Roofing Reflectance

What are the trade-offs regarding home roofing reflectance. I ask because I'm considering using a 40% R roof for most of my home (mostly for asthetic reasons) and a 60% R for a south facing (and not ground visible) large dormer we have -- mainly to reduce heat load in the summer which has been a problem.
Is there a down side to this. Some people seem to want to use darker roofs so the snow melts faster -- others say it's good for snow not to melt and a cooler roof is better in both the winter and summer.
I live in minnesota -- so we get both very cold and quite warm -- but only a moderate amount of snow usually.
Thoughts?
Rob
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x-no-archive:

when the roof is covered w/ snow, it does not matter what color the roof is?
Mark
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I have not paid a much attention to this though people have told me it does -- but I'm not certain how much I believe that. That is why I ask.
Several ways it could make a difference --usually snow pack is not thick or may be none around edges and so I suspect a dark roof starts melting faster there. Also snow must be quite thick in order that there be no penetration through to the roof -- so it'd expect the roof to be a bit warmer under even a modest snow layer. The other potential issue is that the emissivity of a black and a white roof are probably quite different, meaning perhaps you get more thermal radiation from below for the black roof case.
I don't know if any of this matters in practice, or if it's a bad thing to leave snow on the roof as long as possible. I'm personally leaning toward using as light a roof as possible while still looking good -- unless there are some issues with that thinking.
Thanks, Rob
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Rob,
A shingle roof's color is determined by the shingle granule. Granules cover 97% of the exposed area. Opacity of the granule is 100% on a shingle, meaning no UV rays penetrate regardless of color (on 97% of the exposed area).
When talking about heat radiation from below, the issue is resolved through better insulating factors, & or ventilation. All fiberglass/asphalt shingles are black on the under side, plus you have or should have black felt paper.
Apparently a roof is up to 60% of the home's curb appeal, this of course will vary with the pitch of a roof. Go for the aesthetics, don't get stuck with an ugly roof, you'll lose money in the long run.
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Rob wrote:

There's at least one other consideration: Looks.
Light-colored roofs discolor from the crap in the air and show unsightly streaks downstream from vents, chimneys, and other protrusions.
I don't see how snow melt can be affected by the roof color. True, dark roofs absorb more sunlight and get warmer, but if the roof is covered with snow, the sunlight's not reaching the roof...
But, I admit, I don't know much about snow. We get snow - maybe a quarter-inch - once every ten years. 'Course that's cause for panic: they close the schools, drivers collides with as many other cars as they can because that's what you're supposed to do when it snows, people stock up on canned goods and huddle in their family, preachers inform us that the end is nigh, there are the obligatory news reports of kids making one-foot tall snowpeople with film of goofy dogs and perplexed cats trying to make sense of the phenomenon.
Snow in Houston is a truly remarkable experience.
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Yes looks. That's one reason I don't think we'll go more than 40% R max on the visible portion of the roof -- especially on the street side.

Is this also true of metal roofs as well.
Also one good thing for my house, the visible portions of the roof don't have any major protrusions -- through it does have 4 roof vents. All of the stacks and the chimney etc. are on the dormer which is on the back of the house and not easily visible from ground level.
Thanks, Rob
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A high reflectance roof on a south exposure is always a good idea. A metal roof on a south exposure is even better. Given the installation methods for standing seam, the extra ventilation possible between roof and deck should markedly lower AC loads in summer. The downside in northern states is snow sliding off the roof and landing in driveways, on bushes and other places that you'd rather not have it. Something to consider for lowering operating costs of the palace.
Joe
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The US National Bureau of Standards was working 80 years ago on insulation problems (as understood in 1920.) The NBS still exists and it is hard to believe they have given up building research.
--
Don Phillipson
Carlsbad Springs
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What is a 40% R or 60% R roof. By defanition that might mean a 0.6 R". There is no 40R roof except 6" of R 7" Foam insulation as roofing. Are you talking foam.
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On 2/9/2009 7:20 AM Rob spake thus:

Let's look at it analytically.
Ideally, one would want a roof to be totally reflective in the summertime and totally thermally absorbent in the wintertime. Until they make roofs coated with magic crystals, this will remain an unattainable dream.
You need to balance the need for summer reflectance against winter absorption.
Let's look at the worst-case results of having the "wrong" roof surface.
In the winter: - snow may build up deeper than if roof were absorbent
In the summer: - more heat may enter the house
Consider the consequences of each. You say you only get a moderate amount of snow. So long as the roof structure is adequate for the maximum snow load, having more snow sitting on the roof isn't going to hurt you. In fact, it may actually help to insulate the house (think igloo).
On the other hand, having a dark roof absorb heat in the summer will definitely affect you, both in terms of comfort and financially, as you'll need to spend $$$ to get rid of it.
Therefore, I'd go with as light-colored a roof as possible.
Other comments here about appearance were pertinent. Choose a light color that doesn't look ugly and won't reduce the house's "curb appeal".
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If your roof heats up and a melts the snow, it may re-freeze in the gutters, then it will start to build an ice jam and backup. This often happens with homes that have poor ventilation/insulation in the attic. I would go with the lightest color that looks good.
If you have proper ventilation/insulation then I would GUESS that the color would have minimal influence on the roof temperature in the winter but slightly more in summer. Ask a shingle manufacturer.
Cheers, Jim

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Jim2009 wrote:

Good advice, but what looks good NOW may be horrid in a few months due to discoloration from airborne particles.
If considering a light-colored roof, drive around the neighborhood (city, maybe even state) looking for a something similar. This will tell you whether your choice might turn ghastly with time. If you can't FIND a light-colored roof, that might also be a clue.
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Rob wrote:

It's been six or seven years since I did much research about roofing problems. I did run across info about the heat on the surface of light vs. dark roofs. Dark=hotter, of course. I'm in FL, so the fact that cooler light roofs grew more mold was of interest. One thought that I believe I read but can't be sure, is that the surface temp didn't transfer to an important degree to the attic temp; i.e., didn't make much difference in the attic temp on the basis of light/dark shingles alone.
From doing a quick Google search, I think the answer is "It depends" :o) Climate, orientation, roof style, underlayment, insulation, etc.
Search on 'roofing color heat attic'.
Here is a pretty good link: http://www.peaktoprairie.com/?D 6
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If appearance is important to you, then choose what looks good. However, there is some prattle in these posts about 'curb appeal' which in my view is nonsense. If you're into 'flipping' houses or peddling real estate then curb appeal is like auto detailing, something to move the product. But if you intend to live in a house for some years (which may be mandatory given the economy) then paying attention to details that lower the bottom line operating cost will trump curb appeal every time for anyone with common sense. The Mc Mansion mindset is history and we all need to get over it. Change is on the way and it looks very austere.
Joe
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