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Community and Q&A

Metal roofing

David Martin | Posted in General Questions on

I am thinking of replacing my roof so I’m wondering about the benefits of a standing seam metal roof. I’m aware of its greater durability, but I’m wondering about the energy efficiency component. Does the energy benefit come mainly from choosing a light color? Do light colored asphalt shingles offer a comparable energy benefit?

It also seems to make a lot more sense to choose a metal roof if I decide to add solar pv at a later time. Why add solar panels with a 30 year lifespan to an asphalt roof with a 15 year life? That seems like a common practice but problematic down the road when the shingles need replacing.

I’ve searched around the internet and found a lot of the industry’s advertising, but I’m a bit skeptical, especially after reading what Mr Holladay says about the dubious claims made by manufacturers of bubble foils, radiant barriers, and open-cell spray foam (Musings of an Energy Nerd. 2-5-2010).

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Replies

  1. David Martin | | #1

    For example, here's one carefully worded claim from the Metal Roofing Alliance: "A cool metal roof can save 25% in energy costs compared to a dark grey asphalt shingle". (1)

    After thinking about my own question, it seems that the biggest factor over whether there is an energy benefit to metal roofing is whether the roof is vented or not. I figure there would be a negligible benefit, if any, on a vented roof. On an unvented roof, would the benefit be comparable to a radiant barrier on the underside of the rafters?

    1. http://www.metalroofing.com/v2/content/metal-roofing/energy-efficiency.cfm

  2. David Meiland | | #2

    I'll bet it has mostly to do with color, and that a very light colored asphalt roof would perform similarly to a very light colored metal roof, vented or not. Comparing "a cool metal roof" and "dark grey asphalt shingle" is apples to oranges.

    I like metal roofs because they last a long time, look good, are recyclable, and it's very easy to remove a few panels if you need to for some reason.

  3. Kevin O'meara | | #3

    It is more than just the lighter color. The paint that the metal companies apply to the metal, has added chemicals that are desgned to reflect infrared wavelengths. Most metal roof caompanies will list these ratings under each color. There exists a "cool roof" tax credit and a website for approved metal roofs.

  4. David Meiland | | #4

    Looks like you're right. On this site http://tinyurl.com/6qv8kgf they do list many shingle/shake products, but they aren't as reflective as the most reflective metals. For that matter, there's a wide variation in the metal colors too.

  5. Bob Coleman | | #5

    you need to look at the published specs of the roofing material and that will tell you how 'cool' it is
    in some areas it may not be the best if you want some of the suns heat

    there is a reflectance and absorptivity, often lighter colors are an indicator but it is not exactly the lighter color that does it.

    also venting the roof underneath changes things
    there is no magic bullet; its all part of a system that should be geared toward your solutions

  6. David Martin | | #6

    Thanks for the responses. I like the appearance, durability, and that they are recyclable, but I still doubt there is much of an energy benefit over a similarly colored asphalt shingle roof. The main benefit would be in summer months, keeping the attic cooler so that attic ducts carrying A/C are cooler. It seems like a good topic for a research project and that someone would have done some testing, but I haven't been able to find anything on buildingscience.com, ORNL, this, or other brand-neutral sites. Martin Holladay's "Radiant Barrier" article (2-24-2010) seems relevant.

    I'm going to go with the steel roofing because it's a better value due to its durability. An energy benefit would be nice but I haven't seen the evidence.

    Here's an interesting video from Midwest Manufacturing -- https://youtu.be/ueYv9fm_-WI
    Again, they make some claims about roof surface and attic temperatures but they don't give much detail as to what they were actually measuring. I know its advertising, not a scientific experiment.

    What about steel roofing over furring strips? That seems smarter to me.

    And can anyone comment on solar pv over steel roofing vs asphalt shingles?

  7. Aj Builder, Upstate NY Zone 6a | | #7

    David, location?

    As to solar over whatever, the solar will make under roofing last for way longer. Also new 30 year shingles come with lifetime warranty now. They are saying they finally have very long lifetimes figured out.

    You want metal, I say put up metal. All materials have good and bad points.

  8. User avatar GBA Editor
    Martin Holladay | | #8

    David,
    The marketing claim -- "A cool metal roof can save 25% in energy costs compared to a dark grey asphalt shingle" -- is ridiculous. 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 space.

  9. 5C8rvfuWev | | #9

    The "Cool Roof Council" has a rating program that shows solar reflectance and emittance for 25 pages of products from a large number of manufacturers and more types that I'd heard of. The database is searchable ... it will give hard numbers, presumably based on independent tests, for that component. Of course ymmv on materials selection. Here's the link:

    http://www.coolroofs.org/products/search.php

    A local mfg of metal roofing told me that "the best" way to install a metal roof to pick up on its reflectance is to vent beneath the roofing and sheathing with sleepers. It's common sense but I don't have numbers to back up the claim.

  10. David Martin | | #10

    Thanks for all that.

    AJ -- my climate zone is 5A. Isn't it true that asphalt shingles rarely last up to their warrantied lifetimes? And isn't it also true that manufacturer's warranty reimbursements to homeowners are extremely rare because of all the exclusions? Have you ever come across a situation where a manufacturer actually paid out on a shingle claim?

    Martin -- Thanks. You are reinforcing my suspicion that there is little energy benefit to metal roofing, though the IRS does offer a tax credit. I think I would still go with a light color to keep surface temperature down. I have heard and read (don't have time to get a reference) that light colored roofs, if they were more widespread, would reduce the heat island effect -- cities have higher temperatures compared to rural areas. Also, solar pv loses efficiency with rising temperature, so it would not make sense to put panels over a dark roof.

    Joew -- furring strip are the best way to go then. Thanks.

    I'm wondering if the combination I'm proposing, solar pv over furred out steel roofing is problematic. It seems good in theory -- longer life roofing to match long lasting solar panels. But is it not practical -- like it would be impossible to walk on in order to install the solar?

  11. User avatar GBA Editor
    Martin Holladay | | #11

    David,
    An experienced roofer or solar equipment installer will be able to walk around on your roofing, or access your roof using cleat boards, ladders, or staging if the roof is too steep to walk on. An experienced roofer knows how to get around on almost any roof.

  12. Aj Builder, Upstate NY Zone 6a | | #12

    David, read up on recently reformulated shingles. And yes they have paid out thousands of times on warranties. The new 30 year shingles are much better than a decade ago imo. Still you want metal so use metal.

    Read a sample of warranties here;
    http://roofing.owenscorning.com/homeowner/shingles/

  13. Roger Brisson | | #13

    "Martin -- Thanks. You are reinforcing my suspicion that there is little energy benefit to metal roofing, though the IRS does offer a tax credit."

    You need to read more carefully, this isn't at all what Martin is saying. He makes his claim *only* when you add enough insulation or underlayment before putting on the metal roofing. In reality his statement isn't really saying anything of substance at all, since this would be true of any roofing with enough insulation.

    A more useful statement is to say that with all things being equal, 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.

    Metal roofing is incomparably better for the environment in other ways as well. Asphalt roofing is a wastefill disaster, and yes, it's 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. Interestingly, in states like New Hampshire and Maine metal roofing is common because in states like this performance is valued.

  14. User avatar GBA Editor
    Martin Holladay | | #14

    Roger,
    1. You wrote, "He makes his claim *only* when you add enough insulation or underlayment before putting on the metal roofing."

    No -- it's perfectly OK to put the insulation on the attic floor. I'm saying that if your house is properly insulated, attic temperatures don't matter.

    2. You wrote, "A more useful statement is to say that with all things being equal, and standard underlayment, metal roofing performs much better than asphalt in its overall insulating capabilities."

    No. Metal roofing is a conductor, not an insulator. Its R-value is negligible.

    3. You wrote, "The Energy Star label is applied to metal and not to asphalt roofing."

    Wrong again. The Energy Star program, for better or worse, recognizes Energy Star asphalt shingles. Here's one example:
    http://roofing.owenscorning.com/homeowner/shingles/duration-premium-cool.aspx

    4. You wrote, "Replacing asphalt roofing is dirty and messy work, which is why cheap, immigrant labor is used for it."

    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.

  15. Aj Builder, Upstate NY Zone 6a | | #15

    I absolutely agree with Martin.

  16. Randy Garber | | #16

    Don't forget the advantages of metal roofing for rain water collection....shorter flush time & no shingle granules to deal with. Something we all should be working towards and including in long range planning.

  17. 5C8rvfuWev | | #17

    Was the link to solar roof council any help with comparisons? I found it useful when I was tangled in all the company blurbs for leveling the field. Yes, reflectance is the bottom line for a "cool roof" it seems.

    Re: furring and providing a "smooth surface." Locally, roofing is sold in 26 guage and in 20 guage. The heavier material is often described as "structural" and the mfg just shrugs when asked if it needs anything under it. His own (the rep's) is vented w/furring strips.

    In a lot of areas "best practice" seems to be in conflict with the legal CYA for the warranty. You have to make your own call, as you certainly are doing. Good luck.

  18. David Martin | | #18

    After doing further research, manufacturers such as Fabral and McElroy Steel do not recommend using furring strips with an air gap underneath. They say to use insulation board between the strips to create a solid surface. That tells me to create an unvented attic. Otherwise the insulation board on the roof deck would serve no insulating purpose.

    Roger -- Thanks for the advice on reading. I read slowly and read the same things multiple times to be sure I understand what's written. Still, being more careful is always good.

    I agree that the landfill problem with asphalt shingles is significant. So much that the main energy benefit of using a metal roof is that it much more durable and it is recyclable. The benefit is in its full life cycle carbon accounting rather than in the operational period, according to what I understand from reading about carbon accounting. I haven't delved into doing those calculations myself.

    My recent insurance claim for hail damage to my 22 square roof showed a $700 credit for tear off and disposal of a single layer of shingles. To tear off a metal roof would be faster, I think, plus you would get paid at the scrap yard for it, rather than having to pay to dispose it at the landfill. On the other hand, I'll probably be in my 90's when my metal roof needs replaced. I might be a little slower by then.

    The EPA's Energy Star website explains their roofing criteria - the only consideration is reflectance - minimum 0.65 for low sloped roof, 0.25 for steep sloped roof. (1) The roofing material has nothing to do with it.

    Oak Ridge National Labs has a program called "Roof Savings Calculator" which is supposed to shed some light on the subject but after clicking on whether you want to evaluate a residential or commercial project, you get a page which says the calculator has been taken down for further validation. (2)

    It would be a fun project to set up some different roof types on top of my existing roof, hook up some thermocouples, above and below the surface, to an Arduino microcontroller, and see what happens. If I had a big budget, I'd use NI's LabView stuff. If I had time, I'd actually do it, rather than just imagining it.

    Randy -- Thanks for that tip. I want to set up a rain water collection system. Metal roofing would be much better than shingles for that.

    Martin -- I have great respect for your work and read everything you write. Many thanks for all that. Maybe when you finish your Manual J heating load series of articles, you can follow up with a Manual D series on ductwork. Then maybe you can do some roofing articles. Great idea, no?

    1. http://www.energystar.gov/index.cfm?c=roof_prods.pr_crit_roof_products
    2. http://www.roofcalc.com/

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