Application of hemp-lime renders to improve the insulating capacity of building walls
This paper measures the variation of thermal transmittance of solid brick walls triggered by the application of hemp-lime renders in an effort to enhance the insulating properties of buildings. Six renders with different proportions of hemp and lime were fabricated and two selected based on their workability and adhesion. They were applied to the walls and their thermal transmittance measured using the hot box method. Thermal imaging was used to control thermal bridges in the masonry assemblies. When compared to commercial mixes such diathomite and expanded polystyrene, the hemp-lime renders display similar thermal properties that qualify them as good insulators. It was evidenced that the application of a hemp-lime render can halve the thermal transmittance of a solid brick wall. The renders notably increased the resistance to heat transfer of the brick wall. Render 5 [a 1.25: 1 - hemp: NHL3.5 mix applied in a 21 mm depth] nearly doubles the thermal resistance and halves the thermal transmittance of the wall, reducing the U-value from 6.99 to 3.65 W/m2K therefore doubling the insulation provided by the solid brick wall.
It was also noted that the 21 mm hemp-lime renders improved the thermal properties of stone walls, but the improvement is not as notable as in the brick walls. However, a thicker render [40 mm] would greatly improve the thermal performance of the stone walls reducing U-values by c.40%.
The outstanding insulating ability of air gaps was exposed when the hemp-lime renders were applied on a metal lath/mesh set with a 20 mm gap off the wall: here, the U-values lowered by over 30%. Also, it was demonstrated that the hemp-lime renders improve the thermal performance of solid walls to a much greater extent than cavity walls.