yost | | #2

    Hi Dana -

    Looks as though both layers of insulation in this assembly are EPS, not XPS. Given that, why are you de-rating the r-value over time?

    Also, I am not sure why the EPS panel joints need to be taped, if a concrete slab is being cast above the insulation and it will be the air barrier? Important to complete the air seal at slab perimeter, granted.


  3. A B | | #3

    That is correct both are actually EPS
    and I should actually have been more specific the bottom layer (touching the gravel) is also laminated on both sides meaning there are no micropores for it to wick through if moisture were to get trapped in between the bottom and top layers. Which is why I am still leaning towards leaving the bottom layer untaped but making sure the top layer is properly sealed.

  4. Andrew C | | #4

    AB - I think the standard order of layers beneath the slab is crushed rock, then rigid insulation, then plastic vapor barrier, then concrete. The insulation is basically just insulation. The vapor goes on top of that, directly in contact with the concrete. Also, the vapor barrier on top of the insulation keeps the rigid insulation from floating up during the pour. Between the vapor barrier and the concrete (air barrier), there's no need to tape the rigid insulation. That's my understanding.

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