Beware of Using R-Values Alone to Compare Insulation
Last year we wrote a whole series on installing spray foam insulation in our basement. As part of the research for that project, we compared a variety of basement insulation options, including closed cell spray foam, rigid foam board insulation, cellulose, and the staple of most homes — fiberglass batts.
Insulation options are generally compared by “R-value”, which is a measure of heat transfer. R-value is always proportional to the thickness of the insulation, usually as described in inches. For example, most closed cell spray foams provide between R6-R7 per inch. Traditional fiberglass offers R3.5-R4 per inch (and hence, why the traditional 3.5 inch batts installed in walls are R13).
The problem with R-value is that it measures heat transfer in a perfect installation. This perfect scenario would provide no way for air (or heat) to move around the insulation to penetrate the conditioned space.
Obviously, the problem with measuring heat transfer in a perfect installation is that it ignores the reality that heat can move around the insulation in an imperfect installation. This can create a huge difference in performance, especially for a material like fiberglass which is extremely porous and therefore difficult to completely air seal. If a fairly heavy draft is coming through an exterior wall, it is likely it will find a way to get around fiberglass.
Closed cell foam, on the other hand, completely shuts off air transfer, which eliminates drafts moving through the insulation. For heat to move around in closed cell foam, it literally has to move from one sealed “cell” to the next – an extremely inefficient process. And when it comes to insulation, you want heat transfer to be as inefficient as possible.
Our experience with closed cell foam is that it insulates so well, you would have to install 1.5x – 2x the amount of fiberglass to achieve the same “real” insulating properties as the foam. In other words, R13 of closed cell foam performs similarly to R20-26 of fiberglass. This isn’t going to be universally true, especially if the fiberglass is installed properly.
However, it’s not completely fallacious either. Take the example of insulating a rim joist. In this situation, you have many difficult angles that must be completely sealed from air penetration. Air flow is a particularly acute problem in the rim joist, especially for a first floor, because the joist is often sitting on top of a sill plate which is on top of (imperfect) masonry work. The air penetration at this point can be a substantial factor in heat loss.
The same situation applies in an attic when comparing rolled-out fiberglass batts to additional blow-in insulation. Blow-in insulation is better at covering the floor, which eliminates spots that foster convection. If you want an even better performance improvement, closed cell foam in an attic prevents heat radiating from the ceiling of the house from creating air flows in the insulation, a problem that exists with fiberglass or cellulose installations.
As you can see, R-values can be very deceiving, so it is worth investigating further and asking your insulation installer a lot of questions about how radiant and convective heat transfer will be minimized in the installation.
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