New R=1 Hearth Pad

[hubisz]

Hi all...new to the forum, but I've been checking it out lately since I'm putting together a new hearth for a Hearthstone Phoenix stove (yay tax credit).

An old Jotul was hacked a bit to fit there before (top was removed and reversed to fit the stovepipe in the area), and tossed on a piece of slate which was in turn tossed onto bare wood...the stove just barely fit on the slate, and there's a nice brown melted spot of linoleum in front of it where the previous owners clearly almost burned the house down...not exactly to code. *** EDITED TO ADD PICTURE OF OLD SETUP ***

Anyway, I wanted to run my plans by you folks and make sure I'm not messing anything up.

From the ground (linoleum floor) up, this is what it looks like:

2x4's for structure
1/2 inch plywood for basic stability
liquid nails adhesive
1/2 inch fiberfrax ceramic board, R=1.1 per half inch
(same stuff as micore, from what I can see, and sold cheaply around here at $6/ft^2 compared to $25/ft^2 for micore at a specialty hearth store)

?thinset? this is one question I have...how do I bond the durarock to the fiberfrax?

1/2 inch durarock
12x12 slate tile thinset to durarock 1/4 inch spaced, with mortar grout

The hearth size is about 62"x44", with brick masonry chimney right behind it. I know the 62" is kind of wide, but it fits aesthetically.

Tile should arrive next week, and then its time to get to work, so I'd greatly appreciate any input before it's a lot of time (and money) to go back!

 

[edthedawg]

So long as you run your Fiberfrax out past the minimum front and side clearances, you should have no issues w/ the combustibles (everything below the Fiberfrax) in your construction. Pay close attention to the rear clearances needed for the stove, since you have to accomodate those and that may push the stove further into the room than originally planned. Happened to me. If you build to bare minimum clearances, you're kinda asking to get bit... 44" from the end face sounds potentially a little shy, but I haven't enough info to tell you for sure. If you have nothing but masonry (with NO combustibles hiding behind a single brick layer!) then you're probably in good shape.

You can use plain old Versabond from HD - it's a fine fortified thinset mortar. I think you should figure out exactly where the legs will rest and lay in 3 or 4" wide strips of Durock there (North-South along the depth of the hearth), instead of putting the FiberFrax all the way across. That way your tilework in the loadbearing area will not be subject to any flexing. You can do strips all around the perimeter, too, to help stiffen and secure it all. I would expect you could bed the fiberfrax in thinset (top and bottom, if desired) and then use standard ~2" Rock-On screws to sock it all down snugly. Liquid nails has me kinda curious - where is that going? Just to hold the FiberFrax to the plywood? LN has poor durability in a hearth situation - also something i tried and have utterly been unhappy with.

Should be a nice stove. What's your firewood situation? Got 4 - 6 cords split and stacked up last year waiting for you?

(oh - and welcome to the Hearth!)

 

[hubisz]

So long as you run your Fiberfrax out past the minimum front and side clearances, you should have no issues w/ the combustibles (everything below the Fiberfrax) in your construction. Pay close attention to the rear clearances needed for the stove, since you have to accomodate those and that may push the stove further into the room than originally planned. Happened to me. If you build to bare minimum clearances, you're kinda asking to get bit... 44" from the end face sounds potentially a little shy, but I haven't enough info to tell you for sure. If you have nothing but masonry (with NO combustibles hiding behind a single brick layer!) then you're probably in good shape.

You can use plain old Versabond from HD - it's a fine fortified thinset mortar. I think you should figure out exactly where the legs will rest and lay in 3 or 4" wide strips of Durock there (North-South along the depth of the hearth), instead of putting the FiberFrax all the way across. That way your tilework in the loadbearing area will not be subject to any flexing. You can do strips all around the perimeter, too, to help stiffen and secure it all. I would expect you could bed the fiberfrax in thinset (top and bottom, if desired) and then use standard ~2" Rock-On screws to sock it all down snugly. Liquid nails has me kinda curious - where is that going? Just to hold the FiberFrax to the plywood? LN has poor durability in a hearth situation - also something i tried and have utterly been unhappy with.

Should be a nice stove. What's your firewood situation? Got 4 - 6 cords split and stacked up last year waiting for you?

(oh - and welcome to the Hearth!)

Thanks dawg! The rear is the masonry chimney and a 21" deep alcove that has open air on the other side, no combustibles. Rear clearance is thus all good, but I have to watch the sides. With the rear heat shield in place, I should be more than safe, I think (I'm installing wall protection as well, using what I'll have left of the fiberfrax, which should be overkill). I'll post a photo of the empty space later with the blue masking tape I laid down for the outline.

The liquid nails is between the plywood and fiberfrax, since I wanted to avoid using any kind of screws which might create hotspots. I guess the thinset should work fine thought too. I'm not even sure anything would be necessary there at all, since the weight of the ceramic board + durock + tile should keep things pretty snug, but I had read somewhere that everything should be bonded.

That brings up another question though...what do you do about seams between pieces of ceramic board? It comes in 42x48" sheets, so there's going to be at least one seam perpendicular to the 62" dimension. Is the air between the seam good enough insulation? I would run any seams in the durock layer above the fiberfrax perpendicular to the fiberfrax seam.

I was planning on a full sheet of durock over the fiberfrax, so I assume I wouldn't need additional strips of durock under the legs.

I have about a bit of wood that I cut from my lot last fall, and some more coming from a neighbor who's selling. I plan on about 5 cords + what I already have, and see where I am in the spring. Some more dead trees are coming down this fall for next year too.

I've got a propane furnace as backup, but at $500+ a month last year, I'd like to do a little better!

 

[edthedawg]

I'm talking about strips UNDER that full sheet of Durock. So the FiberFrx would not be continuous across, it'd have a strip under the left and right edges of the stove to provide rock-solid support. If you're using a large-format tile, this is likely a non-issue. If you have a small-format tile in mind, you need to be concerned. That's a 400+ lb stove sitting on 4 tiny little legs, likely only evenly distributed to three of them. 100+ psi is enough to do some damage if you have flexy material underneath that full sheet of Durock.

I don't know how flexible FiberFrax is personally - you already have it on hand? As for handling the seams, I'd think you just cut it straight and tight, and get it packed in as well as you can, and it'll work out fine for you. Might throw a layer of Durock underneath it (bedded in thinset, not Liq Nails) if you're really concerned about heat xfer.

 

[hubisz]

Okay, I wasn't quite sure from your post. The fiberfrax undergoes 15% compression (.075 inches for a half inch layer) per 57 psi:

http://thermalproductsco.thomasnet....iberfrax-duraboard/2300-ld-1-2-24-36?&seo=110

Under the tile and durarock, the compression should be considerably less (since it would be more distributed after ~3/4 inch of rather solid material - I'm going with 12" by 12" slate tile), so I'm guessing this is safe.

I like your setup, by the way! How much wood do you go through in a season with the heritage?

 

[DAKSY]

sold cheaply around here at $6/ft^2 compared to $25/ft^2 for micore at a specialty hearth store

$25 per sf??!?!??! We sell 72" x 20" x 1/2" micore for about $20. That's about $2/SF & we mark it up!
Sounds like somebody's price gouging...

 

[hubisz]

$25 per sf??!?!??! We sell 72" x 20" x 1/2" micore for about $20. That's about $2/SF & we mark it up!
Sounds like somebody's price gouging...

Yeah, pretty much. Not sure why they'd go that far...30 sf or so at that price is prohibitive, and they'd never sell the stuff that way. I picked up the FiberFrax Duraboard LD, and it is kind of soft, but I think it should be okay between the plywood, 2x4 supports, and the durarock. I wish I had handled a piece of the micore for comparison.

Here's a pic with the Jotul and the slate removed, with the tape outline of the footprint: