Floors, ceilings and roof insulation - The Hempcrete Topic: Designing and Building with Hemp-lime - page 386

Hempcrete roof insulation between rafters. 80mm wood-
fibre sarking boards will be fixed on top of the rafters to
provide airtightness and extra insulation and to stop cold
bridging down the rafters.
The hempcrete used for roof insulation is a very low-density mix, in order to minimize
weight and maximize insulation. The mix has just enough binder to enable it to support its
own weight and to protect it from rotting and insect infestation. It is not in any way load-
bearing, so ensure that all loads, for example the weight of the roof covering, are placed on
to timberwork.
The mix is prepared in exactly the same way as for normal walling hempcrete, but using
less binder (refer to the individual manufacturer's guidelines).
The hempcrete is placed from above, before the roof covering is put on. When placing
hempcrete for roof insulation you literally just need to place it into the space between the
rafters. Make sure it fills the whole space, but don't pat it down or tamp it at all. This is
because you are aiming to achieve a high air content (for better insulation), and since it is
not carrying a finish it only needs enough structure to support itself.
For more information on cast-in-situ roof insulation, see Chapter 22, page 322 .