How to cool this HOT house???
April 28, 2012 9:26 AM   Subscribe

Just replaced a 50-year-old tar/gravel roof with white shingles. This is in Florida. But now the house is hothothot, heat keeps rising, air-conditioner runs constantly but can't keep up! It's just after noon and 84 degrees in here! The roof is low-hip, the attic is a crawl space insulated to about R-30, there's a turbine on the roof and three passive vents. Still hot! Depressing after writing such a big check. Capable roofer says previous thick layer of tar/gravel probably insulated a lot. But now what's my next step? 1. Would a white latex coating be more cost-effective than a solar-powered vent? 2. Any other suggestions? Whew, and thanks!
posted by fivesavagepalms to Home & Garden (16 answers total) 2 users marked this as a favorite
Install a radiant barrier on the underside of the top chord of the trusses, or on the underside of the roafters, whichever you have. I would try to use the foam boards with aluminum foil on one side, like these. I might be a bit difficult to install now (i think the ideal time/place is under the shingles but above the roof sheathing)but it is possible to retrofit and still get most of the benefits. Also a whole house fan should provide a lot of relief. Whatever you do you want to make sure it doesn't stop the roof/attic space from being adequately ventilated in such a warm/humid climate.
posted by bartonlong at 10:12 AM on April 28, 2012 [2 favorites]

Look into a radiant barrier in the attic, and a powered vent (I ran one for years before we went to R-24 foam in a cathedral ceiling). Improving your ceiling insulation (perhaps with an integral radiant barrier) works also, but the radiant barrier aspect is important, as fiberglass bats are distressingly transparent to infrared.
IAMYRoofer, but I don't think a white latex coating on an already white roof is going to make much of a difference.
posted by the Real Dan at 10:14 AM on April 28, 2012

Make sure the turbine & vents are still open and working.
posted by jon1270 at 10:18 AM on April 28, 2012

He's probably right, the gravel probably absorbed a lot of the radiant heat and then radiated it away again that the new shingles aren't able to do.

The other thing is that while it's usually the case, just because a material is reflective in the visible spectrum doesn't mean it is reflective in the IR spectrum. So unless the new coating is specifically designed to be heat reflective and insulative, it probably won't make much difference.

So yeah, radiant barriers inside the attic are probably the best/cheapest solution.
posted by gjc at 11:01 AM on April 28, 2012

I'd bet your new white shingle roof is actually better at reflecting heat than your old tar-gravel roof, and no worse as an insulator.

However, some scientists claim that a black surface is much better than an equivalent white surface at reradiating the heat it has absorbed, and in your case the tar-gravel roof probably had considerably greater surface area for that reradiation process. The tar roof would also have absorbed the heat from the house better, but this effect was probably attenuated by your insulation.

I'd guess water plays a role in the difference you're feeling as well.

Tar-gravel would hold on to a lot more residual water from rain, and water retention coupled with its ability to cool faster from radiation would mean it collected and kept a lot more condensation overnight than your new roof can.

Then when the sun hit, that water would evaporate, and evaporating water has tremendous cooling power.
posted by jamjam at 11:07 AM on April 28, 2012

Remember seeing "Lawrence of Arabia"? Didn't you think it was strange that all those people were walking around in the blazing sun wearing black?

It turns out that black clothing is cooler to wear than white clothing. True, the white clothing reflects more sunlight, but it also traps IR from your body. Black clothing radiates heat away better.

You just replaced a black roof with a white one, and the same principle is in play.
posted by Chocolate Pickle at 12:02 PM on April 28, 2012

It turns out that black clothing is cooler to wear than white clothing. True, the white clothing reflects more sunlight, but it also traps IR from your body. Black clothing radiates heat away better. You just replaced a black roof with a white one, and the same principle is in play.

Things are more complicated than that. Black clothing is not always cooler than white.

What matters is whether the surface of the clothing is cooler or hotter than the ambient air and whether the air is moving. In still air, a black surface will absorb more heat than a white surface. If the wind is blowing and is of lower temperature than the surface, then a black surface will dissipate more heat than a white surface.

So, for example, a white car will be cooler than a black car sitting in the parking lot. However, the radiator is painted black so that it dissipates heat more quickly when air is passing over it. The difference in the two cases is still air vs moving air.

In the case of the Bedouins, they generally live in very windy places. (Those sand dunes didn't get there by accident.) So black clothing may work to keep them cooler. The opposite would be true in a less windy environment.

Under most conditions, unless you are in an area of steady winds, a lighter colored roof is better than a dark colored roof to reduce heat.

Ideally, you would want a white roof during the daylight hours to reflect heat and a dark roof at night to radiate heat, sort of a switchable LCD surface. However since in summer there are more sunny hours than dark hours, a white roof is the best compromise.
posted by JackFlash at 1:15 PM on April 28, 2012 [1 favorite]

Thank you, and I stand corrected.
posted by Chocolate Pickle at 6:56 PM on April 28, 2012

Capable roofer says previous thick layer of tar/gravel probably insulated a lot

Well, yes, but with Florida building codes, you have that new extra layer of water-proofing material added in now as a requirement, so I wouldn't think the difference would be that great. Of course, all the wind-sheer requirements also mean that your roof is so well-secured that you may be keeping more heat in your house.

We just had a new roof put on, too, and we've had to turn up the a/c, but that's because our a/c needs replacing. Since you have an older house, is it possible yours needs servicing or replacement, too?
posted by misha at 7:39 PM on April 28, 2012

Agreeing with misha, I'd have a capable AC tech come out and examine your AC to see how well it's working. The new roof might not be the cause of your 84°.
posted by exphysicist345 at 9:09 PM on April 28, 2012

How white are the shingles compared to a white coated roof? I have a white-roof (I'm in the desert, and it's routinely 105+ in the summer here) and these things lose efficiency if they get dirty. White shingles are likely not pure white, but more of a white gritty surface. At least if they're the asphalt type I'm thinking of.

That said, I'd be hesitant to coat a brand-new roof just yet until you've tried other things. A whole-house fan would be a very good idea. Passive vents can help, but the fan will move a lot more hot air. Also, as mentioned above, check your AC unit.
posted by azpenguin at 10:29 PM on April 28, 2012

Given the old roof was gravel/tar, I assume this is a flat roof with roll roofing and not 3-tab shingles. I have had success coating whitish roll roofing with this reflective elastomeric material. Though I didn't bother to objectively quantify the temperature difference, my house was noticeably cooler after application. As I have trees overhanging the house that deposit a dark grime, I do have to topcoat every couple of years to maintain the effectiveness.
posted by ferdinand.bardamu at 1:13 AM on April 29, 2012

After studying the problem in the hot sun yesterday I've concluded it's this:
There's an addition on the back of the house, amounting to about a fourth of the square footage. It's flat, with tongue and groove paneling on the ceiling inside and probably little room for insulation between that the the actual roof--maybe six or eight inches. The roofers put gritty composition roll roofing on that section, which amounts to three rooms on the hot south side. If my assumption is correct and there's little insulation there, that's probably my problem--I think this is the part that had thicker tar/gravel before.
So now what?
Take up the roll roofing and put down some kind of insulation board, then replace roll roofing and possibly paint it with the white stuff mentioned above?
Anybody have better insight?
Thanks again!
posted by fivesavagepalms at 8:35 AM on April 29, 2012

Oh, and if so, what kind of insulation board would you expects recommend?
posted by fivesavagepalms at 8:36 AM on April 29, 2012

I think you've reached the point where you need a professional to visit and offer advice.
posted by Chocolate Pickle at 5:58 PM on April 29, 2012

Just learned that the roofer's helper put a nail through an a/c line and drained the coolant. About $800 to repair. Roofer and I agreed (amicably) on a price adjustment. It's now 77 degrees in here! Nice!
posted by fivesavagepalms at 9:14 AM on May 2, 2012 [2 favorites]

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