Need Materials Review - R-Value Specification check and verify...

[Gooserider]

As many of you know if you've been watching the threads, in addition to getting things ready for the Hearth Party, I've been working on an extensive article on Hearth Design and construction for the Wiki. I would appreciate general review and comments, but I need one specific part checked over specifically, preferably by folks with access to good "official" documentation.

One of the things that I've been running into as a problem is coming up with a table of R-values for different possible materials that one might want to put into a hearth pad. I started by cribbing the table that Tom Oyen had over in his Sweep's library, and then adding such things to it as I find in other sources. However I'm finding discrepancies in some of the values, and I want to see if there are folks like Elk or Corie that can verify the correct values, and possibly give better sources for them - Additional items are also welcome. Note that currently it's a bit tricky to edit the table, I had to take a screen shot of it and paste the image into the article in order to make the columns work properly, so it would be best if you could post anything extra in this thread and I can then make the changes on my end.

The items I'm having particlular problems with are:

Ceramic Tile 1/4" 0.020 Tom says R=0.02, Hearthstone says 0.01

Concrete 1" 0.950 Tom says R=0.95/inch, but masonry cement is 0.1 per 1/2", and I don't see that much difference between the two to account for the huge R difference.

Concrete Block empty core 8" 1.000 This value came from a Hearth article, and seems awfully low just from the thickness of the blocks, not to mention the materials in them

Gypsum or Plaster Board 1/2" 0.450 Seems awfully high in light of the values for other cement type boards (also is this the same stuff as sheet rock, and if so, what about the paper backing?)

Horizontal Dead Air Space 1" 0.970 3 1 This value comes from Elk, Hearthstone and ASHRAE says 0.92 per 1/8" if each layer is separated by non-combustibles, however we are already beating this one up pretty well elsewhere, I don't think we need to go into it here...

Rockwool or Fiberglass Bats 1" 3.330 I'm assuming this is the same as "mineral wool" in which case Hearthstone is only giving a value of 3.12/inch

These are the Hearthstone chart values where I found discrepancies with the other numbers...
Ceramic Tile 1/4" 0.01
Mineral Wool Insulation 1" 3.12
Horizontal Still Air 1/8" 0.92

I'm also having trouble getting the chart to reproduce accurately in the post, so I'm attaching a screenshot of it so you can see the whole thing.

Gooserider

 

[nshif]

Per USG Durock has an R value of .26 per 1/2 inch.

http://www.usg-flooring.com/CB399.pdf

 

[webbie]

The ceramic tile figure is so low, that the difference does not mean much. You'd need 50 layers to make a hearth. So it is a moot point pretty much - use a figure 1/2 way in between. It can differ, I'm sure, depending on the type of tile (stone-based, terra cotta, etc.)

It's another story, but I still question whether an R-Value at room temperature is the same as the R-value at 400-500 degree...but you can bet I will find out!

 

[nshif]

From what I can find R value is the ability of a material to resist heat transfer from one side to the other. If a material can resist 5 degress of heat transfer it will do it at any temp. How this is possible from 100 to 900 degres is beyond me but everything Ive read so far seems to support this. It is also interesting that R values are derived from K values so mabey that is where we should be looking.

 

[Gooserider]

Thanks for the link nshif, I've updated my working copy at home, will update the Wiki next time I get a bunch of editing done.

Web, I agree that the difference in the ceramic tile is small, but it could be significant, if someone has a hearth that is right on the edge of acceptable, that .01 difference could be the amount that makes you pass or not. I'm not sure I'd want to see a hearth pushing that close to the limit (unless you think they have a lot of safety margin already built into the number) but I also don't want to be in the position of telling a user (via the chart) that a given combo will pass and then have him run into an inspector w/ different numbers who says it doesn't.

This is why I'd ultimately like to have a source for each value in the chart that comes from the sort of "official" source that an inspector would have a hard time arguing with - I.E. a manufacturers website, a reference manual from a recognized engineering body, etc.

nshif: From what I can find R value is the ability of a material to resist heat transfer from one side to the other. If a material can resist 5 degress of heat transfer it will do it at any temp. How this is possible from 100 to 900 degres is beyond me but everything Ive read so far seems to support this. It is also interesting that R values are derived from K values so mabey that is where we should be looking.

Agreed, on the definition, the difference in temperatures is what drives the thermal transfer, and the R- or K- value defines how easily the transfer will take place, it is the size of the difference that matters, not the absolute temperature on either side (As long as neither side hits the point where the material breaks down, changes state, or otherwise changes it's properties)

In some ways I'd agree with you about using K-values instead of R-values, but K-values are a pain to deal with as they are constants defined in terms of how much heat will transfer through a given thickness of material, which doesn't match the thickness that we would be using in the hearth construction. One of the things that converting to R-value does is let you specify an actual thickness of the layer, so you can get a number for the actual materials you will be using, and can just add up the totals to see what you get. Thus the R-value is much easier to work with for our purposes.

Gooserider

 

[nshif]

Goose I agree that the table should be in R I only metioned K to possibally get a better understanding of why the Rs are what they are and how they are arrived at.

 

[thechimneysweep]

Actually, the R-Value chart that appears on our website originally came from Hearthstone. You can view it in the dealers-only section on their website at http://www.hearthstonestoves.com/documents/TB79 - Insulating Values of Common Building Materials.pdf (Please do not let on who revealed the existence of the super secret dealer section)

 

[webbie]

From what I can find R value is the ability of a material to resist heat transfer from one side to the other. If a material can resist 5 degress of heat transfer it will do it at any temp.

Everything I read seems to disagree with your point - but I have two experts looking into it.

If what you say was true, they would not even specify that building materials were tested for R at an inside temp of 75 degrees...there would be no need to say so!

From the government site for building material tests:
http://www.ftc.gov/bcp/rulemaking/rvalue/16cfr460.shtm#5
"R-values given in labels, fact sheets, ads, or other promotional materials must be based on tests done under the methods listed below........The tests must be done at a mean temperature of 75 deg.Fahrenheit."
The same paragraph discusses air space and requires that air be tested at 50 degrees, with a variable of 30 (so that is 20 degrees to 80 degrees)...

OK, so is R-value is constant, why do they have to make rules specifying the temperature?

Again, I am only guessing and talking about my gut feeling, but the little bit of facts I have so far are showing that these values differ GREATLY with temp. This does not mean the specified materials or hearths are unsafe, but it does mean that the guidelines for them could be wrong.

Here is an article that clearly says that R values change with the temperature (concerns hot tub covers)
http://www.hottubessentials.com/cover_R_Values.htm

As a final piece of todays "evidence" here is a chart from Dupont showing the reduction in R value of one of their foam products - notice the slope of the line - DOWN with temperature increase. Now imagine what would happen if the chart went up another 100 degree or more!

That is the great thing about the wiki - we can find more info and then update it.

 

[elkimmeg]

Its late afe ter a career golf day a couple of point here I think the report should indicate ignition temps of or non combustibility like the point of common fiber glass melting

Again without good questions or concern anw swers may not be researched Many products are made for one purpose or function be find there way preforming additional ones
common fiber glass r value is determined upon density I' canhave a 3.3 r value and higher density HVAC wrap 1"=R4.3 a Rule ot thumb poured 10" concrete foundating = R1.0

To manswer Craig question Fiber glass and tile backed boards are made originally for home building products Like any test or scientific experiment ther is a constant or set of controled conditions the 75 drgrees is set for control testing condition for home building products since that is what the product is intend use why test it for a range not experienced or required now wht about rock wool or ceranic blanket insulation in this discussion Also what about relfective value of either foil faced insulation or sheet metal does the reflective value degrade due to moisture aznd dust /dirt in the air forming a film on the reflective surface?

 

[Highbeam]

Please remember that this hearth site is a place where regular people come to seek information for their regular installations using regular materials to be approved by regular inspectors. For us to dilute or ruin the value of good and applicable information by tring to challenge every notion of why certain things are accepted we are doing the regular people a disservice.

For example, a guy like me wants to follow a plain vanilla chart to find the R-value for durock. I want to find the accepted value of 0.2 per 1/2 inch. That is the correct R-value that every inspector worth his salt will use but if I were to read this thread I would be uncertain about what temperature that the R-value of 0.20 is calculated. 75 or 500?

I'm all for overanalyzing things to the point of silliness just for the fun of it, guilty as charged. For the sake of regular people trying to build legal hearths we must remember to identify the point where the analysis has become less about applicable information and more about academics.

Just my opinion of course, I am getting confused.

 

[elkimmeg]

I agree with highbeam there is a lot to be said by keeping it simple and in layman terms

 

[webbie]

I would say that as long as one uses DuraRock, Wonderboard (or Hardie once we get an answer from them), and then Micore or stuff like that, they will be 100% OK at the temperature ranges we are talking about.

For simplicity (and once all info is in, we can edit the article to START with the simple stuff and then go into detail), I would NOT use air spaces, fiberglass insulation or sheetrock for hearth use. Although there may be safe ways to do so, with other materials available it seems there is little reason to use these methods. Remember that the whole thing started because of the 1/8" airspace being equiv to almost R=1. This brought up the other questions. If we end up finding out some important information, and (for instance) HearthStone and others change their manuals because of it, then all the silliness would be valuable.

That said, I agree that our own Hearth.com articles should eventually start with the "easy way", so the layman does not have to read through the whole thing unless they need to - good suggestion, thanks!

BTW, the amount of heat radiated downward is pretty serious in some stoves. For instance, it used to be (and maybe still is) that many VC stoves as sold (without the bottom heat shield) could ONLY be installed on concrete above BARE EARTH. In other words, no hearth would do! That is a pretty scary indication of the amount of heat radiated down. But if we used NFPA standards for the same exact stoves (leg length), we'd probably ignite the floor. So we should not gloss over this subject either.

Note: See attached for acclaim specs - as noted, there is NO hearth that will do the job except concrete over bare earth. How many people have that? Yet the heat shield is optional. It would be my guess that thousands of these are installed incorrectly - for instance, people who put them in fireplaces as the pic shows probably do not use bottom heat shields. If I were UL (or the test lab), I would not allow such a spec in the manual, I would require that the stove be standard with a heat shield....but that is another story. In any case, this spec should show the seriousness of downward radiation and why NFPA hearth specs mean nothing except in "generic" terms. (no manual available, etc.)

 

[Gooserider]

I thought that these days the VC stoves did come with the heat shields as part of the package - my interpretation was that the shield was "optional" in the sense that one didn't have to install it, not that you didn't pay for it

However on the main point, I agree the articles should be simple and in lay terms, which I'm mostly trying to do in the stuff I write. However I figure that it's essential that those who are WRITING the articles have a much deeper understanding so that we can be sure to include the right information.

Gooserider