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The Latest Twist: Crisscross Build-Down

On a recent house, we tried a new build-down method that doesn’t require gussets. Under 2x12 rafters, we simply used 31/2-inch pneumatic nails to spike 2x3s on edge perpendicular to the rafters (Figure 4).

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Figure 4. The author’s newest method is to spike 2x3s on edge crosswise to the rafters (top left), install a reinforced vapor barrier and strapping (top right), then blow the cavities with cellulose (left).

Again we installed a reinforced poly vapor barrier and strapping (this time, the strapping went parallel to the rafters), and blew the cavity full of dense cellulose. Subtracting 3/4 inch for the vent channel, this gave us a 13-inch thickness of cellulose, including a 21/2-inch thermal break below the main rafters — a good R-49. This particular roof was more a cape than a cathedral: There was a small, flat ceiling at the top, and a kneewall halfway down the roof plane from that. But as in a straight cathedral roof, we insulated the rafter cavities all the way from the upper attic section to the eaves — the space behind the kneewall stayed within the insulated space. This technique was simple and effective, and I plan to use it again. However, next time it involves a kneewall, I’ll do that part differently. Last time, we framed the kneewall before building down the rafters, which created complications with the vapor barrier (Figure 5).

Crisscross Build-Down
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Figure 5. In the case shown, a kneewall complicated the vapor barrier installation (photo, left). To prevent that, the author recommends first insulating the entire roof plane down to the eaves, then building the kneewall. Besides giving a 13-inch-deep insulation cavity, the crisscross build-down also provides a 21/2-inch thermal break below the rafters (illustration, above).

In the future, we’ll build down the rafters first, install a 3/4-inch plywood plate spanning two build-down pieces, then frame the kneewall to that plate. Of all these methods, the only one I wouldn’t use again is the first — to my mind, I-joists for rafters aren’t worth the hassle. Of the other three methods, I’d use whichever one best suits the particular case. Building down with gussets is great for a large expanse of ceiling, where the work goes quickly — and you can get an almost unlimited thickness of insulation. Our new crisscross build-down method is also quick and simple, as long as you plan ahead for vapor barrier and strapping details. And although the foam-face and batt method isn’t the best or the cheapest technique, it gives decent results quickly on a small ceiling. But like I say, I’m still experimenting. If anyone’s got a better idea, I’d love to hear it!