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Community and Q&A

Wall stack-up with recycled polyiso

easyrider470 | Posted in Green Building Techniques on

Just wanting a sanity check on my final wall stack-up.

I am in Bloomington, Indiana 47460

Stack-up is as follows:

2×6 studs with Knauf EcoSeal at all seams and voids

R-19 batt fiberglass insulation

Plywood sheathing

Tyvek attached to sheathing and windows mounted and flashed to the Tyvek

1.5 inches of recycled polyiso fiberglass-faced rigid foam mounted to the sheathing with taped seams

1×4 furring strips over foam for siding to mount to

Vinyl siding with vented plastic between furring strips.

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  1. GBA Editor
    Martin Holladay | | #1

    The only aspect of your plan which may raise some questions is your plan to install vinyl siding on furring strips.

    I think that it's fine to install vinyl siding on furring strips, by some manufacturers don't. For more information on this issue, see Can Vinyl Siding be Applied Over Furring Strips?

  2. Expert Member
    Dana Dorsett | | #2

    The stack-up will work fine in your climate, but it may be worth the up-charge to swap out the R19s for R21 fiberglass or R23 rock wool.

    R19 fiberglass is extremely low density, and only performs at R18 in a 2x6 cavity. This is because it is tested at R19 for labeling purposes at 6.25" thickness, but is compressed to 5.5" in a 2x6 cavity- raising the density (and R/inch) but lowering the overall R. With R21 f.g. or R23 rock wool the as-installed performance is as-labeled. In a 16" o.c. 2x6 studwall the difference whole-wall R between R19 fg and R23 rock wool is a bit over R1.5, and with 24" o.c. framing it's about R2.

    Assuming a 25% framing fraction (16" o.c. studs) and a climate-derated R5/inch average performance of the reclaimed polyiso, your original stackup is going to deliver about R21.1 whole-wall (with the thermal bridging factored in), With R23 rock wool that becomes R22.7, a 7.6% boost in wall performance.

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