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).

	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

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!