The original rafters had to go
This barn annex had stood empty for many years, having been used as an artist’s studio at some point, but had fallen into disrepair. The rafters needed to be completely replaced, so a green oak frame of queen post truss construction was used, which would then become a feature of the vaulted ceiling.
Removing the risk of interstitial condensation
TLX Gold was chosen as the underlay to guarantee against interstitial condensation problems, even if there are high humidity activities 
(e.g. a log burner, bathroom) in the room below. It was fitted draped between the rafters, with the usual 10mm breather membrane sag which allows any water entering the tile batten space - either by penetration through the tiles or of frost melting - to run down to the gutter. Because it compresses to only 3mm under the tile battens TLX Gold does not require counter battening and so the roofline is maintained.
Achieving U-0.18 W/m2K
The U-value target was 0.18 W/m2K, so since the rafters were 150mm deep @400mm centres the buildup required was to add 100mm PIR between the rafters and 20mm below, which allowed the substantial purlins to remain largely exposed.
No vapour barrier was necessary
It is not necessary to use a vapour barrier when TLX Gold is used, because of two factors: water vapour can pass straight out since it is extremely breathable, and because TLX Gold occupies the coldest part of the temperature profile, keeping the air beneath sufficiently warm to prevent a dewpoint occurring. (Condensation risk calculations can be found on our website – it does require there to be a well-sealed ceiling that does not permit moisture-laden air to move directly into the rafter space). This makes it ideal for use where a fully breathable solution is required, or where there is an existing lath and plaster ceiling without a vapour barrier. In this case, because insulated plasterboard with an integral vapour barrier was used as the underdraw, this represented a belt and braces approach to avoiding condensation.
And the result?
A barn conversion with character that isn’t going to cause long-term condensation problems.
