Reflectix R-value claims reasonable?
Hey y’all (implied Wautch thee-us!):
I’ll be moving my beloved ’71 Airstream (the Aerie CAD station) from steamy South Carolina home to the Colorado High Country this summer, and making it my full-time place way up above timberline at 12,000′. Since temperatures there range from 60 degrees summer to -60 winter, I really need to address insulation, as the ol’ gal now has around 1-1/2″ of fiberglass batts with copious mouse tunnels, and gets pretty dang cool in spots even down here. Construction is aluminum frame with riveted stressed-skin aluminum panels inside and out. I figure I’ll have to either drill & remove the interior panels to apply better insulation, or overlay the interior. One method would entail a layer of Reflectix with air gaps on both sides, either via removing the fiberglass and putting it between studs or building inward an inch and a half or so with furring/paneling. Another might be to strip the batts & spray-in foam, which would only work with interior skin removal/replacement. I’ll certainly add an external blanket of hay bales before winter hits this September.
Question: The Reflectix brochure makes claims of anywhere from R-4 to R-16.89, depending on application. That’s just hard to swallow for material less than 1/4″ thick. If I make this stuff my only insulation, am I looking at something that mainly blocks short-wave radiation (even on toasty 60-degree summer days I wouldn’t mind a little solar over-warming to see me through the night), or are these numbers transferrable to longwave radiatioin, as in keeping precious propane btus inside when it’s really cold?
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Replies
Ted,
Don't get bamboozled by advertising claims that discuss long-wave and short-range radiation. What you want is R-value; that's what stops heat flow. Contrary to the misleading claims of some reflective insulation manufacturers, R-value measurements account for all three types of heat transfer -- radiant, conductive, and convective. According to ASTM C518 testing, Reflectix has an R-value of 1.04. That's quite low. If you can successfully construct a wall assembly that includes a 3/4 air gap on one or both sides of the Reflectix, the R-value of the entire wall assembly (not the R-value of the Reflectix) will go up. The increase in R-value is due to the R-value of the air space or spaces. However, this higher R-value depends upon the long-term maintenance of the air space, and will be degraded if the Reflectix ever sags. Moreover, the effect of the shiny radiant surface will be degraded over time by dust. Bottom line: get yourself some rigid polyisocyanurate foam.
I have to correct you... R-value tests ONLY for conductive heat flow. There is no air movement. In fact there is no air. An argon or nitrogen atmoshere is used inside a closed in box so there is no gas movement (convection) or condensation (if air was used).The sample is also in the horizontal position so no convective looping happens like in a FG wall assembly.Radiant heat flow is only through space and is dependant on surrounding surfaces radiation of heat indexes and then the material absorbing the heats index as well. Very complicated stuff for sure. Radiation only happens thru space. When heat hits the next solid it is absorbed then convected. So if you have that R 1 piece of metal bubble wrap in there it will probably work a little less effectively than it did in the lab where they designed "the perfect storm" to somehow arrive at inflated values.Perfect app for urethane foam. I did my work trailer like this and heat it with 1500W heater and it stays 50F + all winter.
MA Spray Foam,
Sorry, you are wrong. Don't feel bad; you are simply a victim of the misinformation spread by the reflective insulation industry. Here are the facts, explained by David Yarbrough of R & D Services in Cookeville, TN, whose lab is the most widely respected tester of insulation products in the country. Yarbrough wrote a letter to Energy Design Update in November 2003, writing, "The R-value used to describe thermal insulating products includes heat being transferred by all three mechanisms [conductivity, convection, and radiation]. The term used in the thermal insulation community is ‘apparent thermal conductivity.’ A formal definition for apparent thermal conductivity is contained in document C168 published by the American Society for Testing and Materials. The term is applied to situations involving the simultaneous flow of heat by all three transport mechanisms. The statement ‘R-values are measures of conductive thermal resistance’ is incorrect, if it implies a limitation, since ‘R-value’ includes radiation and convection when they are present."
If heat can get through a material, by any means, then the extent to which that heat is slowed is measured by the R-value of the material.
"ASTM C518 testing""document C168 "Since I don't have the standards and don't want to pay for them just to get this question answered can you give we a few more details.I was under the impression that the standard for testing was to use a hot plate on one side of the material (or assemblly) under test and a cold plate on the other side and to measure the amount of energy that flows for a given tempature difference.Now if those plates are directly on the material then there is only thermal conductivity.If what is being tested is assembly and that assembly includes an air gap then it then radiant would also be included.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Bill Hartmann,
Unfortunately, I don't have a copy of ASTM C518 either, so to a certain extent I am depending upon the authority of experts whom I trust. If you have a material with integral air pockets -- let's say, corrugated cardboard or a honeycomb sandwiched between rigid sheets, or even a fiberglass batt -- then all three kinds of heat transfer can occur across the material, even if the material is in contact with a hot plate and a cold plate. (And, to be honest, I can't confirm whether or not C518 requires contact.) In a batt, there is a certain amount of heat radiation from one fiber to another fiber; there can be some convection effects within the batt; and of course, there is also conduction. C518 is a valuable test to test the R-value of a material, and it incorporates all three heat transfer mechanisms.
If you are measuring the R-value of a sheet of aluminum foil, it will be very low; aluminum is a conductor. If you measure the R-value of a sheet of aluminum foil surrounded by two 3/4-inch air spaces bounded by drywall on one side and OSB on another side, you are no longer measuring the R-value of the aluminum foil; you are measuring the R-value of a building assembly. This is usually measured according to ASTM C1363. This is a valid and useful test, but it is a building assembly test, not a material test. It doesn't reveal anything about the R-value of the aluminum foil.
Reflectix has an R-value of 1.04 because it consists of bubble-pack surrounded by aluminum foil. The C518 test measurees the R-value of Reflectix. If you make a building assembly with Reflectix, the R-value goes up. You could measure the R-value of the entire building assembly with a C1363 test; but you would need to be careful and not attribute the R-value of the assembly to the Reflectix alone. The air spaces have an R-value and contribute the the R-value of the building assembly.
Finally, if you suspend other types of insulation -- for example, a sheet of one-inch-thick expanded polystyrene -- between two 3/4-inch air spaces, a layer of drywall, and a layer of OSB, the R-value of the entire building assembly also goes up. This increased R-value from adding up a sandwich of air spaces is not a phenomenon that only occurs with reflective insulation; it occurs with any type of insulation. But reflective insulation manufacturers are notorious for blurring the important distinction between a material R-value and a building assembly R-value.
I salute your enthusiasm, but you are really splitting hairs.The fact that convective and radiant transfer occurs within a fiberglass batt is not really important. The test is for conductive heat transfer. As for that particular product, in a conductive transfer test from one side of a batt to the other, it would transfer heat using all 3 mechanisms within itself, so you're technically correct and the R value takes all that into account.But that does NOT mean that R-value is actually accurate for all situations or products and the reflective manufacturers have a point to make when they say the R value of their product is not its primary benefit... it's the reflectivity, which does actually have benefit if you engineer a primarily radiant transfer situation, such as by using airspaces. This is NOT taken into account in an R-value test.That said, I would still prefer real R-value in nearly all cases, and the reflective industry did lie through their teeth about some things such as the prescence of radiant transfer under a slab (where there is none). But in walls, as tested by oak ridge national laboratory, with airspaces (i.e. a primarily radiant transfer situation) the stuff works decently well.But, you'd do about as well filling the airspaces with polyiso or a spray foam, agreed.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
I was looking to you to be the "expert".Now I did find this from the ASTM website for C518http://tinyurl.com/25n7c3"1.4 The heat flow meter apparatus establishes steady state one-dimensional heat flux through a test specimen between two parallel plates at constant but different temperatures. By appropriate calibration of the heat flux transducer(s) with calibration standards and by measurement of the plate temperatures and plate separation. Fourier's law of heat conduction is used to calculate thermal conductivity, and thermal resistivity or thermal resistance and thermal conductance."The best that I can tell from that description is that there direct physical contact between the heat flow meter and the MUT (material under test). So they transfer from the test plate to the MUT is conductive.As far as I can tell Yarbrough is talking about the heat transfer WITHIN the MUT. And frankly who gives a damm? All that is doing is confusing the issue even more than the rediculous claim of RB manufactures.If the MUT was a "thermosbottle" construction the heat transfer might be almost 100% radaiant from one side to the other of the MUT. Or the MUT might be a slab of closed cell foam which is almost 100% conductive transfer.Based on that discription of the test it give ABSOLUTLEY NO INDICATION OF WHAT THE MATERIAL WOULD DO IN AN ASSEMBLY WITH FREE AIR ON ONE OR BOTH SIDES..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Bill Hartmann,
I disagree with you when you write that a material's R-value "ABSOLUTLEY NO INDICATION OF WHAT THE MATERIAL WOULD DO IN AN ASSEMBLY WITH FREE AIR ON ONE OR BOTH SIDES." And I also disagree with your apparent attitude that "who gives a damn" about R-value.
1. We need to define our terms. R-value has been defined and is an eminently useful measurement. I, for one, give a damn about a material's R-value. In the absence of a consensus definition to "R-value," we are at the mercy of manufacturers who invent their own backyard test of heat transfer. (Believe me, many manutfacturers have tried.)
2. In fact, determining the R-value of a building assembly is not that complicated. It can be done by measurements using ASTM C1363, or it can be done by adding up R-values listed in ASHRAE Fundamentals. ASHRAE provides the R-values for air spaces, including air spaces bounded by materials with a variety of emittances, including aluminum foil. ASHRAE also provides a way to calculate the R-value of an outside surface in a 15 mph wind, and an inside surface with still air.
3. The confusion arises when a builder repeats an untruth -- for example, "Reflectix has an R-value of 6." (Believe me, such untruths are repeated all the time.) Reflectix has a measurable R-value of 1.04. If you want to calculate the R-value of an assembly, go right ahead: inside surface; one layer of 1/2-inch drywall, 3/4-inch air space bounded by Reflectix; Reflectix; 1 1/2 inch airspace; 1/2 plywood; wood lap siding; outside surface. Add it up; you have your R-value for the entire building assembly, including the two air spaces. Just don't attribute the R-value of the entire assembly to one component -- the Reflectix.
4. None of these calculations would be possible unless engineers defined R-value; differentiated between the R-value of a material and the R-value of an assembly; and published the results of testing in a useful way.
" And I also disagree with your apparent attitude that "who gives a damn" about R-value. "I never said that."or it can be done by adding up R-values listed in ASHRAE Fundamentals. ASHRAE provides the R-values for air spaces, including air spaces bounded by materials with a variety of emittances, including aluminum foil. ASHRAE also provides a way to calculate the R-value of an outside surface in a 15 mph wind, and an inside surface with still air."If it is that easy why does the federal energy lab do all of those testing on whole wall assemblies.Do you have any reference to calculating the r-value of an assembly with radiant barrier on the inside. I have never seen this. But it has been long time since I needed to calculate an assemble and then it was all solid materials (except for the boundry surface).But now you are saying that with a with the radiant barrier material it is not just the R value of the RB, but also the emittances of the RB.Which is exactly what I was asking about in the first post."Now if those plates are directly on the material then there is only thermal conductivity.If what is being tested is assembly and that assembly includes an air gap then it then radiant would also be included"Which, if correct then just quoting the r-value of the RB does not really give much information about it effectiveness as a insulating product when properly installed in an assembly.And after all that is what the whole purpose of these tests and calcualtions are for..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Bill Harmann,
In ASHRAE Fundamentals -- a hardback book -- consult the following chapters:
1. Chapter 3 lists the emittances of a variety of surfaces (Table 3). Highly polished aluminum has an emittance of 0.02 to 0.04 at 50 degrees to 100 degrees F.
2. Chapter 22 lists the R-values of a variety of air spaces (Table 2). For example, at a temperature difference between inside and outside of 30 degrees (mean temperature 50 degrees F), a 3/4-inch air space bounded by a material with a emittance of 0.03 (that is, highly polished aluminum) has an R-value of 2.91.
Here's the rub: the air space contributes to the R-value. It's not just the aluminum foil that is creating the R-value. A 3/4-inch air space bounded by a material with a white surface -- for example, the paper facing on drywall -- has an R-value of about 0.94. That's significantly less than the R-value of the air space bouded by aluminum foil. The lesson: if you are incorporating an air space into your wall assembly to improve the R-value, emittance matters. Aluminum foil has a low emittance -- it's low-e -- which is good in this application.
The basic problem with reflective insulations is not that they don't work. They work. They are low-e surfaces; if they are used adjacent to air spaces, they can create a building assembly with a useful R-value. The problem with reflective insulation is that it is fussy to install -- very labor intensive. For the R-value you get, it is almost never worth it. It is degraded by dust. It doesn't do anything magical. It doesn't stop a certain type of heat transfer that passes magically through walls lacking a layer of aluminum foil. It's just a pain in the butt, with a few exceptions ( for example, foil faced roof sheathing is useful in Florida for builders who don't want to move the ductwork out of the attic and into the conditioned space where the ductwork belongs.) In almost all cases -- duct insulation, wall insulation, and insulating Airstream trailers -- there is a better product that is cheaper to install and that performs better. And you don't have to create a fussy, delicate air space to make it work.
"2. Chapter 22 lists the R-values of a variety of air spaces (Table 2). For example, at a temperature difference between inside and outside of 30 degrees (mean temperature 50 degrees F), a 3/4-inch air space bounded by a material with a emittance of 0.03 (that is, highly polished aluminum) has an R-value of 2.91."Is that onesided? Is that would you get say adding sheating with foil on one surface?If you have a 1 1/2" air space with foil in the middle would it be twice that amount?BTW, when I was a kid we bougth a house built around 53-54 which had an according foil insulation. The best that I remember it had foil on kraft paper. I am thinking it was 4 or 5 layers. But that was over 50 years ago and with CRS I am not sure."The problem with reflective insulation is that it is fussy to install -- very labor intensive. For the R-value you get, it is almost never worth it. It is degraded by dust. It doesn't do anything magical. "I don't disagree with the comments about fussy or the dust. But never having seen any numbers of what the equivalent R value of a an assembly using a RB is then I have no way of qualify the affect of dust. Thanks for giving we some numbers to work with.But that gets me back to this statement."Yarbrough wrote a letter to Energy Design Update in November 2003, writing, "The R-value used to describe thermal insulating products includes heat being transferred by all three mechanisms [conductivity, convection, and radiation]. The term used in the thermal insulation community is ‘apparent thermal conductivity.’ A formal definition for apparent thermal conductivity is contained in document C168 published by the American Society for Testing and Materials. The term is applied to situations involving the simultaneous flow of heat by all three transport mechanisms. The statement ‘R-values are measures of conductive thermal resistance’ is incorrect, if it implies a limitation, since ‘R-value’ includes radiation and convection when they are present." "R values along does not give the different products won't give the whole story of what the r-value of an assembly using an RB is. You need to also know what the emittance is.And that just saying that the r-value of Reflectix is 1.04 is not telling the whole story..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Bill,
You are certainly right when you wrote, "R-value alone does not give the different products won't give the whole story of what the r-value of an assembly using an RB is. You need to also know what the emittance is. And that just saying that the r-value of Reflectix is 1.04 is not telling the whole story."
Right. Assembly R-values matter. The whole assembly, including the emittance of any surface adjacent to an air space. (This certainly holds true for windows.) But really, seriously now -- how many purchasers of Reflectrix understand that? Many people install Reflectix without an air space.
The problem is that when I see something that I know is not right then I start questions EVERY about the situation. It is all SUSPECT until I can get enough information to put it in it's place..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Bill,
I forgot to answer one of your questions: "If you have a 1 1/2" air space with foil in the middle would it be twice that amount?"
The answer is yes. The R-value of the two 3/4 inch air spaces created would be about R- 5.82.
Reflectix has a measurable R-value of 1.04
Is that with or without the airspace?
'Cuz I know I can hold my coffee cup in my hand, but not the stainless coffee pot. OTOH, I can feel more heat radiating off the cup than off the pot. OTOOH, 1/4" of ceramic has a lot higher R value than 14ga stainless.SamT
There are three kinds of people: Predaters, Prey, and Paladins. For the life of me, I can't see why Prey feels safer from predators by disarming and emasculating Paladins.
Sam,
You're right, of course -- a china cup is much less conductive than an aluminum coffee pot. Less conductive; a better insulator. Thermal conductivity and electrical conductivity are similar. We use aluminum or copper for coffee pots and wires; we use porcelain for coffee cups and old-fashioned electrical insulators.
Reflectix has a measurable R-value of 1.04
Is that with or without the airspace?
Edit: From reply to Bill; ), a 3/4-inch air space bounded by a material with a emittance of 0.03 (that is, highly polished aluminum) has an R-value of 2.91.
. A 3/4-inch air space bounded by a material with a white surface -- for example, the paper facing on drywall -- has an R-value of about 0.94.
So the answer to my OQ is "No, it's not with an airspace."
SamT
There are three kinds of people: Predaters, Prey, and Paladins. For the life of me, I can't see why Prey feels safer from predators by disarming and emasculating Paladins.
Edited 4/7/2007 9:30 pm by SamT
Sam,
That is without the air space. As I explained in my answer to Bill, a 3/4 inch air space bounded with a low-e material would have an R-value of about 2.91. If you add the air space and the Reflectix, you get an assembly R-value of about 3.95.
Sam,
All material R-values are reported without an air space. If you want to report the R-value of a material with an air space, that is no longer a material R-value; it's an assembly R-value. You could also calculate the R-value of other materials adjacent to an air space -- expanded polystyrene plus an air space; fiberglass batt plus an air space; cardboard plus an air space. All of these assembly R-values would be greater than the R-value of the polystyrene, batt, or cardboard alone.
Changing the color or material of the hot plates will change the radiant heat flow. The test is meant to measure conductive heat flow.I get your point tho.Stu
I lined the interior of my van with Prodex radiant barrier/insulation. I did my best to provide an air gap- especially on the roof.
Before insulating, the factory AC would not keep the van cool. After, there is no problem with cooling. The difference is amazing..... but I have no similar benchmark for heating.
I suspect that a spray in foam would give you the best results.
I do not know the specifics regarding the construction of your Airstream, but with the mention of "stressed-skin panels inside and out", I would be looking for structural rivets and adhesives...
Forget about the Reflectix. With shiny metal walls to begin with, a layer of foil applied to bubble wrap isn't going to do much for you!
I was under the impression that Airstreams were made using stainless steel, rather than aluminum. I could be wrong; in this example, it really doesn't matter.
Fiberglass is simply an awful choice of material for anything wi9th thin walls, that moves, or will be occupied by forest critters much of the time. For those applications, there is but one choice: foam, especially spray foam.
A minimal expanding foam should be used, and applied in small sections, to prevent the expansion from popping the walls apart. With metal on both sides, the curing of the foam is also a major concern. Spray foam requires moisture to cure. While it usually finds this moisture in the air around it, in a closed space the foam can easily use up all the moisture before it cures. This results in some of the foam not curing ... which means not insulating.
You can prevent this from happening in two ways. The first is to do small areas at a time ... no larger than 18" square by 2" thick. I also suggest that you add some moisture ... say, a single squeeze of a spray bottle mist into the cavity before you apply the foam.
Another place you need to make a change to is the floor. A LOT of heat escapes there ... and has a HUGE impact on your comfort. I'd put down at least an 1 1/2" of rigid foam, a layer of Masonite, and cover that with padding and carpet.
You'll need to make some sort of insulated cover for the windows. There will also be issues with ice in the door frame and lock. Not a lot you can do there, except preventive oiling ... and keeping a propane torch handy .. on both sides of the door!
, or will be occupied by forest critters much of the time. For those applications, there is but one choice: foam, especially spray foam.
Have you ever seen a carpenter ant colony in urethane foam? They love that stuff. Tunnels and nests all over the place, just under the surface. It's eerie: you can stand there in the basement and hear them chewing their way through it even while you're explaining to the HO that this is gonna cost him big time....
As to the Reflectix itself, I put it in the lower roof of my own gambrel 12 years ago, to boost the R-value I'd lost by venting the 2x6 rafter bays. The claim was an additional R-10 for the assembly when laid over FG batts and under furring.
Based on 22 years of heating bills, I would estimate that is roughly what I got. This place had 4" wall and roof insulation previous to the major re-build, and I know what it cost to heat then...and what it costs to heat now: about the same, but the cubic heated space has tripled.
Yes, I put R-40 batts (and Reflectix) over the ceiling below the upper roof, but it's a much smaller area than the two lower roof slopes. And those contain only 4" batts (plus the Reflectix). Ergo, that shiny bubble wrap is doing a pretty good job.
FWIW....
Dinosaur
DON'T MISS THE FEST!
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....