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2 November, 2009
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Writing on the wall for open panel?
Winter 2007
Published:  23 February, 2008

Open panel systems are more reliant on site skills

The Code for Sustainable Homes is billed as good news for timber building, but Chris Gaze, associate director at the BRE, contends it may not be so good for a certain type of timber frame

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 decided to introduce the Code for Sustainable Homes into England to replace EcoHomes.
The Code is similar in many ways, but more stringent, with a greater emphasis on reducing energy use and the encouragement of renewable generation. For each cate-gory, 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 construction, 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.
Already the Housing Corporation has announced that its next bid round for 2008-2010 will require new developments to reach Level 3 of the new Code and there is talk of making the Code mandatory for all new homes.
For the time being the Code does not apply outside England, but something similar will emerge in Scotland, Wales and Northern Ireland.
So what does this all 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 work that has gone into chain of custody management. 

The Code is a great opportunity for the timber frame industry 

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 Code points. Timber frame external walls, internal walls and roofs will give a good environmental impact performance with A or A+ Green Guide Ratings, contributing up to a further 2.7% points – but it’s still not much. 
The really big impacts are to be found in the area of energy and CO2 emissions. The two parts of interest to timber frame are the Dwelling Emission Rate (DER)_and the building fabric. 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 total, but mandatory improvements must be obtained for each code level.
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 understand the performance needed to obtain level 4 the best experience we can look at is Passivhaus design (Timber Building Autumn 2007). Passivhaus is an eco-friendly philosophy of building from Germany. A Passivhaus building needs U-values of 0.15 accompanied with airtightness of 1m3/hr/m2. 
The U-values are reasonable, but the airtightness levels are 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 of 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 guarantee in open panel timber frame.

Factory-produced closed panel systems are more able to guarantee airtightness, thermal and acoustic performance 

The final area that the timber frame effects is that of sound insulation for walls and floors. Up to three credits (4.68% points) can be obtained for dwellings with airborne sound insulation values 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.
So what is the way forward? For some time I’ve 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 prevalent elsewhere in Europe. 
With the advent of the Code and the possible challenge of every new house being “zero carbon” by 2016 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 Code is a great opportunity for the timber frame industry, but is the industry ready for it?
For more information visit www.bre.co.uk

Keywords: BRE

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