Tackling historic buildings with difficult to insulate roofs

20/08/2021

Insulating historic buildings such as churches, museums, schools, Victorian houses, pubs and agricultural barns comes with many challenges since their roof structures are variable and therefore require different approaches to improving the insulation. Boarded roofs often feature among these, and can be tricky to insulate, since the rafter space, which is generally utilised to fit insulation in between, is separated from the roof above by an imposing board. This board is generally quite thick, taking up a substantial amount of room which could otherwise be better utilised as part of the insulation profile. In old buildings where boarded roofs are common, the rafters tend to be quite shallow, making it impossible to meet building regulation requirements with traditional forms of insulation alone. Furthermore, when dealing with listed buildings, permitted disturbance to the structure of the roof may be limited, if at all.

Gloucester museum uses tlx gold roof for refurbSo, when The Museum of Gloucester needed a major refit, a compromise needed to be met between the roofers and the local building control to improve the energy efficiency of the boarded roof, whilst maintaining a conscientious approach to the constructional constraints of the Grade II listed building. With our technical team working in conjunction with Minster to establish the best solution for the contractor, the use of TLX Gold was agreed since it would provide excellent thermal improvement without considerably raising the roof height.

 

 

The use of TLX Gold multifoil insulation:

Although insulating a boarded roof can be met with difficulties, it is possible to insulate with the use of a multifoil insulation. 50mmtlx gold boarded roof detail counter-battens can be fitted above the boarding, then TLX Gold 2-in-1 insulating breathable membrane draped over the counter-battens and into the unventilated air space provided to achieve better thermal performance. Roofers at the Museum of Gloucester project welcomed the specification of TLX Gold as a single layer used in this way, givingaU value of 0.67 W/m2K. When compared with a completely uninsulated roof having a U value of 3.67w/m2k, this is a substantial thermal energy saving, especially for a listed building where building regulations don’t apply; TLX Gold was able to maximise the thermal efficiency of this construction.

If a significant change to the roofline is permitted, then a continuous sheet of PIR between the board and the counter-battens can reduce it much further. For instance, to meet Building Regulations requirements of 0.18 W/m2K, 85mm of PIR can be used.

How does TLX Gold work to achieve lower U values? 

Reflecting heat: TLX Gold has a shiny under surface which reflects heat and is therefore a very effective insulator, providing that an optimum 20mm unventilated air layer is maintained beneath it. In this way, the 50mm counter-batten acts as a mini rafter creating the space beneath TLX Gold for the maintenance of the air layer. The under surface of TLX Gold enhances the core R value of the air layer next to it. Therefore, the U value of the overall structure is greatly improved when these two layers interact with each other.

Preventing Condensation: You may be wondering, if a 50mm ventilated air space is required below non-breathable felt, why then does TLX Gold require a 20mm unventilated air gap underneath it? And how does TLX Gold prevent condensation? Indeed, whilst the under surface reflects heat, it is also fully breathable, allowing water vapour to pass through it. TLX Gold occupies the coldest part of the temperature profile, so any water vapour entering the rafter space from below can escape through the breathable layers and is transported out of the building before it is able to cool and condense. When installed in accordance with the TLX installation guide, and if a well-sealed ceiling is in place, there is no risk of surface or interstitial condensation.

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