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

dynamically and therefore the rate at which heat is stored and propagated through therender‘s matrix is in a state of constant flux and whether this improves or deterioratesoverall thermal performance is unknown.Hemp is fast growing, a useful organic break crop, economically viable withoutdisplacing a food crop (Rhydwen, 2009), relatively low maintenance, requires nobiocides (ADAS, 2005), little if any fertiliser (Murphy & Norton, 2008, Ronchetti,2007), increases biodiversity (Rhydwen, 2006, Small 2002) and bio-sequesters 500kg orcarbon per tonne of crop produced (Pervaiz 2003), all of which give hemp very strongenvironmental credentials. The overall environmental impact of hemp and binder is thendetermined by the binder (e.g. hemp and clay can be composted but can hemp andcement?) and how the hemp is grown as mention later and discussed in detail by Miskin(Miskin 2010).Hemp and binder (e.g. lime, cement, and clay) is an insulating middle weight matrixthat is strongly hygroscopic with a high capillarity and moderate moisture bufferingpotential (Arnaud 2009, Evrard 2006, Wilkinson 2009). It is applied wet into form workto create a whole wall or to from an insulating render. It is easier to achieve an airtightconstruction using ‗wet‘ materials, such as hemp-binder and the encasing nature of casthemp-binder reduces the potential for thermal bridging; both potentially improvingbuilding thermal performance beyond U-value predictions (Bevan & Woolley, 2008).The thermal and hygric performance of the hemp and binder matrix has beenextensively investigated. In the Haverhill project two hemp homes thermally outperformed two standard cavity wall homes with a significantly lower U-value (0.58W/m 2 .K versus 0.35 W/m 2 .K) over several years yet no definitive conclusions could bedrawn as many cavity walls significantly underperform in practice (Hens 2007, Yates2002, Yates 2003). Evrard and Arnaud have reported all the thermal and hygricproperties of ―Hempcrete‖ (binder lime & cement) and demonstrated its dynamicthermal properties and made estimates of its thermal performance under laboratoryconditions and within computer models (Evrard, 2005, 2006, 2010 Evrard & De Herde2005, 2006, Arnaud 2009). However modelling has been difficult and the results areinconclusive (Arnaud, 2009, Evrard, 2010) as to whether the actual in-situ thermalefficiency of hemp and binder is better or worse than its predicted U-value performance.This in-situ performance is crucial in determining the overall carbon consequences ofhemp and binder in comparison to other insulating techniques. The moisture bufferingeffect of hemp and binder is not fully described as how to assess the potential ofmoisture buffering is still being fully determined (Padfield 2009). Lewis however hasshowed that hemp and binder does buffer acute pulses of moisture (e.g. a kettle boiling)more effectively than DL and that it appears to prevent condensation at the solid wallrenovation interface where he recorded an RH of 55% for a mock hemp renovation on asolid brick wall (Lewis 2009).Therefore there are several questions being raised about the potential for DL tothermally underperform and to handle moisture poorly and there are still knowledgegaps in the understanding of the dynamic hydrothermal performance of hemp andbinder insulating renders. This study was therefore designed to try and add some realworld data to shed more light on these questions and to form the reference for a4