Indicative detailing - The Hempcrete Topic: Designing and Building with Hemp-lime - page 573

Unless hardwood or treated timber is used, the timbers around openings should be protec-
ted with a vapour-permeable membrane, owing to their proximity to the outside face of the
wall.
Airtightness around openings
The area around openings is a potential weak spot in terms of airtightness. The window
manufacturer's recommended methods for closing the gap between the window frame and
structural timber frame should be followed, and, in addition, the internal plasterwork and
external render should both be sealed to the window frame using a render stop bead and
mastic seal, or a frame seal bead, the latter of which is aesthetically far more discreet for
use internally (see Chapter 18, page 245 ) .
Shuttering for an opening
Openings can be created using either temporary or permanent shuttering. If permanent
shuttering boards are used for the walls, then it makes sense for these to be used for the
reveals also.
Where the internal walls are created using temporary shuttering, then doing the same for
the reveals gives a more consistent background for the plaster and allows the hempcrete
to be curved with a nail float after placing. However, the use of temporary shuttering for
the reveals requires the placing of the hempcrete around the openings to be very thorough
and even, so that the corners created in the cast hempcrete are strong and stable. This takes
extra time, since it is an awkward space to physically get into when placing the hempcrete.
For this reason, those inexperienced with hempcrete building may wish to use permanent
shuttering to make the placing of the hempcrete at this section less critical.
Permanent shuttering also makes it easy to support the hempcrete above the opening, since
the permanent board forming the top of the opening is in place to hold up the wet hemp-
crete as it dries out through the board (whereas temporary shuttering has to be removed
for the hempcrete to dry out). However, most permanent shuttering boards are inherently
weaker than temporary shuttering boards, due to their lower density, so may still require
additional temporary bracing to be fixed underneath while the hempcrete is being placed,
to avoid movement.