Roof insulation mix
It’s estimated that about 25% of the heat lost from a house is through the roof. Clearly, effective insulation is a priority in order to minimise these losses. Of equal importance to me was the need to insulate the living space from the solar gains of the tiled roof. I.e. to prevent the roof from being a giant solar collector that could overheat the tiny house in the summer. This problem could be particularly acute in my case because the vaulted ceiling meant there was no loft space to act as a buffer.
As with the rest of the house, hempcrete was chosen as the insulation material.
St Astier, the manufacturer of the lime I bought, suggest a lightweight (less dense) hempcrete for roof insulation and gave some technical guidance on how it should be mixed and used. The recommended mix ratio is 1 sack of lime to 200L of hemp. This is half the amount of lime that is used for the wall insulation mix.
The first step was to nail wooden boards to the underside of the rafters. These boards retained the roof insulation and also became the visible ceiling face from inside the tiny house. There was no separate plasterboard layer. The boards would later be whitewashed with lime. Rustic, cheap and simple!
Learning from my previous mistakes, I left generous gaps between the boards to allow for expansion due to water absorption from the hempcrete. I hoped that, once set, the hempcrete would stay in place with only a minimal amount falling through the gaps.
There were now ‘bays’ into which the hempcrete could be directly placed.
Filling the bays wasn’t complicated but it was physically challenging. The problem was that it was extremely hot and I had to mix the hempcrete, transfer it into large buckets, climb a ladder with the filled buckets, tip the buckets into the bays and then repeat the process! I think it took a couple of days to completely fill the roof (I lost a lot of weight that summer!).
After filling the bays with hempcrete, it was lightly tamped down with a large plastic float. The aim was to compact the hempcrete slightly and to leave an air gap of about 2cm between the top of the insulation and the top of the rafters. This gap was necessary for ventilation.
The rafters themselves were 15cm deep and so the insulation layer was 13cm thick.
Once the roof was completely insulated with hempcrete, a breathable fabric ‘rain screen’ was stapled to the tops of the rafters in preparation for roof tiling.
I was quite satisfied by this method of roof insulation, it’s a simple idea and easy to put in place. The wooden ceiling planks give exactly the kind of look I wanted. Unfortunately, little pieces of hemp do tend to regularly fall through the gaps, so I might have to fit cover strips at some point. I’m also very happy with the effect of the vaulted ceiling, it makes the tiny house feel bigger and more interesting on the inside than the outward appearance would suggest.
The mezzanine sleeping area shown below is very close to the underside of the ceiling. Despite this, it didn’t feel uncomfortably hot or stuffy during the night.