Insulating a boarded roof


Various types of buildings have boarded roofs

Buildings with boarded roofs range from churches, church halls, schools, big Victorian houses and pubs to agricultural barns and garden sheds, and the insulation strategy may be different according to the particular situation.

If reroofing is not being carried out then the only option is to insulate under the boarding, between the rafters, and this requires a 50mm ventilated space to be left under the boarding, (which is not considered to be sufficiently vapour-permeable to avoid condensation when it is the external covering and not ventilated below).

Not possible to insulate under the boarding?

If it is not possible to insulate underneath the boarding because of an existing ceiling or insufficient depth, then the TLX Gold boarded roof solution is ideal. 50mm x 50mm counter battens are set over the boarding (to act as mini-rafters) then the TLX Gold is draped over them in the usual way with a slight sag between the rafters to allow the passage of water down to the guttering. The tile battens can then go straight over, since the TLX Gold compresses to 3mm underneath them. The counter batten depth can NOT be reduced as the 50mm depth is essential to have enough room for the drape, the product thickness and the unventilated heat-reflecting air gap underneath.  

What if the external roof covering is Metal or Fibre cement?

If the external roof covering is metal or fibre cement then these need to have a 50mm ventilated space underneath them, with air flow from eaves to ridge, as they are not considered to be 'air-open' like natural materials. TLX Gold would then need to be used taut and counter battened, by sandwiching between two 38mm x 38mm counter battens.

Insulating a boarded roof using TLX Gold insulating breather membraneJust having TLX Gold by itself over the boarding achieves a U-value of 0.69 W/m²K over exposed rafters – better than the 0.70 W/m²K required for a Listed building. If there is plasterboard under the rafters beneath this improves to 0.60 W/m²K because of the additional enclosed air space beneath. If there is room to raise the roof further, then some insulation board can be set over the boarding under the counter battens. Using 25mm PIR board, for example, would achieve 0.38 W/m²K.

Often house owners will want the insulation over the boarding so that they can have exposed rafters, but unless it is a Listed building this will need to be justified to building control for them to accept a worse U-value than 0.18 W/m²K.

There should be no risk of condensation over the boarding when using TLX Gold, assuming that it is close boarded. If, however, there are ‘penny gaps’ in the boarding then water vapour carried by movement of air can occur, and it would be advisable to use a vapour barrier above the boarding. For the same reason any gaps around penetrations should be sealed, e.g. using polyurethane spray foam.

Can you put insulation board both above and below the boarding? In principle, yes, since the boarding is no longer the outermost layer that BS5250 would require to have a 50mm ventilated space beneath it.

And what about condensation risk?

A condensation risk calculation would show whether or not condensation is likely to occur underneath the boarding. However such calculations are for idealised situations and don’t account for the inevitable gaps between the rafters and the insulation board, which would allow the passage of moisture laden air from below. It would be essential to have a well-fitted vapour barrier underneath if such a build up is used.

We can help you with a condensation risk analysis - call our technical department on 01204 674730

There are various types of room suitable for this application

Garden rooms can range from luxurious cabins requiring the full U = 0.18 W/m²K insulation that will satisfy Building Control, to basic log cabins with no insulation fitted. Sometimes a conventional build up with a vapour barrier internally might seem to be the obvious way to go, but such build ups are designed for situations where is a constant relatively warm indoor temperature, and people inside generating the usual domestic amount of water vapour from cooking, washing, etc. TLX Gold used without a vapour barrier may well be the best option depending on how the building is used.

For example, if the room is used as a home gym, possibly with a shower, a lot of moisture may be generated for short periods of time, but if the room is left unoccupied and unheated for most of the time then the moisture would be trapped internally if a vapour barrier were used, and condensation might occur on walls if not mechanically ventilated. In such situations TLX Gold would allow gradual dissipation of internal water vapour. A garden room used as a home office with only one occupant keeping warm for most of the day, and windows with trickle vents, say, would not have the same problems and a build up employing a vapour barrier would be fine.

TLX Gold has transformed the possibilities for boarded roofs, especially for large community buildings where heating costs have become exorbitant. In reducing the U value from the 3.85 W/m²K of a roof with no insulation down to 0.69 W/m²K, 82% of the energy being lost through the roof is saved.

View all LABC approved Gold solutions HERE

View the case study based around this solution