Consider an RB Product: Made into a triple-layer material, each layer is a double-sided aluminum reflective surface with emissivity of .03, each layer is air separated by a "scrim" of very low-conductive material. Therefore, there are 6 reflective sides making up 3 layers separated by 2 scrims. The scrims force & maintain an air gap between each layer: thus the middle layer has very little surface area touching neither the hot nor cold side and stays dust-free.

Question) Even if the top becomes dusty and the top layer reflectivity is reduced significantly, doesn't the RB still work "nearly as well" as when dust-free to reduce heat gain/transfer through the barrier?

Question 2) Would not the emissivity remain .03, even when laid against a surface such as insulation (as an example of on the floor of an attic)?

I think the RB product you imagine would not perform well in a home and any performance you did get would be from its air barrier and insulation properties. Not much to be gained with the reflectivity and emmissivity. A triple pane window gets most of its extra performance from the air not the low E coating and too much of a coating cant hurt you from decreased SHGC.

I would guess that Question #1 is yes. Nearly as well, depending on how dusty.

#2: No. Please dont ask me to produce them, but studies find an RB must have a free air space to work and insulation is not enough of an air space.

You will find more than enough information to refute or confirm what Ive said here:

florida solar energy center has a faq about rb's
that is very comprehensive.

the dust build up will reduce reflectivity by 50%
building in an air space may work...but needs to
be 3/4". the cost of a material that provides this
would be high.
rb costs vary. I used a single sided foil with white
kraft paper and a nylon scrim. this is the cheapest
material and it performs as well as the most expensive
material.
the reason it performs is the install.
attached to undersides of roof rafters with foil
facing into the attic space.

in new construction there are rb plywoods
like Techshield, and Solarply.
these are installed with foil facing...into
attic space.

this is the correct application.

best of luck

SpringTimeHomes: Thanks for the opinions, the OakRidge site was pretty basic stuff I already knew, but it had valuable references. As I'm reading through the many Florida Solar Energy Center research docs (read 300 pages so far) all testing is single layer products. There is no argument single-layer RB's are less effective and eventually become useless if fully covered in dust.

Two of the Florida articles (and several other individual tests) have strongly implied there is no significant conductive heat transfer between RBS horizontally applied to the attic floor when laid on top of insulation with reflective side facing up. That pretty much answers the 2nd question, even for single layer RBS. I hear the conductive heat transfer counter-argument quite a bit.

Energy_Rater_LA: I've yet to find the reference to the 3/4" air gap requirement, but I'm still going through the info. So far, it appears that statement is incorrect (see below).

Before today, I've read evidence (from neutral research) on both sides of the equation for attic floor applications (discounting greatly, but not discarding, additional evidence from folks who sell the stuff). I've been unable to find dust/conductivity research of the various types of RBs, other than the reference by the retailer/manufacturer of RBS Chips discussing the Tuscon Electric Power Report of 3 different RB products showing correlation between the multi-layer approach resisting the impact of dust.

The Oak Ridge Lab site defines a Radiant Barrier:
If a single reflective surface is used alone and faces an open space, such as an attic, it is called a radiant barrier. Is the focus on single-layer because a multi-layer, multi-surface reflective product is technically defined as "reflective insulation"??

If a one-sided radiant barrier is laid on top of the insulation with the reflective side facing down and touching the insulation, the radiant barrier will lose most of its effectiveness in reducing heating and cooling loads. No discussion of multi-sided, multiple layer facing up.

"Horizontal attic RBS, however, will collect dust on their upper surface and their performance will degrade [only?] to the extent that the upper surface is the operative radiant barrier surface " I added "only": So if middle layers are insulated from dust & conductive transfer by air gap, their reflective properties kick in?

If I could find relevant research on the 1st question above, that would help greatly. still looking...

http://www.fsec.ucf.edu/en/publications/pdf/FSEC-FS-37-88.pdf

more detail than short faq.
you'll have to scroll down to page
7 (of 9) to see where it states
"to be a radiant barrier the foil
must face an air space."

monitoring program for rb's
http://www.fsec.ucf.edu/en/publications/html/fsec-cr-1231-01/

all of the above is from an unbiased educational site
florida solar energy center. no dog in the fight
just consumer awareness for folks getting oversold on
benefits of RB's.
note that rb are for hot climates
for cold climates the savings are very minimal.

this is from a site selling products:
Why Radiant Barriers REQUIRE An Air Space
No matter how you plan to install a radiant barrier, it MUST have at least one air space of at least 3/4 of an inch on either side to be effective at BLOCKING radiant heat. It does NOT matter which side of the radiant barrier the air space is located. The purpose of the air space is to prevent conductive heat transfer.

If a radiant barrier does not have at least ONE air space on either side of it, heat will conduct from the surface touching the radiant barrier, through the barrier, and then transfer to the next surface touching the radiant barrier on the opposite side therefore, giving you no protection against the heat you intend to block.

Therefore, as long as the air space requirement is achieved, a radiant barrier will be effective at BLOCKING radiant heat regardless of your application, i.e. interior/exterior walls, siding, roofing and attic locations, etc.

What Happens When No Air Space Exists
Because a radiant barrier requires an air space on at least one side of itself to be able to BLOCK radiant heat, a radiant barrier CANNOT be installed directly underneath roofing materials where no air space exists.

For example, if you install a radiant barrier on top of roof decking between the felt paper and asphalt shingles, it will NOT provide any benefits as the radiant heat would be transferred through the shingles, through the felt to the radiant barrier, and through the roof decking into the attic space.

the website:

http://www.radiantguard.com/radiant-barrier-101.aspx

if you are going to spend the money to install a radiant
barrier, first make sure that you aren't overspending on the product & underspending on the install. this often happens.
while you can find info on line to support either install
do try to find the educational rather than sales hype sites.
this topic has been discussed many times on this site
if you do a faq you will find some of the posts.

best of luck

Let me keep reading through everything before I reply again and thanks again for taking the time to put down more info for me.

best of luck
best of luck