• The Code for Sustainable Homes will replace EcoHomes.
• All new homes in England must achieve level 3 by 2010.
• Wales has set the target at level 5 by 2011.
• The biggest impact will be felt in the areas of energy and CO2 emissions.

The government is determined to act in response to the Stern Report and, with over a quarter of all carbon dioxide emissions produced by the homes in which we live, they have decided to introduce the Code for Sustainable Homes (CSH) into England and Wales to replace EcoHomes. The CSH is similar to EcoHomes in many ways, but is more stringent, with a greater emphasis on reducing energy use and the encouragement of renewable generation. For each category credits can be won and these are weighted to convert them into a percentage points score.

For those building homes the most important difference from EcoHomes is that an individual dwelling will be assessed before and after they have been built, so making conformance to design specification vital. That’s bad news for traditional masonry construction, but not necessarily good news for traditional open panel timber frame either.

Housing Corporation

The Housing Corporation has already announced that its next bid round for 2008-2010 will require new developments to reach level 3 of the new Code. In 2010 all new homes in England must achieve the Code’s energy use as laid out for level 3. This then changes to level 4 in 2013 and level 6 in 2016. In Wales things are even tougher: all social housing will be Code level 4 “as soon as possible” and all the dwellings will be Code level 5 by 2011. In Scotland the Sullivan Report has recommended net zero carbon dwellings by 2016/2017.

So what do all these changes and potential changes mean for timber framers?

The good news is that the timber industry is in a good position to exploit the green credentials of wood and the hard work that has already gone into chain of custody management.

The bad news is that, even with an excellent custody chain, the timber frame element of the building will only give you around 1% of the total points within the code. Mind you, timber frame external walls, internal walls and roofs will give a good environmental impact performance with A or A+ Green Guide Ratings and contribute up to a further 2.7% points – but it’s still not a lot.

The really big impacts are to be found in the area of energy and CO2 emissions. The two parts of interest to timber framers are the dwelling emission rate (DER) and the building fabric (see tables above). The DER comes straight out of the SAP calculation and credits are awarded for percentage improvement over the Building Regulations’ target emission rate (TER). Not only do you need to score well to contribute towards the overall score, but there are also mandatory improvements that must be obtained for each code level.

Beyond level 3

Achieving level 3 is reasonably OK. However, beyond that requires a lot more work. The fabric and ventilation play a vital part in achieving good results and, as a reflection of its importance and permanence, a further two credits (2.52% points) are available for the building fabric.

The effectiveness of the fabric is measured using the heat loss parameter (HLP) – the total fabric and ventilation heat losses over the floor area of the dwelling. The HLP is taken from the SAP calculation sheet. You must achieve less than or equal to 0.8W/m²/K to be awarded code level 6. To get a good HLP you need four things:
• good insulation;
• good detailing to avoid thermal bridging;
• excellent airtightness detailing;
• a good ventilation and heat recovery system.

To get a handle on what sorts of numbers we are talking about to obtain level 4 the best experience comes from those familiar with Passivhaus design. Passivhaus is an eco-friendly philosophy of building from Germany. To be a Passivhaus building you need U-values of 0.15 accompanied with airtightness of 1m³/hr/m².

The U-values are imaginable, but the airtightness levels are frighteningly tough. When I look at SAP calculations it appears that an HLP of 0.8W/m²/K can be achieved with less airtightness, perhaps up to 5m³/hr/m² as long as the windows and doors are excellent. But 5m³/hr/m² is still potentially a tall order for volume housing and not a performance that I would wish to guarantee if I was building in open panel timber frame.

The final area that the timber frame affects is that of sound insulation for walls and floors. Up to three credits (4.68% points) can be obtained for dwellings where the airborne sound insulation values are up to 8dB higher than Building Regulations and the impact sound insulation values are up to 8dB lower than Building Regulations. Relatively lightweight building systems such as timber frame have to work harder to achieve good sound insulation performance – detailing and conformance to specification are everything and with open panel timber frame so much of this is reliant on site skills.

What is the way forward?

For some time I have argued that there is not enough value added in traditional open panel timber frame to make a decent profit and that the industry should be looking to develop the sort of closed panel systems that are prevalent across mainland Europe.

With the advent of the CSH and the possible threat of every new house being zero carbon in the next seven to nine years, we need timber solutions that can guarantee airtightness, thermal performance and sound insulation. In my opinion the only way to do this is through closed panel timber frame, produced in factory-controlled environments.

The CSH is a great opportunity for the timber frame industry, but is the industry ready for it?