Design fundamentals - The Hempcrete Topic: Designing and Building with Hemp-lime - page 500

building, then an alternative to render for the exterior finish should be considered. Well-
detailed clad walls with a breathable membrane and ventilated air gap effectively eliminate
water ingress (see Chapter 22, page 330 ).
Wall thickness
With regard to the overall thickness of the wall, the U-value of hempcrete limits the min-
imum thickness. UK Building Regulations regarding minimum U-values for new buildings
are continually updated, but at the time of writing, the minimum width of a hempcrete wall
should be around 250mm (see Chapter 11, page 140 ) , depending on the choice of binder.
Of course the maximum width of the wall is governed more by the client's pocket and the
law of diminishing returns than regulations, but it is worth noting that the labour cost of
increasing a 250mm wall to 350mm is comparatively low, and that the bulk of the extra
cost would be in materials alone. Building a 400mm wall does not cost twice as much as a
200mm wall, for several reasons. The shuttering, permanent or otherwise, remains a con-
stant, as does the structural frame. Because hempcrete is a relatively lightweight material
and placing it is a quick business, even doubling the labour of placing doesn't add signi-
ficant costs to the build overall. However, casting walls in excess of 400mm wide will not
produce much of an increase in performance relative to the extra cost, and the thicker the
wall, the longer it will take to dry. For all these reasons, hempcrete walls are usually cast
at thicknesses of around 300-400mm.
Where less stringent regulations apply to the required U-value, it is possible to build walls
thinner than 250mm. This might be, for example, in a garden building, when construct-
ing internal walls, or during the refurbishment of a listed building where protection of the
original fabric and historical character of the building take priority over Building Regula-
tions on insulation. An absolute minimum for a hempcrete wall has not been defined, since
it is a thermal element and so one would usually be seeking to increase its depth rather
than minimize it. In the past, 200mm and 175mm external walls have successfully been
achieved, as have 125mm internal walls and 150mm external walls in the restoration of
historic timber-frame buildings. Of course, the depth of cover over structural timbers must
be considered when constructing thin walls (see page 290 ), so wall thickness will always
be limited to the width of the structural frame within it plus the minimum recommended
cover over this structure.