"reflective foil" term

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"reflective foil" term

Postby cooked » Tue Mar 13, 2012 1:10 am

Hi.
I am worrying about the term "reflective foil". I wonder if the correct term wouldn't be "vapour barrier" and that its main function is to keep humidity (both from the air and from when it rains) out of the roof space, draining any condensed water away to the eaves? This is what we use it for in Switzerland, where it gets both very hot and very cold. One form of vapour barrier is reflective foil.
Shoot me down in flames,
but I have been having difficulties with the concept ever since I started reading about it on this forum for the first time in my life. Reflecting heat back to the roof covering is going to heat it up even more, it will say thank you very much and send most of it back again? Won't it?
http://www.yourhome.gov.au/technical/pubs/fs48.pdf
If this is the case, the most important factor in installing the stuff would be to ensure that no water can drip through the barrier anywhere, which rules out letting most Thais install it for a start..

wajja reckon?
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Re: "reflective foil" term

Postby payebacs » Tue Mar 13, 2012 5:03 am

cooked wrote:Reflecting heat back to the roof covering is going to heat it up even more, it will say thank you very much and send most of it back again? Won't it?

Following on from your thread insulating from underneath cooked I believe Geordie was saying that this neatly trapped hot air between foil and tiles would be pulled
geordie wrote:upwards towards the ridge which should be vented so if you have vented the soffits and the ridge you should by means of convection have a nice flow of air up the inside of the roof making it cooler :roll: :roll: simple eh ??

If you remember I had been suggesting foiling your ceiling boards so as to reflect the heat upwards until Geordie corrected that it had nothing to do with reflectivity. Something about radiants...
cooked wrote:the most important factor in installing the stuff would be to ensure that no water can drip through the barrier anywhere
Cooked, is it the vapour barrier's purpose to bring about condensation so as to create water with the idea that this water then evaporate once more to create cooling?
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Re: "reflective foil" term

Postby BKKBILL » Tue Mar 13, 2012 11:10 am

One common misconception regarding radiant barrier is that the heat reflecting off the radiant barrier back out the roof has the potential to increase the roof temperature and possibly damage the shingles. This is simply not the case. Performance testing by Florida Solar Energy Center[7] conclusively proved that the increase in temperature at the hottest part of the day was no more than about 5 degrees F. In fact, this study showed that radiant barrier had the potential to decrease the roof temperature once the sun went down because it was preventing the heat loss through the roof. RIMA International wrote a technical paper on the subject, where they collected statements from the largest roofing manufacturers, and none said that radiant barrier would in any way affect the warranty of the shingles

This is at http://en.wikipedia.org/wiki/Radiant_barrier

And from the US Department of energy, since they are not selling the stuff I would put a little more faith in this.
http://www.energysavers.gov/your_home/i ... opic=11680
Is there another word for synonym?
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Re: "reflective foil" term

Postby cooked » Tue Mar 13, 2012 1:48 pm

thanks, this is a great site for this kind of thing. I didn't start off with "this may be a silly question, but..." - but I could have done.
As the roof is metal, I am not too worried about heat damaging it. What I was thinking about was a build up of heat under the roof that would have to be got rid of. I don't think that water will be running down the outside of the foil in rivers, I should hope that the barrier would be fulfilling its role as a vapour barrier and at least keep the roof space 'dry', In this case adequate ventilation would also be important. If what I understand is correct, the fact that dust will with time collect on the barrier would have no effect on this function.
As I will be insulating the roof above the ceiling , an additional vapour barrier ( as I now see it) on top of the bats won't cost me a fortune and should make everything hunky dory.
I think there are as many opinions about the way 'reflective foil' works as there are people thinking about it. Maybe if we dropped the term and talked about vapour barriers (as I now do) things would be clearer.
Still ready to discuss this, I am always open to advice.
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Re: "reflective foil" term

Postby payebacs » Tue Mar 13, 2012 2:20 pm

payebacs wrote:
cooked wrote:the most important factor in installing the stuff would be to ensure that no water can drip through the barrier anywhere
Cooked, is it the vapour barrier's purpose to bring about condensation so as to create water with the idea that this water then evaporate once more to create cooling?

Because if it's not then I would have said that the most important factor in installing the stuff would be to ensure that no vapour can pass through the barrier for otherwise what purpose is it serving?
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Re: "reflective foil" term

Postby BKKBILL » Tue Mar 13, 2012 2:50 pm

If you read the two sites posted it is not called a vapour barrier but radiant barrier or reflective barrier. :roll:

A vapour barrier is a tight membrane that is installed on the warm side (facing towards the livable area) of a roof or on the inside of the exterior house wall. The function of a vapour control layer is to avoid moisture that builds up inside a house and find its way to the insulation material and condensing the moisture making the insulation material lose its ability to insulate. Not normally a problem in Thailand

So if you call it anything I would think reflective barrier would be correct.
Is there another word for synonym?
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Re: "reflective foil" term

Postby cooked » Tue Mar 13, 2012 4:42 pm

Ok thanks for explaining, I had a problem with the terminology.
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Re: "reflective foil" term

Postby gliffaes » Sun Apr 15, 2012 7:32 pm

Ive noticed they sell a tape to join this stuff together ie each 1.2 metre wide piece, anyone use this? is it necessary?
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Re: "reflective foil" term

Postby geordie » Mon Apr 16, 2012 12:10 pm

I would just overlap the foil and use the tape to ""tidy the lap i cannot see continuous taping being of any benifit what you doing for ventilation at the ridge/top also the bottom to allow cooler air that should set up a nice convection current between the tiles and foil
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Re: "reflective foil" term

Postby Mike Judd » Mon Apr 16, 2012 12:56 pm

The main thing to remember, is that heat always rises,and if there is a barrier it will build up, while the reflective foil under your roof covering reflects the suns heat back throughout the day,(Stand under any shade and it's cooler than out in the direct sun) Unless there is some means for all the hot air rising in the house and roof space to escape, your house will stay warm when it's cooler out-side in the evening. So the best thing is at least allow the air to escape through the ridge and or better still vented gables. If you are into Air-Con the more insulation in the roof/ceiling /walls the better, but large areas of glass make the Air-Con work harder unless you Double glaze, which is not usual in Thailand.
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Re: "reflective foil" term

Postby cooked » Mon Apr 16, 2012 1:04 pm

...a never ending story. I have recently read (name removed forum rules mod?) that reflective foil deteriorates and shreds after a few years. I now understand (duuuh...) how it works, but as I think I will have to take the roof off to put it in, I am still hesitating. Anybody has his head up there recently? Aluminum should be indestructible. Should be used in combination with roof ventilation to prevent build up of humidity I think, but I would do this anyway.
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Re: "reflective foil" term

Postby geordie » Mon Apr 16, 2012 1:25 pm

cooked you could with paitience :roll: :roll: install it under the roof but not the thin film i would go with the double/triple stuff with insulation sandwiched in it and drill some self drilling screws (hex head roofing screws) and a good supply of wire the sort they use on rebar for tying it double sided tape it to the trusses (the foil) then use the wire to give it support permanently with a couple of turns around the screws you put in for that purpose :roll: :roll: not easy but should be acheivable i have to say i would rather take the roof off its a tad too warm for me to want to go inside a roof here
an option would be spray it with foam possibly cheaper than removing the roof and almost certainly less hassle to you just pity the guys doing it
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Re: "reflective foil" term

Postby gliffaes » Mon Apr 16, 2012 2:50 pm

geordie wrote:I would just overlap the foil and use the tape to ""tidy the lap i cannot see continuous taping being of any benifit what you doing for ventilation at the ridge/top also the bottom to allow cooler air that should set up a nice convection current between the tiles and foil


Geordie Ive bought some of these (4), all I have to do is make a cowling over them sort of "T" shaped with a mesh in to keep insects out for venting or buy their stainless ones, the air will come under the eaves thru various ways and hopefully out thru these chimneys. I have seen full ridge vents with the drytech system but as yet am not sure exactly what I have on order
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Re: "reflective foil" term

Postby geordie » Mon Apr 16, 2012 4:09 pm

excelent but remember the more out the more in so the more efficiently it works i mentioned somewhere before the early landrovers utilised the principal they had a second skin on the roof with a small gap between allowing air through the gap kept them cool while it moved before the days of aircon in the car ??
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Re: "reflective foil" term

Postby gliffaes » Mon Apr 16, 2012 4:35 pm

geordie wrote:excelent but remember the more out the more in so the more efficiently it works i mentioned somewhere before the early landrovers utilised the principal they had a second skin on the roof with a small gap between allowing air through the gap kept them cool while it moved before the days of aircon in the car ??

Being positioned between two hills we do get a good strong breeze most days on the plot of land which Ill take full advantage of. In the morning all is still, by 10 its coming from the west for about and hour an a half but by 12 noon its all blasting from the east to west until about 5-6pm when it subsides.
These special tile vents are 800 baht a piece.
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