Replacing Your Roof? Get Reflective

Replacing Your Roof? Get Reflective

  • cool roof graphic
By Paul Eldrenkamp

The headboard of my bed is on the second floor, tucked into an alcove right under our northwest-facing roof. After a sunny summer day I can feel the heat radiating through the drywall over my head when I go to bed. Many nights the only remedy is to crank up the air conditioning (Hypocrite auteur!, to paraphrase Baudelaire).

Roofs are terrific collectors of solar energy. That's why we put solar hot water systems or photovoltaic panels up on roofs — they catch a lot of sunlight. The earth absorbs roughly 100 watts of solar energy per square foot on a sunny day. Assuming 8 or so hours of nearly full-strength sun on my northwest-facing, un-tree-shaded, moderately pitched roof, that's 800 watt-hours per square foot, or over 300,000 watt-hours total.

Not all of that energy is absorbed into the roof, of course. My shingles are red, which reflects more solar energy back to the sky than black shingles would, for instance, but less energy back than white would (hence Energy Secretary Steven Chu's only partly tongue-in-cheek suggestion to paint all our roofs white). In terms of heat absorption, though, red shingles are much closer to black shingles than to white ones. Maybe 80% of the solar energy that hits my roof is absorbed by the roof, or about 250,000 watt-hours. This is roughly the equivalent of having my two daughters, plus 160 or so of their closest friends, on the roof all afternoon blowing hair dryers (on the highest setting) at our shingles. No wonder our bedroom ceiling heats up.

Asphalt shingles are one of the most heat-absorptive roofing materials available. The little granules on the surface of the shingle mean there's a lot of surface area to catch solar energy. Metal roofs can be far more reflective, for instance.

But asphalt roofing manufacturers are increasingly producing shingles that are more reflective ("high albedo" is another way to put this) without looking as ugly as white asphalt sometimes does. If you're considering a new roof, it's definitely worth looking into one of these alternatives to standard asphalt. In fact, you can get more information at Energy Star and The Cool Colors Projects.

Some of you may be thinking at this point: In the Boston area, our heating energy in the winter is much greater than our cooling energy in the summer (by a factor of about 5 or 6, in fact). Wouldn't it work against me, overall, to have a reflective roof?

In fact, because of the lower angle of the sun and the generally greater cloud cover, solar gain in the winter is much less than solar gain in the summer. There's less solar gain through the roof when you most want it, and much more when you don't.

In other words, it doesn't help you as much in the winter as it hurts you in the summer, in our climate and latitude.

One more factor is something called "peak load." How much total energy a home consumes over the course of a year is one thing; the most power it needs at any one point in the year is quite another. The average power draw of a home may be 3 or 4 kilowatts over the course of a year. But at peak load times — when the air conditioner and refrigerator and freezer and a few other appliances are running full blast — it might be 12 or 15 kilowatts, and pretty much all of that is from electricity. The power companies (NSTAR or National Grid mostly, in our area) need to be able to provide enough electricity to meet the peak demand when that demand arises. This means the overall generating capacity of their supply chain needs to be several times higher than the average capacity needed, which can get expensive. It can also get carbon-intensive: Generally (and simplistically) speaking, they'll use their cleanest sources of energy (gas, hydro, and, yes, nuclear) to meet basic loads, and go to the dirtier sources (coal and fuel oil and older-technology plants in general) only as needed, to meet peak loads.

Installing reflective roofs thus can increase your comfort, save you some money, and help our regional carbon footprint. What's not to like?