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

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moisture content in the solid wall was set high (85% RH equilibrium state) andremained high in both simulations over the10 months. Indian sand stone was used forthe slate wall after multiple tests as it has similar properties. In the DL simulation the airgap retains a high humidity near or at 100%, whereas the RH in the mineral woolfluctuates expectedly between 100 % and 60% as the internal temperature changes. Inthe hemp-lime simulation the RH in remained constant at ~100% as it is still drying andslower than expected due to the cold room conditions.Next a predictive analysis applying a standard sine curve from WUFI for indoor climatewith a mean temperature of 21 o C and RH of 50% with amplitudes of 1 o C and 10%respectively to the DL and Hemp models. During a 3 year simulation a 3 year period itis noted that the wall renovation interface MC varies more than in the verification runand the model for DL shows a slightly upward to constant trend in moisture of the stonewall adjacent to the air gap, whereas in the hemp/lime model in the third year there is amarked dip in the moisture content in the stone wall. This implies that the dry liningmodel tends to retain moisture in the wall, whereas the hemp lime tends to drawmoisture out. Two Additional analyses were conducted, first setting the startingmoisture in the wall to a lower level (RH45%) and a default MC solid brick wall225mm thick and both confirmed these same findings which were more pronounced inthe dryer initial and greatest in solid brick figure 11.The animation outputs highlight well the behaviour of moisture transfer around theinterfaces between different materials in the insulated wall structure and revealed thatsignificant moisture build-up or spikes can occur at the renovation wall interfaces.These spikes are time dependent from application of the new internal insulation. In theun-heated cottage condition the interior wall spike for the dry lining case occurs atmonth 3 and remains till month 5. In the un-heated hemp-lime a spike also occurs at theinterface between the hemp-lime and the lime plaster at month 4 and holds till month 6.Figure 10: WUFI simulation with hi9gh initial moisture content in wall section showing the increasingMC of the wall behind DL left side and the drop in MC of the wall behind the Hemp Render.12
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
Page 1: Dry-lining versus a hemp and lime i
Page 5 and 6: computer model to predict the poten
Page 7 and 8: Where U errQ , U errTns and U errTo
Page 9 and 10: Various hempcrete combinations were
Page 11: Figure 9: The error bars represent
Page 15 and 16: Allwood J. M., Cullen J.M., Milford
Page 17: Miskin, N. (2010) The Carbon Seques

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