mvhr & svp
Fitting the MVHR intake / exhaust & SVP.
We have now fitted the MVHR intake and exhaust ducts through the external wall, as well as the soil vent pipe (SVP). We are trying to have a 'foam free' site and avoid the use of expanding foam altogether, so we had previously just made the wall construction where the MVHR intake and exhaust ducts are to go as normal, with the intention of fitting these ducts later and adjusting the wall insulation to suit.
During the course of the ventilation design process our system designer, Stefan of Heat Recovery Scotland, elected to use an insulated duct, which has a better lambda value than the previous plan of a metal duct over-insulated, and is still condensation resistant. In order to get these ducts, which are pretty large diameter, to align correctly and fit onto the external combination grille they had to pare back the insulation on each duct in the centre and glue them together using a proprietary glue with insulating cover strip. This meant that the whole assembly arrived on site for fitting in one piece, and that each 'duct' still has the correct amount of insulation all around the air.
Here are some photos that illustrate the process.
Firstly, we prepared a rectangular hole as we thought coring two circular holes whose diameters overlap would be pretty tricky, especially considering there are several different materials through the whole wall construction.... We worked from the outside in and the inside out.
Once the duct was in place we re-filled around the perimeter from the inside using the flexible wood fibre insulation (pushing it right into the construction with a long drill bit to get it through the c. 400mm wall depth)... From the outside we then filled in around the duct with cut-down pieces of Gutex (thank goodness for the band-saw!). Although this looks pretty scrappy this will all be covered by an Extoseal flashing later.
Back to the inside, we then trimmed several strips of Intello membrane to fit around the ducts and to cover the Steico Flex insulation that was exposed, and taped these to the wall first. We then taped these to the duct itself, using short strips of Tescon Vana tape to accommodate the radius of the duct.
Unfortunately in order to fully connect the bottom strip of Intello to the wall board we had to remove the piece of plasterboard we had previously fitted to the wall, un-screw the service cavity batten and remove a panel of the insulation, as there wasn't enough room to make a good connection, even with my relatively small fingers! We made sure we taped over the previous screw holes before re-fitting the batten. Luckily this didn't take very long.
Once the duct was fully taped we put the service cavity zone back together again.
We went through more or less the same process with the SVP, although this time we were able to just core a simple hole straight through the wall using a standard hole corer. Here are some photos of this.
From top left to bottom right:
Sunshine! We cored the hole on a slight downward angle to allow any condensation that may form within the pipe to run towards the outside of the building - here you can see the plywood that extends down from our parapet wall construction above half way through the wall construction...
We pre-assembled the pipe with a grommet and taped it on to make sure it didn't move while installing the pipe. The external balloon cage style grille will sit behind our cladding...
We primed the Gutex before sliding the pipe back into position...
We then taped the grommet back to the Gutex using Tescon Invis tape (black and with no branding so that it won't be visible behind the open rainscreen cladding)...
Finally, to avoid any moisture migrating back into the structure we applied external grade silicone between the grommet and the pipe...
The same pipe from the inside, with a 90 degree bend fitted ready to take the rest of the pipe later. We used another Roflex grommet internally and again taped this on to the Spano Durelis Vapour Block board. (On taping auto-pilot I also taped the grommet to the pipe however this isn't necessary to make an air-tight seal!)