Solar heat gain of insulation (not windows) – GreenBuildingAdvisor

It’s great that the industry publishes the R value of different insulating materials (fiberglass, Rockwool, wood fiber, different foams, hemp, sheep wool, cellulose, and on and on), but I’m realizing that R value is a pretty limited data point that doesn’t tell enough of the story, especially when it comes to sunny days versus cloudy days. 

During the last couple of weeks, we’ve had sunny days at about 80 degrees F, and cloudy days with the same air temperature. The difference in how the sun versus clouds affected my house was enormous. I have a decent amount of insulation in the roof (R60-ish; I have to say “ish” because it varies; I’m in the middle of an energy retrofit, and some parts are easier to insulate than others), and with the same amount of insulation across all of these days, the cloudy 80 degree days were comfortable, with the air conditioning running very occasionally, but not much. On the sunny days, the air conditioning ran all day, non-stop. 

We could say that I should add insulation to the walls, which currently have none, and that is definitely part of my plan for the future, but honestly, the uninsulated walls performed well enough on the cloudy days. The only difference that really mattered was the sun.

In building science there is a whole lot of talk of air tightness, R value, and there are even discussions of the effect of wind outside or air flow within the insulation material… and there is plenty of talk of solar heat gain for windows, but there doesn’t seem to be much talk of solar heat gain for insulation, aside from windows. I’d love to see a solar heat gain coefficient (SHGC) data point — I suppose it would have to be SHGC per inch) for Rockwool, wood fiber and all the rest.

A sort of related idea is the concept of thermal mass, where it helps to have high thermal mass outside of the enclosure on a hot day, to absorb the heat during the day, then release it at night, when it is cooler, though if the difference in temperature between day and night is small, all the heat stored in that thermal mass will eventually be passed to the inside of the enclosure.

And I know there are radiant heat barrier products, which do get discussed sometimes, though often dismissively, because they have essentially no R value, which is true, but if I had a huge shade umbrella (or tree or radiant barrier product) over my house on the hot sunny days, my AC would not have run hardly at all, assuming that the shade is as effective as the clouds. I suppose that means that the shade would have to encompass my whole house, to prevent the sun from hitting the walls and windows, and to prevent reflected radiant heat from bouncing off the ground or neighboring houses. 

So it gets complicated, I know, but still: I want a SHGC measurement for insulation, because I can now confidently say that on sunny days, solar heat gain (absorbed radiant heat, or perhaps (?) more accurately the “emissivity” of the insulation) matters a whole lot more than R value. The sun overpowers the insulation far more easily than high outdoor temperatures do.