An important question here is how does other insulation perform 'in-situ' if measured the same way as this product.
If this product can achieve an RT=5 'in situ', that means the overall measured insulative performance is 5 m^2-K/W. That performance includes the affects of convection and radiant heat transfer from the living space to the product, and from the product to the environs on the other side.
But what is deceptive about this, is that if I were to put a simple piece of conventional building insulation that has an RT=5 value in the same circumstance, it would undoubtedly have an 'in situ' performance that is
*better* than 5. Because added to the material's own RT=5, would also be the affects of the convective layers on each side (just like this product), and the radiant transfer to/from the surfaces.
Unless this products RT value is calculated by taking the 'in-situ' performance and *subtracting* the insulative performance of those items common to *all* installations, the RT value is inflated by those other factors. Thus when compared with other materials tested in the more traditional manner, it overstates this product's performance.
They now have TRI-ISO SUPER 10, not 9, so this judgement againast them doesn't stand anymore. They still state that it is equiv to 210mm of mineral wool.
In this case, one would hope they derived by subtracting. Nobody seems to have disputed the result that it worked as well as the rock wool did.
That number includes the other stuff, as the FTC-mandated "system R-value," with a substantial contribution from the foil surface. A non-radiant maker could also legally advertise a system R-value in the US.
Conventional insulation would only be between the floor joists. For 2 bys at 16" centers that means that 12.5% of the area is the insulation value of only the joist. I would think that would be around a US R6 for a 2 * 6. There will be a point of diminishing returns for conventional insulation, just because of that. Don't forget that under most floors you have a maze of plumbing and wiring and that has to be worked around with conventional insulation.
The more interesting question in my mind, is whether this would be a good afterfit for an existing structure. It certainly would be easier (crawl spaces are no fun!) to install and it would be a complete seal. It seems to me that most energy lost is in existing structures rather than new construction. The option of tearing down the old house and building a new does not exist for most people.
I would be delighted to have a US R 16.8 floor in my '20s house. At this point underfloor radiant with radiant bubble looks attractive, not sure which way I will go and I'm open to other suggestions (like foil backed iso).
The FSEC has done radiant barrier work -- here is one report:
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If you go to their site and search on Radiant Barrier, a lot of stuff comes up.
I think that
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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 cooling.
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
5 minutes.
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 NR=Non Reflective
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 basement app.
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.
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.
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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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