Floors, ceilings and roof insulation - The Hempcrete Topic: Designing and Building with Hemp-lime - page 384

A freshly cast hempcrete ceiling with exposed battens.
Roof insulation
Hempcrete roof insulation is cast between the rafters and is equally suitable for flat and
pitched roofs. It is often used in new build but is not usually retrofitted to old buildings,
as the thickness of hempcrete required necessitates deep rafters, and to increase the depth
of the total roof build-up in an old building causes complications with either changing the
roof height externally or losing height from the inside of the room, not to mention the in-
creased load put on the original structure by extra roof timbers. Roof insulation hempcrete
can be cast straight on top of a freshly cast hempcrete ceiling, creating a homogeneous
ceiling-roof insulation slab. If you are not going to have a cast hempcrete ceiling, then roof
insulation is usually cast on top of permanent shuttering formed from a breathable carri-
er board, for example wood wool board. The depth of the cast slab will be defined by the
thickness of the rafters - the hempcrete simply fills the space between them.