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How Insulation Works

5/27/2015 | by Marcus V. A. Bianchi

Thermal insulation has been used in buildings all over the world for many decades, and is a product that builders, contractors and the general public are fairly familiar with. Yet every so often, even very seasoned professionals face a difficult time explaining how it works. And frankly, no one should be surprised; the mechanisms involved are not that simple.

For over twenty years now I have been interested in thermal insulation and how to improve it. I was first exposed to the concepts of how radiation interacts with materials that absorb, scatter (spread) and emit radiation when I took a graduate course on radiative heat transfer at Purdue University. Later, in 2008, I presented some of these concepts in the Westford Building Science Symposium and more recently at the NESEA 2014 Building Energy Conference.

One surprising fact is related to the very significant role of thermal radiation. In an uninsulated cavity, thermal radiation is dominant. Yes, if one holds the two surfaces that compose the cavity at different temperatures, the main mode of heat transfer that takes place between the two surfaces is infrared radiation. This is due to the fact that typically the two surfaces have emissivities that are very high and that the natural convection that occurs, characterized by an air recirculation that takes over the entire cavity, is not as strong as people think.

When we insulate the cavity, we do it to obtain two major effects:

1.To reduce the radiation exchange between the surfaces

2.To eliminate the natural convection

Fibrous materials (and foams) are used for these purposes. Fibrous materials added to the air in the cavity create a porous medium that both increases the absorption and the scattering of the radiation within the cavity, and decrease its air permeability to the point that essentially the air is stagnant in the cavity. With the air stagnant, an insulated cavity has two main mechanisms of heat transfer left: conduction through air and fibers and thermal radiation between fiber surfaces, with conduction through air being dominant. The upper bound of the thermal resistance is determined by the thermal conductivity of air in this case.

For more details please read So You Think You Know How Insulation Works that first appeared in BuildingEnergy Magazine, vol. 32, number 1, page 14 (spring 2014). It is reprinted (or excerpts are re-used) with the permission of the Northeast Sustainable Energy Association.

Related Tags: Performance | Thermal