Image Credit: David Goodyear A drywall lift simplified the installation of OSB on the ceiling. To seal the seams between sheets of OSB, we used acoustical sealant, 3M tape, primer and a J-roller. A self-adhering butyl flashing tape called Blueskin was used on floor-wall and wall-ceiling connections. 3M tape was too floppy to use in the corners, but when folded and cut as shown here Blueskin worked well. After the corner has been primed and the seams filled with acoustical sealant, the folded corner piece can be installed. Seams between sheets are sealed with 3M tape. Top and bottom edges, and vertical corner seams, get a layer of self-adhering Blueskin tape. The job was complicated by the need for 3-inch strips of OSB at the top of the wall. We used a two-part spray foam kit to seal the area behind the rim joists. It took a bit or practice to apply the two-part spray foam around the rim joists. We applied the foam in two lifts to get a finish thickness of about 2 inches. Sealing around ducts for the energy recovery ventilator was one of the last steps in air-sealing the house. Acoustical sealant was followed by a gasket and flashing tape.
Editor’s Note: This is one of a series of blogs by David Goodyear describing the construction of his new home in Flatrock, Newfoundland, the first in the province built to the Passive House standard. The first installment of the GBA blog series was titled An Introduction to the Flatrock Passive House. For a list of Goodyear’s earlier blogs on this site, see the “Related Articles” sidebar below; you’ll find his complete blog here.
In the previous post we were beginning to install the OSB air barrier on the interior side of the 2×8 stud walls. Once all joints are caulked with acoustical sealant and then taped, the OSB will act as both an air barrier and a vapor retarder.
It didn’t take long to decide that lifting these heavy sheets (each sheet of 1/2-inch OSB weighs about 46 pounds) and attaching them to the ceiling using muscle power and a dead man support was out of the question. Our local Princess Auto had a drywall lift on sale so we bought it (see Image #2 below).
We started in one corner of the building. One person on a ladder guiding the other where to push the drywall lift loaded with a sheet of OSB worked great. Our first sheet was cut so that the edge of the sheet could be nailed under the underside of the roof truss. Using a framing nailer armed with 2 3/8-inch nails was the way to go. A hammer and nails was just frustrating. The second run of OSB was offset so the joints were staggered. There were 42 sheets of OSB in the ceiling, which took about eight hours for us to complete.
After the ceiling was complete, the next task was to install the OSB on the walls. Luckily we already had a plan in place for this because we had completed the installation on the main level. The batt insulation had already been installed upstairs so all we had to do was start at one corner and install the sheet goods one at a time.
The trick was to angle the bottom of the sheet against the bottom wall plate, wedge it with a foot and use hand force to compress the insulation while pushing the OSB in place against the studs. Nailing initially (with a framing nailer) at the plate holds the sheet in place. Working our way across and up the sheet with nails on 24-inch centers completed the job. If we overlapped with a window we found that scribing around windows and then cutting to fit worked best.
Sealing the seams
After the OSB was fastened it was time to start air-sealing the seams. The technique is straightforward: simply caulk the seam with a 1/4-inch bead of acoustical sealant and then roll 3M 8067 tape on top (see Image #3 below). After rolling the tape I used a J-roller to apply pressure to the tape and the bead of acoustical sealant. This flattens the acoustical sealant and pushes it into the seam. The walls were completed using the same procedure.
Corners… those darn corners! They are a real pain to work with. The 3M tape is too floppy to work with in a corner. Blueskin, on the other hand, is pretty easy in comparison. First, the corner was primed with Bakor Hi Tac Primer. To make an inside corner I simply cut a square of Blueskin, and ran a half cut through the center. After a simple fold it was ready for install (see Images #4 and #5 below).
There was an interesting conundrum with the upstairs wall. The OSB is attached to the bottom side of the trusses, then strapped with 2×4 before drywall is attached. In order for the ceiling to be the right height for drywall and to allow for about 1/2 inch of play at the bottom, the exterior wall had to be about 99 1/8 inches high. This height is not standard. Because of this, a single sheet of OSB wasn’t high enough to reach to the top of the wall. We had to add a 3-inch-wide strip all the way around the top of the 2×8 wall. This lead to a minor problem: How to seal the upper corner of the wall with Blueskin? My plan was to run the Blueskin 4 inches down the wall and 2 inches onto the ceiling. This would effectively cover two joints at the one time.
Blueskin doesn’t have a split paper backing. Using a straightedge, I could score the paper backing if I cut lightly with a utility knife. 3M tape was too hard to work with in the corners. Blueskin is firmer and easier to place. We came up with a two-person system that allowed us to lay about 8 to 10 feet at a time. We primed the surface with Bakor Hi Tac Primer before applying the self-adhering membrane.
The key here is to work slowly and methodically. Trying to rush leads to a bad installation with a lot wrinkles and folds. Even working slowly allows some folding. With some experience working with the material, we eventually were able to minimize wrinkling, mainly through careful observation as we were applying the tape. Overall the installation went pretty smoothly (see Image #6).
Sealing around windows and doors
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 there’s 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 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 feet), this lead to a 1/4-inch 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 the rough opening should be 3/4 inches larger than the window frame. With framing inaccuracies being anywhere from 1/8 inch to 1/4 inch (especially as studs are drying, cupping, and warping), that’s simply not enough room to play with. A full inch would be better and would lead to a better installation, 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 the 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 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 backer rod in place. We caulked the backer rod 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 inch, the gap was filled with caulk.
Because of the uneven spacing between the windows and the bucks, we needed a selection of backer rod. I picked up backer rod with thicknesses from 3/8 inch to 3/4 inch for the windows. Backer rod should be about 1/8 inch larger than the gap it’s filling in order for it to be effective. The rod should compress enough that it stays in place. Working around the window I transitioned from one size backer rod to another, pushing it in place with my fingers first and then pushing it firmly against the spray foam using a 4-inch broad knife.
It is really amazing how effective backer rod is. 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
Air-sealing the rim joists called for a two-component spray foam (see Image #7 below). 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 we purchased at our local Home Depot. We were equipped with a disposable suit, a respirator, safety glasses, and a set of nitrile gloves.
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 inch to 3/8 inch gives about 1 inch when the foam has fully expanded.
We sprayed the corners first then the surfaces as specified in the instructions. We sprayed a second coat after a couple of hours to ensure we had about 2 inches of foam as specified in my construction drawings (see Image #8 below). Spray foam can suffer from issues during the curing process, which can lead to non-uniform R-values. 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 half the price if you do it yourself.
A few final details
Some final details included sealing around the ERV duct vents (see Image #9 below). First, I applied a bead of acoustical sealant around the duct, sealing it to the OSB sheathing. Next was a Roflex gasket from 475 High Performance Building Supply, 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 area included in the thermal boundary is 5,906.8 square feet. Based on the PHIUS standard, I need to hit 0.05 cfm/sq. ft. for a total of 295.34 CFM at 50 pascals 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.