Indicative detailing - The Hempcrete Topic: Designing and Building with Hemp-lime - page 558

Keeping the work dry can be a challenge when casting
roof insulation. Image: Margot Voase
Hempcrete insulation in a flat roof
When constructing a flat roof with cast-in-situ hempcrete insulation, a vapour-permeable
membrane covers the top of the hempcrete, with a vented air gap above that (see Figure
32 ) . To ensure airtightness and reduce cold bridging within the hempcrete layer, counter-
battens can be used as they are in a pitched roof (see page 327 ) . An air gap is achieved
by running roofing battens on top of the vapour-permeable membrane and fixing the roof
deck to these. Vents can be inserted into the fascia on either side of the roof to make the
spaces between these battens into a series of air tunnels. Battens should be of good-quality
treated or hardwood timber, owing to the potential exposure to moisture and the weight of
the roof deck they are supporting. Once again, a suitably competent person should specify
the spacing of the battens and the depth and type of the rafters.