Thursday, December 5, 2013

Sheathing & CI

When it's not raining too much for them to work, (Sigh! Rain in December?!) the sheathing is going onto the outside of the exterior wall framing.
I had planned to use the "layered" system (left) but the marketplace has filled a need by creating Zip R insulated sheathing

In most of today's building, sheathing is 4'x8' sheets, installed horizontally, to the outside edge of the wall studs. If structural, sheathing provides lateral (forces from the side, like wind) support to the walls. Once the sheathing is installed, those temporary diagonal braces we saw placed around the structure can be safely removed.

Typical structural sheathing might be plywood or OSB (oriented strand board). Non-structural sheathing could be rigid foam board. If non-structural sheathing is used alone, lateral bracing needs to be accomplished with metal strapping or let-in wood braces.

The product being used on the new house next door is a fabricated sheet that integrates the structural sheathing along with rigid foam and a water management barrier. Instead of three different materials, this product - called Zip R - is all of them on one board. Kind of neat - and a labor saver for the workers.

The continuous 1" of rigid insulation that is part of the Zip R provides R-5 insulation value to the wall. But R value is not the whole story... Building scientists have studies thousands of buildings and learned that just having a high R value is not the silver bullet we all once thought it was. Wall studs are responsible for thermal bridging (a connection from out to in and vice-versa) creating a conduit for moisture and heat to travel through the wall. (that's a bad thing) Continuous insulation stops thermal bridging. So do insulated headers. CI also puts the insulation on the outside of the wall, and with taped seams can be a fairly effective air barrier as well. That little ol' R-5 does a lot for the wall as a system. That's why I could then frame the walls from 2x4s (instead of 2x6s) because I didn't need such a thick cavity space for a higher R value in my wall cavity.

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