Envelopes: Four houses on the BRE Innovation Park have made the first steps towards meeting the upper levels of the Code for Sustainable Homes. Stephen Kennett finds out what we can learn from these pioneers
The industry has had almost two years to get to grips with the Code for Sustainable Homes. During this time a few brave firms have tried to design and build code-compliant prototype homes, and four even braver ones did so in the spotlight provided by BRE’s Innovation Park in Hertfordshire. Those houses were unveiled in June last year, and attracted a lot of interest from the media, politicians and the public. The tantalising question then was, how would the designs perform in practice?
BRE is about to reveal some answers to that question in a series of papers. But the good news is that all four prototypes have provided insights into the difficulties of meeting the upper levels of the code.
To refresh your memory, the four houses were the Ecotech Organics House, Hanson’s Ecohouse, Stewart Milne’s Sigma Home and Kingspan’s Lighthouse. Ecotech and Hanson aimed to meet level four, but they did so in different ways: Ecotech, concentrated on the building fabric and Hanson on renewable energy. Stewart Milne used both approaches to try to hit level five, and Kingspan did the same, but more so, to scale level six.
The first paper is all about the building envelope. “That is the logical starting point, especially with new-build homes,” says Christopher Gaze, associate director of manufacturing at BRE.
The most important lesson learned from the homes has been the need to do an airtightness test as soon as you get integrity of the envelope, says Gaze. “If additional work is then required, you do not want to be doing it to a finished building.”
Two strategies for achieving an airtightness were adopted. Stewart Milne went for a floor-by-floor strategy whereas the others opted for a single barrier around the dwelling.
The floor-by-floor approach proved problematic. Air was found to leak out of service penetrations, through fixtures and via the floor cassettes and holes in the air barrier. In contrast, the whole-house approach proved simpler and more robust.
Houses with large volumes in proportion to their footprints, or those with complex constructions such as curving facades, are more challenging to get down to low infiltration levels and can be difficult to carry out remedial work on. Simpler designs with lower ceilings and straight walls are easier. “I’m not suggesting architects just design bland boxes,” says Gaze, “but it is worth keeping things simple until the industry has got to grips with building airtight envelopes.”
Restricting the number of holes in the external wall and gaps around openings was essential to limiting the amount of sealing required. Where joints between panels and gaps between panels and floorplates need to be sealed Gaze recommends using tapes and gaskets rather than mastic, as it is easier to demonstrate in a detail drawing.
I’m not suggesting architects design bland boxes, but it is worth keeping things simple until the industry has got to grips with building airtight envelopes
Christopher Gaze, BRE
Minimising thermal bridging was also critical, although this is an area designers and contractors have more experience of. Thermal bridging mainly occurs at the junctions between the panels, around windows and door frames and services penetrations through the fabric. The junctions of wall panels to floor slabs and wall panels to roofs are particularly susceptible.
There have been a number of solutions to this. On the Ecotech home, when the sole plate was laid, it was topped with rigid insulation and the external walls fixed on top of it. At roof level, traditional eaves have minimal insulation at the point where the roof structure meets the wall plate. One answer to this has been to wrap the insulation around this point. The use of NorDan doors and windows in some homes also helped. “These incorporate insulation in the timber frame, which acts as a thermal break,” says Gaze.
Elsewhere, the freestanding balcony used in the Ecotech home overcomes the problem caused when they canterlever off the main structure, creating a penetration through the building fabric and a potential cold bridge.
All the houses had building elements of high thermal performance. Some in particular had impressively low U-values, such as the structurally insulated panels – 0.11W/m2K – and the Ecotech roof – 0.06W/m2K. However, says Gaze, it is an area where people are beginning to find diminishing returns.
One recommendation is that sustainably sourced doors and windows that address thermal gains as well as airtightness and daylighting be specified. Most of the doors and windows on the Innovation Park come from Scandinavia, although Gaze says UK manufacturers are catching up in this area.
There can be a design conflict between obtaining credits for Lifetime Homes compliance (adaptable homes for people with disabilities), good daylighting and good insulation. The Lifetime Homes standard requires windows to be no more than 800mm from the floor, whereas credits are awarded for the amount of daylight that enters the building and views of the sky. These tend to drive up window sizes, but it is better to have smaller windows to minimise solar gain and improve fabric insulation.
This conflict became apparent on the Kingspan Lighthouse, where some rooflights had to be omitted to obtain the desired fabric performance. Hanson and Kingspan achieved their daylighting credits partly by placing the rooflights in the kitchens where the requirement for daylight was greatest.
The first of BRE’s information papers – on building fabric – will be available this month. It will be followed by papers on energy sources, overheating and ventilation; water use, harvesting, recycling and drainage; and architecture, construction and sourcing.