Looking for thin Hearth but high R-value

[rsiros]

OK... lemme try this again under a new title. My Hearthstone Homestead needs a r-value of 6.6 in the hearth, and no I won't be switching to the taller legs. I am going to pull up an old slate hearth (on 1st floor; not in basement) and work with the space that I'm given under there. After looking at older posts, I saw that fiberglass insulation is the best (3.3/inch) but it can't be compressed. So I will use metal studs on the decking or maybe over the floor joists if possible and/or necessary. I want to throw a layer of flashing in there somewhere too. Here's my thoughts: Decking, Metal studs, Fiberglass insulation between studs, Flashing over studs, Durock, then slate or granite tile. After I get under there, I will adjust the stud cavity depth to get as thick a layer of fiberglass as possible. This can't be solved by just fabricating a simple bottom heat shield, can it? <please tell me it can>
Micore won't work for me as I can't get it; fiberglass is better r-value anyway. Any thoughts or suggestions appreciated. I really don't want to rip up the old hearth!

 

[rsiros]

OK... how about this... just found this statement in my manual: "Install your Hearth Mount stove with 4-inch legs on one of the following:
- A noncombustible floor, such as a slab, cement, or stone hearth. (A noncombustible floor will not ignite, burn, support combustion, or release flammable vapors when subjected to fire or the anticipated heat from your stove.)
- A floor protector with an R-value of 6.6 or more that you obtain from your dealer. (A floor protector is any noncombustible on the floor underneath and extending to the front, sides, and rear of the wood stove.)"
I have always interpreted this as needing 6.6 under the stove, but after re-reading it does say "floor protector"; as in something over an existing floor. My slate "stone hearth" that served the fireplace should suffice, right? I will be contacting Hearthstone tomorrow, but what do you guys think?

 

[Hogwildz]

Does your stone hearth have combustibles underneath it? If it does you need that 6.6R above those combustibles. Slate is not much if any R.

 

[rsiros]

It does have combustible floor joists underneath. I had the same thought, but after re-reading the manual it doesn't mention anything about protecting combustible structural framing under concrete or stone, so I thought that maybe the r-6.6 was for insulating over a combustible floor, and not framing. The thought being that maybe the surface area of the floor would need the r-6.6 insulation. It's not written very well and can be interpreted a few different ways. I agree with, and understand your logic, but am hoping otherwise.

 

[Highbeam]

It is my understanding that fiberglass insulation is not considered a non-combustible. Just because you took the paper backing off the stuff doesn't become micore. Perhaps a listed non-combustible like mineral wool can be found for your hearth. I couldn't get micore either and chose to use an air-space from the hearthstone manual. The metal studs in my application gave 3.5 inches of "horizontal still air" to which I added two layers of durock and then tile. My inspector and I know that my system beats 1.2 as required by my Heritage but you've got a high mountain to climb with the 6.6

 

[nshif]

RS
I kinda agree. They arnt very clear in how they word that. if there was an OR between those statments Id say you were right but Id wait to hear from them

 

[rsiros]

I emailed Hearthstone. I am a little irritated that there is no 800 support # given anywhere. Anyone have an 800# for me to call tomorrow?

 

[Gooserider]

There is similar verbiage in my Encore manual, but they make it VERY clear that when they say "non-combustible floor" they mean something like a cement slab with dirt under it, or a fireplace hearth with no supporting timbers, or similar structure that contains NO combustible framing. I would be very much surprised if Hearthstone says anything different.

Gooserider

 

[colsmith]

Hi, there is a phone number for Hearthstone in our manual, (802) 888-5235, don't know how helpful they will be, don't think we have ever called them.

Have you asked the local stove dealers if they have any Micore they can sell you? That is where we got ours. We couldn't find anybody actually selling it anywhere near us, but since we knew they used it in building hearths that they had to have some. Good luck, that is a heck of an R value you need!

 

[rsiros]

Thanks for the #. I'll try that one today. I emailed them on Sunday. I got an 800 # from my dealer, called Hearthstone yesterday, and left a message. Nothing yet, but I'll be sure to post when I get a response. They apparently want you to go through your dealer, but my dealer is not very knowledgeable. I called five or six other dealers and got several different answers... suspicious of some of their motives.

 

[rsiros]

I spoke to someone at Hearthstone and then received an email. Good support and answered questions without hesitation. I do need the r-6.6 under the hearth so I guess I found myself another project. I wanted to make the hearth deeper anyway, so I guess I'll get started on this a little sooner than I had planned. They recommeded a pre-made hearth from Hearth Classics but I will be making my own. I wonder if they are "bed-mates"... hmmm.

 

[tradergordo]

I know you already called to confirm, but I must admit, my first thought was that the 6.6 number HAD to be a typo! That's crazy high! You would need 6 layers of micore 300 + some non-combustible top with a small r-value to hit 6.6. That seems very expensive and maybe impractical. Your best bet might be to use metal studs and put an airspace gap in there. See this thread:

https://www.hearth.com/talk/threads/8289/

But note there seems to be confusion about the r-value of air spaces.

The wiki article has confilicting information from various sources. 1” of dead air might have an R value of .97, maybe higher if its ventilated. But you'd still need an awfully big air space to reach 6.6! Might be even more impractical than using the micore... are you SURE that 6.6 number is right???

 

[rsiros]

R-6.6 is right per Homestead manual. I'm not sure where they get this # and why, but I've been referred to the Hearth classics website about three separate times... kinda fishy. But anyway, I've found some insulation specialty places in the local Pittsburgh area that sell Roxul (a mineral/rock wool type product), which has an r-value of 4.2 per inch. This seems to be the best r-value per inch material out there. Only down side is that you aren't supposed to sandwich it. Pretty reasonably priced too... much better price than the pricey micore.

 

[tradergordo]

R-6.6 is right per Homestead manual. I'm not sure where they get this # and why, but I've been referred to the Hearth classics website about three separate times... kinda fishy. But anyway, I've found some insulation specialty places in the local Pittsburgh area that sell Roxul (a mineral/rock wool type product), which has an r-value of 4.2 per inch. This seems to be the best r-value per inch material out there. Only down side is that you aren't supposed to sandwich it. Pretty reasonably priced too... much better price than the pricey micore.

I agree, its fishy

Based solely on how they look, and my knowledge of r-values of various materials (again see the wiki article) I do not believe any of the "Hearth Classics" hearth pads that Hearthstone recommended you buy would have a 6.6 R-value. This info however is not listed on their website. I sent them an email asking about it. If their pads aren't 6.6 does this mean the 6.6 figure is in error - or the recommendation to buy one of these pre-fab pads is in error? (or do they actually expect you to do a custom order with this company to produce a special 6.6 r-value pad that they don't normally sell?? If that were the case you'd think they would have mentioned that when they dropped the name!)

 

[webbie]

There is some discussion about air, but at the same time it is ONLY air that represents the actual insulator - in these sheet materials it is the air between the fibers, etc.

6.6 could be gotten in numerous ways, using a sandwich. But are those Hearthboards approved as 6? I would be surprised! Just because a hearth board is UL approved does not mean anything - in terms of R-Value. UL or NFPA rating might just mean that the board has a R=1 value, which does the job for many scenarios.

I suppose if I needed to get 6+, I would do something like this.

1. Sheetmetal on the floor with steel studs sideways
2. 1/2" micore
3. 1-2" airspace
4. Rockwool batts or similar hung above airspace - as per the above post at R=4 for the inch
5. Two layers of durock for weight distribution, and tile, etc.

This would end up at 5-6" deep, so a lower one could simply be sheet metal flat, plus 2" of that rockwook (at 4 per inch) and then a finish.

 

[tradergordo]

This is what the guy from hearthclassics said:

"Hello, our website is being updated. I am attaching our new brochure for your review. It covers our R-value which is R=1.01. Thanks

Bob Robinson
National Sales Manager
PH 866-881-7237
FX 866-881-2449
[email protected]"

But for what its worth, they DO actually have a standard solution that would get you the 6.6 r-value. This is in the PDF brochure he sent me: “Our new low profile base, at 1 1/8″ provides an R-value of R=3.19. Our 6″ and 10″ pedestals can increase your protection from R=1.01 to R=10.”

So with a big old 10” pedestal (that is, a 10 inch air gap) under your hearthpad, you will have an r-value of 10. The r-value with the 6” pedestal for some reason is NOT listed in the brochure...

 

[rsiros]

What do you guys think of pulling up hearth, sheet metal on decking, metal studs, 1.5" Roxul to fill metal stud cavity, one or two sheets of Durock (depending on how much space I'm left with), and slate or marble? Thanks for the links and advice everyone. The dealers around here (Pittsburgh) are pretty clueless about my stove. I was hoping more than anything that someone could logically discredit the r-6.6 rating, but I guess Hearthstone has it's reasons. I was going to re-do the hearth anyway, so I'll be starting a good bit sooner.

 

[tradergordo]

Sounds reasonable to me.

And hey, maybe now this puts to rest the debate/question about r-value for air gaps in hearth pads? Looks like a professional hearth pad manufacturer says 1" of air = r-value of about 1 and apparently its cummulative so 10 inches (under a non-combustible top) gets you an r-value of 10. Although the addiitonal comments from those guys probably only add confusion to the issue:

"My research shows the Homestead at a 2.5 r value unless it is being used as a hearth stove then is goes to 6.6. If you use our 6 inch pedestal we can have an R of 5 and with pad it is 6.01. If you use our 10 inch pedestal we can put 6 layers of insulating material inside the pedestal with the pad you will have an R of 7.01.

Bob Robinson
National Sales Manager
PH 866-881-7237
FX 866-881-2449
[email protected]
"

 

[Gooserider]

The challenge is getting that R-6.6 - which is only needed with the 4" legs - the 6" legs only need a far more reasonable R-2.5, which is still on the high side but is pretty easy to do, even with a flush pad, or close to it. Web - that rockwool insulation is sort of like fiberglass bats - you only get that R-4.2 / inch if you have it uncompressed - which gives a serious question about how to support it. I know lots of folks have done hearths with steel studs and gotten them approved, but I have often wondered what the conductivity of the metal does to the protection offered by the hearth pad. I wouldn't want to use a hot glove with metal studs through it, and this seems much the same deal...

The other thing to keep in mind, and this may be how the hearth pad outfit handles the problem, is that Hearthstone is one of the few manuals I've looked at where there are provisions for "split level" hearths, where you always need the R-6.6 under the stove, but depending on the height difference in the levels, and how far out from the stove the drop is, you can have a far lower value on the area in front of the stove. Thus the "stove on pedastal" approach will work, but that leaves me wondering why not just go with the 6" legs to begin with. It seems like all the solutions mentioned so far take more than the 2" difference between the leg heights to gain that extra R-4 of required insulation.

Gooserider

 

[rsiros]

The stove with six inch legs only clears my lintel by about 1/2". It also interferes with my mantle clearances. I have a hearth that needs re-done, and a mantle that needs to be left alone. So for my purposes, the 4" legs were the way to go. We liked the look of the 4" legs better as well. The r-6.6 is obviously the biggest issue right now, but it's workable.