Incremental Details: Window Bucks and Rim Joist Air Sealing.

It has been another productive week.   The main level is almost framed.  There are a few deficiencies like blocking and window bucks that are yet to be completed but we are moving along (Photo 1-1).

As described in the previous post, air sealing should be incremental: carried out at the necessary step to ensure that the air tight barrier is continuous throughout the whole structure.   The next step in the process is to ensure that the rim joist space is air tight and can be connected to the interior OSB sheathing.  Since the joists are laid on top of the double top plate of the wall, there needs to be some way to connect the OSB to the top plate and to be able to caulk the space with acoustical sealant.  Once the joists are in place it would be too late.  The designer's construction details for the rim joist space relies on two parts:  Acoustical sealant/Tape sealing the OSB to the top plates and then spray foaming the rim joist space.  I had to come up with a way to do this which took a little thought.    I decided to use 3M 8067 tape.  It has a split back so I was able to tape the top using 1/2 of the tape, while leaving the paper backing on the other half and letting it hang down over the top plate (Photo 2-1).  I also taped any but joints out to the edge of the plate where the rim joist will sit.  I used a J-roller to apply firm pressure to the tape to ensure good adhesion.  Once the joists are installed and house shell is complete,  the surface of the rim joist space will be sprayed with foam out over the tape.  This will provide a fairly air tight seal.   I used tuck tape to secure the 3M 8067 tape to keep the paper backing from getting wet.   We'll see how this detail works to shed water and save the paper backing. With this detail complete its on to window bucks.

While the framers were moving along with construction I figured it would be a good time to map out the window buck construction.  I decided to try two different methodologies.  One method was to frame the buck as a single unit and push it into the window opening.  The other was to frame the buck in place.  With foam sheathed walls (3" Type 1 EPS),  the bucks needed to be about 10 1/4" deep.  Sheets of 3/4 spruce plywood were ripped then cross cut to fit the narrowest dimensions of the buck.  The butt joints of the bucks were nailed and glued, then a 2x4 rim were screwed around the perimeter.  The 2x4 rim acts as a backer for nailing the window nailing fin and the Type 2 EPS (1 1/2") was glued to the back of the 2x4s and then the buck was inserted into the opening until the foam was flush to the building (Photo 3-1).  Framing the window buck in place takes about the same amount of time and sometimes requires two sets of hands.  My experience, although limited, tells me that framing the window bucks then inserting them works well for smaller windows.  Larger windows will need to be framed in place.

Photo 1-1.  Main level almost completed.

Photo 2-1.  Tape on top the double top plate

Photo 2-2.  Tape on top the double top plate secured with tuck tape.

Photo 3-1 First window buck installed.

Photo 3-2.  Another view of the buck.


  1. Ronald Trinidad-3M14 August 2017 at 11:56

    Great post! I just wanted to inform you that around the time of your posting 3M released an air barrier sheet membrane portfolio. They are the 3015 Air & Vapor Barrier, 3015VP Vapor Permeable, and the 3015TWF Through Wall Flashing to be able to handle all those scenarios with a single manufacturer.


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