Just saw one of Cecil’s columns about the benefits of a lighter colored roof. Over here in Australia, someone is on their way to a few million bucks, and helping to reduce our dependence on fossil fuels at the same time, because they have a system of pumping warmer , but filtered, air from your roof space down into your house during winter which will cut your heating bills substantially. It’s called Heat Recovery Ventilation. Mind you, it probably won’t work very well in a very cold and snowy winter like New York’s, but in autumn and spring it would work just fine.
Conversely, in summer, after your roof space has cooled down at night, it will pump the cooler air into your rooms so that if you keep your doors closed during those sweltering days, it is just like aircon… to a degree.
And although the set-up cost isn’t cheap, the running costs are a very small fraction of fossil fuel heating and cooling, and after a few years it will pay for itself in reduced power bills. After then, it’s all savings. Unfortunately, it is really only practical for single houses and not multistorey apartment blocks. Sure wish I owned the patent! Google for hrv.com.au to find out more. Makes use of your warm roof when it’s cooler indoors - solar power - and your cooler roof when it’s hot indoors.
That sounds painfully obvious (in a “Why didn’t I think of that?” kind of way), but pretty nifty. I assume running a few fans to circulate the air as needed is much cheaper than running the compressors that (I think) ACs would require. Anyone have a rough guess as to how much a heavy duty fan costs energy-wise as opposed to a central air system for a house?
But after that, he just goes on to talk about asphalt roofs and pretty much sticks to light versus dark colors. I kind of wonder if Hyperglass paint is just a scam and the glass is pointless; the white (or light) paint is doing all the work.
My understanding from what I saw during the research for this column (which I helped with) is that the Hyperglass paint does work well, but there are other paints which work about as well (and which may be cheaper). There are a few places on the Net where different brands, types, and colors of paint are compared and contrasted. See for one example this link: http://eetd.lbl.gov/coolroof/coating.htm#white%20coatings
Metal roofers claim that Kynar based paints absorb sunlight and re-emit the heat as IR into the atmosphere. Even dark colors do this. There is even an energy-savings tax credit for such roofs with Kynar.
Is this really true?
Really, I don’t get it. I pop in to this forum to tell you all about a fantastic new way to reduce your energy bills that will, in the long term, be much more effective than painting your roof and only one of you, the first, gives any indication that you have even read my post. The rest of you ignore the main thrust of my message entirely. You have no curiousity? I must have failed with the title by not calling it “Save big $ by using your roof space as a heater and cooler”, I guess.
Do you have any views about it other than to deride me for drawing your attention to it again?
MeDrewNotYou, this system costs about 10 cents a day to run (ie less than $50 a year) and it does have fans, of course, but the best thing is that the air is filtered and extremely clean.
One thing not covered in Cecil’s article, which is an important caveat, is that the effectiveness of a reflective roof is greatly reduced ironically by having a well insulated attic. IOW: The energy savings from having a white roof will be far less dramatic if you already have the recommended amount of insulation vs. little or none.
Add to that the fact that increasing the insulation will be beneficial in reducing energy costs in both summer and winter and that it’s usually far less expensive overall to add insulation than re-roof or coat an existing one. Also the insulation will require little or no maintenance to maintain its efficacy, unlike a white roof.
As Cecil said, if it’s time to re-roof, and you and/or your HOA don’t object to a blindingly white roof, go for it and enjoy some extra energy savings. Otherwise, you’ll likely spend more on the retrofit than you’ll save in energy usage over the life of the roof.
Personally, as I’m looking into setting up a solar water heating system on the roof anyway, that would be at cross purposes with having a reflective roof.
In the Winter my roof space is anywhere from 50F to below freezing. I know this because I’ve spent long hours up in my loft. And yes my roof is dark brown/black. How does <=50F air heat my house effectively?
First off, it provides absolutely no dehumidification, which is a principal part of air conditioning. I live in an area which is notorious for humidity above 50% combined with 90F+ days. Second, my roof space at night does not get down to less temperature than the ambient air - ever - so I’d be better off just opening a window…which I won’t do, as in the summer here it can be 80F at night (until the very early morning) and 60% relative humidity. Talk about sleeping in a pool of sweat.
So no, I did not ignore your post, I read it and said “I doubt that method will work at any place in my country with even remotely similar atmospheric and climate conditions.” It’s not a fantastic nor unique way to reduce energy bills in the way in which you have described it, for the climate region I (and I’ll hazard, many others) live in.
This is really nothing new at all; similar heating/cooling methods were tried in the 1890’s-1930’s, as I learned in engineering school in my HVAC course. It would be sort of interesting to see what is so unique about this configuration that it’s patentable…link to the patent?
OK, I was going to post this on Friday, but I had to register. I didn’t know the boards were free again.
I think this is another column that will be attributed to “school of” Cecil Adams, as it has two major problems. (IMHO)
The space reference is misleading. Absorption and radiation are characterized by the same constant (emissivity,) so a surface that is good at absorbing is also good at radiating. In space a material that absorbs heat less effectively will reach higher temperatures since there is no convection and it can only shed the heat through radiation. On a roof, the opposite is true and a surface that absorbs heat slower will stay cooler since convection can keep up with the energy absorption before the temperature gets high enough to make radiation a major factor. The column was misleading since it implies that white paint is good because absorbs so little, but it is actually absorbing more than the polished Al.
As mentioned in a previous comment, the advantage of a reflecting roof should be secondary to first providing a well insulated attic space. There are a lot of roof coating products available but most of that is targeted toward commercial buildings with flat, difficult to insulate, roofs. Most residential buildings have ample attic space and this is usually the most effective way to reduce home heating/cooling costs. Optimum roof color should be insignificant once the attic is properly insulated.
Una- Maybe the humidity issue (which I didn’t know/realize AC handles, I just turn the damn thing on ) isn’t as big a deal in the ‘inventor’s’ Australia. Doesn’t do us a lot of good in the Midwest, but… Otherwise naive ignorance fought!
The Lawrence Berkeley Laboratories testing shows that aluminum roofs have a net effect of increasing the inside temperature relative to the white-coated roofs.
If white reflects 0.85 and aluminum 0.61, then the white roof is absorbing more.
Moreover, the aluminum roof has an infrared emittance of 0.25, whereas the best white roof one of 0.91. That’s a very significant difference which you need to consider.
This assumes you have a significant attic space to begin with. While many single-family homes and businesses do, many do not. My previous home had essentially a 6-inch space between the inside drywall roof and the outside shingles. The one before that about 8 inches, maybe 12, with no practical way to get inside and add insulation.
Cecil’s column was focused on a single subject as well - the reflectance of the white roofs versus other colours and materials. Your point is valid, being in effect that a complete energy solution must look at many things, from the efficiency of your HVAC to foliage and landscaping to insulation to roof materials and colour. In this instance, these items were really outside the scope of the column. If several folks write to ask Cecil more questions to research regarding total home energy use et al, then he’ll probably look into it. Or at least I’ll suggest that he does; you never know.
While it’s true absorption and radiation are both characterized by emissivity, emissivity varies with frequency. White paint can be designed to be reflective at optical and radiative at IR. Even thought the white paint absorbs more light than the aluminum, it can be much more efficient at radiating IR, and stay cooler.
I’d imagine there’s also the roundabout way. If your house doesn’t heat up much, you don’t need to run the AC, creating less of a demand for electricity. Lower demand means less is produced, leading to fewer greenhouse gases emitted.
I’m dubious about the purported advantages of painting roofs (rooves??) white. The reflectivity of energy back into space assumes that roofs would send the reflected light straight up, but most roofs are very slanted and would send the light/energy through quite a bit of atmosphere, warming that.
Also, roofs constitute an infinitesimal percentage of the earth’s surface.
I’ll have to look up the actual article Cecil references.
) isn’t as big a deal in the ‘inventor’s’ Australia. Doesn’t do us a lot of good in the Midwest, but… Otherwise naive ignorance fought!