Building the Air Tight Barrier: Window Bucks.

Ensuring that a building is air tight involves some planning...well, actually, a lot of planning!  I've learned that the barrier needs to be defined incrementally as stages of construction are being completed.  With a cross section of your building you should be able to start at some point on the buildings interior (if the air barrier is defined there) and follow around the floor, walls, fenestrations, ceilings and eventually back to the point where you began.  From there you really need to think about how to deal with changes in planarity of the building in order to make the transition with your air barrier.

One of those areas is the windows.  The windows themselves become part of the air tight barrier.  However, they need some way to tie into the air barrier of the building.  For this building, the air barrier will be defined at the interior:  OSB with acoustical sealant at joins and further sealed with 3M8067 tape.  The window bucks will be tape sealed to the OSB.   This change in planarity for the air barrier makes the inside of the buck the air tight barrier; so the windows need to seal against the interior.

I have seen many builders assume that the exterior WRB is an air barrier...and I guess in some cases, you could make it one but with all the penetrations with nails, staples, electrical, I really can't see how it could be air tight.  Builders here are typically using tyvek to flash their window openings.  The tyvek is wrapped around the building and the tyvek is cut diagonally and wrapped to the interior sill, jambs and header of the window opening to form flashings.  When spray foam is applied it is often between the tyvek and the window which would allow a lot of air leakage between the framing members and the tyvek to the interior.  None of the corners of the window opening are caulked or sealed in any way and are not typically tied into the interior vapour barrier.  This makes for a very leaky structure.  Using a solid buck with caulking at the corners and proper tape sealing makes the most sense.

The other question is how to get a good air seal against the buck but also have the window flashed properly.  Since the window drawings in my plans are cross-sectional, the details are hard to visualize.  So it took me sometime to map out my process.   My process is as follows,

For inside corners:

1. Caulk seal all corners and joins in the plywood buck with acoustical sealant.
2. Cut strips of tape the same length as the depth of the buck.
3. using 4" 3M tape, peel off one side of the split paper backing.  Fold the tape and stick to one plane of the buck close to the corner
4.  Peel off half of the paper backing for the remaining half of the tape and use a blue hospital card as in the bottom right of buck in Photo 1-1 to slowly roll the tape into acoustical sealant in the corner of the buck.  Just kidding! Any old card will work!  This gets much easier with time and practice.
5.  Use the card to smooth out tape with light pressure making sure to work outwards to smooth out veins that can form by uneven tape.
6.  For horizontal joins I find it easiest to make a small roll.  Position the sticky part at the interior edge of the buck and then peel and stick as you pull the paper backing towards the outside to unroll.
7.  Once the joins are complete, I roll them with a j-roller to ensure good contact between the tape's adhesive and the wood buck.

Now you have good air tight joins at the corners of the buck!

Most window manufacturers require some sort of drainage plane at the sill of the window.  I have seen this done with a piece of clapboard on the sill.  The big issue here is that you need the buck to be air tight but you also need it to flash to the exterior.  If you use clapboard it involves another step of making the clapboard airtight to the buck.  My plans called for a back dam on all fenestrations.  This is easy to implement.  I mark a line with a pencil, about 3 1/4" back from the outside edge of the buck, to define inside edge of my back dam material.  I picked up some 1/4" thick (3/8" wide) self adhered neoprene rubber gasket for this.  Starting at the one bottom corner of the buck I tape the gasket in place using my guide line. See Photo 1-2.

To ensure  that the taping detail will flash the window to the exterior tyvek (when it gets applied) I staple a 12" piece of tyvek flush to the top of the buck sill and out 8" beyond the edge of the jambs (Photo 1-6).  Now comes the tape flashing detail which will also provide an air tight seal to the buck:

1.  I precut my tape (6" 3M 8067) 5"-6" longer than the sill in order to make a pan flashing (Photo 1-3).
2.  I place a small bead of acoustical sealant at the inside bottom corners of the buck.
3.  I peel off about 2-3" of backer for the part of the tape to be stuck to the sill interior and use my blue hospital card (any card will do!) to work the tape into the corner.
4.  As I peel off the backer I am constantly looking to see that the tape is going straight and I lightly stick the tape to the outside edge of the buck.
5.  Once I get to the other corner, I use my card to work the tape into the acoustical sealant and then back up the jamb (Photo 1-4)
6.  Using my card I slowly work the tape around the back dam material.  As long as you haven't applied any pressure, the tape will reposition a little.

I use a similar procedure for the jambs and header.

1.  I caulk seal the corners again.  I fold my tape along the split backing line and stick as I pull off the backing.  At the head of the jamb, the tape has to be cut at the corner and wraps to the exterior header.
2.  I work the tape downwards using my card Photo 1-5.
3.  Once I reach a corner, I work the tape into the bead of acoustical sealant.
4.  When the interior of the jamb as been tape sealed, I simultaneously pull the paper backing off at the exterior while I wrap the tape to the exterior 2x4s of the building at the same time.

The final taping details can been seen in Photo 2-1, 2-2 and 2-3.  When the windows go in, they will be caulked along the back edge of the nailing fin at the jabs and header with exterior silicone caulk.  More 3M tape will seal the nailing flange to the tape already in place.   A aluminum end dam flashing will be added to the top of each window and the WRB (Tyvek) will tape directly to tape used as the final flashing detail on the nailing fin for a completely flashed opening.

Photo 3 illustrates the current stage of framing.  We are hoping to complete exterior framing within the next week.  Then we will install the roofing materials, windows and doors, WRB, and start insulating.  After insulating we'll erect the interior OSB and start air sealing from the interior.

In my next post I will discuss the sealing details for my doors.



Photo 1-1.  Air sealing the corners and joins in the window buck.


Photo 1-2.  Neoprene rubber gasket stuck on the buck sill to act as a back dam to prevent water from entering the building.


Photo 1-3.  Setting up tape for sill pan.


Photo 1-4.  Taping detail at the sill and jamb.  Note the acoustical sealant at the corner and the back dam material under the tape.


Photo 1-5.  Smoothing out tape on the jamb using a plastic card as the tape roll is being peeled by pulling from below.  Once the jamb interior is complete, the paper backing will be pulled off the exterior side in the same fashion and stuck to the exterior of the building as a flashing.


Photo 2-1 Interior view of buck.


Photo 2-2.  Exterior of buck showing the Tvyek.


Photo 2-3.  This picture shows the corner detail and how it wraps around to the Tyvek at the sill.  You can also see how the jambs wrap around to the outside of the building.


Photo 3.  Current state of construction....on the road to completion of the framing stage!

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