The heat insulating potential of straw, an agricultural residue, used as part of the envelope of unconditioned buildings is the focus of the current work. Using a CFD software with experimentally validated boundary conditions, transient thermal analysis is performed on a 3-D model room of dimensions 3 m3 m2.5 m with vents and heat source representing a naturally ven- tilated indoor space. The thermal performances of a reference uninsulated concrete envelope and a straw insulation-based envelope are compared in terms of quantitative parameters such as indoor air temperature, decrement factor, and time lag as well as a qualitative discussion on the need for occu- pant adaptation, namely closing or opening of inlet vent and requirement of additional heating. The assessment and subsequent recommendation of straw insulation utilization is done for representative summer and winter days of the ve di erent climatic zones of India. A maximum reduction of up to 3.5 Corresponding author Email addresses: aksabapathy@outlook.com (Karthik A. Sabapathy), sateeshg@iitm.ac.in (Sateesh Gedupudi) June 13, 2021 C in the peak indoor air temperature is achievable with inlet temporarily closed when straw insulation is included in the envelope in summer condi- tions. In winter conditions of cold climatic zone, with inlet always closed, use of straw insulation leads to the lessening of additional active heat source requirement by as much as 600 W for the considered model room. Further, the e ects of increased straw insulation thickness, removal of heat source, varying inlet air velocity, and changing the position of the straw insulation to the outside are also analysed for the extreme warm and cold conditions. Keywords: Building envelope, CFD, Natural ventilation, Straw insulation, Transient simulations, Thermal comfort 2 Nomenclature c speci c heat (J/kgK). d thickness of the envelope (m). DF decrement factor. h heat transfer coecient (W/m2K). Ib direct/beam component of the total irradiance on a surface (W/m2). Id di use component of the total irradiance on a surface (W/m2). Ig irradiance on a horizontal surface which is equal to the Global Horizontal Irradiance (GHI) (W/m2). It total irradiance on a tilted surface (W/m2). k thermal conductivity (W/mK). q heat ux (W/m2). Q heat transfer rate (W). Rdi erence in the longwave radiation emitted and recived by a surface (W/m2). Rbratio of the beam radiation on the tilted sur- face to that on the horizontal surface. Rfroughness factor. t time (s). T temperature (K). 3 TL time lag (s). v air velocity (m/s). Vwvelocity of wind speci ed in weather station data (m/s). z height of room (m). Z standard wind velocity measurement height (m). Greek symbols sabsorptivity of a surface. semissivity of a surface. zzenith angle (). density (kg/m3). gground re ectance. tilt angle of a surface (). Subscripts C concrete. conv convection. i inside surface. in indoor. o outside surface. out outdoor. P plaster. 4 rad radiation. S straw. sol sol-air. surf surface. Abbreviations ASHRAE American Society of Heating, Refrigerating and Air-Conditioning Engineers. CFD Computational Fluid Dynamics. DHI Di use Horizontal Irradiance. DNI Direct Normal Irradiance. ECBC Energy Conservation Building Code. GHI Global Horizontal Irradiance. NSRDB National Solar Radiation Database. 1. Introduction 1 India is facing two closely intertwined challenges of growing housing de- 2 mand [1] and persistent portion of consumed energy derived from fossil fuels 3 [2]. With the building sector estimated to be contributing to 39% of car- 4 bon emissions globally [3], burgeoning built-up area in India does not do any 5 favour in her e orts to ght the consequences of climate change. During the 6 lifetime of a building, considerable energy is used up in maintaining ther- 7 mally comfortable conditions [4]. An individual accustomed to a conditioned 8 space eventually begins to feel comfortable over a narrow range of thermal 9 conditions [5]. Such a perception of a narrow comfort range, achievable only 10 5 through active space-conditioning, also plays a role in driving up energy con- 11 sumption in buildings. To slow down this vicious cycle, innovations aimed 12 at optimizing energy use in buildings are urgently needed. In this regard, 13 passive techniques are worth exploring because of their inherent energy sav- 14 ing potential. One of the simple and cost-e ective passive techniques is the 15 introduction of insulation materials [6], of sustainable nature, in the building 16 envelope. Apart from air-conditioned buildings, there are also many build- 17 ings still functioning under natural ventilation or mixed-mode ventilation. 18 A signi cant portion of building occupants in India are dependent on natu- 19 ral/mechanical ventilation or other forms of adaptation to maintain comfort 20 conditions [7]. So, to assess the feasibility of adopting building insulation for 21 unconditioned spaces, more research is necessary to gain better understand- 22 ing of the e ect of insulation on the indoor thermal environment. Energy 23 savings, even to a small degree, can prove bene cial as sustainable bio-based 24 insulation materials are locally sourced and available at exceptionally low 25 cost. 26 Straw is an agricultural residue generated during the harvest of a number 27 of staple food crops such as rice, wheat, barley, and oat. Straw in the form 28 of rectangular bales were known to be used in load-bearing walls of houses 29 as early as in 1908 in Nebraska, U.S. [8]. The large thickness of such bales 30 (between 35 and 50 cm) ensured excellent thermal insulation. The insulation 31 characteristics of straw was given scienti c authenticity when McCabe [9] 32 measured the thermal resistance ( R-value) of straw bales for the rst time 33 in 1993. Since then, there has been a growing interest in the various char- 34 acteristics of straw: thermophysical, structural, hygroscopic, sound, and re 35 6 resistance [10][11]. Studies involving straw-based prototype buildings have 36 showed that a fastidious design with proper plastering and sealing can en- 37 sure minimum moisture related damage to the straw present in the building 38 envelope [12]. In India, it was reported that the annual production of crop 39 residue from 2010 to 2014 was around 521 million tonnes (Mt) with rice and 40 wheat accounting for 121 Mt and 114 Mt, respectively [13]. Kumar et al. 41 [14] reports that 81% of rice stubble and 48% of the wheat stubble are burnt 42 causing pollution in neighbouring towns and cities [15]. The problem of agri- 43 culture residue burning is not unique to India as this practice has also been 44 reported in other countries with sizeable agricultural output such as Brazil 45 [16] and China [17]. Converting the waste straw into a construction material 46 can o er an alternative to farmers who are currently left with no choice but 47 to burn the straw in preparation of the land for the next crop cycle. 48 7 Table 1: Summary of literature with experimental results of thermal performance of straw buildings. Work Building type & location Envelope con guration Ambient conditions Key results Sabapathy and Gedupudi [18] (2020)Experiment on straw test room in Chennai, IndiaStraw cells of 10 cm thickness supported by plywood frame10-day monitoring period in warm conditions (maximum outdoor air temperature reaching up to 43C)Indoor air temperature as much as 4C lower than outdoor air temperature Marques et al. [19] (2020)AnalyticalStraw bale of thickness between 200 and 500 mm protected by lime mortar/gypsum plasterboard/oriented straw boardSinusoidal temperature input for 24 hours with uctuation of 10CTime lag (thermal delay) increasing from about 12 hours to 32.5 hours with increasing thickness Gallegos-Ortega et al. [20] (2017)Experiment on straw bale building in Tecate, Baja California, Mexico400 mm straw bale envelope with 25 mm earth plaster on both sides and supported by wooden beams73-day monitoring period with outdoor air and outside surface temperatures in the range of 8C to 30C and 12C to 50C, respectivelyThe indoor air and inside surface temperature uctuations were only about 4C and 2C, respectively Douzane et al. [21] (2016)Experiment on straw bale building (2 oors house) in Voyennes, France440 mm straw bales with 40 mm thick lime plaster and supported by wooden frame400-day monitoring with outdoor air temperature extremes of 40 C in summer and -10C in winterStable indoor temperature ranging between 20C and 25C maintained Lee and Yeom [22] (2015)Experiment on straw bale mock-up room450 mm thick straw bales with 30 mm plaster and timber support structure169-day monitoring period across three seasons with outdoor temperature ranging between -10C C and 35CIndoor air temperature variation 11 C lower than the same in outdoor air temperature during winter. Indoor conditions similar to outdoor during summer Verbeeck and Ponet [23] (2012)Experiment on three rooms of a straw bale house in Limburg, BelgiumStraw bale envelope with overall mean thermal transmittance of 0.26 W/m2K294-day monitoring period covering summer (maximum outdoor temperature of 35C) and winter (minimum outdoor temperature of -2C) seasonsMaximum indoor temperature 5C lower than outdoor in summer and minimum indoor temperature 6C higher than outdoor in winter Wall et al. [24] (2012)Experiment on prototype house (`BaleHaus') located in University of Bath, U.K.Prefabricated panels made of 490 mm thick straw bales with 30-35 mm line render and softwood timber supportYear-long monitoring period. Indoor heating for one day to assess heat loss performanceThermal imaging of building envelope showed no indications of thermal bridging or building defects Garas and Allam [25] (2011)Experiment on prototype straw bale building in New Cairo City, Egypt450 mm straw bales with 50 mm plaster coating12-day monitoring period with di erence in maximum and minimum of wall outside surface temperatures ranging between 20C and 33CDi erence in maximum and minimum of wall inside surface temperatures ranged between 3C and 5C only 8 Table 1: Summary of literature with experimental results of thermal performance of straw buildings. Work Building type & location Envelope con guration Ambient conditions Key results Ashour et al. [26] (2011)Experiment on straw bale house with multiple rooms in Bavaria, Germany450 mm straw in ll within wooden frames and 20 mm plaster coating261-hour monitoring period with outside surface temperature uctuation over 24 hours as much as 16COver 24 hours, inside surface temperature uctuation does not exceed 5C Holzhueter and Itonaga [27] (2010)Experiment on milk processing plant with straw envelope located in Kyotango City, Kyoto, JapanStraw bales with earthen plaster supported by post and beam wooden structure11-month monitoring period with mean monthly outdoor temperature varying between 4C and 26CThe mean monthly indoor temperature varied between 15C and 26C Whitman et al. [28] (2010)Simulation360 mm straw bales with 30 mm earth renderRepresentative summer (outdoor temperature between 10C and 35C) and winter (outdoor temperature between -6C and 13C) daysIndoor temperature for summer between 23C and 28C and for winter between 7C and 14C Elias-Ozkan et al. [29] (2006)Experiment on straw bale house in Yozgat, TurkeyEnvelope made of straw bales40-day monitoring without internal heating followed by 21-day monitoring with intermittent heatingResults presented for 3 days without heating show signi cant smoothening of indoor temperature. With heating, straw bale provide better insulation 9 1.1. Review of literature 49 The following literature review brie y covers the three topics of interest 50 in the current work: (i) thermal performance of straw based constructions, 51 (ii) passive technique of using building insulation materials in the envelope as 52 well as thermal comfort, in the context of Indian climates, and (iii) some of 53 the transient numerical studies carried out on indoor spaces under di erent 54 ventilation conditions. 55 1.1.1. Straw building thermal performance 56 Several researchers have opted for experimental approach involving test 57 rooms or case study buildings to assess the performance of straw insulation. 58 Various attributes of straw buildings namely structural [26], moisture in l- 59 tration [30][31][32], re, and sound resistance [24][19][33] have been explored. 60 Here, the focus is on literature discussing the indoor thermal performance. 61 The studies pertaining to thermal performance of straw envelope are summa- 62 rized in table 1. With the exception of the study by Sabapathy and Gedupudi 63 [18], the test room or buildings considered in these had the following com- 64 mon features: (a) the envelope consisted of straw of large thickness ( >20 65 cm) in the form of bales/panels or as part of a composite wall, (b) the straw 66 wall forms the load-bearing component of the building envelope, and (c) the 67 straw surface is protected by a coating of some type of plaster. The ability 68 of the thick straw bales to inhibit heat transfer and maintain stable indoor 69 environment is evident from the studies. So naturally, to take advantage 70 of the insulation potential, other methods of incorporating straw into the 71 building envelope were explored. Filling hollow concrete blocks [34][35] or 72 clay hollow bricks [36] with straw, mixing straw with clay [37][38][39] or with 73 10 sand-concrete [40] or with earthen bricks [41] and fabricating high density 74 straw boards [42] were some of the innovations found in literature. All of 75 these require additional processing and manufacturing stages. 76 A simple and cost-e ective way of using straw insulation in buildings 77 is through a retro tting technique in which straw in the form of non-load 78 bearing compacted cells of smaller thicknesses (less than 20 or 25 cm) is 79 xed on to the existing load-bearing walls/roof of a building [43]. The indoor 80 thermal performance achievable in this option is the focus of the current work. 81 1.1.2. Building insulation usage and thermal comfort in the Indian context 82 The use of insulation in buildings, though simple to implement, is not 83 widely prevalent in India. There are, however, studies revealing the energy 84 savings potential of using insulation in buildings in India. Natural materials 85 such as bagasse, leaves, etc. [44], wooden boards, [45] and commercial insu- 86 lation materials such as Extruded Polystyrene (EPS) [46] have been studied 87 for Indian climates. In a review, Gupta et al. [47] noted the suitability of 88 straw as a construction material for buildings in India. The potential for 89 energy and cost savings on implementing straw insulation retro tting for 90 conditioned spaces in Indian climatic zones was evaluated in a recent work 91 [48] by the current authors. The decision to use any material including straw 92 in the building envelope is intrinsically linked to its thermal performance and 93 the highly subjective and localized thermal comfort p