is this a dumb idea? Put a pool (even shallow) on a roof to reflect sunlight. (cool roof)

[suziedemocrat]

I read a lot about passive solar design, and in the South, 'cool roofs' that reflect sunlight supposedly go a long way toward keeping a building cool inside and reducing the need for air-conditioning.

On a recent airline trip, I looked out the window and noticed how much the sun reflected off the rivers and lakes. I thought, why not put pools, even shallow pools, on roofs to reflect sunlight in the summer, then drain the pools in the winter when you want to absorb the sunlight through the roof??

OK - tell me why that's a dumb idea.

[robinlynne]

1. white roof. people are doing it now. for that reason.

[ChazII]

13. Not a home but

school buses in the valley all have white roofs.

[hobbit709]

2. Water is heavy

62.8 lbs per cubic foot. Can your roof take that load. Plus once the water warms up, it will take a long time to cool off-even at night.

[n2doc]

3. leaks and weight

No rooftop pool will stay leaktight forever.

But paint, yes that would work.

[tridim]

7. Grass works too. I'd love to have a garden on my roof! n/t

[DesertRat]

14. Like this one?

[patrice]

4. Is it just reflection that causes cooling?

Or is it evaporation that pulls heat from the interior?

Reflection can be achieved with something much lighter than water and water rationing is a necessity in some parts of the country.

[donco6]

12. Yes. I think evaporation would absorb more than reflection, but

But all the other problems - weight, acidic nature of water over time, etc. etc. Reflective paint is the much more workable solution.

[dmkinsey]

17. My own gazillion dollar idea

involves a roof sprinkler, like in the produce department at the supermarket.
Turn it on a couple times per hour to mist the roof and evaporation would keep the house cool.

[patrice]

18. The mass of water makes it a major heat sink.

But that also makes it necessary to have a heck of a stout building to hold it up, concrete even, or there is a new product as strong as concrete, but made with hemp.

Water scarcity and hence price might become more of a problem with more global climate change.

[babylonsister]

5. Here's an article all about it-it can be a good thing!

http://www.washingtonpost.com/wp-dyn/content/article/2005/09/30/AR2005093000687_pf.html

[jebediah]

6. You'll get a pretty smelly, green roof quickly...

and water is *very* heavy. Lots of mosquitoes. The white roof is the best idea.

[Fovea]

8. Why Drain it?

Why not just cover it in winter with a black plastic (and maybe an insulator layer for night) and use it for solar water heating?

The main drawback is the weight. But for an earth contact house, this might be a reasonable approach. Some earthships use a reflecting pool/water garden inside the front window to disperse light and heat rearward at midday.

[Fresh_Start]

9. radiant barriers on roofs

we added one when we reroofed two years ago

[damntexdem]

10. It's a wonderful, wonderful idea ...

until the leaks start.

[The Velveteen Ocelot]

11. It's not a dumb idea, but I do see a few problems --

First, water is heavy, so the roof would have to be able to carry the weight. Second, most roofs of private homes are sloped because flat roofs tend to leak. A roof "pool" would have to be on a flat roof, like on an office or apartment building, and it would have to not leak. Also, there would have to be a way to keep replenishing the water as it evaporated, and somehow it would have to be kept free of algae and crud.

Maybe the concept could also be executed by using something shiny, like tinfoil? But then you might have bright sunlight reflected into other people's windows.

[jedr]

15. standing water will ruin a roof;

use solar panels to pre-heat water in a closed system and feed it into your hot water tank. More expensive but a much better system.

[TreasonousBastard]

16. Water absorbs heat and...

is heavy, leaky, evaporates, and stinky stuff grows in it.

For years there have been solar water-heating panels that circulate water to preheat the water for the hot water tank and even heat the home, but they're expensive and don't cool the house much in the summer.

There's always been swamp coolers, too, that use evaporation to cool the house, but they're only worthwhile in desert climates.

In northern areas, deciduous trees to shade the house in summer and light, reflective roofs would be preferable to putting a couple of tons of water on the roof.

Best thing would be the roof garden, greenhoused if up north, if the building will support the weight and you've got the money to put one up there.

[patrice]

19. You make another point. The evaporation cooling would work best

only in dry climates.

[AZCat]

22. Which is ironic...

because those climates are the ones where water is usually conserved. We struggle with that here in Tucson, Arizona, because our electrical loads during the summer are outrageously high, but using evaporative coolers just swaps one problem for another. Nobody seems to want to change the way the buildings are actually built.

[patrice]

23. If I were to build again, it would absolutely be earth-bermed at minimum

and underground if possible. I think I'd prefer that even to straw-bale, which I think LOOKS just great and must have fantastic r-values.

Have you heard anything about that new hemp product http://www.treehugger.com/files/2005/12/hemp_houses_for.php ? I haven't checked yet to see how it does in an earth contact situation.

[AZCat]

24. I haven't seen this yet.

Bermed, rammed-earth and straw bale walls all have good R-Values, but depending on your locale one may be superior to another. Here in southern Arizona we tend not do dig much because of the widespread presence of caliche so a lot of conscientious home-builders use rammed-earth or straw bale walls instead of berming. We also get a few adobe buildings, but it is much more expensive and I don't know if the gains are significant (I haven't done a calc for one in a while).

[Tandalayo_Scheisskopf]

20. The guy out back of my apartment...

Had flat roofs, in basic black. He had to have something done, as they were leaking both water and heat. He found a guy who put on a binary polyurethane foam. Went on like tar then expanded. After it cured a couple of days, the guy came back and put a silver sealer on the top.

The guy out back said the leaks went away and the house was about 10-15 degrees cooler in the summer. In the winter, his heating bill went down rather smartly.

[IDemo]

21. You're on the right track, but the water will work better for you underground

In a passive geothermal system, the water is kept below ground, at a depth where the earth stays at about the same temperature year round. By using water at, say 55 degrees, you can both pre-heat air entering your heater in the wintertime, and pre-cool air entering your air conditioning in the summer.

These systems can be a tad pricey to install, but the cost recovery time should become increasingly quick with rising energy prices.

[Ready4Change]

25. Mosquitos?

I'd think silver or white reflective paint would be more effective, and not become a breeding ground.