A Better Exterior Wall System
Like everything else in construction there are hundred different ways to do things. However there are always trade offs. In designing the exterior wall system for our latest project we laid out our requirements and then worked backwards from there to come up with our design. For any green building you must first start with a high R-value and tight air-sealing. So we set our sights on designing a system that we could insulate to R-40 and seal to an ACH50 of 0.60. We wanted to design a system that could meet Passive House certification. That way in the future we could utilize the same system should we decide to go for their certification.
Secondly we wanted a system that was easy to construct. No special tools or knowledge should be required. It needed to be a system that could be easily taught to our subs.
Lastly it was important that the materials in our system were “green”. Many high r-value walls rely on lots of foam insulation. While it is possible to specify eco-friendly foams, we prefer to avoid the use of foam when possible. We did use XPS below grade, but no where else in our design.
Given our requirements we decided to design a system around a double stud wall construction. Below are the details for our 10″ thick wall system.
A key feature of our system was the thermal isolation of the two walls. The only thermal bridging occurs where the 3/4″ OSB is used for a draft stop between the first and second floors and at the roof truss connection. The walls are both constructed with 2×4’s. The inner load bearing wall was laid out 16″ O.C. and the exterior wall was laid out 24″ O.C. By keeping all of the loading on the interior walls we were able to eliminate contact of all of our headers and floor systems with the exterior surface.
To air seal the envelope we relied on a two fold approach. We used Huber’s Zip Sheathing on the exterior walls. This, combined with strategic use of accoustical sealant and the ceiling drywall on the second floor, constituted the primary air barrier. To ensure the integrity of this air barrier we used a blower door to test for leakage prior to the installation of insulation. After touching up all identified leaks we netted the walls with Insulweb and proceeded to dense pack the walls. Our system was designed to make it easy to insulate behind the headers and floor systems (an often difficult area to insulate well).
In addition to the primary external air barrier we also sealed the interior surface of the walls using the “airtight drywall approach”. While there is a great deal of debate regarding the ability of wall systems to either breath to the inside or the outside, we took the approach that vapor drive becomes a non-issue when air cannot penetrate the system from either side. The key to ensuring that all critical joints are sealed is to address them during framing. Many joints are difficult to reach at a later stage, but by having a methodology in place and proper training we make sure to always be conscious of the importance of sealing our framing as we go.
Continuous beads of sealant or tape will take care of all the panel edges on both the interior and exterior side, but the most difficult areas to seal are the electrical and plumbing penetrations. To address these areas we used airtight electrical boxes from Airfoil and plumbing flashing panels from Quickflash.