Indicative detailing - The Hempcrete Topic: Designing and Building with Hemp-lime - page 553

The use of tongue-and-groove wood-fibre insulation boards for pitched roofs and walls is
increasingly popular in the natural building world. Many of these boards perform the same
function as a vapour-permeable membrane in terms of keeping water out while adding ex-
tra insulation. They are also more sustainable than such membranes, since although a pro-
cessed material transported from Europe, they are produced from a natural material, wood,
which acts as a carbon sink. Used as a sarking board, they can be installed over the rafters
to provide a water barrier, and left without a final roof covering for up to three months
while construction proceeds below. The tongue-and-grooved edge makes fitting easy, as
the join between two boards doesn't have to sit above a rafter.
This join also gives a good level of airtightness in the roof, although some manufacturers
also provide tapes to seal the joins. There are many different types of wood-fibre insulation
boards available on the market, so it is important to make sure that weatherproof boards,
specifically designed for this function, are used.
The use of wood-fibre boards also assists in achieving airtightness at the eaves, for two
reasons, as shown in Figure 29. First, it is very easy to introduce an obstruction to the pas-
sage of air at the airtight junction. Second, as described earlier, one of the main reasons
that a good seal at the eaves is hard to achieve when using soft quilt roof insulation is that it
is so lightweight that there is nothing to compress the seal between the insulation quilt and
the top of the hempcrete walls. When fixing the boards down, a layer of quilt insulation
can be trapped between the board and the hempcrete, squashing it down to give a much
tighter seal.
Airtightness in a flat roof
Most modern flat roofs are constructed as a warm roof, with the insulation on top of the
timber deck. Here the airtightness will be provided by the roof covering, which should be
sealed to the airtight component in the walls, usually via the roof deck, with the roof cov-
ering glued to the top of the roof deck and the airtight component in the walls sealed to the
underside. In a hempcrete wall this means sealing both the hempcrete and the render to the
underside of the roof deck. The simplest way of doing this is to use some form of render
stop bead and mastic to seal the render, and hemp quilt insulation sandwiched between the
hempcrete and the roof deck to seal the hempcrete (see Figure 30).