Builders in Britain never used to care much about airtightness. In fact, until fairly recently, a degree of draughtiness was expected of houses in order to maintain their indoor air quality. The modern push to seal up all the cracks and holes in the building fabric in order to keep the heat in is not without its problems in this respect. ‘Build tight, ventilate right’ is the slogan of low energy building but, in practice, this goal can be difficult to achieve.
I’m building a seriously low energy building so – as well as wrapping the entire building in a 300mm layer of insulation – I have had to pay careful attention to airtightness. The Orchard is designed to be an exceptionally airtight building with fresh air provided by innovative heat-recovery cowls supplied by Ventive.
An early decision was to hire Touchwood Homes to supply the timber frame as they have a track record of achieving very high levels of airtightness. This is principally due to how they sheathe the frame once it is up. Rather than just butting sheathing boards up against one another and then covering the wall in a membrane, they use special tongue and groove boards that are locked together with a butyl mastic. This means that the sheathing itself becomes the airtight layer.
High quality windows and doors are also essential, so I opted for triple-glazed Ecocontract windows from the Green Building Store, a company that has long promoted airtight products. We also had to pay careful attention to how we wrapped and jointed our damp-proof membrane on the ground floor, as this doubles up as our airtight layer. Every detail matters: if you want an airtight building, don’t leave it to the end and hope to bodge it with a load of sticky tape.
Despite all our efforts to make the building airtight, I knew that hidden problems were likely to remain. So last week we had the house pressure-tested to identify all our errors and omissions before it was too late to do anything about them.
The pressure test was conducted by Paul Jennings of Aldas Ltd, who knows more about airtightness in British buildings than anyone else in the business. The test involved shutting all the doors and windows, sealing up any openings such as waste pipes – which will eventually be blocked by the traps in our sinks and toilets – and installing a fan in the back door, with a seal all around it. The fan was then turned on and the air sucked out of the house. This depressurisation drew outside air into the building through all the cracks and holes that shouldn’t have been there. It was then easy to spot high pressure draughts either by hand or with a special thermographic meter.
Sure enough, there was one major cold spot through which air was gushing into the building: a junction where the sheathing board had not been properly attached and sealed to the timber frame. Happily, this was easy to fix. Having done some repairs, Paul conducted the official test – assisted by his brother Duncan – and found that we were losing air through the building at a rate of one complete air change every hour at fan pressure. This is a very good result – ten times better than Building Regulations – but not quite as good as the Passivhaus standard of 0.5 air changes per hour that we were hoping for. The airtightness of the building will, however, improve once we have boarded up, insulated and plastered the walls. Right now, I’m just happy to have snuffed out the significant draughts before they disappeared from view.