Sunday, 20 February 2011

Forming an airtight wall-ceiling junction

We have started to create the airtightness layer at various points in the building. In this post, I will describe the sequencing for one of our ceiling-wall junctions. The airtightness layer in this part of the build is formed by plaster on the wall and by Intello vapour barrier on the underside of the roof I-beams.

Stage 1) The Intello was stapled to the underside of the I-beams. Tescon tape was applied to join up each row of Intello. We also taped over the staples, in case the Intello is stretched and a small gap appears around the staples, although apparently this is not strictly necessary to achieve the Passivhaus 0.6 air changes per hour (ach) target.

Stage 2) In order to join the plaster to the Intello, we used Contega PV tape - half adhesive backed, half mesh. We had to use Tescon tape to stick the Contega to the Intello, as the Contega adhesive would not stick properly to the Intello. I had a very helpful conversation with Niall Crosson from Ecological Building Systems where he told me that if the Intello is exposed to humidity, it can affect its ability to stick to the Contega. I guess this must be because of the property of the Intello to vary its vapour permeability according to the ambient humidity.

Stage 3) After the first photo below was taken, an initial parge layer was applied under the Contega. We took care to put a kink along the length of the Tescon tape by pushing it up slightly, to allow for any possible future movement between wall and ceiling.

 Stage 4) Once it had dried, a second layer was applied to encase the mesh and the Contega tape above it. Then battens were screwed through the Intello to the I-beams. The Intello grips the screws and forms an airtight seal around them, which is only compromised if the screw is removed: it is vital therefore not to remove a screw if it has been incorrectly positioned - better to leave it in place. The space above the Intello (i.e. between the I-beams) is to be filled with Warmcel cellulose, so the battens will help the Intello support the weight of the Warmcel and will provide a 25mm service void to the ceiling plasterboard. 25mm is normally too narrow but I didn't want headroom to be reduced any more than is necessary in this attic space.

The remainder of the wall will be plastered later, along with the ceiling.