Tips and Tricks for Exterior Air Sealing

Airtight drywall is an all encompassing term that includes air sealing according to the Thermal Bypass Checklist, the Thermal Enclosure Checklist, and Energy Trust of Oregon's air sealing checklist. The theoretical limit for the airtight drywall approach to air sealing is about 1.5 ACH50. Earth Advantage has learned this by working with the very best production builders in the state of Oregon for over a decade.
For builders that want to get even tighter let's take a look at some lessons learned from the Passive House movement, which has historically required building to a maximum of .6 ACH50. These builders are also adding an additional air barrier (in this case, the primary air barrier) at the exterior sheathing layer. This is typically accomplished by:

1. Taping vertical and horizontal seams on the sheathing, using a fully adhered WRB, or using a liquid applied WRB. (bringing these products in to RO's on windows and doors helps too)
2. Using higher quality windows and doors with better air sealing built in
3. Upgrading the air seal at RO's of windows and doors to high quality tapes, backer rod, expanding foam tapes, AirDam sealant, or a combination there of.
4. Limiting penetrations through the primary air barrier (one way plumbing valves, recirculating range hoods, and balanced ducted HRVs are common)

In regards to the first recommendation (making the sheathing layer the primary air barrier) one of the most difficult challenges is how to tie that layer into the top and bottom of the theoretical 6 sided cube from the sides. At the bottom this can usually be accomplished using the same tape used to seal the seams of the sheathing to the foundation or the fully adhered WRB product (sometimes additional adhesive primer is necessary). At the top side making this connection fool proof can be more difficult because standard hurricane clips used to tie down trusses and rafters create significant holes in any air barrier layer brought from the exterior sheathing to the inside.

One of the easiest solutions to this challenge is to use Simpson Strong-Drive SDWC Truss Screws or FastenMaster 6" TimberLOK Wood Screws to attach trusses or rafters to a double top plate via a screw driven at an angle from beneath the top plates into the trusses or rafters. This code approved methodology eliminates the need for hurricane clips and alleviates a potential air sealing nightmare.
If eaves or overhangs are part of the design and the goal is to bring the air barrier from the sheathing layer to the roof deck (as part of an unvented / hot roof assembly) this can also present a challenge. Many builders in this situation elect to add false rafter tails after this critical air sealing connection has been made.

If you have a design that presents a challenge to effectively air seal contact your green building consultant today. Chances are that you aren't the first builder or designer to come across it.