Radiant barrier question

I hope there are people here who install or have purchased radiant barriers. I am considering this.
My questions: Are they really effective and reduce AC operating costs?
I have read there are two types, sprayed (like a paint) and a film like thin plastic. Which is better?
I have also heard that as dust collects on the surface, that they are less effective, which would seem to be a bad problem. (Who would go up in the attic and vacuum the underside of the roof). Is this true and is it a problem?
What is the approximate cost - by square foot, or however?
Thanks in advance for any information.
Bob-tx
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On Tue, 01 Jul 2008 04:27:58 -0600, Bob wrote:

I just received samples of the following: http://stores.ebay.com/Energy-Efficient-Solutions_Radiant-Barrier_ARMA - Foil-Perforated_W0QQcolZ2QQdirZ1QQfsubZ17724604QQftidZ2QQtZkm
It looks promising. I might opt for this over paint for what I'm doing (hydronic floor heat). If I ever get to doing the roof, I think I'll apply the foil beneath the felt paper.
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This won't work. In order for a radiant barrier to be effective, it must face an air gap. If the radiant barrier is in contact with another material, it will just conduct heat, and its radiant properties won't matter.
So for a roof, you can use foil-faced plywood sheathing (foil facing inside), or you could apply a film barrier to the underside of the joists.
Cheers, Wayne
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If that's true, how can it work on roof sheathing with the radiant barrier facing in towards the attic? If it only works as you describe, it would seem to work backwards, only providing a barrier from heat escaping the attic, not coming in from the roof.
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Radiation is how heat transfers across a vacuum or air gap (or any transparent insulator). In the absence of an air gap, heat will simply be transferred by conduction, which is what normal insulation is designed to reduce.
By being "shiny", a radiant barrier does two thing: first, it reflects most incoming radiant heat, and second, when it gets hot, it doesn't radiate heat very well. If you have a shiny metal roof, you would be taking advantage of the first property: it will reflect most of the incoming solar radiation.
If you instead have a radiant barrier on the underside of the roof sheathing, with an air space below, then you are using the second property. The roof and roof sheathing and radiant barrier will heat up from the sun, but the radiant barrier won't radiate much into the attic.
Of course, where the roof sheathing contacts the roof framing, the radiant barrier won't help, as the heat will be conducted to the roof framing, which will radiate into the attic. I'm not sure if this is a big deal; I think you still get most of the benefit anyway. You could counteract this by painting the roof framing with a radiant barrier, or you instead use a continuous film radiant barrier on the underside of the rafters.
BTW, as previously mentioned, a radiant barrier facing up to an air gap is typically not effective in the long-term, as dust will accumulate and reduce the "shininess".
Cheers, Wayne
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<snip>
Sounds like good info, Wayne - thanks.
Although you did not specifically discuss the effectiveness a paint barrier, I get the impression that it is not very effective.
There are so many newspaper and radio ads for radiant barriers that I am wondering if it isn't a case of companies taking advantage of concerns over high prices of energy, and pushing their product. Either that, or it is becoming a fad.
The dust factor is also a concern - no attic remains completely dust free, so in time, its effectiveness will diminish.
What do you think?
Bob-tx
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I don't really know anything about paint on barrier. It's all a matter of how reflective the surface becomes.

This is a big concern for radiant barriers facing up, they soon become worthless due to dust. But a radiant barrier facing down should be OK, I think. Not too sure about vertical radiant barriers.
Cheers, Wayne
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On Jul 1, 3:29pm, snipped-for-privacy@optonline.net wrote: ...

If the barrier is on the bottom of the roof sheathing, it will reflect heat back into the attic area reducing heat loss during the winter. If it is on the floor, it would reflect the heat back towards the roof preventing it from getting into the living area during the summer. The location of vents and insulation also come into play.
For the best results you need to consider all of those issues especially the ventilation situation and local conditions (more is better). Sometimes radiant barriers are not a good idea. Also as someone noted, they loose a lot of effect if they get dusty, which they likely will if they are facing up.
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snipped-for-privacy@columbus.rr.com wrote:

That's why you face them down and rely on their low emissivity rather than their reflectivity.
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On Jul 1, 7:20pm, snipped-for-privacy@columbus.rr.com wrote:

Which is close to worthless, as the attic should be ventilated so the attic itself is still cold and it has conventional insulation above the ceilings in the living space. Any heat reflected from the roof is going to be close to meaningless in the winter.
I think the main benefit of a barrier on the bottom of the roof sheathing is as Wayne described, by reducing the amount of heat in the summer that radiates off the hot roof sheathing towards the attic. It would also seem to me that it would be more effective if it was stapled to the rafters and reflective on both sides. With a ridge vent at the top, a lot of heat would be trapped between the barrier and the sheathing and then convected out the ridge vent. That would be good for keeping heat out of the house and also for reducing the max temp of the sheathing.
If it is on the floor, it would reflect the heat back

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I agree stapled to the underside of the rafters is more effective since you don't have the rafters radiating into the attic. However, I think the reflectivity on the upper side doesn't matter, since the upper side will soon become dusty.
Yours, Wayne
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I had it installed on my old house - stapled between the rafters in the attic with a few inches between it and the plywood interior. I live in South Florida and saw no significant savings in my A/C bill. However, I did have a leak in the roof and it channeled the water to the soffits which kept the leak from soaking the insulation and ceiling drywall.
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On Jul 3, 3:54 pm, snipped-for-privacy@gate.net wrote:

I think having enough thermostatically controlled fans would be more cost effective than installing a radiant barrier. I also saw a 25% savings in AC costs by blowing in treated celluose insulation over the existing insulation.
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