Indicative detailing - The Hempcrete Topic: Designing and Building with Hemp-lime - page 557

horizontally and the second vertically in line with the rafters. This provides a fix for the
horizontal tiling battens, and also allows the cast insulation to fully cover the rafters, min-
imizing cold bridging and also removing the potential air gaps right through the insulation
along the side of the rafters. Between the counter-battens and the tiling battens a vapour-
permeable membrane or wood-fibre sarking board is required as usual. If using a sarking
board, however, it would only need to be 22mm thick for airtightness and waterproofing,
which is much cheaper than 80mm boards.
The system of having layers of counter-battens sounds complicated, but actually the depth
of a roof containing only hempcrete insulation needs to be at least 300mm in order for suf-
ficient insulation to be achieved. Using a 300mm-deep rafter is expensive compared with
using a 200mm rafter and two 50mm counter-battens, and the latter solution also results
in a more robustly detailed roof in terms of maximizing airtightness and minimizing cold
bridging. This system can also be used with other loose-fill insulations, for example re-
cycled cellulose insulation.
The spacing of the counter-battens will depend on the weight of the roof covering they are
supporting, and, along with the depth and type of the roof joist, should be specified by a
suitably competent person.
Since cast-in-situ hempcrete in a roof is placed from above, there is an issue with keeping
the work dry as it progresses, which applies to the walls too if these have already been cast.
This can be achieved with a waterproof tarpaulin temporarily fixed at the top of the roof
and rolled down each time work stops. However, fixing these coverings down and rolling
them up when work starts again is a time-consuming and risky business, and on larger
roofs the best solution is probably a scaffolding roof over the whole structure.