Skeilings: the limiting part of the thermal envelope


Re roofing more than 50% of the property?

If you are reroofing more than 50% of the roof then there is a requirement to improve the thermal insulation at the same time - to current B Regs standards if possible. If there is a room in the roof there will very often be a sloping section – or skeiling – with voids above the flat ceiling and at the eaves. If there is an existing ceiling below, then the amount of insulation you can put between the rafters will be limited by the rafter depth, unless the roof can be raised by battening out the rafters.

BS5250 - Code of practice for control of condensation in buildings

BS5250 is the main source of advice in these situations. Where possible, the best solution is to run the insulation at rafter level from eaves to ridge. This would mean that the voids are kept warm also, and even if the insulation is limited in the skeiling sections by what can be accommodated, it may be possible to improve the U value of the rafter insulation over the voids.

Using TLX Insulation in a skeiling applicationFor example, if the rafters are 100mm deep, the best that could be achieved would be to use TLX Gold draped between them with 50mm of rigid board between, giving a U value of 0.33 W/m2K with no condensation risk, even where there is no vapour barrier.

(See HERE for the calculation)

In the rafters over the voids additional board could be added beneath to improve the U value in these sections.

Loft converting from the inside?

Conversely if a loft conversion is being carried out from within, or the insulation improved when the ceilings are stripped out, then the type of roof underlay will determine whether or not ventilation is needed beneath it and the amount of PIR that can then be fitted. If there is a bitumen felt in place, or a boarded roof, these vapour-impermeable layers require that there is a 50mm ventilated space underneath, with the ventilation pathway being continuous from the eaves vents to a ridge vent.

Using TLX with 1F Felt

The TLX 1F solution is to use PIR between the rafters with TLX Silver underneath. The 0.18 W/m2K buildup is based on 100mm PIR, set 30mm from the bottom of the rafter to create an unventilated air gap secured with a 38mm deep batten set crosswise, provides both an excellent vapour barrier and a 20mm service cavity.

However the rafters in skeilings are rarely deep enough (180mm) to accommodate this; if the rafter is only 100mm deep this standard method of using TLX Silver, with air gaps both above and below, can be modified. Beneath the upper 50mm ventilated space 50mm PIR can be fitted between the rafters with the TLX Silver directly against it and secured by a 38mm deep batten, achieving 0.31 W/m2K.

Using TLX in a skeiling applicationRather than continuing the insulation and ventilation up along the rafter space into the voids, there is the cheaper option of maintaining them as cold ventilated spaces, with mineral wool on the loft floor and in the dwarf walls.

Using TLX Silver under the ceiling joists

In this case it is of benefit to run TLX Silver underneath the ceiling joists if possible, to provide vapour control and prevent moisture migrating up into the cold loft space (which even though ventilated, still has the potential to experience condensation). Secured with a 50mm batten with the ceiling plasterboard below, the loft roll required to achieve the required U value of 0.16 W/m2K is then reduced from the usual 270mm thickness to 100mm between the joists and 100mm over. 

Using TLX in a skeiling applicationSo under a bitumen felt it’s easy to maintain the ventilation pathway from eaves to ridge. But what if there is a breather membrane instead? This is where the options get rather more complicated…

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