Grandpa used to do it...if you don't know, old timers used to water down the roof on their house when it was hot. Anyone think this really works? Explain the logic?
Evaporation of water cools the roof (just like sweat on skin), which in turn reduces the heat radiated into the attic from the bottom side of the roof decking, reduced attic temp means less heat conducted through the ceiling into the conditioned space.
These days though, water is so expensive and places that need AC more often have it, so there's probably not much call for it now.
Run the water in your tub and jump in. 
A forced air attic fan does wonders. Thermostat and humidistat control if you live up north.
On really hot days coming out of an attic into 100 degrees feels like stepping into AC. So pull that hot air out.
A forced air attic fan does wonders. Thermostat and humidistat control if you live up north.
On really hot days coming out of an attic into 100 degrees feels like stepping into AC. So pull that hot air out.
I actually did this just earlier this evening and it does help. I do it when we get these random heatwaves out of nowhere. We went from rain/60s to 97F in the span of a week and it feels crazy warm. I don't have central air and the house gets pretty nasty since the hottest part of the seems to be 4-8pm. The dinky window AC in my bedroom doesn't seem to cool below 80F and I'm hoping we get some cooler weather for a few days after this. I'm really not liking the summers we have had past few years, with last year being the worst off them all.
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There are two ways water can help cool a roof. First would be simple conduction - cool water on a warm roof lets heat energy transfer go from high to low (so roof —> water), which sheds off (depend on roof style of course), transferring some heat away. The second, and more effective way, would be through evaporation. Like what @freedom_in_4low said, it could (in theory) work a lot like sweat on your skin. if the roof, either by design or materials of construction, were able to hold onto the water for a long enough time to allow for evaporation, the reduction in energy via latent heat transfer (converting liquid to vapor) would be effective. This is why people in the southwest use "misters" so often on patios and why evaporative coolers can be a economical but effective system for cooling a house down.
As @InOmaha said, putting hot air out of your attic will make significant improvements through the use of a simple attic fan (I'm about to install one in mine, even though we have A/C).
As @InOmaha said, putting hot air out of your attic will make significant improvements through the use of a simple attic fan (I'm about to install one in mine, even though we have A/C).
Brianj5600
TJ Expert
How much does humidity affect cooling the roof? It sounds like humidity is high there.
in the same temperature, it would be much more effective in low humidity. same reason there's a lot more houses with swamp coolers in the southwest than in the southeast.
For example yesterday, you would have gotten slightly more cooling effect from water evaporation in Phoenix than you would have in Baltimore, in spite of Phoenix being 20 degrees warmer.
that would be the trick. I suspect asphalt would hold the water longer, but there's a bunch of mass there.
I've had radiant barrier roof decking before and am using it again in the house we're building. I still didn't want to hang out up there but it made it a lot more tolerable when I was up there running wiring or whatever.
I didn't mean this to indicate you actually get the roof cooler in phoenix. There are other factors, such as the radiation gain from the sun, which was likely more intense in Phoenix, and the fact that it was 20 degrees hotter is adding heat, likely more than one would have gained by the extra rate of water evaporation.
It all ends up being a balance where the roof ends up finding a temperature where heat in = heat out.
Heat in = convection from surrounding air, radiation absorbed from sunlight
heat out = convection into attic air, radiation to lower attic surfaces, and potentially water evaporation
Thanks for all the replies..im in the PNW and we have a few hot days here and there. We have central ac but yesterday was nearly 100 degrees and we were struggling to keep inside Temps below 75.
I did clean out the evaporator coils last night. I will look into the attic fan today.
I did clean out the evaporator coils last night. I will look into the attic fan today.
at least around here, 75 is usually what they use as the interior temp to size the unit on.
The American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) publishes a table of weather patterns for basically every major city that looks like this:
A typical house AC is going to be designed for the DB numbers under the 0.4, 1%, or 2% column, which is the temperature only .4, 1, or 2% of hours exceed. If you look toward the bottom of the table you can see the annual, 5, 10, 20, and 50 year extremes. No one sizes units based on the extremes because you'd waste money on the initial purchase of an oversized unit, and that oversized unit would run less efficiently (cycling on for just a few minutes is not enough time for the system to stabilize) for the 99.6% of the time that you don't need to cool the house against a rare extreme. Not reaching below 75 on occasion does not mean you're equipment is undersized or that anything is wrong unless it's happening more than a few of the hottest days of the year. But keeping your coils clean is always a good idea because anything built up on there is insulation preventing heat transfer from taking place.