The FSEC has done radiant barrier work -- here is one report:
If you go to their site and search on Radiant Barrier, a lot of stuff comes up.
I think that www.SouthFace.org has also done work on radiant barrier.
These are good and independant outfits -- I would tend to belive what they
publish. It seems like the bottom line turns out to be about 10% saving on
I am aware of that and have previously read it. It is quite old now, 1999.
At the time the performance in cold climates from other non-comprehensive
tests in the USA was not too encouraging.
Actis claim the equivalent of 210mm of Rockwool. I believe they do have an
effect on cooling when pinned to the rafters of a roof. What is the overall
claim for rockwool equivalent thickness for heating by tests in the US?
There must be some ballpark. No one is going to type in Nicks program, they
read the makers blurb, or test results to confirm the blurb.
I have the impression much of any heat saving is because this stuff is
air-tight. More the draught prevention is making the difference rather than
the reflective qualities of the material itself. I hope I am wrong and it
does what they say. If so my attic gets done out in it. Until something
more concrete in realistic more real world testing the jury is still out and
it stays out of my attic. 35C here means I may have to act with the attic by
next summer - but using what has to be determined.
The second edition (1998) of Pitts & Sissom's Schaum's Outline on Heat
Transfer gives k = 0.023 Btu-ft/h-F-ft^2, ie 0.276 Btu-in-h/F-ft^2, ie
US R3.62 per inch, at a rock wool density of 10 lb/ft^3.
No need to type much. Just save it in a file, remove the headers, and run it.
Or look at the on-line RIMA Handbook and use a calculator, which takes about
That's assumed to begin with, but foil helps. For instance, the RIMA Handbook
says a horizontal foil with E2 and E3 = 0.03 and 3" airspaces 1 and 2 above
and below the foil with E1 and E4 = 0.8 boundaries and downward heatflow and
110 F above and 80 below has E = 0.0298 for both airspaces and an overall
dT = 110-80 = 30 F. Assuming the foil is 80+dT/2 = 95 F, the mean temp in
airspace 1 is Tm1 = (110+95)/2 = 102.5 F, and Tm2 = (95+80)/2 = 87.5. From
Table 4 on page 25 of the Handbook, hc = 0.075 for both airspaces. Equation 3
on page 22 says hr1 and hr2 = 0.00686((Tm+459.7))/100)^3 = 1.219 and 1.124.
Equation 1 says R1 = 1/(Ehr1+hc) = 8.98 and R2 = 9.22, so R = R1+R2 = 18.2,
and dT1 = 30x8.98/18.2 = 14.8 F and dT2 = 15.2. Close enough. We could
iterate if needed, using these new dTs to find new Tms.
No. If we replace the foil above with another E2 = E3 = 0.8 opaque surface,
then E = 0.8 vs 0.0298 for both airspaces, so R1 = 0.952 and R2 = 1.026 and
the overall R = 1.98 vs 18.2, ie 9 TIMES less. If we replace the foil with
IR-transparent polyethylene film, the difference is even greater, even though
there's still draught prevention. OTOH, if we add more foils or move the foil
up so there's only one airspace, that doesn't help much in this case, given
the same overall airspace dimension.
Rock wool would only add 3.62x6" = US R13.13 vs 18.2, using a lot more stuff.
Au contraire. This has been settled science for over 50 years :-) See
Robinson and F.J. Powell, "The Thermal Insulating Value of Airspaces,"
Housing Research Paper No. 32, National Bureau of Standards Project NE-12,
National Bureau of Standards, Washington DC (1954), and
Yarbrough, "Assessments of Reflective Insulation for Residential and
Commercial Applications," Oak Ridge National Laboratory Report ORNL/TM 8819,
Oak Ridge, TN (1983), and
Yarbrough, "Estimation of the Thermal Resistance of a Series of Reflective
Air Spaces Bounded by Parallel Low Emittance Surfaces," Proceedings of
the Conference on Fire Safety and Thermal Insulation, S.A. Siddiqui,
Editor (1990) pp 214-231, and
Yarbrough, "Thermal Resistance of a Air Ducts with Bubblepack Reflective
Insulation," Journal of Thermal Insulation 15 137-151, (1991).
From The Passive Solar Energy Handbook, Edward Mazria 1979 we have this
in Appendix E.6 Resistance values of airspaces
Horizontal, Heatflow Down
Thickness | Season | NR/NR | NR/Aluminum Coated | NR/Foil
3/4 W 1.02 2.39 3.55
1 1/2 W 1.14 3.21 5.74
4 W 1.23 4.02 8.94
3/4 S 0.84 2.08 3.25
1 1/2 S 0.93 2.76 5.24
4 S 0.99 3.38 8.03
Obviously that's all from observations.
What strikes me for my application at hand, insulating under staple up
radiant, is that 8.94 for a single radiant barrier. It sure makes foil
double bubble look good.
One thing though about radiant barriers. It's well settled that the upper
surface of horizontal installations will not retain its low emissivity.
Unless you fancy wiping and cleaning off the dust every year or so, it will
accumulate and lose its effectiveness.
In attics, it's advised to put the radiant barrier on the rafters overhead
so the radiant surface is on the underside. For underfloor installations,
the same thing. The foil goes on the underside to limit the accumulation of
dust that will ruin its effectiveness.
Isn't the reflective surface only effective if there is an air gap
next to it?
I have read some testing reports that show those 3/8" doubvle radiant
barriers under slabs to have a non-measureable difference to no
insultaion at all. Pissed off my concrete guy but I gave him a print
out of the report and now I can't find it again.
That's my understanding. It reduces the amount of heat transferred across
the air-gap by radiant heat transfer.
Since convection is the larger heat transfer mechanism for heat flow upward
(such as in an attic in a cold climate), they are not as effective when
trying to stop heat loss. So you usually only see them touted in situations
to stop the heat flow downward. Such as a hot attic to an air-conditioned
space, or from a heated room downward to an unheated crawl-space/cellar.
Yeah, putting some other material in direct contact with the foil, such as a
layer of wallboard, or a concrete floor pretty much nullifies the affects of
the foil. Of course, if the foil is over one inch of foam board, you still
have the one inch of foam and its insulation value. But not worth paying
any extra to get the foil.
Do you know what the defined RSI rating means? I have heard so many
definitions that none of them make sense anymore. Many try to equate
it with the R factor unsuccessfully and I believe it has something to
do with "reflective..."
Well that puts an interesting spin on my underfloor, staple up, unheated
It looks to me that I have two ways to go:
1) 3 1/2" (R 11 + R 6 or so for the radiant) fiberglass batts with a
radiant barrier wired up with wire hangers or
something similar. An airspace of an 1 1/2" or so.
2) double bubble (triple radiant)
I think the radiant barrier is essential due to the higher temp of the
radiant to ambient.
Originally I had only thought of method 1.
But radiant barrier batts are hard to find. Adding a radiant barrier
to an existing is awkward.
So method two, which is what at least some staple up suppliers
provide, seems plausible. It would be easier to dust seal this and it
certainly would be easier to install.
Have I missed something, or is this really the best app for radiant
bubble? Perhaps the only time it should be used.
With how many foils and what temp? What's the significance of "W" and "S"
with downward heatflow? A winter floor and a summer ceiling?
So up-facing foils may not help much, unless they are well-sealed above.
It seems that Reflectix makes a product with no radiant effect for use
under concrete, and another with 2 foils (not "triple radiant") on the
outside. The inner layers have no foil. Other options are double-foil
polyiso board and double-sided "builders foil" in 4' rolls at 10-20
cents/ft^2 from companies like Innovative Insulation, and more costly
adhesive-backed foil, and OSB with one foil face, which might be found
on the underside of a roof.
Dust sealing the exposed upper foil would be difficult.
Radiant barriers are good for downward heatflow (including a fridge roof),
OK for horizontal heatflow, and poorish for upward heatflow.
As I said before. Government and independent testing has shown that radiant
barriers lose much of their effectiveness if they get a layer of dust over
the foil side of them. The dust raises the emissivity to that of other
non-metallic materials ( > 0.85).
In places like attics, the 'usual' installation of radiant barriers is not
across the floor, but attached to the rafters overhead, facing downward.
This avoids the dust buildup issue. Thus the radiant surface is 'aimed'
downwards to the floor space of the attic. In climates that need a lot of
A/C, this can work quite well. The solar heat gained by the roofing heats
the sheathing and rafters, but the radiant barrier prevents it from
radiating to the attic floor (ceiling of the living space). Testing for
their efficacy in such installations has gone well. Radiant barriers in
this sort of situation can be an inexpensive, easy to install way to reduce
cooling energy needs.
Sadly, for climates needing a lot of heating, the situation doesn't work so
well. Installing the radiant barrier on the rafters does little to reduce
heat loss from the attic floor (living space ceiling). One reason for this
is that attics in cold climates are deliberately ventilated to keep the
attic cool. This prevents ice damage and ice dam formation on the eaves.
Another reason for poor performance in heating climates is that with the
heat flow upwards, natural convection of air from the attic floor to the
radiant barrier far outweighs the radiant heat transfer component, so
reducing the radiant heat transfer does little to reduce the overall heat
transfer (most upward heat flow still happens from convection currents).
So, bottom line. If the direction of heat flow is upward, radiant barriers
don't work well. Either convection outweighs the radiant component, or the
surface gets contaminated with dust and requires cleaning, or both.
For downward heat flow, they can add to the overall insulation if installed
I was disagreeing with "no good in the attic," and you seem to agree
with me, at least in climates where AC costs predominate (like where
I am -- Texas).
I thought the focus was on AC, it being summer and all, but you may
have a point in a broader context.
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form firstname.lastname@example.org.
I always thought the shiny side reflects, so needs to be facing where heat
needs to be reflected back and there needs to be a 1" gap between that and
any other surface. Having it under floors facing down should not be
effective. Yet I have read that some makers say it does not matter which way
it goes, I find that hard to believe.
When heat radiation strikes a surface, it's either transmitted, absorbed,
or reflected. Kirchoff said "It has to go somewhere," ie T + A + R = 1. If
T = 0 (an opaque surface with no transmission), A + R = 1. If the surface
emits as much power as it absorbs, E = A, integrated over the whole spectrum
(R is an energy conservation wash.) So a foil has reflectivity 1-E, which
is large if E is small, ie it's a good heat mirror. It can stay cool because
it doesn't absorb much heat, and it won't lose much heat because it's at
a low temp and it emits poorly.
Big gaps with less still-air conductance are good for downwards heatflow.
A 1-1.5" gap is good for sideways heatflow. Smaller gaps have more still-air
conductance and larger gaps have slightly more "convection conductance."
It should be, if there's an air gap beneath the foil.
We're talking about the FOIL side. Its going to be shiny regardless.
With a crawl space underneath, IT MAKES LOADS of sense. But the
direction it faces is CLIMATE dependent. Cold climates, foil side
faces towards the house to radiate heat back to the floors. Hot
climates, it faces down to reflect back heat from the crawl space.
Foil, insulation, paper, or foil insulation foil are available
In new construction, you can get foam boards for sheathing that have the
radiant barrier foil attached, in some cases to BOTH sides.
www.atlasroofing.com for an example of such. A 2" board will add about
$1.15 sq ft to materials cost of the house and adds R12 to the walls.
Similar boards are available for roofs, in areas that will see water
freeze on the roof.
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