Final Steps in Air Sealing

With the date for our blower door test set within the coming week, we had some major work to complete with air sealing around the windows and doors.   Like all of the details for the air barrier, there is a redundant system for air sealing.  First, the gap between the window and the buck gets a good bead of spray foam.  This will provide some air sealing as well as some insulation to the gap.  Next a backer rod is installed.  Finally the backer rod is caulked to the door jambs/header and the window buck.  Overall its not a hard detail to implement...unless theres not enough space between the window frame and the buck.

When the windows were installed, we worked to ensure there was an even space all the way around.  Don't trick yourself into thinking that framing is accurate.  It is not.  Once a square window goes into a framed opening we quickly saw that the bucks weren't as square as the windows.  We had to rob space at one corner of the window in order to get an even space at the opposite diagonal corner.  with the longest window (9'), this lead to a 1/4" gap at the window corner.  My experience with the window installation is that Kohler doesn't provide for enough space when specifying windows.  They specified that the rough stud opening should be 3/4" larger than the window frame.  With framing inaccuracies being anywhere from 1/8 to 1/4" (especially as studs are drying, cupping, and warping), there is simply not enough room to play with.  A full 1" would be better and would lead to a better install with gaps wide enough to work with.

In any case, I had no other option but to work with the space left.  Luckily, my spray foam gun has tips which can be inserted into fairly narrow openings.  The big issue is seeing how much spray foam you are actually applying to the space between he buck and the window.  Using a flashlight helps but with really deep bucks it was really hard to see.  I was using a low expansion foam (Photo 1-1) so I just had to spray enough so the bead touched the window/door and the buck. After some expansion there was still enough room to push a backer rod in place.  Photo 1-2 shows the foam between the door and framed opening.  Photo 1-3 shows a 3/4" backer rod installed.  The backer rod was then caulked to the door and the stud opening.  For large gaps, we opted to caulk one side of the backer rod to the window/door and caulk the other side to the stud opening.  On gaps smaller than 3/8", the gap was filled with caulk.

Because of the uneven spacing between the windows and the bucks, a selection of backer rods were required.  I picked up backer rods with thicknesses from 3/8" to 3/4" for the windows.  Backer rods should be about 1/8" larger than the gap they are filling in order for them to be effective.  They need to compress enough that they stay in place.   Working around the window I transitioned from one size backer rod to another, pushing them in place with my fingers first and then pushing them firmly against the spray foam using a 4" broad knife.  It is really amazing how effective these backer rods are for sealing around windows and doors.  The gap at the bottom of one of the sills was too narrow to spray foam.  Because the bottom of the window was not caulked (for drainage) we could feel air entering the building from outside (it was a particularly windy day again).  My only option here was to insert layers of backer rod.  Pushing lengths of backer rod into the gap immediately stopped the air from entering so I know it is doing the job it was meant to do.

Air sealing the rim joists called to a 2 component spray foam.  Initially, I was going to get a contractor to do this for me.  However, after exploring my options, I decided to do it myself.  We used a 600 board foot kit that was purchased at our local Home Depot (Photo 1-5).  We were equipped with a disposable suit, a respirator, safety glasses, and a set of gloves (nitrile) (Photo 1-6).  Working in the rim joist space is difficult.  Space is limited which makes it hard to orient the spray gun and also makes it hard to see where you are spraying.  It took some time to get the application right.  Spraying on 1/4" to 3/8" gives about 1" at full expansion.  We sprayed the corners first then the surfaces as specified in the instructions.  A second application was completed after a couple of hours to ensure we had about 2" of foam as specified in my construction drawings.  Photo 1-7 and 1-8 show the final product.  It is a bit bumpy in places but should work fine.  Spray foam can suffer from issues during the curing process which can lead to non uniform R-value.  In places where the foam was protruding, I cut it off to inspect the quality.    The pieces that were cut were randomly sampled.  The inside of the the samples looked almost like XPS insulation.  There were no large voids or bubbles so I am pretty confident that the application was good.  Spray foam is expensive but about 1/2 the price if you do it yourself.

Some final details included sealing around the ERV duct vents.  A bead of acoustical sealant was first applied around the duct to the OSB.  A roflex gasket ( was pulled over the duct, pushed back to the OSB and tape sealed with 3M 8067.   The same application was completed for our wood stove air intake.

In preparation for the blower door test, all duct openings, pipe stubs, etc, were sealed with tape and plastic.   According to my passive house designer, the square footage of the thermal boundary is 5906.8 sq. ft.  Based on the PHIUS standard I need to hit 0.05 cfm/sq. ft. for a total of 295.34 CFM at 50 Pa of depressurization.  This is a hard target to meet but I feel that we have implemented the air sealing details accurately so we are hopeful!    With the air sealing details completed, we feel a sense of accomplishment and a sense of uncertainty...

Photo 1-1.  Low expansion foam attached to a reusable gun.

Photo 1-2. Spray foam around a door frame.

Photo 1-3.  Backer rod installed.

Photo 1-4.  Climatic backer rods are available locally.

Photo 1-5.  Two component spray foam kit.  Just read the instructions and apply!

Photo 1-6.  Spray foam application.

Photo 1-7.  Spray foam between the joists.

Photo 1-8.  Spray foam between the joists.


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