Design fundamentals - The Hempcrete Topic: Designing and Building with Hemp-lime - page 508

Shrinkage
Because it is a wet-mixed and naturally flexible material, hempcrete tends to shrink very
slightly as it dries out. Different binders result in different rates of shrinkage, depending on
their strength and setting times: some slower-setting or weaker binders give a noticeable
1mm or 2mm gap at junctions, while others, for example Prompt Natural Cement, have
practically no shrinkage at all. See Chapter 6 for details of different hempcrete binders.
Except when it is spray applied, hempcrete does not adhere tightly to the structural mem-
bers within the wall. Instead, its strength derives from the fact that it completely engulfs
and sets around them, with its fibrous aggregate tying it together. Therefore any frame
component that extends from one face of the wall through to the other has the potential to
create air leakage, as a result of gaps forming around it as the hempcrete dries. The use
of such elements should be avoided wherever possible, but where they cannot be avoided,
steps should be taken to mitigate air leakage. In the rest of this section we discuss how to
counteract any shrinkage gaps at junctions. Airtightness as a whole is not discussed here,
but is covered in more in depth in Chapter 22 .
Exposed frame internally and externally
This situation may arise in the refurbishment of a historic timber-frame building where the
main timbers are to be left exposed both internally and externally. If upgrading or repla-
cing the panels of material between the timbers with hempcrete, the junction between the
hempcrete and the main timbers is at risk of air leakage. To mitigate this, a binder with
extremely low shrinkage, such as Prompt Natural Cement, can be used. Owing to its ag-
gressive set, Prompt reaches full strength before all the water leaves the material, and is
therefore strong enough to resist the shrinkage otherwise caused by drying. By adding a
timber batten to the junction you increase the surface area of the junction and introduce an
obstacle to the passage of air, reducing it further (see Figure 15, page 276 ). It is possible
to introduce a natural-fibre insulation to this junction to further increase airtightness at the
edges of the panel, but in our experience this is rarely necessary. The use of Prompt as a
binder, together with the batten and a good standard of finishing, is sufficient for restricting
air movement in a historic building that will never, and is not expected to, meet modern
standards of airtightness.