You should not use boards that are not vapour-permeable (or not sufficiently vapour-permeable) such as OSB or ply except as temporary shuttering boards – used to shutter wet-cast hempcrete. There are specialist vapour-permeable racking boards, such as Econic. If possible, it is better to add boards after wet-cast hempcrete has fully dried, as even vapour-permeable boards will significantly slow down the drying process. 

Yes. You may wish to combine the hempcrete with other types of natural insulation such as wood fibre board to reduce the wall thickness somewhat. However, the hempcrete element, making up the bulk of the wall thickness, will provide greater thermal mass than most other natural insulations, and will increase the thermal performance of the building. 

Our team can give a broad per-square-metre of usable floor space* figure for building a brand-new hempcrete house. However, the actual cost will depend on how the building is designed, including design of the structural frame, the other materials involved and types of finishes, as well as factors such as ease of access to the site. If you would like a specialist contractor to accurately price the construction of your building, you will need to have detailed construction drawings, showing full dimensions and all elements of the wall-, roof- and floor-build-ups. We [or our sister companies] can offer specialist services to get you to this point, whether you have hand-drawn sketches on paper, planning drawings for a building not yet designed for hempcrete, or simply a vision of what you want to achieve. 

*include ground floor, first floor and any floors above

Ratios for mixing hempcrete in a large forced action pan-mixer, or spray machine (bags of binder to bales of hemp shiv to water volume) are given in the individual product pages. If you are mixing on a smaller scale, for example in a cement mixer (Belle mixer), you can use “this rule of thumb” ratio to measure mix quantities out by volume (e.g. in buckets). This mix ratio works for all hempcrete binders, with small adjustments to the amount of hemp if your mix is slightly too dry, or slightly too wet.

Small batch hempcrete “by volume” mix:   3-3.5 hemp shiv : 1 binder : 1 water 

If you’re mixing in a Belle mixer, please watch this video.

Yes. Hempcrete, either in wet-cast or precast block form, can be used to insulate existing stone or brick walls internally – internal solid wall insulation (ISWI) – or externally (ESWI). It needs to be used with other vapour-permeable materials – for example lime or clay plaster, lime render and lime mortar. Generally it should not be used internally where the walls are below external ground level. We usually recommend using between 100 mm (min.) and 200 mm (max.) hempcrete internally, depending on the amount of space you can afford to lose from the internal dimensions. Used externally the hempcrete can be thicker than 200 mm if this can be accommodated.   

In comparison with concrete, hempcrete: 

• is much less dense than concrete (at around 300 kg/m3 compared to over 2,000 kg/m3)  
• is not a load-bearing material 
• has a plant aggregate rather than stone 
• has high vapour permeability, as opposed to very low vapour permeability 
• is an insulation and mass material, rather than a mass material. 

Hempcrete is a non-structural medium density walling and insulation n material with a plant aggregate. It is not suitable for use as a load-bearing material, and should not be installed at or below external ground level, or anywhere else where it may be exposed to standing or running water. In some specific circumstances, hempcrete can be used to line out and insulate below-ground rooms. Please read our information sheet to find out more, and make sure your hempcrete installation will not fail through poor detailing. 

For hempcrete (wet-cast or blocks), or for lime plaster or render materials, we usually recommend allowing 5% for wastage/cutting on top of the volume you think you need. However this is obviously at the customer’s discretion. If you are calculating the area of plaster or render needed, don’t bother subtracting the area of any window or door opening under 2 square metres as the area of the reveals will usually make up the difference. For openings over 2 square metres in area, just subtract the balance over 2 m2 in each case.