Staying Cool with a Metal Roof
Metal roofs are durable and recyclable — but do they really save energy?
David Martin is intrigued with the idea of replacing his existing roof with a standing-seam metal roof. It should last longer than the alternatives, he says, and it would be compatible with photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. panels should he decide to add them in the future.
So what’s the issue?
David Martin is troubled by some of the advertising claims he’s seen about metal roofing, specifically a statement from the Metal Roofing Alliance that a “cool metal roof can save 25% in energy costs compared to a dark grey asphalt shingle.”
“I've searched around the Internet and found a lot of the industry's advertising,” David Martin writes in a post at GBA's Q&A forum, “but I'm a bit skeptical, especially after reading what [GBA senior editor Martin] Holladay says about the dubious claims made by manufacturers of bubble foils, radiant barriers, and open-cell spray foam.”
It seems to David Martin that the biggest variable in energy efficiency is whether they roof is vented. “I figure there would be a negligible benefit, if any, on a vented roof,” he says. “On an unvented roof, would the benefit be comparable to a radiant barrier on the underside of the rafters?”
His questions about the energy benefits of a metal roof are the subject of this month’s Q&A Spotlight.
Rating the cool factor
David Meiland also likes metal roofing — for its durability, recyclability, and ease of removal if the need arises. But he suspects that light-colored asphalt roofing would perform similarly, with or without ventilation. “Comparing a ‘cool metal roof’ and ‘dark gray asphalt shingle’ is apples to oranges,” he writes.
Kevin O’Meara says the difference in energy performance is due to more than just the color of the roofing. “The paint that the metal companies apply to the metal has added chemicals that are designed to reflect infrared wavelengths,” he writes. “Most metal companies will list these ratings under each color. There exists a ‘cool roof’ tax credit and a website for approved metal roofs.”
Indeed there is, as Meiland discovers. A products directory maintained by the Cool Roof Rating Council lists solar reflectivity and thermal emissivityAmount of heat radiation emitted from a particular body or material. Emissivity is expressed in a fraction or ratio, with the lowest values indicating low emissivity and the highest indicating the high emissivity of flat black surfaces. values for hundreds of brands of roofing.
Meiland finds that many shingle/shake products are not as reflective as the most reflective metals, although there’s a wide variation in metal colors as well.
“You need to look at the published specs of the roofing material and that will tell you how ‘cool’ it is,” adds Bob Coleman. “In some areas it may not be the best if you want some of the sun’s heat...Also, venting the roof underneath changes things. There is no magic bullet. It’s all part of a system that should be geared toward your solutions.”
And as to the claim of a 25% energy savings?
“Ridiculous,” says Holladay. “That would only be true in a very poorly designed house (one that doesn’t meet code and is therefore illegal). Assuming that the home is equipped with a properly installed layer of deep insulation, roof color doesn’t matter much – unless the ducts are installed in the attic. But as we all know, ducts belong inside the home’s conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. .”
Winning an Energy Star rating
Roger Brisson likes metal roofing over asphalt for a variety of reasons, too, but some of his claims may bear more checking. “With all things being equal,” he says, “and standard underlayment, metal roofing performs much better than asphalt in its overall insulating capabilities; this is why the Energy Star label is applied to metal and not to asphalt roofing.”
He adds that metal roofing is “incomparably better” for the environment than asphalt shingles.
“Asphalt roofing is a [wasteful] disaster,” he says, “and yes, its lifetime is generally much less than what it is rated (at least here in coastal New England). Replacing asphalt roofing is dirty and messy work, which is why cheap, immigrant labor is used for it. And there's no way asphalt will perform as well as quality Energy Star metal roofing over the course of 50 years (the rated lifespan of most metal roofing). This is why in more technologically advanced countries asphalt roofing is virtually non-existent. It is prevalent in the US for one reason: the upfront costs are cheap, cheap, cheap, which is also what you get.”
Holladay is quick to reply, pointing out that asphalt shingles, in fact, can be Energy Star rated, as is the case with Duration Premium Cool Shingles manufactured by Owens Corning.
David Martin adds that the EPA’s Energy Star website notes that roofing material itself has nothing to do with whether it can be Energy Star rated, only its measured reflectance.
As to Brisson’s comments on immigrant labor, Holladay adds this: “This comment does not deserve to be dignified with an answer, since it implies that immigrant workers are only fit for dirty and messy work. My only comment: I used to work as a roofer, tearing off asphalt shingle roofs and installing new asphalt shingle roofs, and I was born in the U.S. Moreover, I have the highest respect for the work ethic and skills of most immigrant workers in the U.S.”
Our expert’s opinion
Here’s how GBA technical director Peter Yost sees it:
All “cool” roof claddings work based on their surface properties: initial reflectance, “aged” reflectance, and emissivity. The difference between the first and the second is how the surface (shiny and smooth out of the factory) degrades to some degree over time (duller and dirtier). In the case of an Energy Star-rated roof product, “aged” means after three years of standardized exposure. Emissivity is important because heat not reflected becomes heat absorbed, and you want high emissivity to re-radiate that absorbed heat.
Energy Star includes in its specs that “roof products can help reduce the amount of air conditioning needed in buildings, and can reduce peak cooling demand by 10–15 percent.” Big difference between claims of 25% reduced energy consumption and possible 15% reduced peak cooling demand.
EPA Energy Star bases its ratings on initial and aged reflectance but also reports emissivity values. Not surprisingly, the best performers are white metal roofs (ACM Regal White metal roofing with initial reflectance – 0.68; emissivity – 0.86) with some asphalt shingles qualifying, but with much lower reflectance values and similar emissivity (CertainTeed Star White shingles with initial reflectance – 0.29; emissivity – 0.90). You can download a pdf or excel file for all qualified products with their performance properties.
NOTE: the paired terms reflectance and reflectivity; emissivity and emittance are often used interchangeably. Though similar, the “-ivity” terms refer to the property of materials in general and the “-ance” suffix refers to the specific value of a specific material.
When it comes to venting (dead end air space between cladding and deck) and ventilating (open ended air space between cladding and deck), there is no doubt that they can have an effect on the energy performance of the roof but their main purpose is increased concealed free drainage (venting) and drying potential (ventilating gives both), not increased energy performance. And no doubt ventilating the back side of a roof cladding (air flow) would be more effective thermally than just venting, but I don’t think their effects have ever been measured.
I suppose making metal roofing with a low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. surface on the back side could help some, but manufacturers are unlikely to fuss with the backside surface of their metal roofing with the uncertainty of that surface facing an empty space.
- Scott Gibson
May 21, 2012 3:29 PM ET
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