Talk:R-value (insulation)

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How does the rate work without factoring time into the equation? In the formula example used the answer is 5 watts per sq m. That doesn't mean anything without time.George Swantner 18:55, 14 September 2007 (UTC)

I've taken the liberty of editing this, even though it's a topic on which I'm no expert. However, it's very clear that at least in New Zealand, R-value is taken to mean SI R, and nothing to do with fahrenheit or BTU. Perhaps the most contentious bit is my argument about thermal conductance through ceilings, so if any physics or building experts are out there, and I haven it terribly wrong, accept my humble apologies.--Limegreen 04:22, 26 May 2005 (UTC)

To avoid confusion, I think Wikipedia should default to SI units. 217.121.144.89 09:23, 28 May 2005 (UTC)
Good changes. I didn't want to appear like some avenging crusader, which probably impaired my clarity.Limegreen 00:17, 29 May 2005 (UTC)


Contents

[edit] F° versus °F

I think it is more proper to use the unit F° rather than °F when referring to temperature differences. One does not see this distinction made very often but it is technically correct. °F refers to a position on a temperature scale while F° refers to a unit of measure. Of course the same holds true for °C and C°. —Preceding unsigned comment added by 12.10.81.212 (talk • contribs) 2005-11-09 16:19:05

I don't think I've ever seen that notation used, but it sounds like something that could be very useful. Can you show any examples of where that notation is used or defined? It's rather difficult to search for on Google, since the degree symbol just gets stripped. —?ospace; font-variant: small-caps;">HorsePunchKid→龜 21:27, 9 November 2005 (UTC)


I've never seen that notation used, neither for the Celsius nor Farenheit scale. A temperature difference is in degrees Celsius; it doesn't make sense to talk about Celsius degrees. If this (distinction) is technically correct as claimed, please state your source. At least for my own part, as an energy engineer, I've never seen it in use. As an alternative to °F or °C, all temperature differences could be stated in Kelvin [K].

[edit] Incredible External Link?

Granted, it may be difficult to get a nonprofit link which is informative in such a mundane (and yet profitable) subject, but I do think that the "'R' fairy-tale" site, as hosted by an organization which seeks to profiteer off of an architectural concept (the "Monolithic Dome"), is not suitable to be linked to by an encyclopedia. Aside from discussing the "ignored" factor of convection on heat flow, the Monolithic Dome Institute does nothing other than to promote the sale of all things "Domephile." And as their financial future is dependant on their ability to promote dome-mania while dismissing weaknesses in their design, the institute does not qualify as a nonbiased source. I would recommend the replacement of those two external links with either non-profit organizations or academic sites.

[edit] "Some other countries" = "U.S."?

I think it would be useful to make it clear that the "some other countries" that use the BTU definition of R-factor include the United States. Also, I think it would be useful to give a straightforward defintion of the U.S. version of R-factor in W/m2KPciszek 19:15, 15 March 2007 (UTC)

[edit] Company Pink

I think of great value would be a section on how this law about the R-Value being the standard for heat transfer for insulation in the US came about. About how "Company Pink" made a deal with congress to get this law passed (which had an insulation with very good R value but quite low conductive and radiative values) which made R-Value the standard. Also how the R-Value rule is keeping many good competitors from being profitable because they have lower R values but much better values for the other two types of heat transfer. I can't give citations on this, but my physics professor ranted for ten or so minutes on it. I'm sure some of you have heard of this. Ergzay 03:12, 21 September 2007 (UTC)

[edit] Reference Spam

It doesn't seem that the single reference link (PDF file) contains any verifiable information and is not directly related to any information on the page (in fact, it contradicts much of it). Moreover, the target of the link is on a commercial site and clearly promotes a product.

—Preceding unsigned comment added by 68.147.142.222 (talk) 19:29, 12 October 2007 (UTC)

[edit] Discussion of what it includes

I just removed this discussion from the article, thinking that it belonged on the talk page. So here it is:

"Correction! The transfer of heat by conduction is specific ONLY to a material's specific resistance to that transfer. The only fixed factor is the material's density which in turn determines its resistance or "R" value. ONLY If the material is porous, could the the R value include convection or radiant transfer potential. The National Fire Service describes CONDUCTION as "heat transfered between two solid surfaces in DIRECT contact with each other that allows heat to be transfered between physical objects." By definition, this would exclude radiant and convective transfers as these means are based on radiance or particles in cirrculation. Simply because the " thermal Insulation Community" dislike competitive products that make insulation more effective, is no reason to rewrite basic scientific principles. "

I think the above is unclear and confused and inappropriate style for wikipedia, but thought it might be more productive to put it here than just delete it. It's true that most solid materials don't have convective or radiative heat transfer inside them, but most insulation materials are not homogeneous solids. I'm also not sure the relevance of the NFS definition of conduction.Ccrrccrr 04:26, 16 October 2007 (UTC)

[edit] Apparent Thermal conductivity

It would be useful to have the analytical method or model used to determine the 'apparent thermal conductivity' of the material quoted by Yarbrough. Is this 'standard' of quoting R-value universally accepted or is it a proposed industry standard. Is the 'r-value' obtained useful for the range of applications and climates in which it is used,

An opinion related to this that was in the article, but belongs hereCcrrccrr (talk) 13:50, 14 December 2007 (UTC)
Ideally, a rating system should properly describe and separately quantify the heat transfer depending on its mode of transfer e.g. conduction, convection or radiation (see below) as they are processes that can be independent of one another.

[edit] Impossiblility

I just deleted this false statement from the article:

It is impossible for one type of material to effectively retard all types of heat transfer.

This is not true. A block of foam blocks airflow effectively, retarding convection dramatically. It has low thermal conductivity, thus reducing conductive heat flow greatly. And it's opaque to thermal radiation. Almost any material is opaque to thermal radiation--windows for thermal radiation need to be made of special exotic materials like ZnS. See, for example, [1]. "Radiant barrier" materials prevent radiant heat transfer through the airspace adjacent to a wall, rather than preventing radiation from going through the wall. Thermal radiation doesn't go through the wall, unless you make a wall out of translucent polyethylene.Ccrrccrr (talk) 14:01, 14 December 2007 (UTC)

Many materials be opaque in the visible light spectra but not in the infra-red and very few to x-rays. Materials can be spectrally selective and emissivity will vary according to wavelength. Even the sun does not emit equally at all wavelengths - that's why it appears yellow. If an object can absorb thermal radiation then it will also emit it. Therefore, they can 'appear' to transmit it. Radiant barriers work partly because they reflect radiation to its source but mainly because of it low emissivity, they won't re-radiate it on its opposite surface. A clay brick is different - it progressively heats up and will radiate its heat into the interior. 219.90.145.245 (talk) 00:12, 15 December 2007 (UTC)

X-rays are much shorter wavelengths than visible; IR is longer wavelengths than visible. The analogy to X-rays is irrelevant. Yes, some materials can be opaque to visible and transparent to IR. But they are hard to find, especially for the thermal (far) IR band. Check out the link above.
Emissivity does vary widely. Yes, high absorption means high emissivity as well. Yes, an object can "appear" to transmit radiation because it absorbs it and re-radiates it--a simple wood wall or a pane of glass are both example in which this would happen. The heat transfer is reduced, but not a lot. That's part of the classic greenhouse effect. Yes, a radiant barrier can work either by reflecting radiation to the source or stopping the re-radiation. However, if a solid object is absorbing radiation on one side, and re-radiating it on the other side, the heat has to get from one side to the other. It does so by conduction. If the solid object includes bulk insulation between the two surfaces, that re-radiation and apparent IR transmission is reduced by the r-value of the bulk insulation. The bulk insulation is not somehow bypassed by the radiation.Ccrrccrr (talk) 01:22, 15 December 2007 (UTC)

No argument here but the surface temperature of your insulator (which is exposed to solar radiation) is going to be much higher than the ambient temperature). R-values being constant and temperature gradient same, the surface temperature of the other side of your insulator will also be high (and also not equal to the interior air temperature). This hot surface will be the source of thermal radiation.

The easiest way to test this is create a foam box. Put a infra-red camera inside it and stick the box in the midday sun. See what kind of thermal radiation is being transmitted. —Preceding unsigned comment added by 219.90.145.245 (talk) 03:46, 15 December 2007 (UTC)

[edit] Overwhelming bulk insulators

Another section removed:

Similarly, bulk insulators can be easily overwhelmed by solar radiation during summer (where black globe temperature far exceed ambient air temperatures[1] but are much more effective in reducing winter convective losses. Often a judicious combination of both materials is required to achieve an optimum solution for a mixed climate.
WBGT is designed for assessing heat stress for people who are outdoors. Figuring out how to apply it to buildings is not straightforward and would be original research. That sounds useful, but it doesn't belong in Wikipedia.Ccrrccrr (talk) 14:15, 14 December 2007 (UTC)
Wet bulb globe temp is not the same as black globe temp. My point being that when insulation is modelled, is it being subjected to the same radiation flux as would experienced during summer. Furthermore, is it measuring the temperature of the ambient air on each side of the insulator or the temperature that is also due to the effects of radiation (black globe temp or mean radiant temperature). This would markedly change the results of any formula on 'thermal conductivity' that you use.Dymonite (talk) 10:00, 23 December 2007 (UTC)

Right--WBGT and BGT are not the same, which was a problem with that original text which linked to WBGT using the name BGT. And yes, a black building in the sun will have an average wall temperature similar to BGT, which will be higher than the ambient air temperature. The implications of this are that light colored exerior finishes, particularly those certified to reflect solar IR (see cool roofs) are advantageous and that more insulation might be needed than what would be calculated based on ambient temperature. That additional insulation may be of any type--radiant barrier or bulk--the high BGT does not have any implication for which type is preferred. Perhaps I should have corrected the paragraph to explain all that rather than deleting it.

[edit] Foam insulators

It seems that ccrrccrr is a strong advocate for foam insulation and it seems the thread is arguing for its use. I think it sounds like a promising alternative. But this product is only new to my country and with most insulation products, you cannot get any info on the lab methodology. What it the emissivity of foam for heat energy? Has this been determined experimentally. If I stood under a block of foam in the midday sun, how much of a temp difference can I experience?

219.90.145.245 (talk) 00:12, 15 December 2007 (UTC)

If you want to know my opinion (something I try to keep out of my editing), I think foam is great for some things but I'm much more enthusiastic about cellulose, because you can get a much higher R-value for the same expenditure with cellulose, and I think that the global warming potential of HFCs in many foams are a serious problem that should be taken more seriously particlarly by people who advocate superinsulation to reduce CO2 emissions.
I certainly hope my edits don't turn this into an advocacy piece for foam--and I hope that they don't undercut the real value of cool roofs and radiant barriers. My goal is accuracy, not advocacy for any particular product. I don't object to claiming benefits for cool roofs and radiant barriers--I just want to make the basis for that claim technically sound. And I want to keep this page a discussion about

R-values, with mention of and links to other topics as appropriate.

I'll try to answer your questions soon.

Ccrrccrr (talk) 13:07, 15 December 2007 (UTC)

If you look at the building insulation materials page and the history of my edits there, I recently restored some disadvantages of foam that had been listed and were removed by someone else. No, I'm shilling for the foam industry!

I don't know the emissivity of foam, but I think it's a safe bet that it's high, like around 0.9. If you used a piece of foam as a porch roof, the top surface of it would get hot from the solar radiation. But you'd want to cover it with something anyway, since it's not resistant to UV damage. If you do that, then it's the emissivity of that cover (paint, shingles, metal, etc.) that matters on the top, and the emissivity of the top surface of the foam doesn't matter. If the top surface does get hot from radiation (black shingles, for example), most of the heat it absorbs will be lost by convection at the top surface, and relatively little will conduct through. The temperature on the porch will not be affected significantly by the solar radiation on the roof--most of the heat will come in the open sides of the porch.

If we wall in the sides of the porch, and insulate them, then the heat gain from the heat flow through the insulation on the roof will become important, especially if, with black shingles, the top surface is hot. Then we might want to reduce that heat flow. We could do that with any bulk insulation (more foam, or cellulose which is much cheaper), or we could do it with a radiant barrier on the bottom of the foam. In a hot climate like most of Australia, a radiant barrier would be a really good choice, because it will give significant benefit at low cost. If you are in a mixed climate, it's less clear what you should do--if you add more foam or cellulose, that will help in both summer and winter, whereas if you add a radiant barrier it will help significantly in summer, but not much in winter. That's because the radiant barrier helps for downward heat flow much more than for upward heat flow.

I hope that helps answer your question.Ccrrccrr (talk) 14:28, 15 December 2007 (UTC)

[edit] Opinions about what data sheets should include

I removed the following from the article because it is an opinion, which doesn't belong in Wikipedia. I agree with some of it, but that's irrelevant. Opinions belong on the talk page.Ccrrccrr (talk) 13:09, 15 December 2007 (UTC)

Data sheets for insulators can be scrutinised for appropriate testing methodology. Such a analysis should specify the type and intensity of heat energy that come from both conductive and radiative sources. It should also document both ambient air temperature and surface temperature on both sides of the material. It should also make clear other specifics pertaining to details about convective gas/fluid movements. A calculated value for thermal conductance and emissivity can then be derived from the data set to quantify the impact of both conduction and radiation through the material.

[edit] Type of heat source used

I removed this sentence:

The measured heat transfer will vary depending on the type and intensity of heat source used in the model.

If you have a conventional bulk insulator, the heat transfer through it comprises radiation, convection, and conduction. The mix of those three does not depend on the mode by which the heat arrives at the surface of the test box. Once the heat arrives at a wall, it is just heat, regardless of how it arrived there, and it's free to proceed through the wall by whatever means is permitted by the insulation there. It's not like heat that arrives at the wall as radiation can only go through the wall as radiation.Ccrrccrr (talk) 13:15, 15 December 2007 (UTC)

[edit] Definition of R value

I added:

R-value is a measure of apparent thermal conductivity, and thus describes the rate that heat energy is conducted through a material or assembly, regardless of its original source.

The higher the R-value, the slower at which heat energy is conducted. It describes the rate of heat transfer and not the total amount that is transferred. It also does not consider the proportion of radiant energy that is reflected away from the surface of a material before being transmitted through it.

Heat energy does not travel through a bulk insulator by radiation. Mostly by conduction and a little by convection (if there is a gas/fluid medium within). Not all radiant energy will be converted into conducted heat - some of it reflected away. Materials with low thermal conductivity only retard the flow of energy (most of it eventually reaches its destination). The most effective means of reducing solar radiation is to reflect a portion of that energy away rather than having a massively thick bulk insulator to deal with the load to slow its passage.


Could you please sign you comments on the talk page? It's hard to follow the conversation otherwise. I can't tell whether the above is two people talking or one. ThanksCcrrccrr (talk) 23:31, 30 December 2007 (UTC)

oops sorry

Dymonite (talk) 02:26, 31 December 2007 (UTC)


We might want to add material on the mechanisms of heat transfer through insulators on the building insulation page--it's a surprisingly interesting topic. Heat does transfer through an conventional bulk insulator by all three mechanisms. A bulk insulator is a large collection of little air pockets. For the moment, imagine a closed-cell foam. Heat can:

  • Flow through the solid material, around the air cells.
  • Flow across one air cell, then through the solid cell wall to another air cell, across that air cell, etc. The heat transfer across the air cell can happen via:
  • Conduction through the air (air's a very poor conductor, but it does conduct some).
  • Convection loops within the air cells (hard if the cells are small, but can happen)
  • Radiation between the walls of the cell.

If the foam is not closed cell, or the insulation is fibrous or loose fill, there can also be larger-scale convection permeating the insulation.

In the ASHRAE handbook there's a great plot of apparent thermal conductivity as a function of insulation density--at low density the thermal conductivity goes up due to more convection, whereas at high density the thermal conductivity goes up due to conduction. There's a point in the middle where thermal conductivity is minimized.

I don't know the exact relative contributions of the three mechanisms in different insulation types--it's hard to make measurements that separate them. (You can do stuff like measure the insulation in a vacuum--if it's not closed-cell--and eliminate convection, so you are just measuring radiation and convection. Then you can figure out how much is rad vs. conv. by looking at the dependence on temperature.) But the nice thing about apparent thermal conductivity is that you don't need to know how much of the transfer happens by what mechanism. If you know the R-value and "you know the temperatures on the two sides," you can calculate how much heat flows, while totally ignoring the mechanism of how it gets through the insulation.

The issue you are talking about regarding reflecting solar radiation away is addressed in the cool roofs article. That is part of what determines the temperature of the surfaces on the two sides of the insulation. That's really important (italicized above). Maybe we need to emphasize that more in the article.

Then there's the issue of rate vs. total amount. To get from rate to total amount, you integrate over time. Ccrrccrr (talk) 13:35, 31 December 2007 (UTC)

I agree that an article on cool roofs and radiant barriers are an important counterpoint to a discussion of R-values. The literature is peppered with R-values and nothing else. The overemphasis on retarding heat transmission through materials diminishes the equal importance of reducing a radiant heat load in the first place. Whilst R values have no place in the description of radiant barriers, there needs to be better standards to describe their usefulness in situations of high solar gain. Better consideration of terms like emissivity and reflectivity need to be considered in the building literature. I think a clear distinction should be made in the article between 'ambient air temperature' and surface temperature. These are two different things. Bulk insulators behave differently with an air temp gradient of 0C (outside) and 20C (inside) vs 20 (inside) and 40 (outside). The air temp gradient may only be 20C in both cases but the surface temp may quite different during summer due to the effect of thermal radiation within the roof space.

Dymonite (talk) 01:33, 1 January 2008 (UTC)

Dymonite (talk) 01:33, 1 January 2008 (UTC)

[edit] "Reader's Comment"

Reader's comment: I Subscribe to the U.S. FTC prohibition to the use of R-value per inch. So this section "Values per inch" is not appropriate (in the USA). Tables or graphs should be developed based on specific available products.

(not my comment--I'm just the editor the moved it here from the article. Note that the article has a discussion of this issue in the section above the list of R-values.Ccrrccrr (talk) 03:22, 8 January 2008 (UTC))

I'm not sure if this is the appropriate place to comment, however, would it be possible to include units in the tables? Yes, it is possible to find them from reading the sections above, but for clarity and completeness it would be useful to have the units explicitly stated in the table. Kmcfar2409 (talk) 15:43, 24 February 2008 (UTC)