How Radiant Insulation Works

On October 4th, 2011, posted in: Technical - Reflective by Comments Off

Let’s start off by defining that radiant insulation is an insulation system where the reflective material faces an open air space. The idea is that a reflective barrier facing an enclosed air space becomes “reflective insulation” with a measurable R-value.

The Concept of Radiant insulation

Standard types of insulation, such as fiberglass, foam, and cellulose primarily reduce heat transfer by trapping air or some type of a gas. Thus, these products or technologies reduce convection as a primary method of reducing heat transfer. They are not as effective in reducing radiant heat transfer, which is often a primary mode of heat transfer in a building envelope, especially in warm Australian climates. Rather, these products, like most building materials, have very high radiant transfer rates. In other words, the surfaces of standard types of insulation are good radiators of heat.

Reflective insulation uses layers of aluminum and plastic (as in the case of RadiantShield) to trap air and thus reduce convective heat transfer.

However, the effect of this is negligible compared to the benefit of a materials reflective foil surfaces, as they are very effective in reducing radiant heat transfer. In fact, the foil used in our reflective insulation materials will reduce heat transfer by as much as 97%

The reflective surface of foil insulation reduces the radiative heat transfer, so the cavity adjacent to the reflective surface becomes radiant insulation. For example, a reflective foil product may be only a 0.5mm thick, and have a material R-value of just R0.05, but its presence will transform the adjacent air space into a radiant insulation system. The point is that the reflective material needs to be installed in combination with air spaces to create these radiant insulation systems.

Typically we find that the total R-value of a building system is improved drastically with the addition of a correctly installed reflective material. In the above example, a material might have an R-value of just R0.05, but if we include the reflective air gap adjacent to it, the total R-value might be around R2.0. Keep in mind that the air gap without the reflective surface might be around R0.15 – so a very significant improvement.

Therefore, reflective insulation is superior to other types of insulating materials in reducing radiant heat. The term reflective, in reflective insulation, is in some ways a misnomer, because aluminum either works by reflecting heat (reflectance of 0.97) or by not radiating heat (emittance of 0.03). Whether stated as reflectivity or emissivity, the performance (heat transfer) is the same.

When reflective insulation is installed in building cavities, it traps air (like other insulation materials) and therefore reduces heat flow by convection, thus addressing all three modes of heat transfer (the benefit of this effect is accounted for in reflective insulation Total R-values).

Remember that in all cases, the reflective material must be adjacent to an air space. Aluminum, when sandwiched between two pieces of plywood for example, will conduct heat at a high rate.