The folly of using air spaces for hearth construction.....

[webbie]

OK, this is new thread since the other one on hearth materials is getting so long...

A comment about using air spaces and R-Values for hearth construction. I would not suggest using the common R-value of air space to design a stove hearth for the following reason - R and K factors differ depending on the temperature and "type" of heat we are talking about!

When R values are calculated in normal house constructions, it goes without saying that they are talking about temperatures of -20 to 140 degrees F. They are also talking about generalized radiation and heat transfer.

BUT

When we are talking about a stove hearth, we are dealing with DIRECT and invisible radiant energy waves, which pass right though air as if it was not there at all. The means it is possible that there is ZERO R value in this application. Let me explain....if the R value of Air was as high as suggested, it would be OK to simply raise the stove 1" up on blocks under the legs and then put a thin tile on the floor - BUT NO WAY WOULD THIS WORK.

Check out this quote from a site discussion heat transfer:

HEAT LOSS THROUGH AIR

There is no such thing as a “dead” air space as far as heat transfer is concerned, even in the case of a perfectly airtight compartment such as a thermos bottle. Convection currents are inevitable with differences in temperature between surfaces, if air or some other gas is present inside. Since air has some density, there will be some heat transfer by conduction if any surface of a so-called “dead” air space is heated. Finally, radiation, which accounts for 50% to 80% of all heat transfer, will pass through air (or a vacuum) with ease, just as radiation travels the many million miles that separate the earth from the sun.

Ok, so that says what I am trying to get at - that radiant heat will not be stopped by air to any degree. This does not mean that air has no place at all in hearth construction, but just that folks should NOT use that R-Value as the guide. As a for-instance, I would think that a hearth built of:
1 sheet of cement board 1/2"
then the hi-hat metal channels
then 2 sheets of 1/2" as the underlayment

would be quite effective, but when all is said and done, why not use 4 layers of cement board instead?

Comments?

 

[webbie]

A real world example of the above - using a common stove as an example.

A resolute acclaim is a stove that creates an intense burn in the rear combustion chamber. This burn not only increases the heat of the cast iron around it, but much of it simply penetrates the cast iron and escapes as direct radiation....

When I first started installing these stoves I always questioned the mantle clearances (HIGH), which were much more than for stoves of similar size. After all, I had been taught that heat radiation is a function of X sq inches of heat surface at X temperature. But after I installed an Acclaim in my own fireplace, I discovered the truth. Holding my hand two feet above the rear collar of the stove, I felt like it was getting sunburned! Not heat off the cast iron, but those invisible radiant waves focused upward. One one hand, it is an impressive display of the secondary and third level burns....this heat used to go up the chimney - but for our purposes here, it shows that radiant heat is NOT as simple as using NFPA or R-Values. When NFPA speaks of clearances, it is for "generic" stoves....which really means unlisted stoves.

Actually, my guess is that NFPA does not do any testing at all, but that the figures there are from past history and are agreed upon (and updated) by committee - including folks in the test business, of course.

Anyway, bottom line on this air thing is that I suggest using it only in addition to other things that would make a hearth safe.

 

[Gooserider]

Playing devils advocate here, not totally disagreeing.

However, it is worth noting that in that other thread, the air space suggestion supposedly came from a stove manufacturer's manual... I was a bit bothered though by the statement that "supporting structures" were allowed in the airspace without defining just what those structures were allowed to be made of, or how much of the space they occupied - to carry it to extremes, if using 9" wide strips cement board for support, spaced 1" apart, thus filling 90% of the "air space" with "support structure" would it still count as an air space, not as a layer of cement board?

While I would agree with not relying on air space alone, it seems like it must count for something, or we wouldn't have clearance specs - of required air space.

Lastly, while it would imply that the "split level" hearth wouldn't offer a significant R-value reduction, an enclosed dead air space should offer significant protection since it is NOT going to be heated by radiant energy - IR is blocked by solid materials, which are heated, and then become radiators themselves, but far less effective ones. After all is not a large amount of our insulation material design based on the idea of creating "dead air space" in very small volumes? (I would have thought the air spaces mentioned would more effective if filled with some form of glass or other insulation material to prevent air convection currents)

Gooserider

 

[begreen]

What about filling the dead air space with vermiculite? It's cheap and easy to find. (R 2.13/inch) Or how about fiberglass insulation with the paper removed? (R 3.14 /inch)

 

[webbie]

In answer to goose, it counts for something, but wall clearances are 100% different because the stoves are starting out as much as 36" from the wall, and only getting as close as 12" or so. Also, very few stoves have heavily radiant rear walls (single with fire right up against it.)

Bottom radiation is MUCH stronger because the coal bed of the fire is below, and many stoves do not have firebrick bottom (grates/cast stoves, etc.)....and the legs are about 6 inches long. That being the case, you can see a vast difference.

Also, with rear radiation, it is clear that there is a LOT of air current surrounding and pushing heat upwards off the surface - the bottom is more of a "soak through" type of direct radiation.

I would not use the HearthStone manual as a source - manuals are written by people like me (I've written quite a few), and we might list R -values and such things and not either test nor think deeper about them.

For more insight on this, we'd have to get to the point of having Corie do a little work for us - that is figuring out the exact type and wavelengths of such radiation, and the best ways to mitigate it. Obviously, the micore and wonderboard offer the best of both worlds, some mass to soak up and spread out the heat and the soft material to insulate well.

 

[elkimmeg]

There is relationship of height of the legs and distance to the floor underneath This is in NFPA 211 the higher or more air space the less the rvalue of protection neeeded the lower the
stove the more thermal resistance needed.. Now in every instance there is a layer of tile and cement/ hardi backer board spaced on say non combustiable material thand enclosed out side creating a dead air space. I believe the radiant heat is delfected and dissipated by the layer of tile and cement board even before it effects the ded air space.

As fpor reduced clearance closures ther is the same factor of radiant he being reflected shiny metal surface of noncombustiable cemeent board dissipating and defusing heat and a steady free air flow behind the 1" air space thatr dissipates heat build up.

This is true to some extent Web has mentioned there is a difference of dead air space and free flowing air space If the sun radiation was not deflected by our attmosphere they would be no life on earth so those gasses in air do have an insulating effect

 

[Highbeam]

Speaking as a person who is planning to use, depend on really, the air space to make the required R value for my hearth I am interested in this discussion. I find it odd that we trust the manufacturer when he tells us that we need a particular R value for our hearth or clearance requirement yet we will not trust him when he tells us that an air space will provide the R-value. Who are we to trust? the inspector tells us that manufacturer's specifications are to be followed. OK, then the air gap is perfectly legal, safe, and proper to use. It will pass inspection, it is per the manufacturer, and it is very common.

Where's the folly?

 

[webbie]

As mentioned, the air space as part of an assembly and where the air space is sandwiched by other materials would seem to be much more effective.

It is 100% possible (I say probable) that HearthStone has not tested a particular assembly - although I would have to read the manual and see what context the air space is mentioned in.

Common sense is the rule of the day. Comparing it to the sun is tough....although those rays burn the heck out of you even after they come though almost an infinite R-Value of air!

Here is a better comparison - say we get a good infrared heat lamp and hold a piece of flat wood 6 inches from it - REAL HOT - now we hold our wood 7 inches from it. Will it be much cooler? NO, because that inch (fully ventilated) is not acting as R-Value, the radiation is simply passing right through it.

Back to the common sense - a stove like an avalon with a high pedestal and a second convection chamber underneath it.....way back, just one layer of ceramic tile did the job for that stove - because all you needed was spark protection, the hearth never even got lukewarm. But a stove with a single wall bottom and legs of 6 inches or so can radiate down heavily.

When you ask who to listen to, I would say that the margin of safety is usually pretty good- but that a lot of the stuff in manuals (formulas) is just a way to make the test lab sign off, and they often don't give real world examples...which they should. It is doubtful that many local inspectors have more info than the manuals and NFPA (which also does not mention K factors as I remember).....

Of course, this is a discussion thread (speculation), but I would suggest not using a bare wood floor on the bottom of any air space in a hearth assembly. Use metal or cement board, then the high hats, then more cement board. Or, if the micore is available, just one later of that and one or two of cement board without and air would suffice for most hearths.

 

[webbie]

What about filling the dead air space with vermiculite? It's cheap and easy to find. (R 2.13/inch) Or how about fiberglass insulation with the paper removed? (R 3.14 /inch)

Some of that stuff might work. But we have to figure out the K factor at a high ambient heat. Again, I would think the factor would vary from the normal room temperature. Vermiculite probably has a good rating, and of course it's tough to get fiberglass in 1 inch or 2 inch thickness. Micore seems like the perfect material....semi-rigid.

 

[elkimmeg]

the problem here is equilvancy The manufactures give a R or k value that has been tested for proper floor protection.. We here are tryinmg to equal ofr go over the minium R value equlvalancy

This is an educational post not who knows most Craig presented the K value aspect and he is right that we need to be carefull here. I tried to uses my Laptop to cut and paste what NFPA 211 says but I have to go back and enter my new pass word. Be patient I will get it done tomorrow.. This question is asked all the time here so the code will be a steping point NFPA 211 even gives examples of acceptiable floor coverage is respect to height and clarify manufactures specs. this is not an arguement post if so I will pass We do not need any right now..

I want to research comercial fire rated essemblies to see if some of those materials can be used in Hearth construction It might be possible to work with other materials accomplish the same objective and simplify the process... I have to understand the heat of radiation requirement to determinee equialvancy

 

[Gooserider]

the problem here is equilvancy The manufactures give a R or k value that has been tested for proper floor protection.. We here are tryinmg to equal ofr go over the minium R value equlvalancy

This is an educational post not who knows most Craig presented the K value aspect and he is right that we need to be carefull here. I tried to uses my Laptop to cut and paste what NFPA 211 says but I have to go back and enter my new pass word. Be patient I will get it done tomorrow.. This question is asked all the time here so the code will be a steping point NFPA 211 even gives examples of acceptiable floor coverage is respect to height and clarify manufactures specs. this is not an arguement post if so I will pass We do not need any right now..

I want to research comercial fire rated essemblies to see if some of those materials can be used in Hearth construction It might be possible to work with other materials accomplish the same objective and simplify the process... I have to understand the heat of radiation requirement to determinee equialvancy

Not an arguement as far as I'm concerned. If I ask questions that look that way, I'm only trying to clarify what others have posted if I think it's confusing, or if there is some reason why I think they may be missing on the logic side.

I'm wanting to get the facts right for my own hearth extension, though its starting to look fairly straightforward - the Encore 0028 seems to have a pretty easy requirement to meet per the manual, so that will be easy. What I'm thinking I should try to do at this point though is figure out what's under the bricks that make up my hearth and just make the extension meet or beat whatever value that gives me. There is no point in doing extra to make the extension better than the existing hearth, since I'm not going to tear that up, but I think it makes sense to have it be at least the same value, then I don't need to worry about any "split level" factor.

Since I'm also working very hard on this Wiki article about hearth design construction, I want to make it as accurate, code compliant and helpful as possible.

Gooserider

 

[Highbeam]

Good example on the heat lamp and 6 vs. 7 inches away. That really helps me picture your point. I interpret the "air space" to be an air space between two layers of something. It's a gap. So I believe that the intent was never to only have an extra inch of space between the stove and a bare wood floor, the intent was to have a layer of noncombustible and then an air gap and then the bare wood floor. Back to the example of the radiant heat lamp, put up two layers of durock between the lamp and the piece of flat wood, and then what is the difference between 6 and 7 inches. Much less I'd think.

On the Hearthstone stove's attached metal tag it requires a noncombustible surface beneath it. The tag itself mentions nothing about the R value of said surface.

It would be simple and cheap enough to roll out a layer of shiny silver sheet metal flashing between the wood subfloor and the hearth to act as a radiant barrier to catch whatever makes it through the noncombustible surface.

This thread is really getting good.

 

[webbie]

The word "radiation" is a good one, because it lets you imagine things such as nuclear radiation which pass right through walls and still do their harm.

As simple as wood stoves sometimes seem to be, it IS rocket science or at least a field where a lot of common sense and background experience have to be put to work. Certainly one should read the manual for starters. But also look carefully at the stove design...length of legs, where the coal bed will be located, etc.

I built a great hearth for my dad long ago for his efel wood stove......we used small wood beams and made a square, then put a sheet of sheet metal down, then 4 inch bricks to set the legs on, then filled it up with marble chips. Worked fine, although I doubt you will find an r value for marble chips.

Let me explain a bit more of my understanding about K factors and even R factors....mainly that:

"Insulation materials usually have K-Factors less than one and are reported at what is called Mean Temperature."

The last two words are the important part. If the R-values reported here and even in a stove manual are reported at the "normal" temperatures, this means AMBIENT (normal) air temperatures! But we are looking for the values at temperatures of 500 degrees and even greater. Why would anyone think they would test house insulation R-Value at 500 degrees? I think they don't. Building materials used for house wall construction are tested at 70 degrees inside and varying temps outside.

And, in the case of this thread name, we need to know the same about air.

My take is that materials such as Micore and Cement board WILL function at or near their rated R an K values because they are/were designed and approved (and tested) with high temps. But AIR was not, to my knowledge, tested to a specific R or K value at high temperatures. If anyone has an actual reference to air being tested in closed situation like this, I'd love to hear about it.

I have a feeling metal and corie may be able to translate this, because we are really talking about plain old thermodynamics.

Summary - only use stated R and K values of HIGH TEMP materials, do not use those of materials tested at room temp and expect them to perform the same at 500 degree!

 

[webbie]

See enclosed for how much a K factor changes by temperature!

About a 350% difference at 4x the temp. Granted that the curve may not be the same fro 75-500 degrees, but hopefully this illustrates the point.

BTW, as you can see from the original post, this stuff is off the top of my head (experience), and only as the thread progresses are we adding any science or facts.

 

[Corie]

Air Properties

Temp (Degrees Kelvin) K Value (W/m*k)

100 9.34
200 18.1
300 26.3
400 33.8
500 40.7
600 46.9
700 52.4
800 57.3


It continues to 3000 K.

Also, be careful Craig when saying that distance has no effect on radiant heat transfer. Distance and its effect on radiant heat transfer is the driving force behind the listed clearances for hearth appliances. If distance had no effect on heat transfer, a stove placed 3 inches from the wall would heat the wall identically to a stove placed 18 inches from the wall, if I'm following your logic.

 

[webbie]

OK, Corie, now translate that for us.

Certainly distance has an effect, my point is that in the case of a heavily radiating flat surface against another, a difference of 1" free air may not provide the R or K value that has been quoted here by others. (R=5-7+ for an inch) - if said stove was going to ignite the wall at 3", it very well might do it at 4" also, although using the HearthStone guide, that would claim an additional R-value of over 7 times that of a hearthboard!

So, real world - using 500 degrees as the temp that a hearth material reaches, what is the effective R or K value of an inch of dead air below that.

We cannot use walls as the example here because of the intense air currents that are in force. It must be direct downward radiation from very close surfaces (6 inches for example).

So, at 500 degrees Kelvin(440 degrees F), are you saying the K value of air is 40.7? And is that for one inch of air, or what?

Note: Corie, I am addressing this thread:
https://www.hearth.com/econtent/index.php/forums/viewthread/8289/

See the post where Hearthstone claims 1/8" of air space on a hearth is .92 R value. That means an R-Value of over 7 for ONE INCH.

"Per the hearthstone manual- horizontal air space gives 0.92 per 1/8”.

That is the part that I think is folly, because of a number of reasons, mostly the temperature (most R-values are calculated at 75 F) and the radiation factor.

If air is .92 per 1/8", then a stud wall without insulation would be R-30! (and that is using room temp stuff).

So, is HearthStone wrong?
Here is the HearthStone info....
(heck, that chart says you can use wallboard!)

 

[Gooserider]

OK, Corie, now translate that for us.

Certainly distance has an effect, my point is that in the case of a heavily radiating flat surface against another, a difference of 1" free air may not provide the R or K value that has been quoted here by others. (R=5-7+ for an inch) - if said stove was going to ignite the wall at 3", it very well might do it at 4" also, although using the HearthStone guide, that would claim an additional R-value of over 7 times that of a hearthboard!

So, real world - using 500 degrees as the temp that a hearth material reaches, what is the effective R or K value of an inch of dead air below that.

We cannot use walls as the example here because of the intense air currents that are in force. It must be direct downward radiation from very close surfaces (6 inches for example).

So, at 500 degrees Kelvin(440 degrees F), are you saying the K value of air is 40.7? And is that for one inch of air, or what?

Note: Corie, I am addressing this thread:
https://www.hearth.com/econtent/index.php/forums/viewthread/8289/

See the post where Hearthstone claims 1/8" of air space on a hearth is .92 R value. That means an R-Value of over 7 for ONE INCH.

"Per the hearthstone manual- horizontal air space gives 0.92 per 1/8”.

That is the part that I think is folly, because of a number of reasons, mostly the temperature (most R-values are calculated at 75 F) and the radiation factor.

If air is .92 per 1/8", then a stud wall without insulation would be R-30! (and that is using room temp stuff).

So, is HearthStone wrong?
Here is the HearthStone info....
(heck, that chart says you can use wallboard!)

A few items to throw into the fray...

1. Your radiation model is OK, except that the human body is a LOUSY temperature sensor - once you reach the point of saying "ouch HOT" there is little abliity to tell how hot. The standard rule for radiant energy is that it follows the same "inverse square" law that all radiation follows - Double the distance, halve the intensity. I'm not sure just how you figure the change for 1" at 6" distance, but I would make a first guess saying that the drop should be that the intensity at 7" would at least 1/6th less than the intensity at 6" - definitely a significant drop, though I'm not sure how much difference it would make in the actual temperature.

2. IIRC the original thread, there were two values offered for the dead air space - 0.92 per 1/8" credited to the hearthstone manual, and 0.97 per inch from Elk. From what I recall of heat transfer stuff, the importance of radiant energy transfer vs convection transfer is the temperature of the radiating object, and the temperature differential between the surfaces. Assuming an enclosed space, the radiant heat from the stove will hit one side of the surface, then have to transfer through the separating barrier - (since it's a load carrying floor, we can assume at least a layer or two of cement board) then start radiating and / or convecting again from the inside surface. That inside surface does not know or CARE what the energy source was that got it to whatever temperature it is at, and that temperature is certainly going to be FAR less than the 5-600* of the stove surface. This means radiant transfer will be less of an issue, and covection more so. Now, part of what makes a dead air space work is if it is NARROW enough that convection currents don't have enough room to set up - this is why double pane windows are so effective as insulators compared to the wider space between a single pane window and a storm. I don't know the exact "magic number" that stops air convection but IIRC, it is around 1/8", while convection has no problem at all with a one inch space. Thus I could almost be convinced that both numbers are correct as long as the 1/8" space is ONLY that wide. (note that the info you posted even SAYS you can't "stack" the dead air spaces, but that each must be separated by another layer of non-combustible material.)

3. Just so that we are comparing apples to apples, Corie, could you convert the numbers you posted into R-values for a specified thickness of air space? (note that I don't think we need to worry about the values for below 300*K, at those temps we'd be trying to keep WARM!

Gooserider

 

[Highbeam]

Oddly, that bit about not being able to stack the air gap is not present in my heritage manual, nor the online version here:

http://www.hearthstonestoves.com/documents/Heritage8021Manual.pdf

Whether gypsum board, plaster board, or wallboard is the same as sheetrock isn't clear to me either. I made the decision that sheetrock's paper is combustible so it didn't qualify for use in the pad.

If the "folly" is based on the hearthstone manual, then perhaps it is time to get hearthstone's opinion. They have very responsive tech folks that might be interested.

 

[Gooserider]

Oddly, that bit about not being able to stack the air gap is not present in my heritage manual, nor the online version here:

http://www.hearthstonestoves.com/documents/Heritage8021Manual.pdf

Whether gypsum board, plaster board, or wallboard is the same as sheetrock isn't clear to me either. I made the decision that sheetrock's paper is combustible so it didn't qualify for use in the pad.

If the "folly" is based on the hearthstone manual, then perhaps it is time to get hearthstone's opinion. They have very responsive tech folks that might be interested.

Strange, maybe I didn't hit the submit button properly - My earlier response to this isn't showing... But I looked and found there is indeed a glitch in the manual - The table has both the numbers for the source reference and the asterisk for the "don't stack" note, just like the shot that Web posted. However the manual PDF only shows the source references, it doesn't have the "don't stack" note, which I think is critical to the value.

HB, you seem to already have contacts @ Hearthstone, do you think you could point this out to them? Tell them how it's caused confusion here?

Gooserider

 

[webbie]

A person would have a difficult time stacking 1/8" airspaces in a hearth. That sounds like machine shop work!

As far as the amount of heat radiated down, see my Acclaim screen shot in other thread - the only approved hearth surface is unpainted concrete about bare earth! That should tell you something. We're not talking about 2 or 300 degrees here!

 

[Highbeam]

A person would have a difficult time stacking 1/8" airspaces in a hearth. That sounds like machine shop work!

I am curious whether an air gap in excess of 1/8" is even worth the 0.92 value? If there is a property of that narrow gap that is damaged by going to a larger gap then maybe the air gap is worth less at 1" than at 1/8". Where does Elk's 0.97 per inch value come from?

 

[webbie]

That is the question! But it is pretty much already answered - at least at lower (building material) temperatures, the rule of thumb is that an enclosed air space of 1 inch is approx equal to R=1

Our concern here is the function of that space at 500 degree or more. That is the open question. And it is a complicated one.....something that the technical people at HearthStone, etc. are unlikely to be able to answer. Why? Because I just heard from one of the foremost experts in the field.....the guy who literally wrote the BOOK....and he doesn't know! But he will find out, and so will we...

Sure, we are beating this to death, but the quest for understanding is what drives progress!

 

[Gooserider]

I must say that I question your assumption of a 500* temperature - to me it seems excessive.

Even someone from the "Burn it 'Till it Glows" school of overfiring is probably not going to exeed 1000*, especially on the bottom of the stove. More realistically, the figures I've seen tossed around here on the forums are more on the order of 6-800* AT THE STOVE - This is at the top of the stove as well, which should be hotter than the bottom because heat rises, not to mention the insulation value of whatever firebricks might be on the bottom, along with ash pans, ash beds, convection air passages, etc.

The heat then has to transfer through several inches of air, following an inverse square law, to reach the TOP layer of the hearth. The heat then has to work it's way down through the various layers of hearth materials. Each layer will block the transfer of some of the heat from the upper layers, so each layer will be cooler than the layer above it. Even assuming just one layer of cement board and a layer of tile (I figure two layers of CB are more likely) before the dead space, you will have a considerable drop between the temperature at the surface of the hearth and the temperature at the top of the dead air space.

In the absence of definite instrumented evidence to the contrary, I'd be willing to bet that if the stove bottom is at 500*, you won't see more than about 300* at the hearth surface, probably less, and no more than 200* at the top of a dead airspace protected by two layers of cement board and a layer of tiles.

Remember when looking at thermal transfer, the ONLY numbers that matter are the temperatures on the surface of each layer. The temperature of the heat source is not relevant.

Gooserider

 

[webbie]

I don't doubt at all that MOST stoves would be fine on nothing but a layer of tile over a sheet of wonderboard. I've installed enough of them to know that I can usually hold my hand on the floor under there. However, my assumption of the 400-500 degrees is based on exactly that scenario I mentioned - stoves with a single wall bottom where the fire is built directly on the cast iron....and, my assumptions were based on TESTING conditions They really fire up those suckers), not what happens in the home.

There is a LARGE margin of safety, because the hearth requirement is that the wood below cannot exceed about 160 degrees. It would take long term exposure at probably 300+ degrees to get things charring heavily, although 250 degrees has been tossed around as a possible long-term ignition temp.....but this would seem low.

But looking at two specific examples....
1. The Acclaim, which can only be installed on unpainted concrete over earth! (without bottom heat shield). They did not pull this out of thin air - the stove must have overheated even the most stringent hearths! The manual says that not even a fireplace hearth (usually 8 inches or more thick) would suffice...because the wood below would be overheated!
2. The HearthStone Homestead with shorter legs specifies R 6.6, that is MANY times what other stove require. In other words, they must have tested this unit and it needed 20 layers of cement board (or equiv) in order to not bring the wood below over 160.

I'm certain you can understand where that meant at least 400 - and probably more, on the top of said hearth.

Older cast stoves like the Vigilant, Defiant, Resolute, Upland, Jotul and many more have single wall bottoms and often no shields. Ash pans in some stoves actually work against them, because the ash pans are empty when the testing is done, and since they often hang very low to the hearth, the means even more heat!

So I am not predicting doom - the margin of safety is large enough (thank goodness for overkill), but I do want to clarify the use of air and also downplay the use of anything like fiberglass insulation and other "tested" materials that are only tested at room temp.

BTW, for a sense of the temps involved, fire up a stove with a big front glass door with small pine lumber as hot as you can - and when it all turns to coal, hold your hand 6 inches in front of the glass (simulating 6 inch legs)......you'll be removing it very quickly! In fact, many stoves with large glass doors require high R value hearths extending in front for that exact reasons....radiation out from the glass and down at an angle!

See pic for the testing lumber used in labs....

 

[Gooserider]

I don't doubt at all that MOST stoves would be fine on nothing but a layer of tile over a sheet of wonderboard. I've installed enough of them to know that I can usually hold my hand on the floor under there. However, my assumption of the 400-500 degrees is based on exactly that scenario I mentioned - stoves with a single wall bottom where the fire is built directly on the cast iron....and, my assumptions were based on TESTING conditions They really fire up those suckers), not what happens in the home.

There is a LARGE margin of safety, because the hearth requirement is that the wood below cannot exceed about 160 degrees. It would take long term exposure at probably 300+ degrees to get things charring heavily, although 250 degrees has been tossed around as a possible long-term ignition temp.....but this would seem low.

But looking at two specific examples....
1. The Acclaim, which can only be installed on unpainted concrete over earth! (without bottom heat shield). They did not pull this out of thin air - the stove must have overheated even the most stringent hearths! The manual says that not even a fireplace hearth (usually 8 inches or more thick) would suffice...because the wood below would be overheated!
2. The HearthStone Homestead with shorter legs specifies R 6.6, that is MANY times what other stove require. In other words, they must have tested this unit and it needed 20 layers of cement board (or equiv) in order to not bring the wood below over 160.

I'm certain you can understand where that meant at least 400 - and probably more, on the top of said hearth.

Older cast stoves like the Vigilant, Defiant, Resolute, Upland, Jotul and many more have single wall bottoms and often no shields. Ash pans in some stoves actually work against them, because the ash pans are empty when the testing is done, and since they often hang very low to the hearth, the means even more heat!

So I am not predicting doom - the margin of safety is large enough (thank goodness for overkill), but I do want to clarify the use of air and also downplay the use of anything like fiberglass insulation and other "tested" materials that are only tested at room temp.

BTW, for a sense of the temps involved, fire up a stove with a big front glass door with small pine lumber as hot as you can - and when it all turns to coal, hold your hand 6 inches in front of the glass (simulating 6 inch legs)......you'll be removing it very quickly! In fact, many stoves with large glass doors require high R value hearths extending in front for that exact reasons....radiation out from the glass and down at an angle!

See pic for the testing lumber used in labs....

I'm not sure the idea that the Acclaim (and many other VC stoves) is to hot to put on ANY hearth is warranted. As has been pointed out many times, the stove maker can't advise on something they didn't test for - It could simply be that they didn't TEST the unit w/o the shield because they didn't feel it was needed... The bare concrete would be OK simply because it is guaranteed non-combustible, so there are no clearance issues with it.

The Hearthstone is a bit harder, although it's worth noting that they are the folks advocating the 1/8" air space. If one built a hearth using the air space, or possibly some of the other materials you don't like, it would be possible to do it with far less than a 10" stack of cement board.

Gooserider