Working on a close boarded roof using multifoil insulation

Working on a close boarded roof using multifoil insulation

The problem?

Making best use of limited depth to achieve a satisfactory U Value without compromising headroom.

A building enduring its second change of use

This urban church was constructed from Yorkshire stone in Edwardian times, before being turned first into a social club, and then more recently into flats. As with many such buildings constructed in the early 20th century, it has a boarded roof.

A close boarded roof can be tricky...multifoil insulation boarded roof

A close boarded roof is considered to be a vapour-impermeable layer, and so the options were to insulate on top of the board, or else insulate below whilst maintaining a 50 mm ventilated space. Where a boarded roof has 2 mm ‘penny gaps’, however, this allows moisture to pass through, and so it’s then considered to be a low resistance underlay as far as vapour permeability is considered, and the usual constraint of maintaining a 50 mm ventilated space under the boarding does not apply.

The boarded roof in this case was in good condition, and covered by a breather membrane from when it was last retiled. To preserve the character, the original purlins were to remain partly exposed. 30 mm of PIR board was used between the purlins, held in place by battens, with TLX Silver then stapled to the underside, creating an unventilated space across which heat was reflected back.

silver multifoil converted to flats

Creating useful air gaps and minimising thermal bridging

A second air gap was created by securing the TLX Silver with battens set perpendicular to the first set, minimising the thermal bridging. Care was taken when taping the joins so as to ensure the integrity of the vapour barrier not to mention airtightness. This still allowed most of the purlins to remain exposed, whilst preventing interstitial condensation because of the extremely high vapour resistance of the TLX Silver. The U value achieved was 0.33 W/m²K – better than the 0.35 W/m²K required by Building Control.multifoil used convertinh church into flats




The solution:

TLX Silver: the insulating vapour barrier with a low emissivity surface to reflect heat back into the building yet prevent overheating in summer. 

Click to view all of our LABC registered TLX Silver solutions

Click to view our (boarded roof) blog based on this solution

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