This build weekend began with another significant milestone: window installation! Our 5 donated windows are a mix of flanged and nonflanged. These types of windows require different types of installation detailing. Flanged windows have (as the name suggests) an integrated nailing flange that runs all the way around the perimeter. When the window is installed, the back of this flange sits against the outside face of the sheathing. Nonflanged windows are attached to the window rough opening by nailing tabs integrated into their frames. Apart from this difference, the installation process for both window types is similar: first, the window opening is lined with a window buck made from ZipSystem sheathing, which is taped with Tescon Vana tape in the corners and around the intersection with the outside face of the sheathing for air sealing. Then, a piece of clapboard siding is nailed in the opening and covered with a layer of Extoseal Encors stretch tape to create a waterproof, slightly pitched sill pan. The tape is lapped over the edge of the window buck and sealed to the outside of the sheathing over the house wrap (for flanged windows) or outboard insulation (for nonflanged windows). This sill pan will drain out any water that may leak into the window, protecting the wall assembly. Another layer of Extoseal goes over the top of the window buck for additional protection from water. The window is then moved into place and centered in the rough opening. Next up is leveling, plumbing and squaring the window. After an initial check with a level, one corner (on the high side of the window) is tacked in place. Composite shims are inserted under the sill to level the window out, and the tab or flange in the opposite corner is nailed off. Next, the window is checked for plumb with a level, and checked for square by comparing diagonal measurements between opposite corners. Any necessary corrections are made by using a small flat bar to pry the window into place before it is fully nailed off. Flanged windows require a few additional details: a bead of acoustic sealant is applied to the back of the flange before install for air sealing, and after it’s nailed in place, the flange is taped over with Vana tape on the sides and top for air sealing and bulk water protection. A buck constructed from 2×4 rips is built around the window to provide a solid surface to butt the outboard insulation up to.

Flanged windows are the most common type used in American residential construction, but they have one major drawback: the nailing flange has to be nailed to the wall sheathing, restricting where they can be set relative to the depth of the wall assembly. This also means that the installation of the flanged windows must be completed before outboard insulation can be installed around them. For the nonflanged windows, on the other hand, the outboard insulation can be installed around the prepared rough openings with their ZipSystem window bucks before the windows are in place.

This created a fork in our critical path. While one team of students worked on window install, another team unwrapped the pallets of Gutex Multitherm that had been waiting under a giant tarp for the past few weeks. Gutex is an innovative insulation product made from wood fiber treated with paraffin to repel water. It has an R-value of 3.6 per inch of thickness; we used the 80mm (~3 1/8″) variety, contributing an R-value of 11.4 to our wall, and eliminating the thermal bridging effect of the wall framing. Gutex comes in board/sheet form similar to XPS, EPS or polyiso foam insulation and can be cut to size and attached with fasteners in the same way these products can. Unlike foam board insulation, it’s made from renewable resources and without CFCs or other environmentally damaging blowing agents. However, working with Gutex is not without its challenges. It’s approximately 3″ thick, which is greater than the maximum cut depth of a standard circular saw blade, and substantial enough that cutting it with hand tools is slow going. Our answer to this was to use a combination of tools: our trusty SawStop table saw for long rip cuts, and a jigsaw with a 6″ blade for shorter cuts and notches with square corners (for fitting around windows and doors.) It has tongue-and-groove edges to lock neighboring sheets of insulation together and prevent gapping, which is great from a performance standpoint but does require pieces to be installed in a specific orientation, limiting reuse opportunities for cutoffs. Not only do pieces need to be oriented correctly, the joints must be staggered between courses-and we also had to remember the 5″ wide removable strip of Gutex that covers the joint between building sections! Since Gutex is a European product, the sheet dimensions were based on the metric system, rather than corresponding to the standard multiples of inches and feet that American-made building products are based on. With all these installation constraints, the pile of scrap grew quickly. And so did the fluffy drifts of wood fiber produced from cutting the Gutex (which, in my experience, is still far preferable to the irritating by-products of working with rock wool or foam board!) The Gutex team made short work of installation, and by the end of the day the entire back wall and portions of the side walls were covered.