Dry-lining versus a hemp and lime insulating render - University of ...

Figure 11: Increase MC content in brick behind DL on left, drop in MC in brick behind Hemp rightIt seems the hemp/lime render is very effective at transporting moisture but to be mosteffective it needs to ―breathe‖. It works better with more porous walls e.g. brick, andwill benefit from ventilation during the drying period. The conditions present in thecottage hemp/lime project at CAT are possibly some of the harshest that might beencountered in the United Kingdom for internal insulation of a solid wall. Thesimulations indicate that even in this harsh environment the hemp lime can be effectiveat reducing moisture load in the wall. However, the prevalent conditions appear tocontribute to abnormally high saturated moisture concentrations at specific points andtimes.The moisture spikes in hemp/lime are time dependent and of limited duration. In DL thespikes are associated with a build-up of moisture in the wall immediately adjoining thewall/air gap interface. Therefore, moisture spikes in hemp/lime can regarded as atransient issue whereas in dry lining it represents a long term problem than in DL whichcould lead to long term problems5 ConclusionsHemp and binder renders take time to dry out and during this process their dynamicthermal performance as measured with the in-situ U-value is worse than anticipatedwhereas the DL is out performing its U-value predictions by 22% most likely due to anunderestimation of the U-value of the slate wall in combination with a small thermalmass effect.The RH behind the DL is in keeping with the WUFI model and is 100% the whole timeindicating that with a standard vapour barrier that moisture will build up behind DL andin the wall. This could induce rot and mould behind the DL and the moisture build up inthe wall increase it thermal conductivity worsening thermal performance and lead to anincreased risk in frost damage in the wall. Although from the monitoring it isimpossible to discern if the hemp render will dry out the adjacent wall this did occur inthree separate WUFI models under differing conditions. This is therefore indicative thatthe Hemp render will actively dry out an underlying wall which should improve itsthermal performance and reduce the risk of frost damage despite the coldertemperatures resultant from the insulation effects of the render.13