The beauty of a metal roof
A standing-seam metal roof makes it easy to mount solar panels, and roofing materials are durable as well as recyclable. The question is: Does metal roofing make a difference on energy efficiency?Image Credit: Scott Gibson
More Q&A Spotlight
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 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…
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44 Comments
Cool roofs are not for high-slope roofs in cool climates
Using a high solar reflectivity/moderate deep IR emissivity approach the steady state and peak roof temperatures under summer sun can can be brought down considerably, and even in a well insulated attic the roof can be a large fraction of the cooling load. The Lawrence Berkeley Nat'l Labs has modeled it fairly well, and has a freebie download spreadsheet calculator for "solar reflectivity index" (SRI) that reports the surface temps under standardized insolation and sky temperature conditions to compare apples-to-apples for materials of given solar reflectance and emissivity numbers:
http://coolcolors.lbl.gov/assets/docs/SRI%20Calculator/SRI-calc10.xls
Roof venting has a far lower effect on peak shingle temps than the SRI, and on medium and high-pitch roofs (greater than 2:12 pitch under cool-roof terminology), convection cooling on the exterior provides an order of magnitude more shingle cooling than any scheme for ventilation under the roof deck can.
(Other D.O.E. cool-roof calculators with energy savings modeling can be found on the LNBL site here: http://heatisland.lbl.gov/resources/5 )
This is all fine and good, if air conditioning is the primary energy hog in the home. But in US Climate zones 5 & up, the reduction in shoulder-season and heating season roof temp increases the average heating load, adding more to the heating costs than it reduces the cooling energy costs under a wide variety of heating & cooling fuel & equipment scenarios. The additional heating load of a cool roof is quite modest, and isn't likely to break the bank, but there's no reason to go out of your way to spec a cool roof finish in a cool climate.
In US climate zones 3 or lower it flips the other way, and for a very modest (or even zero) uptick in cost a cool roof will outperform a radiant barrier, and is "worth it".
The best cool roof "coating" is of course, a photovoltaic panel that shades it. While the albedo of typical silicon PV panels is comparable to lower than that of a cool-roof (and comparable to medium-dark asphalt shingles), the net increase in warming from the more absorptive surface is far less than from the offset emissions of the electricity produced, even in a cooling dominated climate.
Steel roofs are great in any climate from a longevity and sustainability point of view, but cool-roof finishes are best left for cooling dominated climates where they actually do some good.
Just curious...
Who is the expert?
Maybe...
I'm generally not a fan of "geoengineering" but urban heat island mitigation is something that should probably be considered in all climates.
Response to Lucas Durand
Lucas,
Thanks for your question -- the usual identification of our expert, GBA technical director Peter Yost, was accidentally omitted from this article. We've fixed the error.
And thanks for your reminder that cool roofs mitigate the urban heat island phenomenon.
Response to Lucas Durand
In cool/cold climates (US zone 5 and higher) the net annual benefit of cool roof strategies to buildings accrue only to higher density construction or on low-angle & flat roofs, single-family residential. When implemented on neighborhood/regional scale the net annual affect of higher albedo roofing lowers the average temperatures in the micro-climate/heat-island in the winter too, increasing the average & peak heating loads, increasing overall energy use for any less-dense construction within that micro-zone.
It's not a simple model, and in some locations (even in cooling dominated climates) the average air temperatures can increase, due to the effects of a higher albedo on a whole city on rate if cloud formation, etc. (even though the net energy benefit to the buildings is still greater than the modest increase in cooling loads from that effect.)
In cooler climates heat island mitigation via increased urban vegetation & reduction in paved area is probably a better approach than cool-roofs, cool-pavement or similar direct albedo boosting approaches that end up increasing heating loads.
Response to Dana
Dana,
You're right, it is not a simple model...
In many cool/cold climates, during much of the heating season, I would guess that the net effect of high albedo roofing on heating loads would be made insignificant by the presence of snow cover...
Another difference...
It all comes down to the testing numbers, but another reason why I imagine metal roofs would be superior at keeping things cool is the difference in thermal mass. Heavier, thicker asphalt shingles can absorb alot of heat and are continously radiating/conducting some of it to the sheathing and framing, and thereby to the interior. Metal roofs get hot too, but should have substantially less capacity to absorb heat and should cool off faster. The night-time cooling cycle should look substantially different for the two materials.
Metal roofing cooler?
I guess I'm looking here at the 'bigger picture'. When I did the roof (and roofing) rip off, the plan was to super insulate with solid foam sheets. As Martin has said before, you don't want to pass up an opportunity to beef up insulation when it presents itself. So I installed 6 inches of solid foam on top of the decking, 1X4 horizontal strapping (with 10 inch screws), and then the metal roofing onto the strapping. I wanted metal for all the reasons others have mentioned. With the roofing raised, I have ventilation. The point I'm getting to though is how would you attached asphalt shingles? To the foam? To strapping? (lot of sagging.) In other words, to do an insulation retrofit, I assumed at the time asphalt was not an option. And likewise for someone looking at replacing their roofing (assuming it is at least 25 years old) wouldn't they want to upgrade the insulation rating at the same time? BTW, my upstairs 'seems' cooler since we installed, but that just anecdotal comments, and I can't separate the insulation effect from the metal roofing and ventilation effect.
metal roofs Work!
First of all: shingles are very heavy and create a huge thermal storage for unwanted heat. Which is why an attic vent fan often runs well after sundown. I removed FOURTEEN TONS of shingles and installed about 1900 lbs. of metal roofing to replace them.
BTW: my previous shingles were all white. So-called because they are barely white to start with and within two or three years may as well be half-black.
With the previous white shingle roof my attic temps could be over 150º at times in the summers here in southern NJ. But now with the metal roof my attic temp was 79ª F. on a sunny 103º day. I have a probe up there and the first time I saw it I immediately thought an A/C duct had broken and I was cooling the attic space. But that wasn't it - the attic was naturally 79º on a 103º day.
In fact; I reduced my A/C systems from two 3 tons systems to two 1.5 ton systems after the metal roof was installed..
A metal roof (I have aluminum but steel is the same) is 100% recyclable, comes with a very long coating warranty, and could almost certainly be re-coated right in place if after 50 years that becomes required.
With the price of oil having the price of comparatively short lived shingles so high - installing a metal roof becomes a no brainer. My only regret is that I didn't think of it thirty years ago.
PHM
-------
Cool Metal Roof
I live in Key West, FL where it tends to get a liitle hot. The Florida Solar Energy Center has studied this issue. You must have both high reflectivity (> .70) and high emissivity (> .75).
I can say from practical experience that nothing beats a white metal roof reflectivity (> .80 - .90) and high emissivity (> .80 - .90). Galvanized and Galvalume metal roofs are hot roofs. They have medium relfectivity and poor emissivity ( .15). Painting a bare metal roof with white house paint dramatically improves preformance. Radiant barriers, air spaces and added insullation all helps but nothing improves performance like keeping the heat out of the structure to begin with. A truely cool roof prevents the roofing, roof desk and rafters from heating.
Response to Rock Termini
Rock,
Plenty of people have installed rigid foam insulation on top of roof sheathing, followed by asphalt shingles. But you have to install a second layer of roof sheathing -- either plywood or OSB -- on top of the foam. This can be done by screwing the sheathing directly above the foam (with long screws), or with intervening ventilation channels (usually created with flat-ways 2x4s, installed perpendicular to the ridge).
Cool roof in hot climates
There are few green building choices smarter than a cool roof in hot climates. The best way to beat the heat is to not absorb it in the first place. Use light color metal roof with overhangs to shade the walls on the south and west sides.
I can tell you that my experience is that any metal roof is cooler than asphalt shingles. Even the dark metal colors retain less heat than the relatively light shingles. As my Florida friend stated white metal is the best and the University of Florida has done some good work on quantifying what works.
Asphalt should be against code in the gulf south. They are perceived to be the cheap solution but they are environmentally disastrous; not recyclable and they create death traps out of houses when your AC is not available. Look how many deaths in Katrina were from heat in houses under asphalt roofs.
Folks who add metal roofs in my area see their AC bill drop in half or less. I have no problem believing a 25% reduction because it depends on thermal mass. The metal roof is raised on 1x4 slats so direct conduction is reduced. They are ventilated so convection is your friend. And the radiation out of the back side is never more than the radiation from the top side so a metal roof will cut radiant heat by a large amount in any color.
No I don’t’ sell metal roofs. I own several houses which I rent to folks visiting the area. I want all of my properties to be houses that don’t kill people just because the electricity gets interrupted in the summer. Cool roofs are the best first step.
Nice article - Question about concealed fastner systems
I don't know if this is the right place to ask the question: I have workshop going up with 4 inch horizontal battens on a 14:12 roof pitch for metal roofing. Will the battens telegraph through the metal roof? Also, is standing seam best applied with a cleat system or through the metal itself. Both are concealed systems. Thank you!
Response to Narendra Patel
Narendra,
Q. "Will the battens telegraph through the metal roof?"
A. Only when the weather is cold. Frost patterns can telegraph because the temperature of the roofing above the air space is sometimes colder than the temperature of the roofing above a furring strip.
Q. "Is standing seam best applied with a cleat system or through the metal itself? Both are concealed systems."
A. As far as I know, most standing-seam systems use clips that do not require fasteners to penetrate the steel. (See photos below.) However, through-fastened steel roofing is common; it is different from standing-seam roofing.
To "cool roof" advocates
"Cool roof" solutions are similar to radiant barrier solutions -- they work best on poorly insulated houses.
If you are getting a lot of heat flux downward from your roofing to your ceiling, something is wrong with your house. The usual flaws: bad air sealing or insufficient insulation.
In new homes with adequate insulation, there really shouldn't be much heat flow from your roofing to your ceiling. However, if you have an older house with insufficient insulation, cool roofing may make sense.
To Martin, GBA Advisor
In hot climates cool roof choices almost always make sense when there is no cost penalty, i.e. you choose white metal roofing over gray, green or brown metal roofing, etc. In the vast majority of homes the rafters attach to the top plates and the top plates are insulated from the interrior by a little sheetrock. You can insulate your attic to the ridge and this does not change. There will still be heat bridging into the perimeter of your exterrior walls. Why allow your roof deck, rafters, top plates and insullation to absorb so much more heat with a poor choice of roof color. There is a reason we wear white shirts in the South.
Response to Ross Williams
Ross,
I agree completely with your statement, "In hot climates cool roof choices almost always make sense when there is no cost penalty."
Nevertheless, I'm going to continue to advocate for good roof design and good insulation details. There is no reason that framers of new homes should be using conventional roof trusses rather than raised-heel trusses; nor should framers of new homes be installing sawn rafters directly on top of top plates.
We've known about the need for deep attic insulation at the perimeter of roofs for at least 30 years. At GBA, we advocate good insulation details, and that includes deep insulation at the perimeter of an attic. It's essential that builders begin to get these details right, no matter what color of roofing they specify.
Cool Roof Contemplations
This article is timely, as we are researching options for replacing our terra-cotta colored asphalt roofing shingles with metal. While we have decent attic insulation, and I've undertaken considerable effort to ensure the ceiling plane between house and attic is much more airtight than average, the "cool roof" aspect of a metal roof still sounds appealing, since our HVAC ducts are in the attic. We also live in a climate where the cooling season seems to be expanding with each passing year.
Reducing heat gain to the attic via roofing material choice is a large driver behind our interest, but so is durability, since we plan to be in this house a long time. Hailstorms in spring and fall are not uncommon, and I understand metal roofing materials fare better through hailstorms and high winds (we can get both from the same storm system). From this article, concerning the cool roof aspect, I've learned to look for reflectivity and emissivity information, which until now was missing from my research. Therefore, thank you for writing this blog, and for the links embedded within.
In response to Mr. Holladay's comment: "'Cool roof" solutions are similar to radiant barrier solutions -- they work best on poorly insulated houses", what I'm faced with is an adequately insulated ceiling plane pierced by marginally insulated metal HVAC ducting in the attic. Also, reducing heat gain into an attic at the source to me makes sense even if the ceiling level insulation minimizes heat flux into the conditioned space, especially in my climate where nighttime temperatures can remain elevated long after sundown, which reduces attic cooling potential overnight.
I also like the comment above about how a house should not cook its occupants should the electricity or HVAC fail during the summer. Every summer our local news will carry a story about someone being hospitalized or dying from heat exposure inside their house with failed or no a/c. Invariably, if an exterior shot of the structure is shown, it will have asphalt shingles. This has always seemed ridiculous to me; that a structure overheats so badly without a/c that it can injure or kill its occupants. Does not meet the concept of "shelter" in my mind, which I hold as the primary purpose of any building.
Response to Cameron Taylor
Cameron,
I agree with you completely: if you have ducts in an unconditioned attic, then cool roofing makes sense. That's why I was quoted in the article as saying, "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."
Response to all y'all, especially Mr Gibson and Mr Holladay
Ok, ok, ok. Enough already.
If y'all are trying to make me feel guilty for learning so much from this website and not paying for any of it, well, it worked. My finances are a bit strained at the moment so coming up with the money to afford the membership fee isn't that simple. So I've decided to cancel my Netflix membership and become a paid subscriber to Greenbuildingadvisor. I've learned a ton here and looking forward to having access to all the rest of the valuable information that comes with being a member. I should have done it sooner.
You guys are great. Thanks, again.
I've done a lot of research on this metal roofing thing, and have a few more random questions and comments. I'll have to post them a bit later.
Response to David Martin
David,
Thanks for the vote of confidence. I'm glad you decided to subscribe to GBA.
Don't hesitate to post further questions -- we're here to help.
Sound, heat, and UV
I feel like I've become a metal roof salesman with all the conversations I've had with people about it recently. I'm sold on them because of their durability, lightweight, recyclability, etc. They beat out other materials in total cost of ownership over a long time frame. I don't like the industry making dubious claims about their energy efficiency. I think exaggerated claims do a disservice to the industry.
You often hear people of the "green movement" make the "save the planet" claim. That may get people excited but the environment is in much more trouble than that. I think the entire system needs, and will get, drastic reform one way or another. Until then, its good for individuals to continuously make better decisions to lead a less damaging lifestyle.
Sound
One preconceived notion that many have is that metal roofs are very loud in heavy rain or hail. True in a barn, but I think that's a myth in an insulated home. I'm interested if there is a difference in the sound properties if the metal roof is raised on purlins. Rock Termini, what is your experience? Anyone else?
Heat
Metal roofs will expand and contract more with changing temperatures. That is one reason why exposed fastener types are less durable. The sheet will move around the fastener, causing deterioration.
UV degradation
The reason the coating deteriorate is due to ultraviolet light breaking down the polymers in the coating. Lower quality coatings will deteriorate faster. I think color choice comes into play here. True that lighter colors are cooler, but don't they degrade faster? Does anyone know? Dana, it seems like you've done or read a lot of research in this area. Can you answer that?
McElroy Metals has a really good handbook called the "McElroy Metal Retrofit Guide" . It's free from their site but you have to register to get it. It has good illustrations and doesn't seem to be full of industry hype.
Response to David Martin
David,
Directly above my bed is an insulated sloped ceiling / roof assembly. The roofing above is steel.
The sound of the rain on the steel roof above my bed is louder than it would be if I had an asphalt shingle roof (even though the roof assembly is insulated).
I like the sound. I don't hear it anywhere else in the house.
Response to Martin Holladay
Whereas radiant barriers reflect radiated heat back to the roof deck raising the temp of the roof deck & shingles to radiate more heat back to the sky, the convection & conduction around the attic back into the attic still raises attic temps. Cool roofs radiate the heat back to the sky without the intervening complexity of roof deck- the surface of the roofing stays cooler, the roof deck stays cooler, and attic stays cooler, since the roof deck isn't running as hot. Cool roof materials do work measurably BETTER than radiant barriers, and are a sufficiently cheap improvement that independent of insulation levels or where the ducts are located, they're cost effective at almost any R-value.
The LBNL site is chock full of data to back this up, but the better performance/lower cost of cool-roofing is the reason why they are prescriptive under California Title 24, whereas radiant barrier and (usually much) higher R values are allowed as alternatives under the code where the building owner or contractor is unwilling or unable to use a suitable cool-roof material. Peeling 10-20F off the peak roof temperatures places a much lower delta-T across the insulation layers. Even at R50 cool roofs can produce a measurable cooling-energy savings compared to a low-albedo roof, even if it comes without a noticeable difference in comfort for the occupants.
Metal roofs are probably the most expensive (first cost) cool roof solution, but they also have a favorable life-cycle cost which shouldn't be ignored. From a long term sustainability point of view metal is a good deal, but cool roof shingles shouldn't be dismissed- they run cooler (increasing shingle life) and come in many colors other than white/off-white, often with zero up-charge. (In CA it's probably hard to find anything BUT CRRC rated cool-roof shingles.)
Response to Dana Dorsett
Dana,
Cool roofing makes sense in areas of the country with high cooling bills. It makes the most sense for homes with low levels of insulation and for homes with ducts in the attic. Because the incremental cost of cool roofing is low, it's a relatively inexpensive and sensible product to specify in those climates.
Cool roofing can reduce energy use in some, but not most, U.S. climate zones. As noted in one of the documents from the LBNL website, "Prior research has indicated that savings are greatest for buildings located in climates with long cooling seasons and short heating seasons, particularly those buildings that have distribution ducts in the plenum [attic]."
I haven't yet found any document that backs up your statement that "Even [when a roof is insulated] at R-50, cool roofs can produce a measurable cooling-energy savings compared to a low-albedo roof." I'd be interested to see a link to a document that backs up that statement.
building in Guatemala
This is about using two-seperate-layers of metal roofing, I am building a one-story-3-bedroom-wooden-house on three-foot-high-pillers, the narrow-end of the house is facing true-west.
A wrap-around-porch of eight-feet. I am considering for the roof a moderate-pitch using corrigated roofing over wood strips then a series of 2X4 spacers over this and under a top-roof of commercial-metal-roofing kept open at the ridge-line for air-flow between the two-layers of roofing.
I will not air-condition this home, but will have a heater for those damp cool winter nights. The ceilings are heirring-bone-pattern-wood with 3-inches of vermeculite over a plastic-membrane.
Any and all comments are welcome.
Home insurance
I live in Central Texas and installed a metal roof on my house twelve years ago over the existing single layer asphalt roof. One of the deciding factors in choosing metal was that my insurance company reduced my insurance premium considerably because of the roof's resistance to hail damage, which is a common insurance loss in this area. The savings in insurance premiums has just about paid for the roof at this point.
Roger Brisson likes metal
I was not very happy at all to read what Roger Brisson said about cheap labor. In my area and just like the rest of the country immigrant works are in ALL lines of work. Not just the roofing industry Mr. Bigot. The going labor rate is the SAME no matter what color your skin is and if you have an accent or not. I have myself paid the going rate to all types of people. And if Roger Brisson is a contractor then he should know that. Not very professional.
Update on Cool Roof Contemplations
Well we did it. We now have ~4,000 sq. ft. of "solar white" standing seam metal roofing on our house. Architecturally, it looks outstanding, since it goes well with the mid-century modern look of the structure. As for the "cool roof" aspect, just two weeks ago we had daily highs over 100 degrees; now daily highs are in the eighties after a nice cool spell with gentle rain. The past two days I've been on the roof in late afternoon measuring surface temperatures with an IR thermometer. Standing seam in full sun (with ambient air around 84) read ~94 degrees. Neighbor's solid black asphalt roof read 122. I haven't measured attic temps yet as I need to revive those sensors, and will do so soon, along with installing a data logger up there.
We chose the standing seam option for the high reflectivity/high emissivity traits in addition to potentially adding solar cells at a future date. In my previous post I mentioned our HVAC ducts being in the attic, with no intention of moving them inside or altering the existing HVAC in any major way. While our ceiling plane is well insulated, and I've sealed every ceiling penetration to the attic I can find, the duct heat gain with our old terra cotta colored asphalt shingles, in spite of a spray-on radiant barrier added several years ago to the roof decking, was definitely reducing the actual delivered capacity of our a/c to each room. Not enough to cause us to be uncomfortable in summer, but enough to increase operating cost.
One thing I should note is that I realize we may see a slow payback with our roofing choice, much due to what I've outlined above. Honestly that does not bother me much, as we plan to be in the house a long time and will not be faced with reroofing costs for the remainder of our stay. In addition, the wind resistance of our new roof is much higher than before...we live in high wind/tornado alley, not to mention almost without fail in spring (and sometimes fall) we get hailstorms. Our old roof was damaged by hail, which prompted this entire project. The new roof may suffer cosmetic damage with large hailstones, but the water shedding integrity of the roof will not be compromised.
One benefit of our new roof we did not expect was that it has made the interior of the house more quiet from exterior noise. We live in the city, about four blocks from a major interstate. When the wind is from the north it carries the freeway noise with it. Before our new roof went on I could clearly hear it from inside the house, and I attributed it to our old single pane windows we haven't replaced yet. But now I'm sitting in the same rooms with the same old windows, and with freeway noise blasting us full on, but the room is noticeably quieter. Such a discovery is delightful, as it dovetails with my views on "shelter" not just being from weather, but from other exterior assailants such as urban white noise. Even when prop or jet airplanes fly over the house, more of the sound enters through the windows than it does from overhead (whereas I used to literally be able to know when the plane was directly over the house, since I could hear it through the ceiling).
Bottom line: we're already enjoying the new roof with the short time we've had it, and are looking forward to monitoring its long term performance in terms of energy reduction, comfort increase, architectural enhancement, and increased protection against severe weather.
White elastomeric roof coatings
I am an avid green roofing supporter. The many advantages are numerous. If you use the reduce, re-use, and recycle thinking it makes so much sense. Think about all the roofing materials in the US that are torn off and replaced with new materials that came from a heavy industrial process leaving a negative green foot print.
I work in the commercial roofing industry and have gained a behind the scenes view point on green roofing. If all companies would take their existing traditional roofing system like EPDM, TPO, hypalon, BUR roofing, or rolled roofing and bring it up to a spray applied white elastomeric roofing system it would dramatically effect the heat island effect caused by traditional dark colored substrates that hold heat, not reflect it. (only if the current substrate is in sound condition or close to sound).
Once brought up at about 50-70% of a tear off replacement, all they have to do is keep it on a maintenance cycle that would lower their capital expnese over the course of time. It's a win win for the earth and their pocket book. A professional system applied by a qualified contractor comes with extendable warranties in 10 and 15 year periods.
It's amazing to be on these roofs applying white elastomeric roofing and walk from a finished section onto the old black roof sections. The extreme difference in temperature makes you a believer real quick. You can always learn more here: http://www.commercialpaintingservices.com
The advantages of using these types of systems on metal roofing is their able to flex with the large amount of thermal shock a roof receives from daytime high temps. They completely encapsulate your old roof with a monolithic single ply of elastomeric rubberized coating that is a water based formula manufactured with energy star ratings and green manufacturing process.
More info from Cool rating Roof Council can be found here: http://coolroofs.org/documents/CoolRoofsforArchitectsandRoofingSpecifiers.pdf
Standing Seam Metal Roofing
I live in Kihei, Hawaii (Maui County) and am about to replace my aging, asphalt, 3-tab shingle roof with standing seam metal. I'm planning to remove the old asphalt roof. The roofing contractors recommend that I put down a peel and stick waterproofing underlayment on the existing 5/8 inch plywood decking followed by a radiant barrier and two layers of 1x4 wood battens, one layer laid vertically and then a horizontal layer on top of that. Then the metal roofing will be affixed to the top horizontal layer. Kihei is a desert microclimate so rainfall and humidity is not an issue here and I'm a mile from the ocean. There was mention in this thread about a similar installation. I'm wondering if the radiant barrier and two layers of 1x4 battens will be cost effective. Or should I just put down a new waterproofing underlayment on the 5/8" plywood decking and install the metal roofing over that. Thanks for your help on this ! Bruce
Response to Bruce Lowrey
Bruce,
There is no way we can determine cost-effectiveness without more information. That calculation depends on the incremental cost of the radiant barrier installation, the thermal characteristics of your building envelope, your local energy costs, the annual run time of your air conditioner -- none of which we know.
A radiant barrier will help reduce heat transfer from your metal roof to your attic. This could also be accomplished with a layer of insulation.
Response to Martin Holladay
Thanks for the feedback, Martin. You're right that I need to do a more detailed cost analysis once I get proposals back from the roofing contractors. My house was not insulated when I bought it so I installed R-30 on the attic floor last year. Even though that was a sub-optimal installation because of the tight, four foot high attic space, wires and pipes, it helped a lot But the house was still hot so I installed an attic fan in one of the soffit eaves. Since reading your comments on this blog I now realize that was a mistake and will turn it off once I get the new metal roof installed. The walls of the house will be insulated when I replace the siding in a few months but there will be no insulation for the floors. I'm also adding new thermally insulated windows, PV panels and a solar hot water heater once the roof is done. I run my high-efficiency, mini-spit AC an average of 6 hours a day but only to cool half of my 940 sq. ft. house and my electric bill averages $130.00/month. It will cost me approx. $2,000.00 to add another layer of insulation in the attic. I'll wait to see what the estimates are but from what I've read on-line, it isn't worth it to install the metal roofing over a radiant barrier and 1x4s especially since I'm planning to choose a light roof color. I'll follow-up with another post once I know how much the radiant barrier and 1x4s will cost.
Response to Bruce Lowrey
Bruce,
I'm not convinced that you need more attic insulation or a radiant barrier. In Hawaii, it's hard to imagine that you need more than R-30 attic insulation.
If your house is hot:
1. The source of the heat may be solar gain through windows. The solution is to shade the windows on the exterior or to replace the windows with low-solar gain windows.
2. The source of the heat may be internal gains (electric appliances and people inside the house).
3. Part of the problem may be a high rate of infiltration and exfiltration (air leaks in your floor, walls, and roof). If that's the case, the solution is air sealing work.
4. Maybe you just live in a hot climate and have to run your air conditioner to stay cool.
cool roof design
Would not a metal roof work great over a cool, vented roof, the likes of which Joe Libsturek talks about with tightly sealed sheetrock ceilings? I think ventilating the roof, using a synthetic underlayment and attaching a standing seam roof, would be great for keeping the attic cool, no matter what the color is? There is no need to do foam underneath, correct? Just the metal, then the underlayment and the sheathing with vented backside and plenty of cellulose or blown in fiberglass above the ceiling (no attic hatch to store Xmas bulbs!) correct?
Response to Joe Suhrada
Joe,
You are describing a typical attic. This approach is very common.
I agree that this is simple -- not rocket science. Most attics are vented; if you have plenty of insulation on the attic floor, you are all set.
Response to Martin Holladay
Aloha, Martin. Many thanks for all your advice. I will abandon the idea of a radiant barrier and more insulation and just install the metal roof over the underlayment and 5/8" plywood decking. One thing I'm still debating is whether or not a ridge venting system for metal roofs is worth the investment. PoodleHead Mikey says his attic is 79 degrees F on a 103 degree F day after installation of his metal roof. So I'm beginning to believe the ridge vent may be overkill for a metal roof. Any advice on this is appreciated. A Hui Hou, Bruce
Response to Bruce Lowrey
Bruce,
Here on the mainland, most roofers who install metal roofing over an unconditioned attic routinely install a ridge vent. It shouldn't add much to the cost.
That said, attic venting is often overrated. Here is a link to an article with more information on the topic: All About Attic Venting.
Response to Martin Holladay
Aloha, Martin,
Many thanks for pointing me to your excellent article on attic ventilation. After reading it, I don't believe that it's worth the additional cost to install a ridge vent. Keep up the good work in keeping us well informed on building science.
Wishing you continued success in all your endeavors,
Bruce
Stuck on analysis
We're east of Houston, just now getting ready to do our post-Harvey renovation. I bought this house eighteen months ago. It has a 2nd layer asphalt roof which is not 10 years old. There are plenty of leaks which I assume all stem from neglected flashing (e.g., two chimneys) and some shoddy carpentry where someone added a low-pitched awning. I'm still deliberating whether to address the leaks only or replace the entire roof with white metal. On the one hand, I'm ready to put our $400 light bills and clammy discomfort behind us. On the other hand, it pains me to have a roof removed so early in its life.
If I do the metal, I'm wondering whether it makes sense to add rigid insulation above sheathing. I've read enough on here that I'm leaning toward leaving my unconditioned attic unconditioned. But it makes sense to the model in my mind that we can amplify the benefits of a reflective roof by insulating right where the rubbermetal meets the roadsun. Will someone please talk me down?
Ben
Response to Ben
Ben,
Whether or not adding metal roofing lowers your energy bills is complicated, but in general the answer is, "Not usually, and not significantly, unless there are problems with your home's insulation R-value or duct location."
You haven't told us whether your unconditioned attic has any ducts. If it does, that's a problem.
If you think that your "$400 light bills" are due to high air conditioning costs, you need to figure out ways to reduce that cost. Solutions might include duct sealing efforts, improvements in duct insulation, or converting your attic from an unconditioned attic to a conditioned attic. Note that creating a conditioned attic is so expensive that some homeowners find it cheaper to abandon their duct system and instead install new ductless minisplits.
If you decide to convert your unconditioned attic to a conditioned attic, adding rigid foam above the roof sheathing is a good way to proceed. For more information, see these two articles:
Creating a Conditioned Attic
How to Install Rigid Foam On Top of Roof Sheathing
Thanks Martin
We have R6 ducts in the attic. The $400 bills last summer were just enough to keep the house on the cool side of unbearable. After Harvey, we removed two feet of drywall and batts from about 30% of the downstairs exterior walls. This winter I had one $800 and one $600 bill, both from running central heat almost exclusively at night. So the attic is probably a major factor, but insulation appears to rank right up there as a cause for my home's poor hvac performance.
This is not our forever home. With that in mind, I gathered from my studies on this website that the benefits we should enjoy from a conditioned attic won't justify the expense. It also sounds like I need to hold off on doing a metal roof until I have a compelling reason to remove the existing roof. For now, I need to see how much it will cost to stop the leaks at both chimneys and at the rolled-roof awning and probably where the downstairs roof and upstairs Hardie plank intersect. Surely that won't cost a fraction of a new roof.
I definitely need a new HVAC system. I'm afraid that every HVAC contractor this far east of Houston is tooting along the same as they have since 1972 when they built this house. If I can find a contractor who is up on things, it will be easy enough. If not, I've got a LOT more reading to do.
I apologize, but on the topic of roofs, I have a lot of questions:
We have a steel frame tiny house on wheels under construction, and plan to live in climate zones 5-7.
1. What of an unvented, unventilated cathedral roof that has no attic? Would white aluminum standing seam (or interlocking shingles) be a good choice? With that, should we leave an air gap somewhere?
2. We have a height limit for our steel frame tiny house on wheels, but we still must "out-sulate" with polyiso rigid foam sheets (as many as we can fit). Do we use furring strips, and if so, what material would they need to be and should they be vertical or horizontal?
3. Is the peel-and-stick membrane necessary with Zip System roof sheathing? Is that instead of ice & water shield?
4. The insulation beneath the Zip System roof sheathing would be rockwool batts. Do we need a radiant barrier or vapor barrier beneath that? Again, still no need for air gap with this type of roof?
Overall, does this roof assembly make sense, or are we newbies making some big mistakes?
Thank you for such informative articles!
Thanks for the great article and the comments make you think and sometimes shake your head. I like when Bob Coleman said "There is no magic bullet. It’s all part of a system", which is true especially when you're building a green home. https://www.lenexaroofers.com/residential-roofing.html
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