The render on a hempcrete building has multiple functions. Traditionally hempcrete buildings have always been lime rendered. If not they’ve been clad.
Hempcrete is a vapour permeable building material. It performs exceptionally with good overhangs, good drainage, careful detailing around openings and it requires an external render.
The render not only provides fire resistance and weather-protection, it also governs the rate of the moisture transfer – the balance between the moisture coming in and the condensation getting out of the building – therefore it minimises risk to the integrity of the frame from moisture. As a flow on from that, the render impacts the relative humidity in the building, potentially impacting on the likelihood of moulds.
Render also has an acoustic function. We know from acoustics testing that sound will transfer through a hempcrete wall that is unrendered on both sides. It doesn’t really make sense to miss out on the sound dampening effect of the render. Well-built hempcrete homes are renowned for being quiet, highly comfortable spaces.
We also know from research that there is eventual deterioration of the porous structure of the hurd in repeated wet/dry and freeze/thaw events. Again it simply doesn’t make sense to ignore this and omit the render if you are after long term durability for your home. What the research tells us is that you will also eventually get a deterioration in thermal and acoustic performance if you don’t prevent repeated saturation events.
Not maintaining the integrity of hempcrete walls, especially in locations where there are wet months will lead to deterioration of all aspects of hempcrete’s performance in the long term. This includes on the thermal performance of the building which will have an eventual flow on effect of increasing energy costs and the need for increased energy consumption.
How long does that deterioration take? Based on what we have seen on a hempcrete build built using European materials in Tasmania where there were rendered walls but no overhangs, and where moisture got into the walls during the build, the answer is, in as little as 15 years. On this build the frame rotted out.
Even if you can take an engineer or certifier on a winding path to consider an externally unrendered hempcrete building, unless they have researched the specific products that are being used they should never recommend that render is unnecessary externally unless there’s a published body of research on that specific product that firmly supports that case. To the best of our knowledge there has been no hempcrete research that suggests that.
In Canada where there is consistently higher rainfall they specify a 20mm thick render or a vented rain cladding to manage the moisture transfer in and out of the building.
There is a weighty amount of evidence to suggest that even though a building might look alright to the home owner, that they mostly have little idea of what’s going inside their walls until there is a fairly major problem. Home owners trust that their building will work well for them. They don’t regularly monitor moisture transmission rates or deterioration of the frame.
They assume that they are being well advised by everyone using the materials.
It is really important to follow the instructions of the product manufacturer. They are the people who have invested in research on the materials so that durable hemp homes can be built. Your certifier’s approval assumes that that is what you are doing.
Many articles and publications on hempcrete give this same advice.