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).
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
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).
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
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
But like I say, I’m still experimenting. If
anyone’s got a better idea, I’d love to hear