Hemp solutions

with Saint-Astier® limes and mortars

Insulating hemp/lime concrete
Hemp is a natural material suitable for making light weight insulating mortars. Its density varies between 110 and 150 kg per cubic meter. The strands are between 5 and 25 mm long. The thermal conductivity of hemp in bulk is 0.05 W/mK.

Particular attention must be paid to ensuring that hemp is not subject to dampness due to stocking conditions of the raw material or capillary action in hemp mortars. Adequate damp proof courses should be in place if constructing walls or insulating/draining ballast base in concrete).

The qualities of Saint-Astier® Natural Hydraulic Limes are ideal for making hemp mortars. A special binder, Batichanvre®, has been prepared for making hemp mortars. Combined with Saint-Astier® NHL, Batichanvre® contains 25% less additions than other products that mix hydrated lime and hydraulic binders (cement, pozzolans).

Mixing in paddle mixers
Introduce the hemp and the sand. Whilst turning, spray with water mist until the hemp dampens (colour change). Introduce the Batichanvre ® and continue to spray until an homogeneous mix is obtained. Total mixing time is about 5 to 10 minutes.
Mixing in drum mixers
Introduce about 35 litres of water and the Batichanvre ®. Mix for 3 to 5 minutes to obtain a milky paste with no lumps. Add the loose hemp and mix for about 5 minutes to obtain the required mix adding the rest of the water. The completed mix should be rather dry and lean. Total mixing time is about 8-10 minutes
Timbers thickness
4 cm
6 cm
8 cm
10 cm
12 cm
Minimum hemp mortar cover
7 cm
8 cm
9 cm
10 cm
12 cm

1) Mixing ratios
Dosage > 1 bag of Batichanvre® (25kg) + 100 litres of hemp + (optional) 5 litres of coarse sharp sand (5mm down)

Water addition > 35 to 40 litres

2) Application
Directly onto soil > On a well levelled and compacted soil, stabilised if necessary, construct a 15-20cm ballast base. Soil can be stabilised by rotovating 200kg of NHL 5 to 1m3 of damp soil. Extra water can be added to dry soil. Do not saturate to avoid a longer drying time. Do not use any plastic sheeting.

On wood > place a 2 cm layer of hemp directly on the surface without plastic sheeting.

Apply the hemp concrete in layers of 5 cm maximum and level with a rake. Compact by tamping. The final layer will be levelled with a straight edge, compacted and floated with a wooden float. The minimum thickness of the hemp-lime insulating concrete is 10 cm. At this thickness, the thermal conductivity of the concrete will be between 0.09 to 0.10 W/mK depending on the degree of compaction. The compressive strength at 90 days will be about 1.8 N/mm2. The yield is 900 to 950 litres of mortar per m3 of hemp + 10 bags of Batichanvre®, depending on compaction.

Tiles, wooden floors or carpeting can be laid on an additional 4 cm lime concrete screed which should be suitably finished after the total drying of the work (up to 90 days depending on thickness and climatic conditions). Keep area well ventilated and avoid forced drying.

Wall construction
Mortar dosage, mixing and water addition: same as hemp lime concrete

Loft insulation mortar
This lightweight mortar does not have high mechanical performance.

1) Mixing
In paddle or drum mixers (same procedure as described in the hemp-lime concrete section).

2) Dosage
1 bag of Batichanvre ®) + 200 litres of hemp + 35 to 40 litres of water.

3) Consumption
1m3 of insulating mortar requires 1.05 m3 of hemp and 200 litres of Batichanvre®.

4) Thermal conductivity
0.06 to 0.07W/mK

5) Mortar density
150 to 200 kg/m3

6) Application
Loosely apply about 2 cm of dry hemp between the floor timbers. Place the mortar above this layer at a thickness of 15 cm min./25 cm max. Make sure that the hemp mortar level is about 1–2 cm over the final required level to allow for compaction. Lightly compact the mortar to bring it to the required level. If a floor is required, make sure that there is a space of about 2 cm between the top level of the insulating mortar and the floor boards.

External hemp renders

1) Mixing
2 bags of Batichanvre® (or Tradéco®) + 100 litres of hemp
Water addition > 50 – 55 litres

2) Application
On solid background, properly prepared, apply a stipple coat as follows: 1 volume of NHL 5 to 2 volumes of coarse sharp sand. Let it dry for 2–3 days (more in damp or cold conditions). The hemp mortar is applied at a total thickness of minimum 4 cm, in maximum coats of 2 cm (30–90 minutes interval). If a finishing mortar is not required, the last coat should be applied 3–4 days from completion of the previous coat. Application of finishing mortar has to be on a dry hemp mortar. Wait 90 days or more depending on the weather conditions (the hemp render must be protected from rain, frost, direct or strong sun or wind).

In external work, use Saint-Astier® ready-mix EcoMortar® WP as a finishing mortar. Available in a range of colours, this mortar has very low capillarity, ensuring that the hemp mortar is adequately protected. EcoMortar® WP has a maximum granulometry of 1.3 mm and is applied at a maximum thickness of 8 mm. For straight work it is suggested to apply 10 mm and scratch it back to 8 mm before floating it with a sponge float. Highly compacted metal float finishes are NOT recommended. The waterproofer in EcoMortar® WP will not have a negative effect on the mortar breathability as it is not film forming.

If site-mixed mortar or coarser finishes are required, applied manually or sprayed, use well graded coarser sands with Saint-Astier® NHL 2 or Tradeco® at a 1 : 2.5 (binder: sand ratio).

Hemp renders should be kept clear from the soil (15cm minimum) to avoid moisture penetration by capillary action. Protect external work from adverse weather conditions.

Maximum areas allowed: 3m2 for wooden frames and 9m2 for walls

In external work, if lime paints are used, mix Uniprotect on the final paint coat.

Internal hemp renders/plaster

1) Mixing
For a natural finish is > 2 bags of NHL 2 (or Tradeco®) + 100 litres of hemp + 60 litres of water.
Consumption > 25kg of NHL 2 (or Tradeco®)+ 50 litres of hemp for 1m2 of render at 5cm thickness.

2) Application
On a properly prepared background apply a stipple coat as follows: 1 volume of NHL 5 to 2 volumes of coarse sharp sand.

The hemp-lime mortar can be applied wet on wet (after 12 hours or so) by laying on or casting on. The thickness of the render averages 5 cm (up to 8 cm). It is applied in layers of 2-3 cm at about 20 to 90 minutes interval between each pass. The last coat (2 cm) to be applied 24 – 48 hours after the previous coat. This coat will be floated to achieve the required finish.

If a decorated finish is required use Batichanvre® instead of NHL 2 and proceed as above. The decorative finish (lime wash, EcoMortar® Stucco, etc.) can be applied after a drying period of 45-90 days.

3) Thermal conductivity of the above renders/plasters
Between 0.13 to 0.14 W/Mk.

4) The density of these mortars
Between 0.75 and 0.95 kg per litre, depending on the compaction.

In all cases the surfaces should not exceed 15m2 without movement joints. Walls should have a damp course to avoid moisture transfer by capillary action.

See the technical data sheets & further information: Batichanvre®, Tradeco®NHL 2, NHL 5, LimePaint, UniProtect