Achieving an energy-efficient house is no longer just about being able to sell the vision of a warm, cosy home that’s frugal on the fuel. It’s vital to meet increasingly onerous Building Regulation requirements.
It’s increasingly common to take a wholehouse approach to calculations to achieve the required U-values. And for roofs, mineral wool insulation is a relatively inexpensive option, so generously deep layers are often used to make up a significant element of the whole-house numbers.
That’s understandable. Unfortunately, however, the insulation thickness dwindles to next to nothing at the narrow point of the eaves. The result can be potential cold spots. In the worst scenario – an upstairs bathroom, where warm, moist air meets such a cold spot at the junction of the roof and ceiling – mould develops. The homeowner complains to the house builder and costly remedial action ensues.
The solution is to do away with this ‘pinch point’, where insulation tapers out, by raising the rafter member so that the depth of insulation installed into the eaves matches the rest of the roof. This means using what is usually called a ‘Bob-Tail Truss’. It’s rarely seen in this scenario, but we strongly recommend it to optimise eaves thermal performance.
Truncating traditional triangular trusses this way also means the mineral wool insulation in the roof space can be brought right through to meet the cavity wall insulation, ensuring a cold bridge isn’t created here either.
Roof designs we usually receive at Pasquill don’t allow for Bob-Tail truss configurations. So we’d recommend that, if possible, they are configured into the overall structure of the roof scheme early by the architect. Our roof truss designers can then design the best solution for cost-effective sizing and configuration detailing at this stage, before one of our branches starts manufacture.
Thermal bridging will always occur in buildings, but if we can help minimise it with solutions like this, we can help house builders reach their holy grail – optimum thermal performance and zero carbon homes.
