Indicative detailing - The Hempcrete Topic: Designing and Building with Hemp-lime - page 552

Airtightness over the roof
The vapour-permeable membrane generally used on top of the roof insulation provides a
second line of defence against any moisture that penetrates the roof covering. Any mois-
ture that moves out of the building via this membrane, which condenses as water droplets
on the underside of the roof covering, is also then kept out by the membrane. The mem-
brane, when joined with tapes designed specifically for the purpose, can also be used as the
airtight layer in the roof build-up, and should be sealed to the hempcrete and the render (or
cladding membrane). The longevity of tapes in this situation is still largely untested, and
in our experience it is very hard to get the overlap of membranes to lie perfectly flat for
easy taping. For those who share our concerns, an alternative to using tape for airtightness
at the roof membrane is shown in Figure 29.
Figure 29. Good and bad airtightness detailing at the eaves.