Floors, ceilings and roof insulation - The Hempcrete Topic: Designing and Building with Hemp-lime - page 377

Tradical ® , Batichanvre ® and Prompt Natural Cement - can be used to create insulating
floor slabs, but Prompt has the advantage that you can walk on it after 20-30 minutes.
Our recommendation is that, when detailing a floor that includes a hempcrete slab, the
hempcrete should be designed so that it is 'out of the ground', i.e. with the bottom of the
hempcrete slab sitting above the outside ground level. In practice the self-draining sub-
base layer will prevent water from building up in the slab, but if the design allows for it,
placing the hempcrete above ground level is the most robust detail.
As much, if not more, than when casting walls, good drying management (see page 264 ) is
essential when casting a hempcrete floor slab. It is important not to leave anything sitting
on the floor while the hempcrete is drying. In contrast to walls, the hempcrete in a floor
slab has open air on only one side of it, so most of the moisture has to dry out of the top
of the slab, and it is therefore important that nothing impedes this. As with walls, it is use-
ful to keep the windows and doors open as much as possible to maximize venting of the
air outside the building. The use of a low-level fan to encourage movement of the air just
above the floor can help. Once the hempcrete is strong enough, you can walk on it while it
continues to dry, but remember not to leave things sitting on it that might slow the drying.
You will get some idea about when a hempcrete floor will be strong enough to walk on
from the type of binder used, and the advice of the manufacturer, but each case should be
considered in its own right. Ultimately, the best way of testing a floor is to gingerly try
putting your weight on it. Expect this period of time to be longer than is normally required
for the initial set, since a) a floor only has one side to dry to; and b) taking down shuttering
on a wall and having it hold its own shape is very different from a floor taking the load of
people walking on it.
Although good planning should aim to prevent this, inevitably it will sometimes be neces-
sary to carry out further works in the room while the floor slab is drying, or before a pro-
tective floor finish has been applied. In this situation, it is advisable to temporarily cover
the whole floor with a solid sheet material such as plywood or OSB to protect it effectively
from dust, dropped tools and the like. However, if the floor is still in the drying stage, re-
member to remove the protective sheets again as soon as the work is finished , so that it can
continue drying out.