The sarking system places the thermal insulation above the roof structure, between the purlins and the tile, instead of underneath as in conventional interior insulation.
Why sarking is chosen
Its main advantage is leaving the timber structure (beams, purlins) completely visible from inside, a finish highly valued in refurbishments of attics and loft conversions with original exposed timber. In addition, by wrapping the whole structure from outside, it eliminates the thermal bridges that insulation fitted only between structural members can leave.
When it pays off compared with interior insulation
It is especially recommended when you want to keep the timber structure exposed, when the work is being done from the roof itself (complete tile replacement) and it is more practical to work from outside, or in projects where the thermal bridge of a heavily exposed timber structure is a real energy-efficiency concern.
Cost compared with conventional insulation
Sarking is usually more expensive per m² than standard interior insulation, both because of the specific material (high-density rigid boards) and the labour, since it is done from outside with the roof open. It is mainly justified when you are already working on the roof for another reason (replacing the tiles, for example).
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