Question: How can you make insulation more energy efficient without increasing thickness?
Answer: OK, here we go. Heat is transferred in three ways: conduction, convection, and radiation. Fiberglass insulation takes an air cavity and reduces the convection transfer of the energy by the air inside that cavity from the hot to the cold side. When we add fiberglass to the cavity we break up the one large air space in to many small airspaces in between the fibers. This makes the convective cells smaller and makes both the convection and radiation paths more difficult.
So how do we increase the resistance to heat flow in the same thickness? One way is to increase the number of fibers to make the convection cells even smaller. This is what happens with the insulation for a 3-1/2 inch wall going from R-13 to R-15. The amount of glass in the wall is increased in the same volume, which increases the density and hence the R-value.
Another way to do this is to remove the air from the cells. This cuts down on the convection that can take place. We see this in vacuum thermos bottles where they draw a vacuum in the thermos bottle walls to keep soup hot or milk cold.
Another way is to add a gas that has a lower density, which reduces the convection in the cells. This is not practical in fiberglass but can be done in foam with a lower density blowing agent.
Sorry about the long answer but it was necessary to answer the question. You must know I don't like to type this much with my furry paws!
