Radiant Barrier:Dust & Conductive xfer

ceburySeptember 16, 2011

I'd like to ask two very specific questions regarding Dust & Conductive Heat Transfer.

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)?

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SpringtimeHomes

Iam no physicist or building scientist but it seems like a lot of wasted time and effort have been put into radiant barriers in housing with the exception of Low E windows.

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:

Here is a link that might be useful: Oak Ridge National Labs Radiant Barrier Info

    Bookmark   September 16, 2011 at 5:46PM
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energy_rater_la

putting radaint barriers on the attic floor
is a waste of time and money.
if you visit sites that are not selling you products
they will tell you the correct install.

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

    Bookmark   September 17, 2011 at 10:19AM
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cebury

I already have a RBS installed and I totally understand it's not cost-effective for many folks AND even worse it's very easy to improperly install if directions are not followed exactly for the site. It is interesting they are now able to easily predict "typical" cost savings based only on climate, attic size, and whether ducting is in the attic.

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

    Bookmark   September 18, 2011 at 1:05PM
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energy_rater_la

when you post can you briefly describe your install?
and where you area located?
was it diy or a pkg deal?
hope the info posted helps.
rb's are often misrepresented and installed.
the savings are fairly accurate based
on my own analysis software.

best of luck
best of luck

    Bookmark   September 18, 2011 at 5:07PM
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