Urgent changes to the way U-value and moisture calculations are applied to pre-1919 buildings have been ordered to avoid inappropriate energy-efficient retrofit works under the Green Deal and ECO.
Amendments to SAP’s underlying data and guidance on condensation are already under way to take forward the recommendations of a new report on retrofitting solid wall properties commissioned by the Department of Energy and Climate Change.
In addition, the review team is drawing up a “guidance tool” for retrofit projects, the use of which is likely to be built into the PAS 2030 specification for Green Deal advisers and installers.
The changes are being progressed urgently as SAP-based calculations will form a key part of Green Deal assessments for properties.
The Sustainable Traditional Buildings Alliance, formed in November 2011 and including the CIOB, published the Responsible Retrofit Report on September 28. It looked at thermal performance, air quality, moisture and overheating in older properties.
The report compared the assumptions underlying SAP and other design protocols compared to evidence of how older, pre-1919 properties perform in practice, and found wide gaps that could lead to inappropriate interventions and negative unintended consequences.
Sustainability expert Neil May, co-author of the report, said: “These gaps are becoming more critical with our ambitions for carbon and energy reduction. We’ve been funded to do quick fixes to some of these design conventions, but there are still lots of gaps in our knowledge.”
The report found that solid wall properties tend to have better thermal perfomance than is assumed in SAP calculations, with the result that installing solid wall insulation could save less energy than current modelling suggests.
“That could be important if you’re looking at payback under the Green Deal,” said May. The table of recommended U-values for older properties used in the SAP software will now be revised.
Retrofit projects also rely on BS 5250, the Code of Practice for the Control of Condensation in Buildings. But May said that the standard was originally drawn up for new-build timber-frame buildings, and fails to mention driven rain, rising damp, or moisture moving through materials via capillary action.
“BS 5250 requires a vapour barrier [when installing wall insulation], but if the wall is already wet, you’re preventing it from drying,” said May. The report therefore recommends using the wider-ranging BS EN 15026 standard on condensation.
These changes and the new guidance tool are due to be ready by February 2013. According to May, the guidance tool would operate at different levels, for use by everyone in the Green Deal supply chain, from homeowners to installers to building researchers.
May said: “There is a huge urgency to this, but there is also a fantastic opportunity for the construction sector to engage in a vital learning process. Traditional buildings are a quarter of our building stock. If we can understand how they use energy better, that will be a fantastic thing for the country.”
