Image Credit: All photos: Matt Risinger Most of the wall sheathing on this house is OSB, except for a 2-foot-high course of pressure-treated plywood at the base of the walls. The gap between the top of the concrete foundation and the bottom of the wall sheathing was taped with a 12-inch-wide peel-and-stick product, Carlisle CCW 705. The Carlisle CCW 705 covers about 1 or 2 inches of the concrete slab to air seal the gap. The peel-and-stick tape also keeps water off the bottom of the wall sheathing. Notice that we apply a primer (which darkens the sheathing) before adhering the Carlisle CCW 705. Before installing the Siga Wigluv tape, make sure that you have the right primer (Dockskin primer), a tape dispenser, and a J-roller. We used Siga Wigluv 60, which is 2 1/4 inches wide. Before taping an OSB seam with Siga Wigluv, it's a good idea to install a coat of Dockskin primer with a mini-roller. Siga Wigluv covers the seam between the pressure-treated plywood and the lowest course of OSB wall sheathing. The wall sheathing is almost fully taped. After all of the wall sheathing seams were taped, a layer of Tyvek Commercial Drainwrap was installed over the OSB as a water-resistant barrier (WRB).
Every house needs four control layers. In order of importance, these layers need to provide:
- 1. Water control
- 2. Air control
- 3. Vapor control
- 4. Thermal control
The building codes have dictated the levels of thermal control and vapor control that builders must adhere to, and nearly every builder in the U.S. knows off the top of their head the R-value of the insulation in their walls and attics.
But until recently, the building codes haven’t addressed air control. The 2012 IRC now dictates that builders in my climate zone (Zone 2) need to achieve a blower-door test result of 5 ach50 or less, which is actually pretty tight. This new requirement will make quite a few builders need to research air-sealing methods in order to reach this level of airtightness.
Codes need to do a better job addressing water control
As an aside, it’s interesting that building codes barely touch on water control, which is of course the most important control layer on any house. What difference does it make what R-value your wall has if water is getting into that wall through a leaky window or a poorly sealed plumbing or HVAC penetration?
In my quest to build ever-tighter houses, I’ve decided that my goal is for every house I build to get as close to 1 ach50 as possible. Not every house I build will get to this standard, as I’m sometimes limited by the architecture or the weatherstripping of exterior doors. I’ve found that sliding glass doors, custom doors, “store front” doors, and other architecturally cool doors can really kill my blower-door test results.
Air sealing approaches
If you’ve followed my blog, you know that I’ve used a ton of different methods to air seal inside a house: spray foam, Owens Corning Energy Complete, caulk, etc. Using these methods, I’ve been able to get most of my houses below 2.5 ach50. But to reach my goal of 1 ach50, I’ve decided to take a page from the Passivhaus playbook and tape my exterior sheathing seams.
After reading Martin Holladay’s “Backyard Tape Test” article from the March 2013 issue of Fine Homebuilding, I thought that Siga Wigluv was the clear winner as the best tape for OSB sheathing seams. Martin’s article says that it’s best to use a primer, and I fully agree.
I bought my Siga tape (along with the necessary Siga accessories) online from Small Planet Workshop. I recommend using a J-roller to roll out the tape and ensure good adhesion. To see a step-by-step demonstration of the installation process, check out the video below, as well as the series of photos and photo captions.
Unfortunately, I’ve been waiting on the rear and front doors for this house, so I’ve not yet been able to perform the blower-door test. I’m expecting this house to get very close to my goal of 1 ach50.
I’ll post an update to this blog post once I get the blower-door results. Overall, I’m really happy with this Siga tape and I think this method of addressing air control is superior from a “best practice” standpoint.
Matt Risinger is a custom builder and whole-house remodeling contractor in Austin, Texas, specializing in architect-driven design and fine craftsmanship. His company, Risinger Homes, uses an in-house carpentry staff and the latest building science research to build dramatically more efficient, healthy, and durable homes. You can read more examples of Matt’s writing at Matt Risinger and the Green Building Blog.