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Flash-and-fill hybrid plus external insulation?

MeriMeri | Posted in Energy Efficiency and Durability on

I designed a 2×6 dense-packed cellulose wall, fully sheathed in plywood, plus 3″ of external Roxul Cavity Board. Climate Zone 6.

I’m trying to consider alternative wall types due to the added labor costs of external insulation, specifically the complications with fastening and window detailing when getting upwards of 3″.

I’m trying to wrap my head around the Flash and Fill system described by Building Science Corp.

The document concludes by saying, “Addressing the thermal bridges would improve this wall construction.” Here are my thoughts….

1. Use 2×6 sill and top plates but stagger frame 2×4’s @ 24″ o,c, Fill the wall with 3 1/2″ of closed cell spray foam and 2″ of dense-packed batts. This eliminates thermal bridges except at the top and bottom of the wall.

2. Use the wall as described in the documents. 3 1/2″ dense-packed cellulose + 2″ closed-cell foam then add 1.5″ of Roxul to the outside of the sheathing. Roxul would not create the double vapor barrier around the sheathing but should address all thermal bridging. I hear that at 1 1/2″ there are no concerns with fastening and window installation details.

Any other ways to improve this Hybrid Insulation wall type?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Any other ways to improve this Hybrid Insulation wall type?"

    A. Of course there are other ways. There is no limit to the ways to build a wall.

    Spray foam is expensive. In this type of flash-and-fill wall, the point of the spray foam is to address air leakage and to raise the temperature of the first condensing surface above the dew point, therefore addressing the "cold OSB" problem. It's an expensive wall type, because spray foam is expensive, but it works.

    Both of your suggested variations on the flash-and-fill wall would work, too, but they add even more complexity and cost.

    It seems to me that if you want to reduce the cost and lower the complexity of the wall, the easiest approach is a double-stud wall insulated with cellulose. (To reduce the air leakage rate, just tape the sheathing seams.) It sounds as if you are already willing to consider a staggered-stud wall, which is fairly complicated. Why not just build a double-stud wall instead, without any spray foam or Roxul?

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