EcoHouse Research

Abigail Woolf (MEng, 2013)

 

Insulation is vital in reducing heat loss from inside a building. It is important to be able to maintain a comfortable internal temperature throughout the year, even when external temperatures are low. The primary function of insulation is to minimise heat transfer through radiation and convection, whilst introducing minimum thermal conduction, through a large number of gas-filled pockets.[6] Thermal conduction is the crucial heat transfer mechanism to reduce as it is the predominant mechanism through which heat is lost through the walls and ceiling of a structure.

 

 

Thermal properties of common materials have been extensively researched and values are widely available.[7,8] Additionally, thermal properties of commercially available insulation are easily obtainable from manufacturers.[9,10,11] Research into alternative insulation materials is increasing; recent papers have investigated the thermal properties of a range of agricultural by-products, such as sugar cane bi-product, cotton stalk fibres, and flax and hemp fibres.[12,13,14] Other organic matter has also been investigated as an alternative insulation solution including sheep wool and textile waste.[15,16]

 

 

Thermal properties of unconventional and inhomogeneous insulation materials are less well documented. Due to their unconventional nature, these materials are not commercially used as insulation and a thorough literature review has shown no previous research had been done into the thermal properties of unconventional and inhomogeneous materials.

 

 

This project focuses solely on the calculation of thermal conductivity, an intensive property, and therefore one which does not change with size, which is a more applicable comparison when testing materials of different thickness outside of the laboratory.

 

The main objectives of this project are:

- Design a test-rig that uses low-cost equipment and materials, all of which must be available in Brazil, and requires no specialist knowledge to construct.

- Develop a simple experimental method that produces results to an appropriate degree of accuracy.

- Build two prototype test-rigs; one in the UK and then one in São Paulo to evaluate ease of manufacture; accessibility of equipment; experimental method and ultimately the ability to differentiate between potential insulation materials.

 

Two methods were tested; the most accurate was chosen for further development. The greater requirement for user input was compensated for by the reduced testing time required, and the structured testing method. It was important to see which aspects of the design and method has the greatest impact on results; in order to identify any simplifications which could be made before testing in São Paulo. Investigations into sealing the box and using a fan had little impact on the accuracy; therefore these design adjustments were not incorporated. The change of temperature gradient effected the maximum and minimum temperatures, which was then included in calculations.

Preliminary testing in both the UK and São Paulo produced results accurate to one significant figure, which is adequate for the environment in which it will be used. Given the environment in which the test-rig will be used, it only needed to be sufficiently accurate such that the insulating properties of potential building materials could be reliably understood. The method of time ratios chosen allowed results to be output graphically, and a line of best fit used to find the thermal conductivity. The regression coefficient was used to assess reliability.

 

In order to assess whether the test-rig met the design objectives, a second prototype was constructed in São Paulo. The test-rig satisfied the design objectives, which was confirmed by the construction of the test-rig in São Paulo. The materials chosen were low-cost and accessible, allowing the test-rig to be made with ease. A change in thermometer meant that the method had to be modified, but this did not impact results.

 

Thermal conductivity of loose material varies significantly with density of packing; a comment which was made by the user in São Paulo. Possible extensions of research include assessing a means of determining when benefits of tightly packing insulations materials begin to cause structural building issues associated with excess structure-mass. Likewise, the materials can be tested in different formats; for example newspaper could be shredded, scrunched, or mixed with another material. Different combination of materials could be tested, to see which amalgamation has the best thermal conductivity. This is especially relevant in favelas, where test material is likely to be from many different sources. Using waste products to manufacture insulation panels would be a useful extension to this project.

 

This project has reached a positive first-step in the development of a test-rig which allows the calculation of the thermal properties of inhomogeneous and unconventional insulation materials, commonly found in the developing world, to a suitable level of accuracy. It has been demonstrated that the design and method is truly practical “on the ground” in the developing world. There are a number of interesting pathways for further study and the additional possibility that this project can become part of a broader tool to increase awareness of building materials, and the impact they have on quality of living.