Roof Ventilation system?

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Re: Roof Ventilation system?

Postby fredlk » Tue Apr 26, 2011 4:19 pm

geordie wrote:In thailand it is now becoming common to use foil or metalized film to stop radiant heat and therefore no air can escape as easily as it would with a tiled roof
heat builds up even on a roof with no foil its a requirement to let the heat escape through vents keeping the house cooler hopefully

Here are the vents on my house - the roof itself is sealed and so any hot air will escape through these:
SDC13576_resize.JPG
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Re: Roof Ventilation system?

Postby Roger Ramjet » Tue Apr 26, 2011 5:17 pm

Fred,
I'm sorry to tell you this, but your won't work like you stated because you have a "dome" at the top of your roof with cool air coming in through the holes in the soffits/plaster board at the base level, so the heated air will be trapped at the top with the cooler air keeping it there so that it cannot circulate. You might like to study the physics of it all here; http://www.suite101.com/content/underst ... nts-a63549 because otherwise you might have a major problem with condensation during the hot and steamy months.
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Re: Roof Ventilation system?

Postby geordie » Wed Apr 27, 2011 2:06 am

Fred rodger is correct but knowing you had a specific look in mind when you desingned the roof the soffit vents will have to do :!: the warm air trapped there will be handy on a cool night
but the roof should still be able to breath a bit soffit venting is quite common over here
i would think condensation is highly unlikely give that you have polystyrene foam under the metal roof however without the foam it could have been a problem when you get a sudden drop in temperature at night and have a lot of heat stored although i would have thought it would form on the cold surface outside the roof
hmm confused myself now
anyway if it became an issue just lift a cople of the ridge caps and put a 5-10mm gap under them you would barely notice
my comments may be wrong but never deliberately
If it aint broke, dont fix it
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Re: Roof Ventilation system?

Postby Ians » Wed Apr 27, 2011 1:40 pm

fredlk wrote:
geordie wrote:In thailand it is now becoming common to use foil or metalized film to stop radiant heat and therefore no air can escape as easily as it would with a tiled roof
heat builds up even on a roof with no foil its a requirement to let the heat escape through vents keeping the house cooler hopefully

Here are the vents on my house - the roof itself is sealed and so any hot air will escape through these:
SDC13576_resize.JPG


Sorry to say, but hot air rises so how can it escape thru a vented soffit ? Why is you roof sealed - it would be better to have some sort of venting system to allow the hot air to escape.
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Re: Roof Ventilation system?

Postby fredlk » Wed Apr 27, 2011 1:57 pm

Ians wrote: some sort of venting system to allow the hot air to escape.

I'm happy with it as it is and I doubt there will be any hot air under a white metal roof, but let's wait and see how it functions. I'll report back.
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Re: Roof Ventilation system?

Postby Ians » Wed Apr 27, 2011 7:16 pm

fredlk wrote:
Ians wrote: some sort of venting system to allow the hot air to escape.

I'm happy with it as it is and I doubt there will be any hot air under a white metal roof, but let's wait and see how it functions. I'll report back.


Would you like to bet a beer or two that there will be hot air under your tiles. :)
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Re: Roof Ventilation system?

Postby fredlk » Wed Apr 27, 2011 7:29 pm

Ians wrote:Would you like to bet a beer or two that there will be hot air under your tiles. :)

Tiles? :?: :!: :?: :shock:
(I don't drink.)
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Re: Roof Ventilation system?

Postby Ians » Wed Apr 27, 2011 8:17 pm

fredlk wrote:
Ians wrote:Would you like to bet a beer or two that there will be hot air under your tiles. :)

Tiles? :?: :!: :?: :shock:
(I don't drink.)


Sorry metal roof - thinking about tiles for my own build.
I drink a little so it's for you to buy and me to drink.
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Re: Roof Ventilation system?

Postby fredlk » Thu Apr 28, 2011 7:29 am

fredlk wrote:I'm happy with it as it is and I doubt there will be any hot air under a white metal roof, but let's wait and see how it functions. I'll report back.

When I have moved I'll measure the temperatures at eye-level (seated) and under the ceiling at midday and at midnight.
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Re: Roof Ventilation system?

Postby Ians » Thu Apr 28, 2011 9:19 am

fredlk wrote:
fredlk wrote:I'm happy with it as it is and I doubt there will be any hot air under a white metal roof, but let's wait and see how it functions. I'll report back.

When I have moved I'll measure the temperatures at eye-level (seated) and under the ceiling at midday and at midnight.


That will be an interesting observation.
Do you have insulation on top of the ceiling?
Initially I was to use metal sheeting on the roof but in the end opted for tiles (al.foil under) as I think they will better suit the house style. To counter the anticipated slower "cool-down" after sun-set I have incorporated vents in end gables, I will install insulation on the bedroom ceilings to slow the heat build-up during the day - ceiling height is 3 metres. However, I belive that ceiling insulation is a two edged sword as it slows down heat transfer in both directions, therefore each room with an ouside cavity wall will have 2 wall vents venting directly into the cavity to assist in removing the rooms hot air especially at night as the roof space cools.

Guess the best we can hope for is to try and maintain the lowest temperature (without using the air-con ) that the room can achieve during any cool down over night.

Likewise, for interest I will monitor room / roof space temperaures during the day and the night .
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Re: Roof Ventilation system?

Postby fredlk » Thu Apr 28, 2011 9:26 am

Ians wrote:Do you have insulation on top of the ceiling?

The ceiling is very close to the P.U.Foam. I would be throwing away money if I did have.
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Re: Roof Ventilation system?

Postby otis-a » Sat Apr 30, 2011 12:27 pm

tend to agree with ians
there are not any perfect emitters nor black.bodies adsorbers of radiation
technical lingo emissitivty cannot =1
energy not reflected is adsorbed as heat
my guess on best available technology is 20% conversion to heat

-the eve vents cannot be as effective as high point vents but
given enuf circulation perhaps 80% as effective as high point vents
the common thai roof tiles provide cracks \'\ that act as air vents. /'/ in between the overlaps. The eve vents allow drafting of cool air to up flow via chimney effect.
.
That is one method. Doubtless there are other means such as solar ventalators. Kiss
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Re: Roof Ventilation system?

Postby dozer » Sat Apr 30, 2011 2:44 pm

the common thai roof tiles provide cracks \'\ that act as air vents. /'/ in between the overlaps.
Just a note; this is only true as long as roof reflective foil is not used (since it seals the roof and doesn't allow the air to vent). You would have a similar situation on any hip roof with reflective foil (on a gable roof the air vents through the gable vents).
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Re: Roof Ventilation system?

Postby fredlk » Sat Apr 30, 2011 2:58 pm

otis-a wrote:my guess on best available technology is 20% conversion to heat

I forget the exact figure, but I seem to recall it was 92% reflective for a white roof. When I was looking for the best and coolest roof I looked at zinc and other silver-coloured roofing, but was surprised at how white out-performed all others and so I went with the white.

"Many considering the potential of reflective roofing are concerned about color. The Florida Solar Energy Center has evaluated the solar reflectance of some 37 different roofing materials, with the measured data showing that white roof materials generally exhibit the best performance. They are highly reflective across the solar spectral bandwidth, while being highly emissive in the far-infrared region--this is another way of saying they strongly reflect solar heat and any heat they absorb will readily re-emit to the cooler sky temperatures. It may seem a bit counter-intuitive, but silver reflective aluminum paints do not perform nearly as well as a simple white coating. This is because, although the aluminum flake paints have a high solar reflectance, they also have a low infrared emissivity--they tend to hold whatever heat they absorb--negating part of the reflectance properties."
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Re: Roof Ventilation system?

Postby otis-a » Sat Apr 30, 2011 5:40 pm

hi mr. fred;
am not questioning the 92% number when new. however over time dust & other nonreflective stuffs will accumulate to reduce reflectivity.
API standards for fugative fumes from covers this topic as do various environmental reg's (most of which will reference either the API or US EPA regs & standards. when fumes are referenced the vent rate is
Qr * Hv. or heat input from rad. times heat of vaporization.
Qr is thermal input from sun times (1-ë) times exposed effective area & (by memory ) emissivity (ë) recommendation seems to be around 80% or 0.8: (for best operational conditions) time permitting i will dig up a table
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