Heat and Energy – timber and thermal performance

ONE THING is for certain – there are no bypassing thermal performance requirements for buildings. Professor Sean Smith, Director of the Institute for Sustainable Construction and Professor of Construction Innovation at Edinburgh Napier University, explains.

The recent changes to Part L announced by Communities and Local Government (CLG) and the drivers by Department for the Environment and Climate Change (DECC) through the revised SAP for thermal performance of buildings will inevitably change our future constructions. Of particular note are the changes required to party walls for attached housing through the need to address cavity thermal bypass.

Cavity bypass has been discussed for many years by academics and industry as potential weak point for thermal losses in new build. The excellent work undertaken by Leeds Metropolitan University in evaluating the thermal loss mechanisms has finally put “some flesh on the bone”. However, the consultation by DECC and CLG did receive responses from across the sector querying the potential U-values of open cavities, wall leafs, fully filled cavities and sealed and filled solutions. But despite this there appeared to be no movement on the final values which appeared in the Part L and SAP values due for implementation in October 2010. This is very concerning.

The new U-values for party walls will not take into account any differentiation between insulated and non-insulated party walls, unless there are fully filled cavities. Thus there is no incentive to utilise a higher insulating “wall leaf” for the party walls. There is no differentiation whether the party wall leaf has mineral wool between studs, such as lightweight metal and timber walls, or if the designer uses aircrete, cellular or dense block. There is also no differentiation between sealing the wall face with render or using only gypsum board on dabs. This negates the effectiveness of different materials and how heat may transfer through the wall into the cavity space. As a result the U-value of 0.5 is for all cavity wall types and has left the industry feeling that this is certainly “one for all BUT not all for one”.

The use of one set of U-values for all forms of construction is unhelpful and also confusing for the industry. Furthermore the use of non boundary or unclear definitions of full fill or effective edge sealing allows some product manufacturers to claim performance or benefits with no effective fallback for the developer or specifier.

The impact of the party wall bypass U-values chosen and the possible “solution scenarios” is significant. Given that over 70% of new homes are attached and also must be compliant with Part E (or Section 5 in Scotland) any solution must be compliant on both fronts of L and E. The blockwork industry with Eurisol have been active in looking at solutions and compliance, however, the lightweight frame twin party walls are currently lacking in tested solutions.

All industry sectors will wish to achieve 0.0 U-values, but yet with just 3 months to go before inception of the new thermal requirements no industry sector has fully completed their cycle of testing or development for such solutions. To put this in perspective the decision for a 0.5 U-value for an empty party wall cavity came from a few test houses, less than 6. For Part E and Robust Details every specified wall in the handbook had to complete 30 tests and prior to RD being accepted over 1,400 homes were built and tested with the innovative designs. It almost appears cavalier that with the Part L cavity bypass issue “so many will be affected by so little”.

So what might happen in October with the new Part L and new Section 6: Energy in Scotland? Will the entire industry move to solid walls or single leaf party walls overnight to accommodate a 0.0 U-value? Will the sector try to achieve some headway with solutions, which lack clarity in definition from Government, only to find through post occupancy that they have not met the intended performance? Who is liable for any court cases that ensue should home occupants or their clever consultants identify non compliance?

To make matters worse for the industry, under the 2010 Part L requirements starting in October, the effective increase in insulation performance by addressing the cavity bypass issue will not be included in the 25% Part L improvements. This appears to many that the lack of inclusion is increased costs without due respect to this important issue and a slap in the face for the industry sector most affected by the economic downturn.

If ever there was a time for the Government regulator for “better regulation” to get involved this is a prime example. There are three months to go before October but design drawings are already being formulated. The industry needs clear definitions of “fullfill and effective edge sealing” to come forward, more precise U-values need to be evaluated. And even better, a six-month delay on this specific issue until all material sectors are ready.