Floors, ceilings and roof insulation - The Hempcrete Topic: Designing and Building with Hemp-lime - page 383

If hanging the shuttering below the battens, rather than messing about with 25mm
spacer tubes, use 50mm screws and a tape measure to hang the boards at the right dis-
tance below the battens, and double-check across the ceiling using a spirit level.
Wearing the appropriate PPE, mix the hempcrete using the normal wall-mix ratio, ac-
cording to the manufacturer's specification, and place it around the battens, working
from above. Tamp it down fairly hard to ensure the total filling of all the small corners
around the battens. This hard tamping is very important, since gravity makes it difficult
to repair patches of loose or missing hemp in a ceiling once the shutters are removed.
The well-tamped layer of hempcrete should be placed in one go across the whole ceil-
ing, and should provide at least 25mm coverage above the battens.
As soon as this layer is completed, mix the low-density roof insulation mix that will go
above it, and place this all in one go across the whole ceiling as soon as possible, to en-
courage good bonding between the two layers.
Leave the hempcrete for 50 per cent longer than normal to take its initial set (refer to
the individual manufacturer's guidance) to make sure it has really gone off properly be-
fore the shuttering is removed.
Remove the shuttering boards very gently. Remember to move the boards horizontally
first before letting them drop, to break the contact with the hempcrete, which was
tamped down hard on to them. By doing this you avoid pulling away any of the freshly
cast hempcrete that might be sticking to the top of the boards.
You now have your cast-in-situ ceiling. Follow good drying management (see page
264 ) , and when the hempcrete has dried sufficiently, plaster finishes can be applied.