Designing and constructing a tight building enclosure is the most important component of an energy-efficient home. Without it, the rest is irrelevant. We hear a lot about various factors that contribute to a tightly built home, such as insulating the walls and including insulated windows, but not much about the exterior sheathing.
However, traditionally installed sheathing can be a source of air infiltration and energy loss. Here’s why. Take a look at the graphic on the left below, courtesy of Norbord, showing the normal horizontal installation of 4 X 8 sheathing. The blue arrows indicate the communication between conditioned and unconditioned space through the seams. (Spacing structural panels is an installation requirement but the resulting horizontal gaps permit air infiltration, even when factoring in house wrap with taped seams because house wrap is designed to breathe.) The photo on the right shows these gaps in an actual installation. Not a pretty picture, is it?
Fortunately, there are much more efficient sheathing options. Let’s take a look at two products that will allow you to sidestep the majority of this leakage:
TallWall and Windstorm: Norbord’s TallWall and Windstorm sheathing products attacked the air infiltration issue by eliminating the horizontal joints altogether. By installing tall sheets of exterior sheathing in 9 and 10 foot heights cut to a width where the vertical seams fall directly over a stud, normal air infiltration can be reduced by up to 60% (according to testing done by the National Association of Home Builders Research Center). See the photo below, courtesy of Norbord. In addition to reducing air leakage, TallWall and Windstorm offer the following advantages:
- Lowers labor cost 60%
- Reduces waste
- Eliminates the need to block seams in high wind areas
- SFI or FSC certified
- NAHBRC Green Approved
- No added urea formaldehyde
- Increases wall strength by 38% according research performed at Virginia Tech
For more information, visit http://www.norbord.com/na/wall-sheathing.
Zip System: Another way to eliminate the open seams in sheathing is to specify Zip System™ wall sheathing. The combination of a water-resistant coating and seam taping contributes to both moisture control and energy-efficiency. This product is a common choice here in the central Florida area.
Here are a few more Zip System perks:
- 180-day rough dry-in
- SFI certification
- No urea formaldehyde
For more information, visit http://www.zipsystem.com/
You may be wondering if it’s worth incorporating sheathing products such as those mentioned above if you’re utilizing open-cell foam insulation. In my opinion, the answer is yes. First of all, all open-cell foams are not alike. The thickness required to create an air barrier varies between products. Secondly, the mix has to be correct. Can you be assured that the installers will provide the recommended thickness consistently and that the mix is correct? I’ve been on sites where there was an obvious problem with the mix causing the foam to shrink and pull away from the studs. Inconsistent thickness issues are more common that mix problems.
As always, The Difference is in the Details.