Experimental investigation of the hygrothermal performance of single and multi-layer wall systems in the context of Iran’s construction techniques

Student thesis: Doctoral ThesisPhD

Abstract

The general awareness about anthropogenic climate change and destruction of the planet earth ecosystem has resulted in worldwide efforts to reduce GHG emissions (the main cause of global warming phenomenon) and a more sustainable approach towards the use of natural resources. Building sector as one of the largest consumers of energy and natural resources has great potential in reducing \co2 emissions and contributing to a more sustainable environment. The main focus of almost all building regulations aiming to reduce energy consumption, is on the reduction of the building fabric heat loss/gain by mandating the use of thermal insulation materials and/or thermal mass effect. This highlights the importance of building fabric in controlling indoor environment and hence energy consumption; therefore implying the importance of appropriate choice of materials for building fabric design as well as accurate investigation of its performance. The former cannot be done without considerations of embodied carbon of construction materials in an era when strict sustainability criteria are required by credential systems. And the latter requires a holistic approach devising a dynamic hygrothermal (as opposed to steady-state and only-thermal) study considering simultaneous heat and moisture transfer and interactions of adjacent layers in a multi-layer system. In a mutual order, the motive to abide by sustainability criteria in building design has resulted in an increased interest in use of natural materials increasing the need to adapt a dynamic approach to study the varying behaviour of natural materials.
Iran is amongst the 10 major contributors to CO2 emissions. Despite requirements of the national building regulation, Code 19, most buildings lack a robust fabric design in modern structures. They are mainly built of slender single layer blockwork made of hollow fired clay, LECA or AAC, plastered internally with gypsum plaster and externally with sand-cement render. In rare cases where use of thermal insulation is not ignored, PIR boards are used to minimise heat flow through the envelope. Poor wall design alongside the harsh nature of the climate prevalent in most parts of the country makes it difficult to provide thermal comfort without excessive use of air-conditioning systems. To improve wall construction design, an investigation was firstly done on the hygrothermal performance of construction blocks in single-layer constructions. This was done in an experimental set-up built in the laboratory, combining the hot-box and in-situ methods of measuring thermal transmittance. In the next stage, application of non-conventional materials having lower embodied carbon was proposed replacing PIR insulation with expanded cork board insulation, gypsum plaster with earth, and cement with lime render. The hygrothermal performance of these alternative multi-layer assemblies was investigated under dynamic periodic boundary conditions representing hot dry and hot humid weather condition using the experimental set-up. The result was compared against the conventional multi-layer wall system.
The result indicated different performance of single-layer blockworks in steady-state and dynamic condition from theoretical assumptions with lower thermal transmittance values being reported under dynamic conditions than in steady-state. PIR insulated specimens under hot dry condition resulted in the least discrepancy from the steady-state assumptions. While hygroscopic wall systems and the presence of moisture resulted in more discrepancy. LECA-Cork wall system resulted in significantly lower thermal transmittance values than calculated, under both hot dry and hot humid conditions making it the best candidate for wall design next to AAC-PIR. Plotting the temperature and humidity profiles of the layers of the wall systems, showed the difference in humidity profiles of different wall systems despite having similar temperature profiles indicating the importance of considering the moisture element in building fabric’s thermal performance evaluations.
Earth is known for its high moisture absorption capacity. Earth plaster in Iran was traditionally mixed with plant-based aggregates such as straw to improve its mechanical strength. The role of these plant-based aggregates in improving moisture buffering capacity of earth plasters has not been thoroughly studied. In this study, earth was separately mixed with three plant-based aggregates (wheat straw, wood shaving, rice husk) and a mineral aggregate (diatomaceous earth) known for its high moisture absorbent properties. An investigation was carried out using 4 different moisture buffering protocols on the improvement of moisture buffering capacity of earth plaster after the addition of the aggregates. The result indicated that wood shaving and diatomaceous earth result in the highest improvement in moisture buffering capacity under a dynamic test protocol known as NordTest Moisture Buffer Value which is widely accepted as the most realistic protocol representing a real-life scenario. On the other hand, unlike other mixes, addition of wheat straw reduced the MBV which can be justified by simultaneous reduction of water vapour permeability of earth-straw mix as the lowest permeability values were recorded for straw mixes.
Date of Award18 Nov 2020
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorStephen Lo (Supervisor) & Sukumar Natarajan (Supervisor)

Keywords

  • Building envelope
  • Hygrothermal performance
  • Natural materials
  • Embodied carbon
  • Indoor air quality
  • Earth plaster
  • Moisture buffering capacity
  • Diatomaceous earth

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