Let's talk radiant Barriers

I have been reading about everything I can find on radiant barriers, but still have a few questions and concerns.
I am not talking new construction here, where radiant barrier can be installed under the roof sheeting. I am talking about existing homes, and then, primarily the spray on type.
Here is what I think I understand - -
Sprayed on radiant barrier will cut down 65 to 75 percent of the sun's radiation, - but I really don't know how much cooler that makes the attic.
Sprayed on radiant barrier effectiveness depends primarily on the people who do the work. They can thin out the paint so it is much less effective, they can spray it on to thin as well, and not get the complete underside of the roof. Most homeowners would never know they didn't get a good job. This is a major concern.
Dust on a radiant barrier surface decreases its effectiveness - the more dust, the less effective. Although there generally is not a lot of dust floating around in most attics, it can be stirred up easily. For example, adding more insulation is a dusty procedure, and if there is a previously installed radiant barrier, you will decrease its effectiveness - to some varying extent.
There is a company that advertises radiant barriers for half price (whatever that is) and free added insulation. If they spray their cheap radiant barrier (about which I would have concern about how it was mixed/thinned) and then add the insulation - instant dust.
This company has been advertising for months with some fantastic deal or other and will only quote prices through an in home salesman. I smell high pressure here.
Has anyone used this company? Has anyone used other companies to spray a radiant barrier, and how satisfied are you?
I know I am asking a lot of questions, but I sure don't relish the idea of getting ripped off. Thanks,
Bob-tx
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On Mon, 15 Jun 2009 15:14:56 -0600, Bob-tx wrote:

Not all radiant barriers are sprayed on. Some are in sheets. I had it done to a house we are rehabbing and the difference is amazing!
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How noticable was the difference, where was it used.
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Bob-tx wrote:

Staple some mylar "space blankets" to the underside of your roof rafters and be done with it.
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Or builders foil, as said staple to underside of rafters, leave a small gap between rows for vents, it's gotta breath.
Any radiant works MUCH better with an air gap. If applied to a surface, conduction is the path of the heat
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If the laws of physics apply here, radiation impinging on the roof will heat up the roof and sheathing as it passes through into the attic. There it is met by the 'barrier, which is simply an aluminum (paint or whatever) reflective surface. So it passes through the sheathing and shingles a second time on its way out, thoroughly cooking those materials. Soffit and ridge vents are installed to permit air heated by such radiation to be escorted out the peak of the roof. Maybe I missed something here, but it seems to me the net effect would be to shorten the life span of the roofing quite a bit. Seems more logical to get the radiation reflected away before penetrating the shingles and sheathing, like using bright white shingles in the first place, or painting the roof roof with aluminum paint, something not very stylish in today's McMansions. Over all, the proposed 'radiant barrier' might be like duct cleaning, high priced and marginal unprovable benefits. Whatever, good luck.
Joe
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neutralexistence had written this in response to http://www.thestuccocompany.com/maintenance/Let-s-talk-radiant-Barriers-378805-.htm :
Bob-tx wrote:

Hey Bob,
Let me see if I can shed some light on the questions. First of all the paint type radiant barriers(term used loosely) have a "reflectivity" of no more than 75% which "technically" is not, by definition, a radiant barrier. A radiant barrier, technically defined, will have a reflectivity of at least 95%.
I highly recommend a foil sheet type <a href="http://www.raflect.com ">radiant barrier</a> with a reflectivity value of 97% to 98%. While the installation is a bit more challenging, the results will be much better.
To address you question about dust and radiant barriers. While it is true that dust can degrade the effectiveness of a radiant barrier, by purchasing a double sided radiant barrier, you eliminate this problem. Also, even if you do have some dust on the single sided radiant barrier, it is still more effective then the paint type radiant barriers.
As far as installation, I highly recommend stapling the radiant barrier to the bottom of your roof rafters. This will leave a gap (usually 5-1/2") for air to move from your soffit vents up to your ridge vent, effectively moving the hot reflected air up and out of your attic causing your whole attic to be cooler.
Cooler attic means lower strain on your HVAC system causing lower energy bills. I also recommend getting a perforated type radiant barrier so you do not run the risk of any condensation issues which could lead to structure rot and/or mold.
Anyway, if you have any other questions let me know.
Adam
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Sounds like good info. Thanks Bob-tx
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http://www.thestuccocompany.com/maintenance/Let-s-talk-radiant-Barriers-378805-.htm
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I'm going to resurrect this one.
It's damned hot here in Central Texas and I bought radiant barrier sheeting to install in the attic. Problem is, I can't get to most of the places where it would need to go. Why not install it ON TOP OF THE SHINGLES? It would be kind of temporary but, to get us through the summer, seems like it would be a good idea. I was thinking of Styrofoam sheets that I could staple the sheeting to and then install these on the roof with some space between them and the shingles. I don't mind crawling around on the roof but I can't get myself in all the attic spaces.
Also, I Googled radiant barrier paint and sure enough there is such a thing. I was thinking of repainting the outside of my house.
In reality, all this would go on the back. The front has nice big shade trees and it's very protected.
Was toying with the idea of installing a sprinkler system on the roof to cool it down during the day...?
Brad
On Jun 16, 2:02pm, adam.beaz_at_yahoo_dot snipped-for-privacy@foo.com (neutralexistence) wrote:

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