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

Dry-lining versus a hemp and lime insulating render for internalthermal renovation of a stone cottage in West Wales, includingembodied energy assessment, interstitial wall monitoring, In-situ U-Value and WUFI modelingMarion Wright 1 & Naomi Miskin 1 & Andrew Flower 1 & Ranyl Rhydwen 11 University of East London & Graduate School of the Environment,Centre of Alternative Technology, Machynlleth,WalesEmail: ranyl.rhydwen@cat.org.uk, njmiskin@hotmail.com, marionwright@hotmail.co.uk,andrew@andrewRflower.comAbstract:The evidence regarding global warming and biodiversity losses suggest there is anurgent need to rapidly reduce carbon dioxide (CO 2 ) emissions and to prevent furtherbiodiversity depletion. Buildings and the materials used to build, maintain and renovatethem account for a significant part of the problem. The manufacture of buildingmaterials tends to have a high energy input, involve toxic processes and create harmfulwaste. The UK housing stock is energy inefficient, with the majority of homes requiringrenovation to improve their thermal efficiency. Therefore materials used have to beeffective, have a low embodied energy and ideally improve biodiversity.Stone walled houses are a particular problem for internal renovation due to lack ofspace and concerns about the control of moisture in the home and within its‘ walls. Poormoisture control leads to ill-health, mould growth and structural damage. The standardapproach to internal thermal renovation is to use dry-lining and improve air tightness,however there are concerns regarding an increased risk of internal condensation as airtightness improves and interstitial wall moisture ingress, either due to membrane failureor liquid water surface diffusion. These risks are of particular concern with regards tomaintaining the integrity of heritage buildings in the UK, many of which fall in to thesolid, stone walled category.Hemp and binder is an insulating matrix, which used as an internal insulating render canprovide improved thermal efficiency in both the heating and cooling season although itsoverall thermal effectiveness remains unreported. It also potentially controls bothinternal and interstitial moisture fluctuations due to its hygric properties reducing theneed for increased air flow to prevent surface condensation. This paper focuses on areal-life case study of the renovation of a solid stone walled cottage, outlining themethod used in the upgrade and reporting the first 18 months of data logged consistingof direct wall heat flux, in room, external and in wall temperature and relative humiditymonitoring. It also includes estimates of the embodied energy (from manufacture,installation and disposal), in-situ U-Values and the results of a WUFI model.1 IntroductionThe hemp-lime Cottage 2 renovation project was driven by 4 issues:1

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

computer model to predict the potential for moisture accumulation in the renovationwall.3 Methods3.1 MonitoringThe study consisted of the monitoring two test renovation areas comprising applied to asolid-walled slate cottage in Mid-Wales. There is a DL wall with a vapour barrierbehind plasterboard and mineral wool insulation with an air-gap before the originalmasonry and a wall with a hemp lime internal render (see Figure 1). The ingredients andproportions used in the Hemp/Lime mixture were as used in the aforementionedHaverhill development. The proportions of the mix were: 37.5kg Lime Binder (ratio: 2Hydrated Lime: 1 NHL 3.5) 5-60 litres Water, 200 litres Hemp shiv. Figure 1 shows themethod used for applying the Hemp-Lime render to the stone wall.The 2 walls were constructed adjacent to one another on the north facing rain shelteredwall, with the renovations being of equal thicknesses. Platinum ResistanceThermometers (PT100's) error (+/- 0.3 o C) and direct voltage output capacitanceRelative Humidity Sensors (Honeywell HIH-4000's) error (+/- 3 RH% 0-100%) wereplaced at intervals throughout the thickness of each test wall with mirroring distancesbetween sensors within each wall. On the internal surface of each wall, Heat Flux Plate(Hukseflux HFP01) errors (+/- 5%) placed in direct line with the sensors and plasteredinto the walls to minimize resistance, absorbance and deflection errors. A diagram ofthe experiment is visible in Figure 1. Table 1 shows the U-values of the walls.3.2 Embodied EnergyEmbodied energy (carbon) is a measure of the energy (CO 2 emissions) associatedwith a product over its entire lifecycle, from ‗Cradle to Grave‘, i.e., supply ofmaterials, processing and manufacture through lifetime, maintenance and end oflife disposal impacts, derived through Life Cycle Analysis (LCA) assessment(Anderson et al, 2009). Embodied energy and carbon data was obtained from avariety of literature sources, but predominantly from the ICE database 1 (Hammond &Jones, 2008). The ICE values are derived from LCAs and other literature sources,however are quoted for cradle-to-gate rather than cradle-to-grave.1 ICE - Inventory of Carbon and Energy; a database available from the University of Bath Sustainable Energy andResearch Team5

Figure 1: Left the Hemp-Lime being tamped into shuttering, right schematic of wall and probe positionsInformation on the energy requirements of installation and initial drying, timescales and labourrequirements for hemp-binder was taken from the cottage renovation. Due to the wide variancein the gathered data several scenario where created to give the impression of the spread of thedata, these are described in detail Miskin 2010 however only the mid range is reported here.Table 1. Calculated U-valuesThe hemp layer resistance is calculated from the conductivity readings taken using an ISOMET 2104 whichhas an accuracy of 5% and the other resistance values are taken from standard literature3.3 In-Situ U-valueBaker has developed a method of estimating an In-Situ U-value for a building‘s wallfrom the measured internal wall heat flux and the internal and external temperaturereadings (Baker 2011) when the temperature difference between inside and outside is atleast 10 o C using equation 1.i=tU Value = 1 / (∑ T ins (i) – T out (i) / ∑Q (i)) + R ins (W/m 2 .K) Eqn. 1i=0 i=0Q = Heat Flux (W/m 2 ), T ins (i) = Inside Surface Air Temperature ( o C), T out (i) = Outside Temperature ( o C),R ins = Internal Surface Resistance (0.13m 2 .K/W)To calculate the confidence interval (+/-δU) for the U-value measurements the errorterms were calculated using equation 2 as used by Baker (Baker 2011).δU = (U-U errQ ) 2 + (U-U errTins ) 2 + (U-U errTout ) 2 + (S.D) 2 Eqn.2i=t6

Where U errQ , U errTns and U errTout are the U‐values calculated when applying all errors dueto heat flux, internal surface temperature and external temperature, respectively and U isthe original U-value3.4 WUFI modelThe interstitial moisture control modelling investigation was developed in the moistureand heat transfer programme WUFI, from Fraunhofer Insitut. The full development ofthe models is covered previously (Flower, 2011). Two models were based on the twowall sections (Fig 1) and extended simulations performed. The first 10 months ofrecorded data from the cottage provided verification of the models ability toqualitatively represent the patterns of RH and temperature fluctuations (Fig 2).Figure 2: Simulated Data (red) versus real data (blue) in the 36mm mineral wool showing how thegeneral trends are well captured by the model4 Findings and discussionFigure 3 shows that the temperature gradient across the DL wall is steepest over theinsulation whereas the hemp appears to acting as if homogenous. On the warmest daythe external and internal temperature are both higher than the inside the centre of thewall by 5-6 o C and thus heat will be being drawn into the wall from the inside andoutside surfaces whereas steady state would only predict a minor flow from outside toinside.From figure 4 it is evident that in both test walls, condensing moisture has beenoccurring constantly in both warm and cold conditions. This should be expected in theHemp/Lime wall as the materials are still drying. However the construction of the DLwas dry and despite the Vapour Barrier the RH behind the DL is constantly ~100% andhas been so for the entire period of data collection suggestive the moisture is beingdrawn into the space from the slate wall.The conditions behind the DL test wall are as predicted and as the moisture presentcannot re-evaporate back into the interior environment as it can through theHemp/Lime, it must penetrate the thick stone masonry in order to escape, thusincreasing the risk of frost damage to the masonry, interstitial rot and mould growth.4.1 Embodied CarbonTables 1, 2 and 3 in figure 5 summarise the embodied carbon calculations for hempcrete7

and DL. Full details of the calculations and assumptions can be found Miskin (2010).The embodied carbon of hempcrete represents a ‗most likely‘ current scenario, wherethe hemp is grown in monoculture using typical farming practices and the lime issourced from a centralised facility. Whilst the embodied carbon figure is negative, thisfigure could be much lower (therefore substantially more carbon sequestered), iforganically grown hemp and lime from local kilns were available, and better again if aclay binder were used (Miskin, 2010).Figure 3: Temperature profiles Coldest and Warmest Day, slate wall is not to scale.Figure 4: RH in DL and Hemp renovations, note how the 36mm hemp starts to drop RH indicating thesurface of the hemp has dried.The walls of the cottage that were insulated covered an approximate area of 50m 2 .Based on the figures in Tables 2 and 3, the embodied carbon for insulating the cottageentirely with hempcrete is -4 kgCO 2 m 2 and entirely with dry lining is 903.5 kgCO 2 m 2 . Itis likely that the embodied carbon in the DL would be paid back by savings in CO 2emissions from heating within the lifetime of the renovation when compared to hemp ifsteady state values are used to calculate the energy losses. However, assuming that thehemp can be demonstrated to have an equal or close thermal performance to DL, thecarbon dioxide emissions savings gained through using hemp-binder become significantif it were to become widespread. This is demonstrated by assuming that all the UK‘s 7million solid walled buildings were insulated using hempcrete.8

Various hempcrete combinations were considered (Miskin, 2010), including hempgrown organically, using a low impact model (Rhydwen, 2006), use of a clay binder,lime produced locally, centrally and assuming various rates of re-carbonation of lime.Three dry lining solutions were also modelled for comparison (mineral wool, EPS andPUR).Figure 5: Embodied carbon TablesThe best case hemp-binder scenario could potentially sequester over 10 million tonnesof CO 2 , whilst the worst case (mineral wool) would emit nearly 10 million tonnes. Themost likely hemp-binder scenario however is only just carbon negative. It is obviouslyunrealistic to assume that all solid walled homes would be insulated using the samematerials; however the comparison demonstrates the potential of hemp-binder andindicates the magnitude of initial emissions if only synthetic materials are used figure 6.Figure 6: CO 2 emissions from insulating the external walls of all UK solid wall houses using best, worstand most likely hemp-binder scenarios and dry lining solutions.9

Figure 7: The number of wind turbines for equivalent carbonIn order to put the carbon sequestration potential of hempcrete into context, the numberof 2 MW wind turbines that could be installed for the same amount of carbonsequestered if all solid walled UK homes were insulated has been calculated, as shownin Figure 7. An average 2MW wind turbine can provide enough electricity in one yearfor over 1,000 homes (BWEA, 2007). Therefore the 22 thousand turbines that could bebuilt with the equivalent amount of carbon sequestered under the best case hemp-binderscenario is more than required to power the UK‘s 22.2 million homes (CLG, 2009).Even the 900 wind turbines that could be built with the equivalent carbon sequesteredunder the most-likely hemp-binder scenario would generate enough electricity to power4.5% of UK homes.4.2 In-Situ U-valueAs shown in figure 8, the Dry Lining (DL) section of the wall has a 22% lower in-situU-value than calculated (Table 1) (figures 8 & 9). However the calculated U-value islikely to be overestimated as the slate wall is treated as a homogenous slate whereasmass walls can contain up to ~30-40% mortar (Baker 2011). This can‘t account forwhole effect as to reduce the overall U-value of the wall to the in-situ value theconductivity of the slate wall would need to be only to 0.5 W/m.K., considerably lowerthan found for solid walls (Baker 2011). Therefore it seems reasonable to assume thatthe dynamic mass of plaster and plasterboard is lowering the effective U-value of theDL lining wall to some degree as well (figure 8 right side).Figure 8. Left: Hemp_Calc. and DL_Calc. are expected U-values for walls using there dry conductivitieswith the error days representing a 5% error, Hemp_Meas. and DL_Meas. are in-situ U valuesmeasurements. Right: This figure shows a time where the heat flux from the DL wall is negativeindicating there has been thermal storage in the plasterboard and plaster. The shaded area shows theperiod of negative heat flux.10

Figure 9: The error bars represent the 5% range reported for the heat flux sensors and all dry conductivitymeasurements. Qw = Winter Heat Flux total, DL = Dry Lining, SS = Qw-Value calculated from SteadyState U-value where Q = - (U-value x (Tout-Tins)), Meas = Measured in-situ value.The DL wall clearly outperforming the Hemp wall (figures 8&9). The Hemp wall isperforming with a U-value effectively 42% worse than predicted. The most likelyreason for this is that the Hemp wall is still drying. This is conclusion is collaborated bythe hemp wall‘s in-situ falling overtime (Figure 10) as compared to the DL which isreasonably static suggesting the hemp wall is still drying.Figure 10: Change in calculated 14 day U-value overtime. This figure shows the change in average U-value for the Hemp and DL walls. The grey shadows are the 95% CI. It can be seen that whereas the DLU-value is relatively static the Hemp U-value slowly declines throughout the winter. The shaded areashows a period after none occupancy when internal RH rose as temperature fell.These results have implications for evaluating the carbon budgets for both wall types, asthe DL is more energy efficient than thought highlighting how using steady state U-value models to estimate thermal performance could lead to significant errors whenassessing actual thermal performance. Clearly at present the high RH behind the DLdoesn‘t appear to be causing any deterioration in thermal performance although thismay change over time if moisture accumulates as the WUFI model suggests. Cottage 2will be continued to be monitored to see how these 2 alternative renovation techniquescontinue to perform and to hopefully establish an effective U-value for the Hemp wallonce it is fully dry.4.3 WUFI modelThe simulations that matched the first 10 months of real recorded data from the cottageconfirm that the model gives a reasonable representation of reality (figure 2). The11

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

It is also clear that hemp renders will sequester carbon into the renovation wallshowever based on U-values alone a DL method will still save more carbon by fueldisplacement in the long term. However as illustrated on the large scale the carbonsaved into the hemp wall could be utilised to offset renewable energy constructionswhich will then save more carbon than just the carbon saved by the increase in thermalefficiency improving the long term carbon savings associated with hemp and limerenovations. Further the U-value is unlikely to be fully representative of the true thermalefficiency of the hemp renders as it doesn‘t include thermal bridge reduction, moisturebuffering effects or the drying of the external wall. It will therefore be interesting to seehow the hemp render behaves once it is fully dry. Therefore the main conclusions are;1. Dry Lining causes moisture to accumulate in the wall and space behind it whichwill lead to rot, internal structural damage and increased frost damage.2. Hemp renders are not only insulating but also dry out the external wall and handlemoisture well suggestive that they reduce the risk of rot, mould and frost damageovertime.3. Hemp renders sequester significant amounts of carbon dioxide which could beused to offset the embodied carbon of renewables in a win win scenario4. U-values can under predict actual performanceThe cottage will continue to be monitored and the results reported, a post occupancyevaluation is under way and further modelling work is planned to try and establish thefuture thermal, hygric and moisture buffering performance of the hemp render. All thistogether with work initiated already looking into the composting potential of hemp andbinder should allow an inclusive environmental and performance of assessment of hemprenders to be established. Finally from the results it appears that for internal renovationthat hemp and binder is a viable option that not only improves thermal performance butalso handles moisture effectively. Its wet application is a problem and creating a dryapplication from pre-formed blocks is another area of future research that is underway.6 AcknowledgementsA warm thank you is extended to the CAT volunteers Simon and Murdo who prepared theCottage for renovation, Carole for her tireless hemp and lime mixing, all the other CATvolunteers who tamped and carried the hemp into place and especially to Arthur Butler the CATengineer who was instrumental in ensuring that all the probes and data-logger where correctlyinstalled.7 ReferencesADAS (2005) UK Flax and Hemp production: The impact of changes in supportmeasures on the competitiveness and future potential of UK fibre production andindustrial use, ADASCentre for Sustainable Crop Management for DEFRA.14

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