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concepts · updated 2026-08-01

Unglazed solar collectors

confidence: medium volatility: cold verified: 2026-08-01fresh

An article by Steve Baer in the NMSEA SunPaper (March/April 2003) arguing that removing the glass from a solar collector is not a compromise — it is often an improvement. Baer reports months of personal shower-water testing with unglazed Skymat and Energie Solaire absorbers through icy January weather in Albuquerque.

An article by Steve Baer in the NMSEA SunPaper (March/April 2003) arguing that removing the glass from a solar collector is not a compromise — it is often an improvement. Baer reports months of personal testing: unglazed absorbers heated his shower water through icy January weather in Albuquerque.

The shower test

Baer tests collectors by heating water for his shower — a 30-gallon plastic drum above the shower, gravity-fed, with thermosiphon lines dropping to a rooftop collector. Two configurations:

  1. Skymat™ blow-molded polypropylene (2 × 10 ft² panels, unglazed): warm showers through “icy clear January weather (high 46°F)” — “not hot, as I didn’t need to mix with cold.”
  2. Energie Solaire selective-surface stainless steel (single 10 ft², unglazed): even better. Baer had already seen this absorber “stagnate above boiling with no glazing on a cold, partly cloudy, breezy November day.” The selective surface absorbs visible light but doesn’t radiate infrared — the opposite of white paint.

Why unglazed can beat glazed

The counterintuitive claim: “unglazed collectors are more efficient than glazed, at temperatures close to ambient.” Glazing blocks airflow and sky radiation, but it also reduces the sun reaching the absorber. Energie Solaire’s own performance charts show that in New Mexico’s 1,000 W/m² sun, the unglazed selective-surface collector outperforms the glazed version all the way to 36°F above ambient.

Practical benefits of removing the glass:

  • No fire risk — the absorber can’t overheat enough to ignite
  • Can lay directly on composition shingles — the roof serves as back insulation
  • No collector box needed — no sides, no insulation, no back-plate
  • Much cheaper than a boxed glazed collector
  • If the angle isn’t ideal, just use a bigger absorber

Baer concedes the honest counter-case: a skeptic with no back-up water heater “is right to ignore my arguments and glaze his collector,” because glazing wins during very cold, windy, hazy weather — exactly when hot water is most needed. But weighing expense, overheat protection, and the shorter life of tanks full of very hot water, he judges the unglazed heater the better choice for Albuquerque, especially with a back-up heater (he has none). He also recalls Zomeworks’ spiral polyethylene-pipe collectors of thirty years earlier, which survived freezing but bloated “like a snake that’s eaten a rabbit” when pump failure let the glazed boxes overheat — and wonders whether removing the glazing would have saved them.

The IR thermometer as education

Baer recommends a $30 Radio Shack infrared thermometer as an essential tool. A walk through a parking lot becomes “a lesson in the effect of colors and orientations on stagnation temperatures.” Black cars and cats reach 90°F above ambient. Green gets almost as hot. White surfaces facing the cold sky drop below air temperature even in bright sun. At night, car roofs drop 20°F below ambient.

Summer nights with an IR thermometer are just as exciting for the energy prospector as winter days. Just as you were delighted to find free heat in the sun, you will find free cool in surfaces pointed at the night sky.

This connects directly to the Sunmen manifesto’s emphasis on IR measurement as the beginning of a “Sunman’s education.”

Historical context

Baer notes that solar pioneers Harold Hay, Ray Bliss, and Harry Thomason built heating and cooling systems exploiting these principles decades earlier. “Somehow their work was ignored. Giant budgets and brilliant minds moved in other directions.” The article is a call to return to low-budget, cost-effective projects — armed now with cheap IR thermometers that let anyone verify the physics.

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