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Re: white rooftops = lower cooling bills
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Newbie
 
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2013/8/4 3:10 Last Login : 2013/10/4 2:04 From Jersey City, NJ
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Add to that the fact that "green roofs" are typically of what they call the "extensive" type and are built with just a thin layer of soil and used low-growing ground cover type plants that have little or no woody parts, so if they do happen to blow off in heavy winds, its like leaves falling to the ground. Not all have had great success rates with plant survival and often need replacements, but it is an evolving field. You still have some runoff reduction with a thin layer of lightweight soil on a roof.
Posted on: 2013/8/4 15:43
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Re: white rooftops = lower cooling bills
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Home away from home
 
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2005/11/12 17:04 Last Login : 5/7 14:26 From Downtown JC, VVP Area
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Quote:
A 5-gallon bucket will do almost 2 coats on a 12.5 x 35 roof, and more than a full coat on a 25 x 45 roof. Want to buy yourself another 5 years? Hit the roof with tar on ALL spots that look the least suspect. Wait a week. Paint this stuff on and relax but check it annually since you know it's getting near the end.
Posted on: 2013/7/12 21:24
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Re: white rooftops = lower cooling bills
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Home away from home
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2005/11/12 17:04 Last Login : 5/7 14:26 From Downtown JC, VVP Area
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If you decide to to a tear-off of your asphalt roofing and replace it with a membrane rubber-type coating, the common practice is to use no vapor barrier on the bare wood roof surface, just nail down closed-cell insulation foamboard (comes in different thicknesses and basically thicker is better but slightly more expensive) then the membrane is installed with a big propane torch to REALLY seal the seams. Then apply silver/white/whatever coating and plan to replace it every 5-7 years. Maintained like that, a membrane roof will outlive you! There's probably no insulation at all between your rafters if it's the original inside top floor ceiling. Foamboard nailed on top is a lot cheaper than ripping up all the roof boards, filling between the joists (from the top) and then re-sheathing with plywood before you install the membrane. Been there, done that twice...
Posted on: 2013/7/12 21:14
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Re: white rooftops = lower cooling bills
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Home away from home
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yes...
I will try to dig out the info and arrange to email it to you... Have to be over the weekend though.. I am under a deadline here at work.
Posted on: 2013/7/12 16:07
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Re: white rooftops = lower cooling bills
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Home away from home
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I'd be nervous about that kind of weight on a regular floor! I've seen too many massive old timbers sagging. Back to roof insulation: so you're saying for the typical old flat roof they no longer say vent, just insulate with cellulose and no vapor barrier?
Posted on: 2013/7/12 15:36
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Re: white rooftops = lower cooling bills
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Home away from home
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When my place was renovated, the roof was replaced by engineered 2x10's, so it can hold what a regular floor holds. One of these days I might get enough free time to finish the rooftop hydroponics and greenhouse.
Posted on: 2013/7/12 15:23
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Re: white rooftops = lower cooling bills
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Home away from home
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Quote:
Quote:
I like how both of you missed the part of the original comment that I was chiming in with that specifically mentioned "large scale developments". Your brownstone ain't it, don't worry about your 2x4 support beams. Green roofs are mostly for skyscrapers since duh, that stuff is heavy.
Posted on: 2013/7/12 15:02
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Re: white rooftops = lower cooling bills
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Home away from home
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ASHRAE dropped venting requirements for attics. After doing real-world testing and analysis, it was found venting actually causes moisture problems rather than removing them.
Super-insulated residential buildings (where the code official permits this to be done) are built without venting. ASHRAE also changed their approach to vapor barriers, which is something the code has not caught up with. If you use a good insulation like cellulose, no vapor barrier is then installed on the interior walls. Somewhere on my computer I have the ASHRAE study for the walls.. and maybe the attic..
Posted on: 2013/7/12 14:55
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Re: white rooftops = lower cooling bills
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Home away from home
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I thought venting a roof was currently code? You insulate the joists of the apt ceiling at least 10" of cellulosic and the attic area vents clear.
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Posted on: 2013/7/12 14:35
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Re: white rooftops = lower cooling bills
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Home away from home
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Here is an excellent product I use on my flat roofs:
http://spec-chem.com/shopping-cart/pr ... ve-white-plastic-coating/ It is water based and is a lot more effective than the traditional silver coat. It keeps my roof deck area cool enough that I can walk on it in bare feet on a hot sunny day. If you have a really well insulated roof assembly (don't use fiberglass or rock wool... neither are good insulators in this type of application), say a non-vented roof with R-40, white roofs don't make that much of a difference energy bills wise. White roofs have a huge effect if you have a poorly insulated roof though. I still use it on my well insulated roofs because it makes the roof deck last longer. You don't get as much movement (thermal growth) from the roof deck hitting 140 F+.
Posted on: 2013/7/12 13:19
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Re: white rooftops = lower cooling bills
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Home away from home
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I agree with you that a white roof will have a net positive benefit on a building?s energy usage, and certainly the utility bills, but greenhouse gasses and soot in the atmosphere will absorb some of the reflected visible light and infrared on its way back to space, just like they did on its way to Earth. Reflectivity is not totally benign. Unlike a layer of snow?which is a good insulator because it?s full of air?there shouldn?t be much of change in a roof?s thermal transmission characteristics associated with white paint. (I suppose it would have an effect on emissivity though.) In winter time, the solar heat gain on a white roof would be reduced just as it is in summer, while the heat transfer out of the building through the roof would be unchanged. So now, we have the same amount of heat escaping, but less heat being generated. (Then again, if white paint reduces emissivity, less heat would escape the building.) Technically, that should result in greater heating demand, but whether it really does in practice, and if so how much, is another story. One would also have to look at how roof heating in winter affects convection within the structure, and the impact it would have in infiltration through the envelope, which we know is a major source of heat loss.
Posted on: 2013/7/12 13:16
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Re: white rooftops = lower cooling bills
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Home away from home
Joined:
2007/9/1 14:14 Last Login : 2021/3/15 20:01 From Downtown - H.P. area
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Quote:
What's the coverage of this stuff on an asphalt roll roof that's never been painted? At $79 per bucket, it could end up being a waste of money, since my roof only has 1-2 years left. At some point I plan on replacing it with a IB Roof Systems membrane, where the material itself is white.
Posted on: 2013/7/12 13:15
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Re: white rooftops = lower cooling bills
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Home away from home
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Quote:
Do not forget to retain a good lawyer just in case those things fly off in a strong wind and hurt someone.
Posted on: 2013/7/12 2:33
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Re: white rooftops = lower cooling bills
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Home away from home
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2005/11/12 17:04 Last Login : 5/7 14:26 From Downtown JC, VVP Area
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SW29,
We could probably get into a long debate over a couple of beers on this but, simply put, diurnal temperature variation is a function of conduction, meaning solar radiation from the absorbing surfaces (roads, roofs, bare dirt, etc.) being passsed to the air levels above it but this air-to-air transfer of heat is VERY inefficient. Reflectivity causes NO harm and no foul. The silver paint reduces the heat in the rafters in the summertime without question. But to say it results in heat loss during the wintertime is basically wrong. Any snow on the roof is actually an insulator from heat loss (and the roof color doesn't mean squat at that point) and if there's no snow and your roof is black then yes you get some winter heat in the rafters but that's where all the heat from your furnace is ending up anyway, so your boiler is still cranking away to keep the 1st floor comfortable. And I cracked up on the green paint joke and I agree with Cory about roof strength before considering a couple of tons of soil and vegetation.
Posted on: 2013/7/12 0:17
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Re: white rooftops = lower cooling bills
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Home away from home
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It was a joke. But seriously... the gains from a white roof you see in summer won't match the heating losses in winter. Not to mention extra moisture, and a hotter atmosphere (see the Stanford study). Green roof? Hope your 100+ year old rafters are sistered.
Posted on: 2013/7/11 23:37
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Re: white rooftops = lower cooling bills
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Home away from home
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Quote:
By green, I'm pretty sure he means that you plant stuff on the roof. Grass, some small brush and a few flowers help insulate/cool incredibly well, reduce noise pollution, reduce the heat reflection, reduce the loads on HVACs, sewage, gutters, etc. Most water gets stored in the substrate so there's not a lot of maintenance needed either.
Posted on: 2013/7/11 23:30
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Re: white rooftops = lower cooling bills
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Home away from home
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Quote:
I don't see how painting the roof green does anything?
Posted on: 2013/7/11 23:20
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Re: white rooftops = lower cooling bills
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Home away from home
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Quote:
http://www.climatecentral.org/blogs/w ... y-increase-global-warming http://www.huffingtonpost.com/samir-i ... green-myth_b_2901288.html http://news.stanford.edu/news/2011/oc ... -heat-islands-101911.html http://www.fastcompany.com/1790991/pa ... ur-roof-white-doesnt-work
Posted on: 2013/7/11 23:16
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Re: white rooftops = lower cooling bills
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Home away from home
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Quote:
link?
Posted on: 2013/7/11 23:06
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Re: white rooftops = lower cooling bills
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Home away from home
Joined:
2005/11/12 17:04 Last Login : 5/7 14:26 From Downtown JC, VVP Area
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Really an easy DIY job if you can carry a 50 lb. bucket to the roof or have a stronger friend. And it's maybe $70 for the materials vs. 500?+? for a roofer to do it. Buy a 5 gallon can, schlep it up to the roof, stir it with a broomstick or similar long pole for about 20 minutes (this stuff is THICK, so stir it a LOT), then pour a puddle and use a paint roller on a long threaded-end pole. Takes about an hour to do the whole thing on a typical roof. It lasts about 5-7 years before you need to re-coat it. It does make a difference in the heat on your top floor and cooling costs. Works great for both asphalt and rubber type roofing material. Done this many times over 27 years and 3 houses...
Posted on: 2013/7/11 21:34
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Re: white rooftops = lower cooling bills
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Home away from home
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Another thing is green roofs on large scale developments.
Posted on: 2013/7/11 21:29
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Re: white rooftops = lower cooling bills
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Quite a regular
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Just had ours done - it's life-changing (I don't call our 2nd floor "the inferno" anymore). We also added roof vents and insulation, so all together a huge difference.
Posted on: 2013/7/11 21:21
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Re: white rooftops = lower cooling bills
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Home away from home
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2012/9/18 3:58 Last Login : 2021/9/23 15:07 From Between Thought and Expression
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Huh? Been doing it for years...White roof silver coating.
Posted on: 2013/7/11 21:04
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Re: white rooftops = lower cooling bills
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Home away from home
Joined:
2007/9/1 14:14 Last Login : 2021/3/15 20:01 From Downtown - H.P. area
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Anyone done this themselves? I have a black asphalt roll roof that makes the top floor hot as Hades and am considering doing it.
Posted on: 2013/7/11 20:47
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Re: white rooftops = lower cooling bills
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Home away from home
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The problem with white roofs is that the light reflects off the building and into the sky, leading to increased heating of the atmosphere. This "double diurnal heating" situation causes people to use more air conditioning, and can actually cause more thunderstorms because of convective instability.
Posted on: 2013/7/11 20:36
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Re: white rooftops = lower cooling bills
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Just can't stay away
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The developer of my building did, but it's all peeled and cracked now and looks pretty ugly. I'll have to redo it.
Posted on: 2013/7/11 20:30
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white rooftops = lower cooling bills
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Home away from home
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why aren't more cities and homeowners doing this?
http://www.takepart.com/article/2013/03/19/white-roof-project
Posted on: 2013/7/11 20:20
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