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What material makes the best insulation is really down to what is required of it. If you’re at one of the poles in a research station and it is 50 degrees below zero, the relative humidity outside is nearly zero and you don’t see the sun for months at a time then something synthetic is going to be your best bet.
However, we in the UK live in a temperate and wet climate. The sun might shine strongly one minute and then rain pour the next, the relative humidity varies between 50 and 100% and the average temperature varies between zero and 25 degrees Celsius. In this situation insulation needs to do more than just slow the passage of heat. It needs to be able to deal with moisture and for occupant comfort, it would help if it could absorb heat too.
Insulation is often regarded as a very one dimensional product. Lower conductivity = better performance = better product to use. However, because of the amount of moisture inside and outside our buildings and the ways in which this moves through the building fabric, insulation needs to be able to cope with moisture. It should be able to absorb and release it without changing it’s thermal performance. In doing so it prevents accumulations of moisture in sensitive components such as timber studs or rafters.
It is also beneficial for insulation to not only slow the flow of heat but actually absorb it too. Our buildings are becoming ever more thermally efficient and are more at risk of over heating, whether from occupant behaviour or from solar gains. This means that to stabilise the internal environment heat needs to be stored in the materials in the walls, floors and ceilings of our houses and allowed to slowly release.
Whilst we are technically advancing all the time we still have a very long way to go to be able to produce materials as complex as plant fibres. These materials not only form excellent insulation materials but they also have a much higher specific heat capacity and so store many times more heat than synthetic materials. In addition to this there is research to show that these fibres use water as a ‘phase change’ material and are able to store and release heat energy by allowing water to change from liquid to vapour and back again in their pores.
Wood fibre is one of the best insulation materials as it combines high heat capacity and high density and is able to transport and store moisture very effectively. It is also a very effective insulant but able to store large amounts of heat energy, ensuring stable internal environments within buildings. Hemp, flax, straw and cellulose all do this too but normally to a lesser extent as they tend to be made in to lower density products.
For another perspective on the importance of thermal mass in buildings read this article.
This post is based on my experience of living in a certified Passivhaus building using, of course, Unger wood fibre for insulation and thermal mass and EBB clay boards for thermal mass and acoustic insulation. What is your experience? Do you have a house insulated in a different way? How does it behave?