Trial and error

Making period homes highly energy efficient is one of the most challenging jobs facing the industry. Robert Prewett, partner in Prewett Bizley architects, has bravely taken on the job of refurbishing a terraced home in Balham, south-east London, to near Passivhaus standards. This means that a leaky, 100-year-old three-bedroom, solid-walled property will be heated with a single towel rail. To achieve this, the house has to be highly insulated and airtight: the team are targeting 0.6m3/hr/m2 which is 17 times better than the Building Regulations demand. Prewett is posting a fortnightly diary on progress at www.building.co.uk. Here are excerpts from the second month on site.

Building services
As well as substantially improving the performance of the building fabric, we have also targeted the design and installation of the building services.

Mechanical ventilation with heat recovery is vital to achieving the performance levels needed for Passivhaus buildings. It is possible to recover up to 90% of the heat that’s normally lost when stale air is exhausted out of the house.

While the large loft would seem the obvious place to install the ventilation unit, we would have had to make access easy and safe as well as taking the supply and extract ducts through the roof. We also felt that the long duct lengths could lead to poorer efficiencies. Instead we are installing the unit towards the centre of the home and running the supply ductwork within the joist zone of the floor and within a thickened stud wall - see the blue pipes in the diagram. The air is then pulled through the house and extracted via the kitchen and bathrooms.

We are installing ducts that are about 2.5m long with 50mm insulation. Users of Passivhaus planning software will be aware that designs with long intake and exhaust ducts within the building envelope reduce estimated system efficiency. This can be mitigated by insulating the ductwork, but this needs to be done with care to avoid trapped air pockets or any gaps that allow air or moisture onto the ducts’ surfaces, which can result in interstitial condensation and poor heat recovery.

The following had to be borne in mind when designing the ventilation system:

• Locate the air delivery and extract points to maximise air movement and avoid dead areas where it may stagnate
• Use rigid galvanized steel ducts to reduce friction
• Short flexible lengths of ducting can be used to attenuate noise transfer so “cross talk” doesn’t occur between rooms
• Allow good separation of the terminals to avoid stale air finding its way into the intake.

Another area we’ve been tackling is the domestic hot water system. If we get the design of this wrong, we could lose enough energy to heat the entire house, so we’ve enlisted the help of Nick Grant of Elemental Solutions to specify the final system. We’re locating the boiler and hot water tank next to each other, and close to the bathroom, WC and kitchen to minimise pipe runs. The measured incoming water pressure is just over 3 bar; we believe the water pressure at the tank will be a little over 2 bar. Given that sort of pressure and short pipe lengths, it is possible to use small bore pipes - the logic being that the volume of water in the pipe is small and will dissipate little heat when standing by. When a tap is opened, the hot water will arrive quickly so users won’t need to waste lots of lukewarm water waiting for the hot water to arrive. We intend to insulate all of these pipes to further reduce heat losses.

Airtightness testing
The previous Passivhaus diary discussed the importance of airtightness. The day of reckoning soon arrived on site. We wanted to test the work before any “closing up” with plasterboard is done so that defects can be remedied before it is too late or too costly.

Paul Jennings of Air Leakage Detailing & Awareness Services 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’d been. At this stage, the triple-glazed windows to the rear had not yet been installed so these openings were sealed up with boards and tape. Similarly, the sash windows to the front had not been snagged yet so these were sealed using polythene. Isolating these components meant we were just testing the main building fabric of walls, floor and ceilings.

Paul took a series of readings. After some time he informed us that the house was achieving a rating of just below two air changes per hour. That’s not bad: before work started, it was about 14. Compared with most tests in the UK, this result would usually be seen as excellent, even for new-build projects. However, that is still substantially more than our target of one air change an hour. Only by really limiting air leakage can one move into the realms of Passivhaus performance.

We discovered the following problems:

• The interfaces between new work and the existing fabric wasn’t perfect; one such case was the end of a beam where it sat in the wall
• Cracks had appeared between new and old plaster owing to shrinkage; large amounts of air could be felt infiltrating along these faults.
• There were holes in the old plaster made by fixings that had been removed
• There were holes in the oriented strand board used as the principal airtightness barrier
• There were defects in the plywood. The window boxes for the triple-glazed windows were formed using good-quality shuttering plywood, but surprisingly imperfections on the surface showed signs of leaking.

After using the fan for several days and surveying every inch of the house using our fingertips to locate the slightest leak we conducted a second test with Paul Jennings. The good news was that we made it. We achieved 0.8 air changes per hour and now are reasonably confident that we can hold it at this if not better. Although that is still above the 0.6 for full Passivhaus compliance, it is good enough to satisfy the EnerPHit Passivhaus house standard that we hope will be ratified soon.