This is how we did the airtightness tape on the windows and doors in our house.

We’re not experts — this is just what we did.

First, we cleaned down all the window frames so they were free from dust, dirt, and debris. Then we closed up any gaps around the windows with expanding foam. We left it overnight to dry and cut off any excess once it had set.

We used Siga Fentrim IS20 (150mm), as we have a large window depth and needed the extra width so the tape could wrap fully around the frame.

When you open the roll of tape, you’ll see a small sticky strip folded over. Press that into the window frame. You’ll notice we applied all of it directly onto the frame — note: this was a mistake. We should have used less on the frame, because there was a large gap between the window frame and the wall. When the plasterer was finished, he could only come out about 10mm, so we ended up having to go around and trim approximately 5mm off each window frame.

If you’re going to drywall slap around the windows, you won’t have this issue. Rationel window frames are quite slim, so we couldn’t afford to lose more of the frame to drywall.

When sticking the airtightness tape to the window frame, make sure it’s firmly pressed on so there’s no way for air to pass through. Roll off the backing gradually, pressing the airtightness tape tightly into the wall crevices and working it in as much as possible. Also, make sure to apply extra tape over any straps coming out of the wall — if left untaped, these can cause a cold bridge.