Second Try: Building a hearth using a thin air space from a wire mesh layer

[Geoff John-West]

I am sorry that my first attempt at getting feedback here was high jacked by my daughter pulling a practical joke on me.

I am at the point of building my hearth using a thin air space and wire mesh.
I am installing a new Hearthstone Mansfield and would like to get an R1 rating although the manual is nonspecific about this so I got the R value from their other manuals.

Here is my construction layers:
R.1 1/4” Fiberrock underlay over 3/4” spruce plank subfloor
1/4” ventilation holes drilled in the Fiberrock (in the gaps between the plank subfloor) to the unheated basement
R.9 1/8” gauge tight knit wire mess (old fencing with 3 square inch air pockets when lying flat)
Sheet Metal
R.2 1/2” Durock, thin set and screwed through the metal into the subfloor
R.1 Slate tile 4’ x 4’ total area

1) Do you think this air space of the wire mess will provide R.9?
I have read extensively here and haven’t heard of anyone putting wire mess down to give the air space insulation. Hearthstone lists as R1 for an air space (like that of the air space in double glazed windows). This air space would be in place of using Micor.
The only draw back I see is the floor may have a little too much flex with the wire mess for the tiles to hold well. But we will have a 500 lbs stove holding things down.

2) Are the air holes drilled between the cracks in the plank subfloor, a good idea for ventilation this might even increase the R value of the wire mess layer to R1.43 as written in the article here on R and K values?

Thanks for any comments
Geoff John-West Edmonton Canada

 

[Hurricane]

I do not think the holes are necessary, I would start from the sheet metal and go up from there. Heat rises and I assume you are talking about under the stove.
I am no expert though

 

[johnn]

Dear: Bizarre Wire Mesh Threadster::I really think you should stop your daughter from shopping at Wall Mart,,,and yes, allow her nails a bit longer. :coolsmile: Got your hands full with that one-huh?(cute)
I really cant add up your R or K and say," yeah " go ahead. I just wouldn`t feel comfortable estimating the wire at .9, and configuring it with air holes,,rate it for 1.43. Just not enough engineering background to theorize. If one side of the Fiberrock was 300 dgr. and the opposite side was 100dgr. then it would be an insulator,,,I dont see where exposing the wood floor to the 300dgr. heat source by drilling holes through the Fiberrock,,to be productive. Even with an air exchange( limited as it would be)
I look forward to other views as im open to new things. good luck

 

[edthedawg]

I still think it's very over-optimistic for 1/8" airspace to = R0.9... It's more like 1" to get that kind of R-value. I can look thru some old posts - this is covered in a number of old threads.

Air holes drilled into the plank subfloor are a horrible, awful, no-good, BAD IDEA. you are doing nothing to increase your clearance to combustible surfaces (the un-drilled wood flooring) so you add ZERO to any insulative values. From a structural POV, you are dangerously weakening the floor where you need it most. Like you said - it's a 500+ lb stove sitting there!

Hopefully a little more discussion can happen this time around. a somewhat-to-scale sketch might help explain things further, as I could very well be misinterpreting your description...

 

[Highbeam]

Okay, Hearthstone gives horizontal STILL AIR a R value of 0.92 per 1/8". I used this and I even called them about it to make sure that it works if I use 3 inches of still air. The trick is that this air space must be a dead air space. Hence the "still air" requirement. Don't drill holes below it.

I believe that your wire mesh idea will cause trouble when you tighten the screws from above when you install the durock for the tile. The durock is pretty flexy and I would worry that the wire mesh being round will gouge into the soft wood floors as well as the durock giving you less than 1/8". FUrther, I propose that the durock will dip into the area between the wires if a screw is placed in the middle of one of the wire squares. If you go this route, at least use the big 1/4" hog panel mesh for more margin against the compression.

I used and believe in the whole air space theory for insulation but you want to do it right. You'll never get a 1/8" air gap by using a 1/8" wide wire mesh.

 

[edthedawg]

HB - he's (allegedly) putting the Fiberrock stuff between the wire mesh and the spruce plank flooring. Then sheetmetal between the wire mesh and the 1/2" Durock. Probably still an issue w/ the wire mesh burrowing into the fiberrock below.

I'm gonna search out the dead-air discussion I recall reading... despite HS's statements to the contrary, I remain in disagreement w/ 1/8" vertical "dead air" being R 0.92.

 

[edthedawg]

https://www.hearth.com/talk/threads/19391/ - one discussion on air-space / hearth building (there are a lot of them)

https://www.hearth.com/talk/threads/8339/ - this is an interesting one, but not much on the airspace issue

https://www.hearth.com/talk/threads/8289/ - this is the one I've been thinking of - Highbeam, it's your own hearth construction thread! See elk's post #13, stating airspace R-value = 0.97/inch, opposing HS's 0.92R/0.125"

 

[Highbeam]

This is one of those cases where the owner's manual for that particular stove wins. I mean, if the owner's manual said it only needed ember protection then the inspector would buy that too. Hearthstone was willing to write me any kind of clarifying letter that I or the inspector needed.

Didn't elk get banned?

I actually have started to think that the hog panels, or even a bunch of #2 rebar might be a great idea. To this day, I don't feel really confident about the spaces between my supporting structure under the raised hearth. Actually, I am more concerned about the steel studs folding up like a deck of cards.

 

[begreen]

Highbeam, I honestly believe you could put a pair of Heritages on that hearth without a single worry. You did an excellent job of creating an interlocked, substructure. With the solid, doubled durock + tile lid, this is acting now as one unit. IMHO, it will not fail on you. You can personally knock me on the noggin if it does.

construction thread:
https://www.hearth.com/talk/threads/9004/

 

[Highbeam]

Thanks BG, I'll keep that in mind. I apologize for the poor picture quality in that thread. I have since replaced my camera with a better one.

 

[LLigetfa]

In my former home, I had installed a special hearth pad made by RSF that reduced the requirement from 4 inches of masonry. I was a multilayer sandwich or solid sheet metal and diamond stretch lath. If memory serves, there was sheet metal then lath, more sheet metal, more lath and another layer of sheet metal.

The lath was the type that is sheared and stretched at an angle as shown below versus the flat style.

(broken image removed)

 

[mayhem]

The Mansfield manual specifically states a required R value of .8 in front of the doors. Logically you would just build the whole hearth to meet or exceed that value so you're definintely in the clear and you leave no room for your inspector to misinterpret the manual and give you an argument.

Personally I would rather not use an air gap as an insulator in my hearth. While I'm sure its perfectly safe, I just don't trust it as much as a precisely measured sheet of micore or durarock or other insulative construction material. Thre is also the issue of strength...in order to make an air gap strong enough to hold 800lb of stove, pipe, tools and wood you're going to have to reinforce the hell out of that air gap...those structural supports are going to have aheat transmittal value, you need to know what it is before you can properly address the issue. Complexity and possibly cost go up.

When I built my hearth I used 6 layers of durarock on top of a 2x4 frame. I forget the precise values of what I used, but I essentially exceeded the R 0.8 requirement of my stove by around 40-50%, leaving myself the option of being able to change the stove out later and not have to redesign my hearth for it. I woulnd up with a relatively tall hearth, which some people don't want, but my wife and I happen to like it that way. Micore was not available within a 60 mile radius of my location, so I just went with the materials that were local.

Just my two cents...many othere here have built wonderful hearths using air gaps...its just not for me.