Barndominium Part 3-Windows

This post first appeared on the Green Building Advisor website.

When I first started planning the water and air control details for the barndominium project, I knew the windows were going to be one of the tougher parts.  Window framing for a typical post and frame building consists of a simple 2x wood frame extending horizontally between two posts, which in the case of this structure are six feet on center, with two vertical boards fastened between the upper and lower horizontal frames.  Once the basic frame is constructed inside the wall cavity, horizontal wall girts are placed over the framing, a kind of window buck, if you will.  My original thought was to have the window buck placed around the window rough opening, flash the buck so I could extend the window flashing back to the Majvest WRB, then add the needed wall girts for fastening the cladding.  In my mind, this wouldn’t be a difficult assembly to build, the framing crew didn’t agree.  They insisted on not changing their usual assembly of having the exterior wall girts installed, then for me to have to figure out how to extend the window flashing to the WRB.

There are two different window sizes in the barndominium project, the first is a roughly 3 foot wide by 3-6 tall triple pane casement window.  The second is a 5 foot wide by 2 foot tall fixed class triple pane.

Working through the detail.

Because we are using Siga’s Majvest WRB along with Siga’s tapes, I used Siga’s Ultimate Guide to Window Flashing publication as a guide to how to flash and seal the window rough openings.  Unfortunately, there were no details showing how to flash a window when horizontal wall girts are present.  There are details showing how to flash a window buck, that’s what I based the first design from.

My window detail includes pieces of Siga’s Majvest SA, which is a self-adhered WRB, attached to the window buck, then cut the self-adhered WRB so that it extended into the rough opening and also extended onto the mechanically fastened WRB.  The issue was I had to cut around each of the horizontal wall girts to make this connection.  I could see right away where water might move to places it shouldn’t be.  The WRB isn’t an air barrier in this installation, so I’m only worried about water.

There were two solutions to fix this issue, the first would have been to cut pieces of Siga’s Wigluv tape to seal the top and bottom of each of the intersecting horizontal wall girts where it met the window buck. This would have required making an inside corner out of tape and lots of time to seal both the top and bottom side of each of the horizontal wall girts.  A second option, and the one I chose, leave the Majvest SA uncut around the wall girts.  Any water that managed to find it’s way behind the edge of the Majvest SA would have to move nearly 12 inches horizontally before it reached the window framing.

I followed Siga’s instructions from the window flashing guide to complete the taping and sealing of the window rough opening using Siga’s tapes.

The second, wider but shorter window did not have as many horizontal wall girts present, only one at the bottom.  This simplified the connection of the Majvest self-adhered to the mechanically attached Majvest.  On these windows, I did make an inside corner out of Wigluv tape to keep water from moving behind the wall girt next to the window buck on the bottom wall girt.  I left the bottom of the wall girt/window buck open to allow any water that should end up behind the flashing to free drain out.

How I made the inside corner using tape, start with a square piece of tape, I used the 4-inch-wide Wigluv tape, it’s best if the tape has a 50/50 split back.  Fold the tape in half along the split back, then peal back half the release tape on half of the folded half.  Fold the exposed tape diagonally from the fold to the tapes edge.  Unfold the tape to create the inside, watertight corner.  Thanks to Siga’s factory rep, Yuxeng Yang for the lesson on how to make the inside corner.

The window rough openings were tied to the interior air control layer by sealing the interior Majrex smart membrane to the window framing using Siga’s Rissan tape (the green tape in the photo).  The windows were installed per the manufacturer’s instructions and the exterior taped to the rough opening to complete the water sealing detail.

To air seal the window to the interior air control layer, Siga’s Majrex, I chose to use another tape, Siga’s Fentrim 20 tape designed for interior and exterior air and water sealing around window perimeters.  When using on the exterior, this tape would typically be used on non-flanged type windows.  When using on the interior, the tape works great as the main air seal between the window and window frame.  I prefer using this tape on windows that do not have a jamb extension installed at the factory.

I first installed a backer rod between the rough framed opening and window.  The rough openings ended up a little larger than I typically see, I used a 1.25-inch backer rod to fill the gap.  Once the rod was in place, I installed the tape along the perimeter making the air seal between the frame and window.  The Fentrim 20 tape has a split liner and is pre-folded to simplify the installation on a window.  A bit of a tedious process, but it turned out well.

Once all the tape was in place, the finishing crew adds the extension jambs and finishes the window trim, which in the case of the shorter windows, were all metal.

The choice of tape

Why use tape instead of a minimal expanding window foam or caulking to air seal the window rough openings?  The issue I have with spray foam is you cannot see how well of a job you are doing.  I’ve seen a lot of windows that have been spray foamed leak air during blower door testing, usually around shims and areas too tight to effectively add the canned spray foam.  I also worry about water getting stuck in the space between the framing and window where it can damage moisture sensitive products.  If, or a better description, when a window leaks, we want the water to freely drain down to the sill where it can exit at the unsealed exterior sill area of the window.  Spray foam may hinder that movement of water down.  In my opinion, backer rod and caulking is a better choice, but this project had a very large space to span.  The tape worked well.

As I said at the beginning of this post, the window details were a challenge.  Had I been involved in the framing of the project, along with the water and air management strategies, I could have changed the sequencing to better accommodate details.  These circumstances had us finding solutions on the fly, I prefer solving these issues at the planning stage, not during construction.  This project has had enough of these “challenges” that I’m learning many lessons on how to make changes to the process of constructing a building of this design.  Something I will discuss in the final post of this series.  We have a couple more posts until then, up next is the insulation installation.