There's more to insulation than R-value

New Life Journal, Dec, 2004 by Isaac Savage

How do we measure how well insulation works? Most immediately think of the R-value, or the material's resistance to heat flow via conduction.

Unfortunately, we as a building industry (and homeowners) have been slightly misled when it comes to judging the performance of insulation. For instance, when the manufacturer of a fiberglass batt determines the R-value of their product, a laboratory test is run in an extremely controlled setting. More importantly, the fiberglass batt is in a container that is 100% encapsulated on all six sides. There is absolutely zero air flow in this laboratory wall-cavity. These conditions are never seen in an actual house though, as there is never a vacuum inside the wall, and often times there are not six complete sides to the cavity (as in the subfloor or attic).

Building scientists across the nation are beginning to use a different vocabulary when talking about the performance of insulation. More and more emphasis is being given to the insulation's resistance to airflow.

Think about how heat is transferred through the cavity of a wall in the wintertime. Since heat moves from hot to cold, the general direction of heat flow will be from inside to outside. Let's say that this wall is composed of 5/8 inch sheetrock, 2 x 4 studs, plywood sheathing, house wrap, and siding. The 3 1/2 inch space between the sheetrock and the plywood is typically filled with an insulation that allows air to circulate through it, like fiberglass batts.

As heat transfers through the sheetrock, via conduction, the air on the inside surface of this sheetrock begins to absorb the heat from the sheetrock, causing it to rise inside the fiberglass-filled cavity. As this warm air rises, the air on the opposite side (the cold side) of the cavity begins to fall. Since heat moves from hot to cold, the upward moving warm air wants to transfer this heat over to the outside of the wall assembly. As this flow of air transfers the heat to the outside of the home, it begins to create a convective loop inside the wall. Imagine a water-wheel inside the wall that is constantly dumping heat in a circular motion to the outside. This is the result of airflow within the insulation. This type of airflow will be there even if you build the tightest house on the planet ... This also means that the airflow will be magnified where insulation is installed on a subfloor, kneewall, or ceiling, because one side of the insulation is exposed to a large volume of unconditioned air. Insulation performance in these locations suffers greatly.

So, what does this mean? It means that resistance to air flow is just as important (and possibly more) as R-value when determining the effective performance of insulation. This also means that when you're shopping for insulation, there's more to consider than which type gives you the greatest R-value for the dollar. Unfortunately, there is not an industry standard that measures the effective performance of insulation. So, you'll have to do some homework.

In terms of resistance to airflow, fiberglass is the least resistant, wet-blown cellulose is next, then dense-pack cellulose, and spray foams are the best. Fiberglass and cellulose are both fairly straightforward, but spray foams deserve further explanation.

There are two types of spray applied foam insulation: open-cell & closed-cell, otherwise know as "half-pound" and "two pound" (referring to the density). The main difference between the open and closed cell foams is their vapor permeability-how well they "breathe" vapor. Both types are resistant to air flow, so we are not talking about that type of "breathing." We are talking about water vapor.

Open-cell foam allows vapor to move through at a much higher rate than a closed-cell foam. This means that if (or more realistically, when) the wall gets wet, it will be able to dry to both the inside and/or the outside. This is seen by many builders, architects, and building scientists alike as a positive attribute in this climate. The reasoning is the same when talking about plastic vapor barriers in our region. Vapor barriers are typically not recommended due to the fact that the direction of vapor drive shifts quite frequently in our climate. By creating a wall that breathes vapor, it can absorb vapor and pass it through to the dryer side.

A vapor barrier within a wall creates a much less forgiving wall system, because it can only dry to one side, typically the outside. This also makes the likelihood of condensation within a wall assembly more likely. What if the outside is also the side with the higher level of moisture? Then you end up with a wet wall until the outside conditions become dry enough to allow the wall to dry. Building Science recommends that no vapor barrier be used in our climate for this reason.

As you can see, there is more to consider about the performance of insulation than the R-value alone. Different insulation types may be needed depending on the wall assembly location and materials involved. As long as you consider the full realm of insulation properties when making this decision, your wall should perform as expected. When these properties are ignored, the results could be a moisture problem, a headache, or an indoor air quality issue. As a homeowner, it is your responsibility to fully understand how each component in the house affects the others, including your choice of insulation.