Cooling metal roof

georgemdn · May 14, 2016

First of all i must say that i have lack of knowledge about physics. I have this silly idea to spray my metal roof in a humid area.

No lack of groundwater and im intending to sprinkle my metal roof with groundwater during solar peakhours. The roof is total only 30m2. I understand that thermal conductivity of metal is such that my roof will be cooler underneath if on top flow of cooler water is realised. By cooler i mean a temp difference of at least 10 degree C. So if i put a fan under the roof can the roof function as a sort of radiator and the fan blowing cooler air to the room?

Hope my question is understood.

Simon Bridge · May 14, 2016

You are thinking that the water carries heat off the top of the roof, cooling it down ... then the fan blows under the roof... um...
Perhaps you are thinking that there are holes in the roof - water flowing over the roof, and the fan sucks hot air through the cool roof, cooling the air in the process?

For the metal to cool anything, it has to lose heat faster than it gains it.
I don't think this process will have a noticable difference but you can easily rig up a proof-of-concept prototype on a smaller scale to see.

billy_joule · May 14, 2016

georgemdn said: ↑

First of all i must say that i have lack of knowledge about physics. I have this silly idea to spray my metal roof in a humid area.

No lack of groundwater and im intending to sprinkle my metal roof with groundwater during solar peakhours. The roof is total only 30m2. I understand that thermal conductivity of metal is such that my roof will be cooler underneath if on top flow of cooler water is realised. By cooler i mean a temp difference of at least 10 degree C. So if i put a fan under the roof can the roof function as a sort of radiator and the fan blowing cooler air to the room?

Hope my question is understood.

I've been in roof cavities that are over 50 deg C, (outside air temp ~30 deg C). Almost all that heat entering the cavity is conducted through the tin, which is too hot to touch. I often hose down my concrete patio & timber decking when it can gets too hot for bare feet, it works well and I think your plan could will work.

Coincidentally, I was installing simple heat recovery systems, which was just a fan or two and a series of ducts that would blow roof cavity air into the house. The fan would come on depending on the the set point, house temp & cavity temp. Generally, in summer that'd be at night when the cavity is cooler than the house, in winter it'd be the middle of the day if the sun has warmed the cavity above the house temp. I've lived in 4 houses with them, their effectiveness varies from house to house, depending on construction and environment, most of the commercial units (like the link above) are wildly overpriced for what they are.

georgemdn · May 14, 2016

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billy_joule said: ↑

I've been in roof cavities that are over 50 deg C, (outside air temp ~30 deg C). Almost all that heat entering the cavity is conducted through the tin, which is too hot to touch. I often hose down my concrete patio & timber decking when it can gets too hot for bare feet, it works well and I think your plan could will work.

Coincidentally, I was installing simple heat recovery systems, which was just a fan or two and a series of ducts that would blow roof cavity air into the house. The fan would come on depending on the the set point, house temp & cavity temp. Generally, in summer that'd be at night when the cavity is cooler than the house, in winter it'd be the middle of the day if the sun has warmed the cavity above the house temp. I've lived in 4 houses with them, their effectiveness varies from house to house, depending on construction and environment, most of the commercial units (like the link above) are wildly overpriced for what they are.

Thanks for your reply. Interesting.
Continiously sprinkling metall roof with (cool) water during sunny hours, thus not on basis of avaporation, will certainly cool the metal roof. Here an example of someone in thailand :

If the temperature of that wet roof is substantially cooler than the inside temp of the room, can for example ceiling fan(s) under the roof cool down the room when there is no attic at all. Can the colder roof function like a radiator?

georgemdn · May 14, 2016

Forgot to say that the guy of the example youtube vid circulates water. My plan is watering the roof with cooler groundwater.

Simon Bridge · May 14, 2016

If the temperature of that wet roof is substantially cooler than the inside temp of the room, can for example ceiling fan(s) under the roof cool down the room when there is no attic at all. Can the colder roof function like a radiator?

In principle, yes - it will act as a heat-exchanger.
You just need to be carrying heat away from the roof faster than it arrives... you should do better than the guy in the vid since you don't need to recirculate.

georgemdn · May 14, 2016

Simon Bridge said: ↑

In principle, yes - it will act as a heat-exchanger.
You just need to be carrying heat away from the roof faster than it arrives... you should do better than the guy in the vid since you don't need to recirculate.

Thanks again for your reply. I intend to repaint my metal roof with white reflecting coating. After coating i want to implement this silly idea of continiously spraying the roof during peak hours with a solar powered pump only when the panel gives a certain volt or amp, assuming thus its hot outside :)

If you think that the roof will work like an heat exchanger i will certainly give it a try.

Assuming outside temp with ful sun is 37c. Inside temp cause of passive cooling=31.
Groundwater is 24c. Roof is 30m2. What can the result be?? Volume of water not important for me cause its groundwater

Kevin McHugh · May 15, 2016

Don't forget about rust. Water and metals = oxidation. Especially with hard ground water.

georgemdn · May 15, 2016

Kevin McHugh said: ↑

Don't forget about rust. Water and metals = oxidation. Especially with hard ground water.

I have zinc aluminium coated with i think waterproof paint. Im not sure. But when i will (re)paint it white there is a special waterproof paint available.

Ground water will be filtered first. Im pumping now ground water to my watertank which is filtered. Incoming pipe is at the same level as the highest point of my roof. With small adjustments i can sprinkle the roof :)

Kevin McHugh · May 15, 2016

OK, evaporative cooling it is then

georgemdn · May 15, 2016

Im thinking of CONTINIOUS spraying the roof when its very very hot. Evaporation is a bit different. You sprinkle with time intervals.

Kevin McHugh · May 18, 2016

You will have both effects going on, evaporative cooling and themal conduction.

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Cooling metal roof

georgemdn

Simon Bridge

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