Help calculating R Values for building hearth

[tradergordo]

I just bought a new freestanding wood stove. I am now in the process of gathering information before building my hearth. One thing I'm struggling with is calculating R values. According to my manual and many others I've found online, I need a minimum R value of 1.19 to pass inspection and meet code. I have seen others here use durock or wonderboard which are readily available from Lowes & Depot. But I don't understand a couple of things.

I have found several sources on the web that list various materials and their R-values. For example, see:
https://www.hearth.com/articles/64_0_1_0_M1.html

Durock & Wonderboard have an R value of .52 PER INCH. This stuff is generally sold in 1/2" thick sheets. Since the minimum requirement is 1.19 and 1.19 divided by .52 = 2.29, that seems to me to indicate that I would need 2.29 inches of durock or wonderboard, or 5 half inch sheets piled up on top of each other (and since these are only sold in 3'x5' dimensions and my hearth is bigger than that, I could need to purchase 10 sheets). I have seen others here using just a two sheet thickness (one inch) which has me confused. Am I calculating this right? If not, can someone please explain it to me?

Thanks!!

p.s. Of course this is ONLY considering the durock layer. The tile I will put on top has some r value, but I also read you should not even count the tile towards your total because you never know for sure what its value actually is unless you can find some specific rated name brand, and then you have the issue of the grout.
p.p.s. I know micore 300 has much better R values, but I'm having trouble finding this material locally. Should I just make an extra effort to find that stuff?

 

[begreen]

What is underneath the durock? The easiest way to build up R value is to have an airspace. Look at DonCT's hearth in the pictures section for a nice visual example. He used metal studs to frame the hearth pad then overlaid two sheets of durock on top of that frame.

 

[MountainStoveGuy]

If you build your hearthpad out of steel studs, you dont have to worry about layering all that durarock. Your calculations are accurate. Airspace like bgreen said is the key when making the pad out of lumber. I have seen it done sucessfully with steel shims layed on top of the studs, but its a pain in the butt to attach your subsurface to the studs that way. Micore is super expensive.

 

[tradergordo]

OK guys, thanks. So air space is key. And incidentally, Don's very nice visual example is where I actually started!

But how do I calculate the R value of an air gap?

Thanks again.

 

[MountainStoveGuy]

.95 = 1/8 inch

 

[DonCT]

I could have gotten away with just the 2 layers of Durock and the metal studs, but since I had the 2 sheets of 5/8" gypsom board down, I just laid the Durock on top. I'm waaaaay over the minimum, but I would rather have too much than not enough. The air gap is what gives me the best R value, as the gap is 3 1/2".

Atleast for my city, they use the stove manual for all clearances and R value calculations. Which stove did you buy so we can look at the manual and see what it gives for R values?

 

[tradergordo]

Wow - I didn't realize a little air gap provides almost all the R value you need. So I guess you could just use some metal studs (3.5" air gap) and a single sheet of cheap durock? How exactly do they define an airgap? I mean do the studs have to be placed a certain distance from one another?

FYI: I just called around and did find a "sort of" local source for micore 300, its not as bad as I was expecting, you can get a huge 4x10 sheet for $33. On a square footage basis, that's pretty much the same price as Home Depot / Lowes' price for durock. What that would let me do is just put the hearth directly on my carpeted floor without studs or an elevated platform (my carpet is a thin, tight, burber so I don't think it will be a problem going right on top of it?).

p.s. The stove I bought is the Vermont Castings Dutchwest Large. Install manual:


(broken link removed to http://www.vermontcastings.com/catalog/elements/files/2006/30002291_Dutchwest_NC_Lg.pdf)

 

[KP Matt]

Regarding air gaps, to get the full R Value (1.43 per inch according to the article mentioned earlier) doesn't it have to be ventilated? DonCT's set-up (very nice btw) doesn't seem to be ventilated.

Also, someone had asked about the framing of the metal studs - does anyone have any insights into the optimal distance between studs to achieve the desired strength yet still achieve ventilation?

Perpindicular framing won't allow for ventilation, will it? And wouldn't too much steel actually conduct rather than insulate?

And if you do ventilate, would you screen the ventilation holes so that the air gap doesn't become a haven for insects, dust bunnies, bits of bark, etc?

 

[elkimmeg]

My suggestion is to use High hats these are metal framing members about 1.5" Spaced 12" on center, then dura rock over them, screwing the dura rock directly to the high hats. The dura rock is the cement base for tile instalation. I have also seen the 12 inch on center space filled with fiberglass insulation 1.5" of fiber glass = R5.5. AS an inspector I would never allow the pad to be built on top of carpet. I would require the carpet to be removed. Metal , though non combustiable, is a good conductor of heat to your carpet. Another spin place fiberglass insulation down on the wood floor and then install the metal high hats allowing it to fluff up inbetween high hats

You could use one sheet of micore on the bare wood floor then dura rock screwed through both.

If you make the pad bigger than the required distances. For instance ,16" in front of the loading door and you make it 18 or more inches you then can use wood moldinge for your finished edge. As long as the wood is the required distances
you also could rip down 2/4 material 1.5 " and use it as your outside frame providing it is beyond the required distances. It can be tiled or wood molded for finish appearances

 

[tradergordo]

Regarding air gaps, to get the full R Value (1.43 per inch according to the article mentioned earlier) doesn't it have to be ventilated? DonCT's set-up (very nice btw) doesn't seem to be ventilated.

Also, someone had asked about the framing of the metal studs - does anyone have any insights into the optimal distance between studs to achieve the desired strength yet still achieve ventilation?

Perpindicular framing won't allow for ventilation, will it? And wouldn't too much steel actually conduct rather than insulate?

And if you do ventilate, would you screen the ventilation holes so that the air gap doesn't become a haven for insects, dust bunnies, bits of bark, etc?

Yea, I have ALL the same questions. I'm also concerned a platform/air gap design will be more likely to crack my tiles (support studs might have to be directly under the stove legs?) or worst case (and probably unlikely) the platform could collapse. It seems a whole lot easier to use a non-platform design, definitely less expensive, and possibly safer. I'll probably end up just buying the 4x10 half inch micore 300, cut it in half, double it up, making a once inch micore base with R value = 2.33 which is almost double the minimum required. THen I'll tile that, and add a nice stained finishing border around the outside edges (which could be wood since its outside the non-combustible only area, or I might use tile or something else?).

 

[DriftWood]

Hi
Read your stove manual, it will guide you through this process.

MICORE® Brand 300 is soft you can dent it with your thumbnail. Modulus of Rupture 350-450 lbs./sq. inch
Tensile Strength Parallel to surface 225-275 lbs./sq. inch
Perpendicular to surface 20-30 lbs./sq. inch.
r-value 1.09

http://www.acoustics.com/specs/USG_product/usg_spec.pdf#search=’micore 300 spec’

MICORE® Brand 300 Board from USG is a superior substrate for fabric and vinyl-covered wall panels, office
dividers and tack boards. An excellent core for chalkboards, stove boards, and similar applications, MICORE Brand
300 Board offers outstanding resilience, superior machinability, high “k” factor, heavy density and highest surface hardness of any MICORE

½” Durock is hard. Indentation strength psi 1” dia. disc. @ 0.02” indent 2500 psi.
Uniform load psf (studs spaced 12” o.c.) 50 psf max.

(broken link removed to http://www.usg.com/USG_Marketing_Content/usg.com/web_files/Documents/Prod_Data_and_Submittal_Sheets/DrckCement_Board-Submittal_Sheet_CB399.pdf)

Composition and Materials DUROCK Cement Board is formed in a continuous process of aggregated portland cement slurry with polymercoated,
glass-fiber mesh completely encompassing edges, back and front surfaces. The edges are formed
smooth—patent No. 4,916,004. The ends are square cut.

Description
DUROCK® Brand Cement Board provides a smooth, sound base for glass and ceramic mosaics; ceramic and quarry
tile; lugged tile; and thin stone and thin brick. Suitable for application to wood or steel framing spaced 16
o.c. in
new construction and in remodeling. Board is ideal for use in partitions, walls, floors, soffits and ceilings in wet or
dry areas. It does not deteriorate in the presence of water so it is highly durable in high-moisture areas such as
baths, showers, kitchens and laundry rooms. Also adaptable for fences, fireplace fronts, mobile home skirting,
agricultural buildings, UL-listed wall shield/floor protectors, garage wainscoting and exterior finishes.

Limitations
A. DUROCK Cement Board is designed for positive or negative uniform loads up to 50 psf. For complete information
on the use of DUROCK panels in exterior systems, consult your USG sales representative.
B. Maximum stud spacing: 16” o.c. (24” o.c. for cavity shaft wall assembly); maximum allowable deflection, based
on stud properties only, L/360. Maximum fastener spacing: 8” o.c. for wood and steel framing; 6” o.c. for ceiling
applications.
C. Maximum dead load for ceiling system is 7.5 psf.
D. Steel framing must be 20-gauge or heavier.
E. Do not use drywall screws or drywall nails.
F. Do not use 5/16"DUROCK® Brand Underlayment for wall or ceiling applications.
G. Do not use DUROCK Cement Board with vinyl flooring.

My 2005 Hearthstone Heritage needs 1.2 r-value hearth pad. 5 sheets of ½” durock
OR
1 sheet of ½ “ durock+1 sheet of ½ “ MICORE® Brand 300 Board to cover the oak flooring in my house.

I put ½” micore 300 down first, then ½” durock and finished the top with stone tile.

Do have a air space the height of your metal framing?

Filling this gap with mineral wool is another way to boost the total R-value of your hearth pad.
1” mineral wool insulation = 3.12 R-value (ASHRAE Handbook of Fundamentals).

 

[Nokoni]

I wanted a low hearth because my stove is in a fairly small area. Anyway, I searched for Micore. If you look on the Micore website it will tell you who distributes it. I found a place in Illinois that was willing to ship a sheet to me. He cut it in four and shipped it. Yes, it was expensive, $60.00 (that included the shipping) but for what I wanted it was worth it. I put one big piece of slate over it and I really like the way it looks. Slate was reclaimed from an old school chalkboard. Low to the ground and smooth. You can find micore if you really want to use it. Good luck.

 

[Rhone]

Make it simple and get a piece of Micore 300. It is soft but not cork board soft it's a light weight particle board. The fireproof hearth extensions and mats usually use it inside.

Cut to size, then you need something on top of it like 1 sheet of durock, and then tile to the durock. You certainly shouldn't (or can't) tile to Micore 300, I'm pretty certain you can't put underlayment on top of it either as it'll eat it. Just get a piece of Micore 300, durock on top of that, and tile, you'll be done.

 

[tradergordo]

Why can't I tile directly to the micore? I've seen others post that they have done it. I understand its a little soft, but will that matter once there are big tiles on it? I guess it doesn't matter, I can do micore to durock to tile, its just more of a pain - have to buy two sheets of durock and cut/splice them to cover the hearth surface, that leaves a seam. Do I just use thinset mortar to attach the layers? I've read that screwing the layers together is not a good idea as the screws transfer heat?

 

[DonCT]

Have you thought about buying a premade pad? If you're not replacing/building a raised hearth, I would think that would be the easiest, most economical solution.

 

[tradergordo]

I could have gotten away with just the 2 layers of Durock and the metal studs, but since I had the 2 sheets of 5/8" gypsom board down, I just laid the Durock on top. I'm waaaaay over the minimum, but I would rather have too much than not enough. The air gap is what gives me the best R value, as the gap is 3 1/2".

Atleast for my city, they use the stove manual for all clearances and R value calculations. Which stove did you buy so we can look at the manual and see what it gives for R values?

Don - I didn't realize you actually left the drywall (gypsom) on there, I thought you removed it. That's another thing that is "controversial". Most things that I've read consider drywall "combustible" (does yours have a paper wrapper?). If it is considered combustible, then you can forget about your air gap, the actual hearth pad consists of just those 2 half inch layers of Durock, which would not pass code. Either way though, there is probably zero chance of any real world problem - from what I understand these latest requirements are serious overkill. I read one post about a guy who said he used a newspaper to prop up one of the legs on his stove, it sat there for 10 years. He finally removed it, and could still read the articles in the 10 year old paper...

 

[tradergordo]

Have you thought about buying a premade pad? If you're not replacing/building a raised hearth, I would think that would be the easiest, most economical solution.

The ones at the local stove shop cost about $500. I can build one for less than $50.

 

[Rhone]

Well, I recommend you see what Micore 300 is. I've too seen people who made their own hearth using Micore 300 and put underlayment on it and then tile but I can't see how a tile won't pop off later. A decent sized log falling on the hearth (if tiling to the Micore) should be enough to cause the Micore to vibrate or bow and crack the grout/pop a tile or more. Micore 300 is like, a condensed card board and to me springy material which is not good for tile. Tile likes materials that don't give. Anyway I'd think the underlayment would eat it or at a minimum weaken/warp it. You know how weak wet cardboard is compared to dry and how much weaker cardboard is that got wet and dried, I think Micore would do likewise.

I'm not an expert on the stuff, but I've had a piece so know what it's like first hand. I recommend you get a piece and do likewise. I threw my sample of Micore 300 away.

 

[tradergordo]

Well, I recommend you see what Micore 300 is. I've too seen people who made their own hearth using Micore 300 and put underlayment on it and then tile but I can't see how a tile won't pop off later. A decent sized log falling on the hearth (if tiling to the Micore) should be enough to cause the Micore to vibrate or bow and crack the grout/pop a tile or more. Micore 300 is like, a condensed card board and I think springy material which is not good for tile. Anyway I'd think the underlayment would eat it or at a minimum weaken/warp it. You know how weak wet cardboard is compared to dry and how much weaker cardboard is that got wet and dried, I think Micore would be likewise.

I recommend you get a piece and experiment with it. I threw my sample of Micore 300 away.

I trust you. I guess I'll junk the micore idea and just do the metal stud elevated platform thing with durock on top. Would appreciate any responses regarding "ventilated air space" as was asked about previously. Should I just leave the back open? Or put holes in it, or what? And do I need to try to get the studs directly under the stove legs or does it not really matter?

Thanks!

 

[begreen]

Good question. I'd double up the durock for that reason. It will add strength and rigidity to the surface. That way if you go to a heavier stove with different leg placement, you won't be worrying. You could also put the metal studs on 12" centers for greater stiffnes. If you want extra rigidity, you could also pop-rivet a layer of sheet metal on top of the studs, before the durock.

I'm not sure I understand the question of ventilated space. Are you referring to the hearth pad or a non-combustible surface behind and/or on the sides of the stove? The hearthpad doesn't need ventilation. The surface behind the stove will greatly benefit from having a gap at the bottom of the surface and open at the top to let air circulate behind it. My install will be a corner hearth with a non-combustible surface behind it, so I'm planning on leaving a 1" gap at the bottom. If your install has the stove at the recommended distances from non-combustible surfaces, then this is not required.

 

[tradergordo]

Yea, I was talking about the air space within the hearth. The hearth.com chart shows an R value for "Air Space (ventilated)" only. Can anyone point me to a source that gives an R value for an unventilated air space?

I just want to be 100% sure whatever I end up building will pass inspection...

 

[elkimmeg]

The info you are looking for, can be found in window manufacturing, where double/ tripple glazing calculates insulation properties of dead air space between the glass panes

"In theory, an inch of air can achieve an insulation value of R-5.5. Realistically, however, the best air-filled insulations only achieve R-4.5 to R-4.7 "

 

[tradergordo]

The info you are looking for, can be found in window manufacturing, where double/ tripple glazing calculates insulation properties of dead air space between the glass panes

"In theory, an inch of air can achieve an insulation value of R-5.5. Realistically, however, the best air-filled insulations only achieve R-4.5 to R-4.7 "

That's why this is all so darn confusing. Too much conflicting info! The hearth.com chart gives ventilated air space an R value of 1.42 per inch and you are talking about unventilated air spaces at 4 or greater per inch?

 

[KP Matt]

The stove and hearth pad with a ventilated air space situation is not analogous to the insulated window with an air space. The hearth pad can vent air back into the room, which we can assume will never heat up to dangerous temperatures. So there is another factor other than the heat source and the combustible surface. With a window the heat that ends up in the air space either must go back inside or outside.

 

[elkimmeg]

Apples vs oranges debate

Ventilated air space is used for clearance to combustiable wall enclosures. The free air space allows air movement to
remove heat and create a cycle of contineous air movement. I do not know where a R-value can be determined for free air space
Especially flowing air. I understand it ability to move air thus reduce heat build up.

Dead air space can be calculated for it resistance of heat transfere. After all it is the air spaces in fiberglass insulation or bubble wrap, is largely responsible to achieving its R rating. The dead air space between panes of glass the u values can be converted to r values. Your own protection pad can satisfy code with the proper u value.

I have followed the post and you received good advice. If purchasing a 4/10" piece of microcore and doubling it up with dura rock cement board on top then tile you are set. You are right about screws having the ability to transfere heat. Thats why they should not be placed directly under the stove or too close to the glass loading door, the areas of the most intense heat.

Here what to do. Produce a plan or scetch detailing the manufactures size requirements and yours proposed hearth demention
copy the manufactures r-value of the microcore down and and present it to your local inspector. Get approval of you process
before conmensing the project and ask for his inspection schedule. He may want to see the screw placements and materials in place before finishing. Make note of the screw placement as I outlined. If you need the official language for screw placement, I will look it up in my 2003 NFPA 211 code book. The more prepared you persent yourself to the inspector, Things will progress smoother. Be organsied, drawings and material labeling is important and a copy of the stove manufactures specs of what is required. I am willing to bet your inspector will be quite impressed.