The Fairfax City Zero Energy Home: Building Envelope Discussion
Our last post for this project was back in June, and we have come a long way. I want to focus in this post on our decisions about the building envelope.
The exterior wall framing of the house is 2×6 wood stud construction, filled with densepack cellulose insulation. Our original plan was to clad this frame with the ZipSystem sheathing we used on the modular Passsive House. That would then be covered with 5” of Roxul mineral wool batts and clad with a rainscreen of high pressure laminate panels. We liked the Fiberesin Stonewood panels because they were the most affordable. We proposed to anchor the rainscreen furring to the framing through the Roxul with special long galvanized screws – a system which has been used on the west coast, sometimes through as much as 10” of insulation. After adapting that to some degree so that our engineer would sign off, we got push back from the subcontractor who was going do the installation. In the end, we simply could not get installation costs down to a reasonable level.
We switched to the EIFS system we used on our Rockville Passive House, and will only be using the Stonewood for accent areas. Working again with AIrd, Inc. we asked them for the finest, crispest finish they could provide and they have not disappointed. They applied their “Limestone” finish with integral coloring over 5” of EPS foam. In the end we have a better product for less cost. One reason for that is we substituted ½” plywood wall sheathing for the ½” OSB ZipWall system. Instead of using tapes for our air sealing we now have the STO Stoguard liquid applied air water sealer over the entire wall, which is part of the EIFS system.
Here are some pictures. I’ll start with a sequence showing the various stages of clothing the building, then follow with some details…
EPS foam went on, the second component of the EIFS
Window corner showing Stoguard covering Tescon Vanna air sealing tape. (The blue is temporary tape attaching the protective polyethelene over glass.)
Preparing the EPS panels for attachment to the wall. They comb the adhesive vertically to hold
the EPS off the wall, creating a drainage plane and capillary break behind it. Thicknesses of up
to 12″ of EPS can be applied this way.
The south canopy is put in place after Stoguard coating is applied over the plywood. Note the
high density foam spacers behind the steel which create a thermal break at the wall plane.
EPS boards after installation, before leveling
Leveling the EPS. It reminds be of the way wooden boatbuilders fair the hull with “longboards” –
boards just like this covered with sandpaper.
An EPS board after leveling and after the accent joint lines are cut in.
Rough stucco at a window sill
EIFS work is now complete and carpenters are working inside applying trim and flooring. In a couple of weeks the another crew will apply the high pressure laminate rainscreen system at the indentation at the front door and at the projecting breakfast bay.