Questions on Working with Micore. Please help. : )

[badger1968]

Hello All--
I'm building a simple hearth extension with dimensions of 1' x 4'. The R-Value must be 2.5 or higher. To accomplish that without a ridiculous amount of layering I have decided on using Micore 300 due to it's high R-Value (1.1 or so). I will be using three layers to achieve my required R-value.

I have four questions I was hoping to get help with regarding this material:
1. What is the best way to attach the Micore to the subfloor which is plywood?
2. What is the best way to attach the layers of Micore to each other?
3. Is it OK to tile on top of the Micore? I understand Durock would be better, but I don't want to buy a huge sheet of that stuff just to use four square feet of it--plus it would add yet another layer. This extension is not going to bear much if any weight so I'm not too concerned with load bearing properties.
4. Is it OK to use an adhesive/grout mixture for the tile or does the adhesive in the mix detract from the inflamability?

All advice welcome. This site and the helpful folks on it are great. Thanks so much.
:cheese:

 

[begreen]

Micore has great insulation properties, but is soft. You must put a layer or two of Durock on top of the micore as a backer for the top tile in order to increase rigidity to support and distribute any weight placed on the hearth. While the stove may not be on it, someone may step on the hearth or sit on it at some point in the future. You want zero-flexing of the tile surface. There are special screws designed for Durock or Wonderboard. You'll need them in an extra long length to go through the layers of micore.

The tile question just came up. The suggestion from a pro is to use latex modified thinset:

https://www.hearth.com/talk/threads/20259/

I have a question (what's new ) Is the current hearth raised high above where the extension is about to be added?

 

[badger1968]

Thanks for the help BeGreen.

As for your question

Is the current hearth raised high above where the extension is about to be added?

No. The current hearth is flush with the subfloor. This will create the unusual situation where the hearth extension will be 1 1/2" higher than the existing hearth. I'm planning on putting tile around the edges and do some nice bordering so it won't be so obvious it is raised.

By the way, is having the extension higher than the hearth a bad thing? (Never thought of that till now). :bug:

 

[fossil]

It's not clear to me why the hearth extension needs such an R-value, if the appliance doesn't rest on it. If it's just to meet the 16" or 18" requirement for floor protection from the door, then all it needs to be is non-combustible. Guess I don't completely understand the installation. Rick

 

[begreen]

I don't know how code would cover this, but do know that some stoves radiate a lot of heat in front of the glass. The floor can be hotter in front of some stoves than underneath them. Our Jotul was like that.

That said, it seems like having the extension higher than the stove is going to exacerbate the problem and to me it will look a bit goofy. Normally I would build up the existing hearth to match the extension, but I need to understand the current situation first. Can you describe the current setup in detail? What is supporting the current hearth and how is it constructed? If you can post a picture that would help too.

 

[badger1968]

Thanks for all the feedback.

It’s not clear to me why the hearth extension needs such an R-value, if the appliance doesn’t rest on it

According to Hearthstone Homestead owner's manual I need "non-combustible fllor construction or hearth extension with an R-value of 2.5 or listed to UL 1618 starting 4" from glass and extending 12" away from the stove."

Can you describe the current setup in detail? What is supporting the current hearth and how is it constructed? If you can post a picture that would help too.

OK. Here is a picture:
http://s334.photobucket.com/albums/m417/badger1968_photo/th_IMG_2124.jpg

The stove will be partially recessed into the fireplace. The hearth bricks extend into the basement so I was told I was golden as far as placing anything on top of the bricks. So I'm all set there. The extension is being built over the plywood sub-floor which I have exposed. This is the area that needs a 2.5 R-Value and is roughly flush with the hearth.

Again, many thanks for advice. I get so many conflicting opinions at the stove store--when I can talk to them--they are VERY busy selling pellet stoves. And, as we all know, the staff at Lowes and The Home Despot usually aren't very helpful.

 

[fossil]

What's the depth from the top surface of the existing hearth extension down to the plywood subfloor? Rick

EDIT: Another question...is the existing hearth extension top surface exactly even with the top surface of the actual hearth inside the firebox, or is the extension just a bit higher?

 

[badger1968]

Hi Rick

What’s the depth from the top surface of the existing hearth down to the plywood subfloor?

I may be incorrect in my terminology, but the plywood subfloor is in the foreground of the picture (the 1' x 4' rectangle beyond the hearth surrounded by carpet on the remaining 3 sides). I removed the carpet, pad, and an old layer of linoleum to get at it. It is roughly flush with the existing hearth--in other words the depth is 0". I hope this is the correct answer, but I may be misunderstanding you or using improper terminology.

Edit--the surface inside the fireplace is exactly level with the existing extension.

 

[fossil]

Will the stove still fit the way you want it to if the entire horizontal surface from the back of the existing firebox to the front of the required additional hearth extension is raised some from where it is now? Rick

 

[badger1968]

Will the stove still fit the way you want it to if the entire horizontal surface from the back of the existing firebox to the front of the required additional hearth extension is raised some from where it is now?

The stove is 25 1/2" tall and the clearance of the firebox entrance is 30" by 30". So, yes, it can be raised, but I don't want to cut it too close. I don't like where this is going! :roll:

Am I going to have to surface the entire area (existing firebox, existing hearth, and proposed hearth extension) to do this right?

 

[begreen]

That's what I would suggest. It will give you a nice continuous surface. You're going to have to get micore and durock in sheets larger than the extension anyway, so look at it this way, less waste! You can put a black metal backer plate to cover up the hole in the fireplace. That way it will not be apparent that the hearth is raised.

 

[fossil]

There are more ways than one to do this "right". I'm just thinking that if you were to come up with a plan that provided the required R-value for the hearth extension, then simply extend that same horizontal surface all the way into the firebox, then top the whole thing with a nice layer of quarry tile or something to blend/contrast pleasantly with the existing fireplace masonry, you just might come up with an overall pleasing setting for your new stove that doesn't look like it was cobbled into the house. No "step" in the hearth extension in front of the stove, just a flat floor. Sounds like you've got at least a couple of inches of wiggle room to work with. Now's the time to be thinking this all the way through. Really not all that much more work/expense, in my mind. Rick

 

[fossil]

Another question: You said in an earlier post that the stove will be partially recessed into the fireplace. Do you want it installed that way, or would it be OK if the stove sat completely proud of the front face of the masonry structure? To install the type of back trim piece BG mentioned, the stove needs to be out of the firebox. If you install it that way, you don't have to raise the floor of the firebox, as it will be hidden from view. If you bring your yet-to-be-determined & built hearth extension out a few more inches, would that buy you anything in terms of overall flexibility in designing the installation? Rick

 

[badger1968]

Yep. You're right Rick. Doing it the way you suggest would be the most pleasing aesthetic. My poor house (built in 1930s) had endured years of amateur "improvements" before my wife and I bought it in 2002. We have been trying to eliminate the many "Frankenhouse" features that we got when we bought the house and smooth out and improve the aesthetic. Going the easy route this time would be a step backwards.

So here's what I think I need to do:
1. Using a combination of Durock and Micore raise the entire surface level 2"
2. Use more Micore in the most sensitive area over the plywood to ensure adequate R-value (2.5), but
3. Make sure the surface to be tiled on is Durock
4. Tile the entire surface using Thinset as the mortar

Am I missing anything or making any large errors with this plan?

Thanks Rick, I really appreciate the help.

EDIT: Thanks again Rick and BG--Bringing the stove completely out of the firebox isn't a great option because of the size and shape of the room it will be in.

 

[badger1968]

One more question . . .
Screw this sandwich down (and together) with Durock Screws? Which are also known as masonry screws? Which may not be good for Micore--so I should use drywall screws for the Micore?
oops. That's about a million more questions. ;-)

 

[fossil]

I've never worked with Micore before, but from what I understand, I wouldn't use it anywhere I didn't have to use it. Great insulative properties, but not so great structural properties. Raising the floor of the existing hearth & extension I'd do with Durock & tile...you don't need any additional insulation there. Durock can be found in both 1/2" & 1/4" thickness, if that's any help to get the thing all flush. I'd only use the Micore where I needed the extra insulation over the plywood. If it's a problem with the height, then a layer of sheet metal sandwiched between the Micore and the Durock should seal the deal. I'm sure it'll take some research & thought, but that, at least, would be my going-in proposal. It ain't rocket science, but it ain't simple, neither...and it really needs to be done right, whichever of the many possible right ways you choose to go. Good luck with it, and keep us posted. Rick

 

[fossil]

The proprietary screws that USG sells along with its Durock products are designed to penetrate the cement board with minimal damage and then thread themselves firmly down into the substrate, which is invariably wood. I don't think it's going to hurt the Micore a bit to have these screws pass through it on their way to where they're going to anchor themselves. Rick

 

[badger1968]

Thanks a million Rick! Off to the Home Depot.
I will post pictures as it progresses. Have a great day!

 

[fossil]

I've never tried driving a Durock screw into masonry...I doubt it would work, as they're made to find wood. I've had good success with "Tapcon" screws for masonry. Blue anodized, lots of sizes/head styles. Some packs come with the required masonry bit included, but the bits are usually hanging right there on the wall with the screws in any case. Requires a pilot hole drilled, then the fastener driven in...carefully, so as not to strip the masonry. Where you're setting Durock down on brick, these will surely work better than trying to use Durock screws. Rick