Metal Roofs and Energy Efficiency: The Midwest Number

A reflective metal roof does lower your cooling bill. The brochure number is 15 to 30 percent off cooling costs, and in the right climate that is real. I want to give you the version of that claim that holds up in Fort Wayne in February, because an energy saving you were promised and did not get is worse than one you were never promised.

How it works

Two properties matter. Reflectance is how much of the sun's energy bounces off the surface instead of being absorbed. Emittance is how quickly the surface gives back the heat it did absorb. Asphalt shingles score poorly on both: they soak up heat all afternoon and hold it into the evening. A painted metal panel with reflective pigments scores well on both, and the Department of Energy's cool roof guidance describes the effect: a cooler roof surface, a cooler attic, less heat driving down through the ceiling, and an air conditioner that runs less.

Attic temperature is where you feel it. On a summer afternoon the air under an asphalt roof can run far above the outdoor temperature. Under a reflective metal roof it runs noticeably cooler. That is not a claim, it is a thermometer.

Why the number is smaller here

Here's the tradeoff. The 15 to 30 percent figure is a cooling-season number, and it is largest where the cooling season is long. In Indiana, Ohio, and Michigan we heat for more months than we cool. A reflective roof saves you real money in July and August and does approximately nothing in January, when a little absorbed heat would actually help. Over a full year, the percentage of your total energy bill is smaller than the cooling-season percentage, and anyone quoting the big number for a house in Auburn or Carmel is quoting another climate.

That does not make it zero. Summer peaks are when electricity is expensive and the AC works hardest, so the saving lands where it is most felt. It is a real benefit. It is a modest one.

What actually moves the bill

If energy efficiency is high on your list, the roof surface is third on mine.

• Attic insulation decides how much heat, in either direction, gets through the ceiling. This is the biggest lever in the house and it works under any roof.
• Ventilation moves hot air out in summer and moist air out in winter. A badly vented attic under a reflective roof is still a hot attic.
• Roof reflectance is the third. It matters, and it matters most on a house with a lot of roof area and a lot of sun.

A metal roof does one more thing for ventilation: it is the natural time to get the soffit intake and ridge exhaust right, because the roof is open. Most of the energy benefit homeowners attribute to their new metal roof is that, plus the reflectance.

Color and pigments

Lighter colors reflect more. That is true. Modern PVDF finishes also carry infrared-reflective pigments across most of the color range, so a dark bronze today reflects more solar energy than the same color did 20 years ago. The gap between a light roof and a dark roof is smaller than it was. Pick the color for the house. The colors article covers what holds up.

Insurance and value, briefly

Energy is one of several lines that do not appear on an asphalt quote. Others are the insurance discount many carriers offer for an impact-rated roof and the effect on resale. None of those is a reason to buy a metal roof by itself. Together with the lifespan they are why the 30-year math works out the way it does. The cost article puts them in context.

Who this matters for

If you are in a long-term home with a large roof, a hot attic, and summers you feel in the electric bill, the efficiency case is real and it adds to the rest. If you are hoping the roof pays for itself on energy alone, it will not, not here, and I would rather you know that. Insulate and ventilate first. Then let the roof do the rest.

Now you have the honest number.