A residential project involving hemp as a building material is targeting Passive House certification in New Zealand. Hemp is a new material for the Sustainable Engineering team and it’s interesting to go diving into the techie details.
As with other natural building materials, hemp is not a standardised product. The exact mix of the hempcrete and its reasonably expected thermal conductivity are crucial things to get right when assessing thermal performance. As with all things Passive House, when a designer is uncertain, make a conservative estimate. Also, offer proof as to why that estimate is conservative.
Start by knowing the mix that will be used and its dry and wet density. Then track down appropriate test data or commission some testing. I’ve included some initial research below. Start there, move into more detail as needed if the project’s mix doesn’t match the data below.
In October this year, an appendix on hempcrete was added to the US residential building code used by 49 of its 50 states and will be also be included in the 2024 International Residential Code (IRC). The Code-permitted values are:
This table is for hemp-lime mixes and it’s not appropriate to use this data for a mix that includes cement. It also needs to be converted correctly: remember that imperial and metric R-values are not the same!
Here’s a conversion to kg per cubic metre and thermal conductivity in W/(mK):
Cross checking these values from Abdellatef Y, Khan MA, Khan A, Alam MI, Kavgic M. Mechanical, Thermal, and Moisture Buffering Properties of Novel Insulating Hemp-Lime Composite Building Materials. Materials (Basel). 2020 Nov 6;13(21):5000. doi: 10.3390/ma13215000. PMID: 33171950; PMCID: PMC7664188.
“In contrast, there was a positive correlation between density and thermal conductivity of the hempcrete [9,10]. Thus, studies have reported thermal conductivity in the range of 0.06−0.11 W/m K for hempcrete mixtures with dry densities ranging from 200 to 400 kg/m3 and hemp/binder ratios of 1:1, 1:1.5, and 1:2 [11,12].” In Table 4 below they used various mixes attempting to maximise the hemp content. Notice they are fairly close to the 320 kg/m3 and 0.094 W/(mK) values from Table AY106.2 above.
Potentially useful resources: