Increasing air flow rate in a ventilated roof

[Todrianth]

Hello!

I had an idea to increase the air flow rate under a metal roof by painting the back layer of the metal roof sheets that are at the upper part of the roof in black (see pic). Therefore the upper part should be slightly hotter than the lower part (since I'm guessing radiation passes through to the back of the metal roof) therefore creating a higher buoyancy. Sorry if this is a stupid idea, I'm not that good at h&m transfer that's why I'm asking here . Any cheap ideas on how to increase the air flow are welcomed . Thanks!

 

[willard3]

Hot air rises all by itself.

 

[chicopee]

I would rather paint the undersurface of the steel roof white to lessen heat transmission thru the wooden support if that is your reason to increase the air flow.

 

[MintJulep]

To increase flow increase the gap between roof and tiles.

 

[Todrianth]

Thanks all for your responses!


I know but wouldn't airflow be increased if the upper part would be hotter than the bottom part as opposed to constant temperature over the entire roof?


You're right! To increase the temperature difference even more I can paint 6/7 in white (bottom) and 1/7 or less in black (upper part) and also decrease the heat transmission as you said on 85% of the roof.


So maybe I can get thicker or just put two vertical wooden battens on which the horizontal battens sit, therefore increasing the air gap.

Another thing, since I need to put anti-condensation sheet all over the roof I was also thinking of putting 1/7 aluminum foil(AF) at the upper part as-well. I wanted to put AF all over the roof but the roof producers told me that the wood has to "breathe" through the anti-condensation sheet, so therefore I cannot block it with aluminum sheet all over due to mold formation. But I'm guessing that narrow and long parallel rectangles of aluminum sheets with space between them should let the wood underneath release it's moisture through there and still reflect most radiation in the back of the black paint part of the metal roof.

 

[trashcanman]

How do you know how much air flow you will get (CFM)? A much simpler and cheaper method is to install a roof exhaust fan and be done with it. You will at least have a qualified number for air flow.
Even if the gravity ventilation works, it is totally solar dependent and will vary during the day and almost stop at night or on cloudy days. It will also vary with local winds.
How would you even test it to see how much air flow you get?

 

[MisterE2]

Why would you need to? That type of roof tile is designed to have minimal contact with the roof and not to expand to knock the other tiles off.

 

[IRstuff]

What's the actual objective? Why do you need more air flow, which is not even possible in many similar roofs?

Furthermore, do the math. For such a restricted air flow channel, you'd be lucky to get 50W/m^2 of cooling, as contrasted with a possible maximum heat load of 800W/^2.

TTFN
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