The zero-carbon house aiming for U=0.11 W/m2K

The zero-carbon house aiming for U=0.11 W/m2K

Energy-saving was top of the list

This new build house had been designed with big picture windows in order to make the most of the stunning views over the countryside, but incorporated energy-saving features such as a ground source heat pump and solar panels. When the insulation requirement for the warm roof was considered, the architect had thought that 150mm deep rafters @400mm centres was a reasonable specification that would allow sufficient room to accommodate the required amount.

Enter the SAP assessment

However, because of the extensive glazing, the SAP assessment indicated that a U-value of 0.11 W/m2K for the roof was required. It was not possible to alter the other fabric specifications of the wall and floor, so a build-up achieving U=0.11 W/m2K for the roof became essential.

Fortunately, a build-up of 125mm PIR between the rafters and 90mm below achieved the desired U-value – though this meant that the headroom was at its absolute lowest point. Alternative build-ups with PIR over the rafters were not possible as the roof height could not be increased.

Factoring bats into the equation

But the architect had not considered the bat mitigation measures that were triggered by the house’s proximity to the stream and woodland nearby. Because bats might decide to use spaces under the tiles to roost, he was advised to use an insulation build-up using bitumen (1F) felt, as modern breather membranes are considered to pose a risk of entrapment to bats, whose claws can become entangled in the polypropylene fibres.

1F felt simply wasn’t an option

However, when the insulation company came back with the build-up using a 1F felt, the 50mm ventilated space that is required beneath a vapour-impermeable membrane such as bitumen felt meant that, instead of a conventional 25mm gap being left at the top of the rafters to allow for the breather membrane sag, not only was more space required for the ventilated space but more insulation board too.

Using two 2x PIR comes with its own complicationstlx batsafe used with pir insulation

When two insulation boards are used, the thicker one must always be on the colder side otherwise condensation could occur at the interface between the two. So, in the 150mm deep rafter, the most insulation that could be fitted consistent with this was to have the 50mm ventilated space with 100mm PIR between the rafters and 100mm below. But this was unacceptable in terms of headroom, and only achieved 0.12 W/m2K!!

How to get down to 0.11 in the available space?

The answer was to use TLX Batsafe, a breather membrane with mesh attached to both sides which enables the bats to grip onto without any risk of becoming trapped. The original spec of 125mm PIR and 90mm below now became possible and the 0.11 W/m2K target was achieved.

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