The architect behind the retrofit of an Edwardian property using Passivhaus principles reviews the results of the first airtightness test…
Last week we reached a milestone in our refurbishment of 64 Midmoor Road when we carried out the first airtightness test. In the first couple of blogs I discussed at some length the measures we have taken to improve the airtightness of this property. In broad terms we are using a combination of dry and wet systems - the dry being oriented strand board (OSB) over insulation and the wet being existing and new plaster to areas that are insulated from the outside. All joints between OSB are taped and all junctions between the wet and dry systems are bonded together using another type of tape.
Finally, after about three months on site we were ready to test the house. Quite deliberately we wanted to do this before any ’closing up work’ with plasterboard is done so that any defects can be remedied before it is too late or too costly.
So last Wednesday Paul Jennings of Air Leakage Detailing & Awareness Services (ALDAS) with whom we have worked with before arrived and set up his fan in the front doorway, switched on his laptop and tapped his pressure gauges repeatedly until he was ready to determine how successful we have been. By the way at this stage the triple glazed windows to the rear had not yet been installed so these openings were sealed up with OSB and tape. Similarly the sash windows to the front had only been just fitted and not snagged yet so these had also been sealed using polythene. Isolating these components meant that we were just testing the main building fabric of walls, floor and ceilings.
After an hour or so Paul switched on the fan and set about taking a series of readings which were fed into his laptop. After some time he was able to inform us that the house was now achieving a rating of just below 2 air changes per hour (ach). That’s not bad given that before work it was in the region of 14. Compared to most tests in the UK, this result would usually be seen as excellent, even for new build projects. However, that is still substantially over our target of 1 ach. Only by really limiting air leakage can one really move into the realms of Passivhaus performance.
So we spent some time with Paul surveying each room hunting for air leaks using our hands to detect the slightest air movement and checking with a smoke gun.
We discovered the following problem areas:
- Interface of new work with existing fabric, one such case being the end of a beam supporting the spine wall. Due to the rough nature of the timber and the way it had been set into the wall, making an effective seal has not worked.
- Junctions between existing plasterworks and preliminary patching. Cracks have appeared between new and old plaster due to shrinkage and large amounts of air can be felt infiltrating along these faults.
- Holes in old plaster due to fixings that have been removed. The walls were peppered with holes like this and wherever they have not been filled air movement could be detected.
- Holes in OSB. Occasionally holes have been left in the OSB layer where a fixing did not hold properly. While most of these had been taped a few remained and caused significant leaks.
- Defects in plywood. The window boxes for the triple glazed windows were formed using good quality shuttering plywood. Rather surprisingly, imperfections on the surface also showed signs of leaking.
Since then we have continued to reveal further problems by pressurising the house with an old fan that Paul left us with. The cracks in the plaster work seem to make the biggest contribution and there is a considerable amount if one measures the total length of crack - perhaps 25m. If a crack width were say on average 0.5mm wide then that equates to about 12,500mm2 of area (a hole 11cm by 11cm) , which in air tightness terms is substantial. All walls like this will have to receive a further two coats of plaster to ensure this problem is resolved in the short and long terms.
We are now concluding remedial works to the areas identified and intend to test again this Thursday. Hopefully by the next blog I can report more progress and lessons learned.