Application of hemp-lime renders to improve insulation of walls

Abstract

This paper measures the variation in the thermal transmittance of solid brick walls triggered by the application of hemp-lime renders in an effort to enhance the insulating properties of building fabrics. Six trial renders with different proportions of hemp and lime were fabricated and two selected based on their workability and adhesion. The renders were applied to solid brick walls and their thermal transmittance measured using the hot box method. Thermal imaging was used to control thermal bridges in the masonry assemblies. The results were compared with a control brick wall with no render. When compared to commercial mixes such diathomite and expanded polystyrene, it was seen that the hemp-lime renders display similar thermal properties that qualify them as good insulators. The results evidenced that the application of a hemp-lime render can halve the thermal transmittance of a solid brick wall. The renders investigated notably increase the resistance to heat transfer in 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 also improve the thermal properties of stone walls however, the improvement is not as notable as in the solid brick walls. Though it was noted that a render twice the thickness (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 by including a 20 mm gap between the renders and the wall: when the hemp-lime renders were applied on a metal lath or mesh set with a 20 mm gap off the wall, the U-values of the walls lower 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.