As thermal performance requirements get ever tighter, the construction industry is getting to grips with new SAP calculations, PSI values and novel construction products, to achieve reduced CO2 emission requirements. The need to construct buildings with zero carbon emissions is now only a few years away, but innovation in construction historically moves at a snail's pace. Timber frame construction already has many of the answers needed to meet the bold zero carbon goal, which is why it continues to improve its market share as building regulations tighten.
Within certain parameters, designers have freedom to choose how they achieve the CO2 emission requirements of the Building Regulations. By specifying well insulated key elements such as walls, they have greater freedom and the ability to relax other areas of energy saving or energy producing building specification – or go on to achieve a building that far exceeds current target CO2 reductions. For a building designer, the choice of a timber frame wall, with excellent thermal performance, is an obvious way to deliver that design freedom.
Timber frame construction can offer external walls with high thermal insulation for a relatively slender thickness - and timber frame walls do meet their theoretic U-value. Over recent years, we have seen the timber frame industry moving from 90mm to 140mm external wall studs. This provides additional space for installing insulation. Reflective breather membranes have also been introduced to effectively block infrared radiation, enhance the thermal performance of the airspace, and consequently increase the overall U-value of the construction.
Nobody wants thicker walls: firstly they reduce the useable floor area of a building, secondly they cost more to construct and thirdly more energy is used to manufacture more materials. Novel insulation products, reflective vapour control layers and air tight membranes all deliver better thermal performance. With the insulation and the timber structure of your typical timber frame house sharing the same sectional space within a wall, brick clad timber frame external walls' thickness can be kept below 300mm, while delivering U-values of 0.20 W/m²K or less and maintain a cavity. Choose a cladding such as timber, brick slips, or render on battens instead of brickwork and external wall thickness can be reduced still further - along with the amount of embodied energy used.
The requirement to test the air permeability of new dwellings was a major change in Approved Document L 2006. The air that we warm up in our houses during the winter leaks out at an alarming rate. By reducing air permeability, we reduce the amount of warm air escaping, so reducing the volume of air we need to heat, reducing our fossil fuel bills and reducing CO2 emissions. Canada and Scandinavia are places that suffer extreme winter conditions and their choice of building systems must therefore address the issue of keeping a howling gale of minus 30°C outside, while maintaining a comfortable environment inside, without using vast quantities of carbon based fuel.
Those countries which have championed the benefits of airtight buildings for many years also happen to be those which have championed timber frame for just as long. This is not a coincidence. Tried and tested techniques and details from overseas are now becoming commonplace in the UK. The polythene vapour control layer which is typically used in timber frame construction now fulfils a second important role in the UK, that of an air barrier. The current default of 10 air changes per hour/m² of external wall area at 50 pascals pressure difference can easily be achieved with timber frame construction. Indeed, the Canadian R2000 timber frame house system prides itself on achieving just 1.5 air changes per hour. The UK construction industry has a lot to learn, although some manufacturers are rising to the zero carbon challenge.
When air tightness levels below 5 changes an hour are anticipated, it is recommended that a mechanical ventilation heat recovery system (MVHR) is installed as part of the construction process. Warm air extracted from rooms such as kitchens and bathrooms is ducted through a heat exchanger, which collects the heat from outgoing air and uses it to heat fresh incoming air. This allows plenty of fresh air for the building occupants and moist stale air is extracted at source. It takes less than 100 Watts to run a system, considerably less than the heat energy it recovers.
Well-insulated, airtight timber frame buildings work well in summer, keeping daytime heat out – as proved in certain areas of Canada and the US where summer temperatures are stifling. Unlike their traditional masonry equivalents, their lightweight structure does not absorb heat well, so it cannot contribute to the excessive and uncomfortable night time temperatures many people experienced this summer as masonry walls release their stored up daytime heat. In winter, timber frame building's quick heating response time can be appreciated. Without a heavy energy absorbing internal structure to heat up first, occupants benefit from a fast heating response, on demand, while benefiting from low heating bills too.
The 2010 edition of Approved Document L will come into effect from 1 October 2010. For the first time, calculations for thermal bridges are required, or poor backstop values have to be assumed. Thermal bridging occurs in all construction types and is caused by areas of reduced insulation, or where an element passes through the insulation. Thermal bridging increases space heating requirements by allowing heat to flow more easily through the thermal envelope. In winter, this creates low surface temperatures around the thermal bridge, leading to an increased risk of surface condensation.
Wood has a lower thermal resistance than the insulating materials placed between the framing members. Therefore greater heat flow occurs through studs, plates, rails and joists than in other areas of the external wall or roof structures. This increase in thermal conductivity is referred to as thermal bridging. In general, thermal bridges can occur at any junction between building elements or where the building structure changes. Many other building materials have lower thermal resistance than timber and could be significant factors in thermal performance. For example, a steel post within a timber frame external wall would create a significant thermal bridge unless it was detailed not to bridge from the warm side to the cold side.
The property that describes the heat loss associated with a thermal bridge is its linear thermal transmittance ψ. This is a property of the thermal bridge and is the rate of heat flow per degree per unit length of the thermal bridge that is not accounted for in the U-value of the plain building element or elements containing the thermal bridge. TRADA Technology provide publications and consultancy to help the construction industry understand and apply these new considerations to exceed minimum Building Regulation requirements.
The new Part L will also bring party walls under the microscope for the first time. Until now, they are assumed to have 'perfect' thermal performance, achieving a notional U-value of 0 W/m²K. This is based on the fact that both sides of the wall are within the heated envelope of a building, so there should be no temperature differential from one side to the other, and therefore no heat loss.
Research conducted in 2005 - 2007 by a team at Leeds Metropolitan University, as part of the Stamford Brook field trials, showed that masonry cavity party walls allow a significant amount of air movement with the cavity. This air movement can transfer heat from the party wall cavity to the outside, leading to far greater than predicted heat loss from the building.
The project went on to calculate the actual thermal performance of the party wall based on the excess heat loss and concluded that the U-value of the party wall was between 0.5 and 0.7 W/m²K. In a terraced house, where the area of party wall could be as large if not larger than the external wall, this represents a significant excess heat loss.
Further research then showed that by closing the party wall cavity with insulated cavity barriers, the effective U-value could be reduced to 0.2 W/m²K, and by fully filling the cavity with mineral wool insulation, the U-value was reduced down to 0 W/m²K, i.e. zero heat loss.
The revised SAP 2009 software, due to be released later this year, includes Table 3.5 covering U-values for party walls based on this research. The table states that a solid party wall can assume a U-value of 0 W/m²K because there is no cavity to allow the movement of air. An unfilled open cavity will be assigned a U-value of 0.5 W/m²K, an unfilled cavity that is sealed against air infiltration will be assigned a U-value of 0.2 W/m²K and a fully filled and sealed cavity will assume a perfect 0 W/m²K.
So what does this mean for timber frame buildings? Currently timber frame party walls contain acoustic insulation in both leaves between the timber studs, and insulated cavity barriers around the perimeter of the party wall cavity. This should allow a U-value of 0.2 W/m²K to be assumed for the party walls, based on the criteria set out in SAP 2009. However, this is not a particularly desirable option, because it is still far worse than a perfect wall with no assumed heat loss. As such, it is likely that thermal insulation will need to be installed between the studs as well as in the cavity between the two leaves of the wall, allowing zero heat loss to be assumed.
Achieving this on site may require some changes to current practices. Installation of the insulation between fully sheathed party walls during erection may slow the build progress, as additional time would be needed. Also, ensuring that the insulation remains dry and does not become dislodged during the rest of the build may be difficult.
The simplest solution to these potential problems will be to use unsheathed party walls or only sheathe one leaf. With open or partially open party walls the insulation can be easily installed once the building is made weathertight. It is also possible to perform site checks. As with masonry cavity walls, if fully sheathed timber frame walls are used and the insulation installed during erection, subsequent checking and snagging would be very difficult.
Timber frame is ideally suited to buildings with repetitive layouts such as flats and care homes, so the new Part L requirements may bring some changes in detailing and order of work, but it will also bring valuable overall improvements which occupants will benefit from, not least valuable savings on fuel costs.
Because much of a timber frame building is constructed offsite on a clean, efficient and well managed production line, the quality of the end product can be enviable. This helps on site by reducing material waste, simplifying and speeding construction methods, plus delivering a building to the client that performs well against the new regulations. Using a truly sustainable material, offering high insulation values, quality construction and good air tightness, new timber frame buildings are well placed to meet the evolving needs of planet earth and its occupants.
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