Window Airtightness and Flashing

As we get closer to the house being rendered, finally, we have been looking more closely at any gaps remaining between our windows, doors and frame. Airtightness is a key to maximising the passive solar gain from our north facing windows and the thermal capacity of our hempcrete walls. So sealing the gaps around the doors and the windows needs careful attention.

The first step is to fill the gap. A very cost effective solution is to use closed cell foam backing rod. Normally used to fill gaps before caulking, I used a rod that was 3-4mm bigger than the gap and pushed it in with a spatula.

Install Foam Gap Filler Backing Rod with a Scraper

The next step is to tape over the gap. The tape acts as both weatherproofing or flashing as well as adding to the airtightness. I used ProClima Extora tape. It has a polypropylene fleece cover that the render can adhere to and the adhesive side sticks well to both wood and masonry so it attaches well to the hempcrete. In fact, when I've had to peel it back, it pulls hemp chips out of the wall. One concern is where the tape covers some small soft patches. We will see whether the render can be pushed under the tape in these spots to back it up.

Extora Flashing Tape installed around Brewery Window

Taping the windows where the timber reveals extend out of the wall was quite easy. The doors (and some other windows) are inset and will have a rendered reveal. The tape was harder to install, especially above the doors. Fingers crossed it will work out.

Extora Flashing Tape installed around Brewery Door

Steel flashings or drip caps will be installed over the windows that have full depth timber reveals. Unfortunately, I didn't make provision for fixing these to the wall so hopefully they will glue to the wall. Normally the steel flashing would extend 100mm above the window, but, rendering over that could be a challenge. So I decided to reduce it to between 30-50mm. It will be embedded under the render layer, so it should provide a suitable barrier. As with all the steelwork, we'll be using bitumen paint to protect it from the alkali environment.