Eco construction


Roof insulation

Preparing the surface:

If the interior underside face is sensitive to humidity, an anti-humidity shield is necessary by dusting a 2 cm-layer of hemp chaff with Prompt natural cement. The other side must be resistant enough to bear the weight of the hemp concrete.


Building Hemp Granulate PNC Tempo* Water**
200 litres 25 kg (1 bag) 1 bottle 44 litres

* Tempo dosage for a open time of around 15 minutes depends on temperature:
 If t° < 15°C: 1 bottle per bag or 1 capful per 2 litres of Prompt natural cement
 If t° > 15°C: 2 bottles per bag or 1 capful per litre of Prompt natural cement
 ** Adjust for the required consistency – Avoid using too much water, which is detrimental to durability.

Mixing methods: 

  • with a mixer: The order for introducing components is as follows: Building Hemp Granulate + ¾ water + Tempo +  Prompt natural cement + the rest of the water
  • concrete mixer mixing (capacity 350 to 400 litres with central core if possible)

The order for introducing components is as follows: ¾ water + Building Hemp Granulate + Tempo + Prompt natural cement + the rest of the water


The mix must be homogenous, with the granulates well covered by the binder, with no lumps. The open time for the mix is approximately 15 minutes, according to the Tempo doses recommended above.

  • pour the mixture between the beams and spread with a rake without compacting,
  • leave a gap of at least 1 to 2 cm between the hemp concrete and the upper part of the beams to allow good ventilation, which is necessary for the drying process,
  • at least one side must be covered with a permeable water vapour shield.  (Figures 12-13 below).


Depending on settling you will need the following per 1 m3 of hemp concrete used:

  • 1,000 to 1,100 litres of Building Hemp Granulates
  • 110 to 150 kg of Prompt natural cement

Technical characteristics: 

  • Density: 280 to 320 kg/m3 depending on settling
  • Compressive strength: > 0.05 MPa depending on settling
  • Thermal conductivity: 0.05 W.m-1 K-1 at 0% relative humidity
  Thermal resistance
(m2 KW-1)
Thickness (cm) 10 15 20 25 30
Roof 2.00 3.00 4.00 5.00 6.00

Consult our Application Guide, available for download.