Practicalities on a hempcrete build - The Hempcrete Topic: Designing and Building with Hemp-lime - page 447

aggressive take-up of water also reduces drying times - another significant advantage, es-
pecially at colder times of the year. All of this allows Prompt to be used successfully in
applications where minimum temperatures are not guaranteed over such extended periods
as would be required for other binders. However, this does not amount to a blanket cap-
ability of the product to work in any conditions. When working in low temperatures, it is
important to ensure that the supplier or manufacturer has signed off on the product's use
after being given accurate information about the type of application, including the minim-
um temperatures expected and over what period, and the thickness of the material you are
casting.
As well as thinking about the type of binder to use when working at lower temperatures,
take other sensible precautions where practicable. For example, consider casting earlier on
in the day to allow plenty of time for the hempcrete to take its initial set before night draws
in, bringing falling temperatures. If you are really concerned about a sudden unexpected
drop in temperature, then it is sensible to cover freshly placed material with hessian to trap
some air against the surface of the wall and keep frost off, although if you find yourself
doing this then you really are pushing the limits of what you can do with the material, and
it might be less trouble in the long run to take a few days off until the weather warms up
again.
Good drying management
The drying time, as discussed in Chapter 16 , is one of the most important considerations
when working with hempcrete. It is vital that a freshly cast hempcrete wall is given suffi-
cient time to dry out before finishes are applied (see Chapter 16, page 215 ). In this way the
water introduced when mixing is allowed to leave the wall quickly, so that a) there is no
risk of the natural materials within (hemp and timber) being exposed to excessive moisture
and so being at risk of rotting, and b) the thermal performance of the wall reaches its full
potential as quickly as possible.
Since the finish for the wall is also going to be vapour permeable, the hempcrete below
would eventually dry out through it, but most finishes will reduce the speed of drying con-
siderably. In an ideal world, a cast hempcrete wall would be left unfinished, at least on one
side, for as long as possible until it has had plenty of time to dry, and indeed it is not un-
common for the interior side to be left unfinished while the exterior is completed. But in
many cases, and certainly on commercial builds, there is a great deal of pressure to apply