Hearth extension

ohnodoc · Dec 17, 2008

Advice please regarding insulation requirements for a hearth extension.

Finally I have decided to put a wood-burning stove insert into our fireplace so that we can keep more of the heat from our fires. The problems is that our 15.5 inch tile hearth on a cement base in front of the fireplace is not enough for the requirements of the stove or our insurance. I have leftover matching tiles from the original construction and had planned on extending the tile another 8.5 inches. After two scary discussions with someone at Home Depot and a wood stove salesperson explaining the minimal R-factor with tile and wonderboard, and that if the insurance company decides that the R-factor was not good enough, they would refuse to pay for a house fire, I pretty much decided not to take on the project.

Then when I ordered our Lopi Freedom Bay insert yesterday the very experience salesperson said that all of those r-factors are not needed on the extension over 10 inches from the front of the glass. He said that all I needed was a non-burnable surface....i.e. that tile and wonderboard would be sufficient. He said that I could just buy a micor extension, cut it to size and put in under the tile if I wanted to be safe.

Why is a layer of micor extension considered sufficient when often in these discussions it sounds like I would need a couple of feet of insulation?
Where to find Micor....I am worried that if I go to a ceiling store, that I would get the wrong stuff and the insurance company would wash their hands of me.

Is there a formula for decreasing r-factor requirements the further from the stove that the flooring is located?

Thanks

Bob

DAKSY · Dec 17, 2008

Hey Bob...

I have leftover matching tiles from the original construction and had planned on extending the tile another 8.5 inches. After two scary discussions with someone at Home Depot and a wood stove salesperson explaining the minimal R-factor with tile and wonderboard, and that if the insurance company decides that the R-factor was not good enough, they would refuse to pay for a house fire, I pretty much decided not to take on the project.

You can do it, but you'll need to put the Micore down first, then Dura-Rock or Hardee-Backer, then thinset, then tile. The total of everything will meet your minimal R-factor

Then when I ordered our Lopi Freedom Bay insert yesterday the very experience salesperson said that all of those r-factors are not needed on the extension over 10 inches from the front of the glass.

VERY EXPERIENCED?!?!? WTF?!?!? MINIMUM hearth protection -in the UNITED STATES is 18 INCHES in front of the glass - unless specified by testing authorities (in the install manual!)

He said that all I needed was a non-burnable surface....i.e. that tile and wonderboard would be sufficient. He said that I could just buy a micor extension, cut it to size and put in under the tile if I wanted to be safe.

WOW! If this guy's an expert, then his local fire department is probably pretty busy...If I told people that those were all they needed to be safe, & the owner of the company got wind of it, I'd be fired on the spot. If that guy's company actually does installs to those criteria, he probably doesn't have insurance coverage...

Why is a layer of micor extension considered sufficient when often in these discussions it sounds like I would need a couple of feet of insulation?<.

Micore is a mineral fiberboard & is non-combustible & doesn't conduct heat...Dura-Rock is non-combustible, but it WILL conduct heat.

Where to find Micor....I am worried that if I go to a ceiling store, that I would get the wrong stuff and the insurance company would wash their hands of me.

Find your local hearth shop - or at least one where the "expert" doesn't work at...
Or you can Google for Micore or USG...If you want to be sure you get the right product, ask for Micore 300...It's approved for stoveboards...

Is there a formula for decreasing r-factor requirements the further from the stove that the flooring is located?

Again the hearth protection is specified in the appliance install manual...NFPA 211 will tell you that anything 36" away IN ANY DIRECTION is safe...

*Whew*

edthedawg · Dec 17, 2008

Everything stated so far is pretty much correct. Except the part about not doing it yourself - you've already pretty much determined it'll get done wrong if you have any of the "experts" in your immediate area try to do it.

If you have the finish materials, the minimum clearances/dimensions needed, and the necessary R-value for floor protection under/around the stove, then i would recommend you take some photos, post them here, maybe include some sketches of what you want/need, and we can all discuss it.

You CAN do a hearth extension yourself - it ain't hard.

ohnodoc · Dec 17, 2008

Thanks so much so far.

I will take some pictures when I get home; then post them. I really appreciate the help.

One question that still puzzles me is why every wood stove store I have been to shows me a hearth extender which is about a half inch thick that can be "laid down in when using the stove, then put in a closet in the summer. How can such a small item be up to code when I need to put down 4 layers to meet code?

Bob

chainsawdad · Dec 18, 2008

Bob,

I recently hunted down some micore from Kamco to build a hearth pad with an r-value of over 2, and I see that there is a Kamco in Londonderry NH (not sure how close that is for you). Here's a link to their site: http://www.kamcoboston.com/default.asp
However, one tip if you want to get it there is to call ahead, because they don't stock all of their materials at all of their stores, but they can have it sent over on a weekly track from their main store.

ps I've no clue about the thin ul listed pads...been wondering myself what magical materials they contain...

ohnodoc · Dec 18, 2008

Enclosed is a picture of the fireplace showing the white/green tiles. The heath extends 15.5 inched. The wood stove insert will extend onto the hearth 4 3/4 inches. With the 18" clearance needed, I must have 22 3/4 inches total. Since each row of tiles give me 4.25", two more rows will give me the adequate clearance. "What I need is sufficient r-factor for the part of the hearth that is over 10 inches from the stove front glass.

My plan is to cut out enough of the carpet and then start the layers. I am not sure how much depth I have, but it appears to be only one full inch if I keep the tile extension flush with the current hearth....which I greatly prefer to do.

Another problem is that our house has radiant heat...which means that there are plastic water pipes running through the floor under the plywood. I really do not want to screw into one of those pipes! So, could I glue the lowest layer to the plywood instead of screwing it down.

Thanks

Bob

ohnodoc · Dec 18, 2008

BTW....In the above picture, the two rows of three tiles are loosely set on the carpet in front of the hearth to give an idea of how much I hope to extend the hearth..

Bob

ohnodoc · Dec 18, 2008

One more piece to this puzzle: there is no plywood under our carpet!

When I lifted up the carpet next to the hearth, I found a "sand/cement like smooth layer that the foam and carpet was sitting on. Clearly this is the material that the radiant heat tubing carrying water runs through. THis makes my above question even more pressing: How to I attach the Micore to this hard-pack sand/cement layer since screws would probably not hold; and they may puncture the tubing within. Can I glue or use tile adhesive to attach the Micore?

With the lack of combustible materials in my "subfloor" (ok, so there is plywood under the sand/cement/tube mixture....but it is not visible from the top), can I simplify the layers and keep my hearth extension flush with the hearth and still meet code requirements? Would a layer of Micore, then tile be sufficient?

Thanks

Bob

PS Gratitude goes out to chainsawdad. Your info about Kamco in Londonderry NH was very accurate. They sell Micore, it is not in stock, but they can get it quickly. Unfortunately, Londonderry is nearly a 2 hr drive, But this may be the best option if I really need it to keep my home insurance valid.

Hogwildz · Dec 18, 2008

ohnodoc said:
One more piece to this puzzle: there is no plywood under our carpet!

When I lifted up the carpet next to the hearth, I found a "sand/cement like smooth layer that the foam and carpet was sitting on. Clearly this is the material that the radiant heat tubing carrying water runs through. THis makes my above question even more pressing: How to I attach the Micore to this hard-pack sand/cement layer since screws would probably not hold; and they may puncture the tubing within. Can I glue or use tile adhesive to attach the Micore?

With the lack of combustible materials in my "subfloor" (ok, so there is plywood under the sand/cement/tube mixture....but it is not visible from the top), can I simplify the layers and keep my hearth extension flush with the hearth and still meet code requirements? Would a layer of Micore, then tile be sufficient?

Thanks

Bob

PS Gratitude goes out to chainsawdad. Your info about Kamco in Londonderry NH was very accurate. They sell Micore, it is not in stock, but they can get it quickly. Unfortunately, Londonderry is nearly a 2 hr drive, But this may be the best option if I really need it to keep my home insurance valid.

What is under the existing hearth? I am thinking if you have a lightcrete type substrate there already, you have a non combustible surface and no need for micore. Set the tile directly to the cement surface that is there.
That way your extension meets up with the existing hearth?

chainsawdad · Dec 18, 2008

Hi again,

BTW, as I understand it the R-value required for a hearth surface depends upon the specifications of the model. If the Lopi Freedom Bay manual simply says it needs a "non-combustible" surface, then there is no R-value requirement and tile should be fine. The fire inspector will do his/her inspection based on the specs of the Lopi FB, and I can't see why an insurance company would require more safety than required by law. I'm no expert (just someone who has been trying to figure out installation R-value requirements myself), but I would ask your insurance company whether they need any documentation beyond the fire inspection certificate, and I can't see why they would.

If for some reason you do end up needing to use micore, my guess is you could attach it to the cement floor with tile-setting mortar. When I was putting together my hearth pad, I used mortar with acrylic (for added bonding strength) to bond 2 half-inch layers of micore to a piece of plywood. I also screwed them down, but my guess is you could weight down the micore while the mortar sets if necessary. This article on hearth.com has good info on the R-value of various materials if you do need to go there: https://www.hearth.com/econtent/index.php/wiki/Hearth_Design/

Best'o'luck!

ohnodoc · Dec 18, 2008

What is under the existing hearth? I am thinking if you have a lightcrete type substrate there already, you have a non combustible surface and no need for micore. Set the tile directly to the cement surface that is there.

The current hearth is on a layer of concrete that I can see from the underside when in the basement. IN the basement, plywood is seen beyond this initial 16 inches. Thus, my hearth extension is not on the same material. What I will be covering is plywood, then the radiant flooring material which is some sort of hard packed sand that looks like concrete, but is somewhat less solid. or has less glue or cement added.

Bob

ohnodoc · Dec 18, 2008

The following is all that is stated in the manual about the hearth. No R-value is mentioned. Perhaps the fact that it is an insert where most of the heat is inside the fireplace lowers the requirement?????

Hearth Requirements
• Must extend 16" in front of the insert and 8" on both sides
• Must be non-combustible and at least .018" thick (26 gauge)

edthedawg · Dec 18, 2008

Wow you were really boned until that last send. No R-value specified? Then it sounds like you can just cut your carpet back carefully and use THINSET - not mastic, you have to use mortar! - to bed the extra rows of tiles. This is fine even if the tiles wind up sitting over the not-so-concrete portion of your flooring.

To be honest, I can't decipher WHAT exactly is going on with the floor systems from your descriptions above, but if you don't have an R-value requirement, and you can get the new tiles to sit flush w/ the existing ones, then sounds like you can go for it. Remember the tiles will sit slightly higher when bedded in mortar - use a notched trowel to apply it.

Edit: Add Durock/Wonderboard in 1/2" and/or 1/4" layers to fill the 1" depth needed to make the tiles sit flush. Bed the layers in mortar. Recommend using Versabond - buy a bag, mix some in a bucket w/ water til peanut-butter consistency, and trowel away.

One suggestion: You may wish to add more protection (dimensionally in front of the door) than the "minimum" as embers can and will shoot pretty far out of the stove sometimes. One thought is to go an extra 1 or 2 rows (so 3 or 4 new rows total) and put the corners at 45* angles. You can slice square tiles like those in half really easy with a cheap tile cutter from HD.

Good luck and keep posting pix of the build!

ohnodoc · Dec 19, 2008

Clarifications:

1) Lopi apparently never states an r-factor...just a state inspected approved hearth is all they require. It appears the default UL is 0.86 r-factor which is the amount found in 1/4 " of asbestos. Thus I need 0.86 r-factor.

2) A "knowledgable" chimney cleaner/inspector/teacher of fireplace inspectors in NH was concerned that my plan of a hearth extension could crack at the point that I start the extension. Since the current hearth is on concrete, and the extension will be on the radiant-heating/gypsum-layer that is ultimately on plywood, he thought that over time with settling, moisture,etc. the different materials would separate. This seems like a minor concern to me. However, it would be nice to find a very good adhesive to hold the Micore and Duroc to the edge of the current cement so that a crack will not occur there in the future.

What would be a good adhesive between cement and Micore, cement and Duroc/wonderboard?

How should I attach the wonderboard to the Micore?

What should be my shopping list when I go to Home Depot or Lowe's? All I have now are the tiles?

chainsawdad · Dec 19, 2008

My admittedly non-expert suggestions:

I'm not sure why you would need the wonderboard at all, unless for adjusting the height, since it seems you could lay the tile directly on the micore. I would use one of the "high-bonding" tile-setting mortars that are available at Home Depot or Lowe's to attach the micore to the floor (weighted down), as well as the tile to the micore. If you do need to adjust the height with wonderboard, I think the same bonding method would work there as well.

I agree that cracking along the joint would not be a big deal, since you could always re-grout the joint even if it were to crack.

ohnodoc · Dec 20, 2008

My understanding is that Micore is like working with cardboard; so I worry that walking on tile directly on the Micore could crush and disfigure. What do all of you think of the structural strength of tile directly on Micore?

Regarding “high-bonding” tile-setting mortars, I checked the HD site and am bewildered. The options include: 1) SpeedSet Fortified Thin-Set Mortar, 2) LevelQuik RS Rapid Setting SElf-Leveling Underlayment, 3) AcrylPro Ceramic Tile Adhesive, 4) VersaBond Fortified Thin-Set mortar, and others. Which would all of you recommend?

Bob

ohnodoc · Dec 20, 2008

FROM A PREVIOUS POST:
To be honest, I can't decipher WHAT exactly is going on with the floor systems from your descriptions above, but if you don't have an R-value requirement, and you can get the new tiles to sit flush w/ the existing ones, then sounds like you can go for it. Remember the tiles will sit slightly higher when bedded in mortar - use a notched trowel to apply it.

MY FURTHER THOUGHTS/RESPONSE:
The flooring was made for radiant heating...3/8 inch tubes with heated water flow through the floor on the subfloor. Then a mixture of sand, gypsum, and Portland cement in roughly equal portions is made into a slurry that is poured over the tubing and is about an inch thick above the tubing. I found the retired builder who did our house and he explained what was in the flooring.

Thus I will be adhering the Micore to the above mixture, then eventually tile.

How thick is the layer of thinset that is recommended?
What is the difference between mastic and mortar?
Is acrylicpro better than thinset?

Thanks
Bob

chainsawdad · Dec 20, 2008

Well, you're right that micore has little structural strength, but since it is going onto a rigid surface I think it should be okay. I laid two half-inch thick layers of micore over a plywood base for a hearth pad, and there is no give whatsoever in the tiles mortared directly onto the micore; it's a really solid surface, although I did use 12" ceramic tiles (1/4" thick), which may make it more solid than your thinner 4" tiles would...don't know. Anyone else?

G-rott · Dec 20, 2008

Micore will have enough durability in the center or field of your hearth after you tile over it. Adding a layer of dura rock will help. Any problems you encounter will be at the edge, there I would cut back the size of the micore and build up with dura rock a few inches wide. The thin set mortar can be used to even out the edge where the materials meet.

I would consider taking out the existing hearth(tiles only) if I had the clearance and installing an additional layer of dura rock or another backer board that would span the gap between the concrete hearth and the flooring material. It could be 3/8 or 1/4 inch. It would help keep the joint from telegraphing through the new hearth.

Garett

ohnodoc · Dec 20, 2008

In the attempt to make my extension flush, I need to calculate all layers to the finished surface. What I am unclear on are:
1) If I use a 1/4 inch trowel for the thinset, how thick will the layer be after the tiles are laid down? A 3/8 inch trowel?
2) What is the thickest that I can make the thinset?

Thanks

Bob

wolfkiller · Dec 20, 2008

Your Morter bed will raise your tile 1/16" - 1/4" depending on trowel used amd how you set them. When trying to match a existing row I bring both trowels and experiment. You can push the tile down into the morter a little with a larged trowel. If you set a tile and it is not quite right pull it up and add or remove some morter. Remember tile is never perfect and looks great with minor flaws.

ohnodoc · Dec 21, 2008

What are good ways to cut Micore given that I have few tools. I will rent a water saw to cut tiles from a local hardware store. I have a Skill saw with normal wood blades. I have a jig saw. I have a table saw. Is the dust toxic to breath?

chainsawdad · Dec 21, 2008

A utility knife and straight-edge is all you need to cut the micore. Just score the board and then make several more passes to get all the way through with the utility knife, and you'll end up with a clean edge and no dust.

Cedrusdeodara · Dec 21, 2008

Wow,

This is a very timely thread. I too am extending my hearth this weekend. I need to add another two rows of 6"x6" slate tile to meet my setback requirement of 18" for my insert stove.

Thus far I have cutout the required area into the oak hardwood floor, down to the subfloor, which is an older hardwood floor. I purchased a sheet of 1/2" Hardibacker and I had planned on doubling up the hardibacker (two layers screwed to the subfloor), then applying thinset and slate tiles above. My stove brand (Napoleon) does not list a required R-factor, just an "approved non-combustable hearth pad". Anyway, now after reading this thread and seeing the R-factor table (https://www.hearth.com/econtent/index.php/wiki/Hearth_Design) I don't think I'll have the needed R-factor.

I'm guessing that without a stated R-factor, I should use the common 0.86 factor??
Each layer of Hardibacker only provides 0.26 from the R-Factor table, that is 0.52 for the two layers of Hardibacker, plus slate tiles that are 3/8" thick. I couldn't find a value for the slate, but 1" slate only provides 0.1 r-factor, so I guess at best the slate may only be giving me only about 0.042 value (my estimate). That brings me to only 0.56 R-factor.

Here's my question: I planned on doing a layer of thinset between the two layers of backerboard, and another layer of thinset beneath the tile (with back buttering the tiles as well). Do these two layers of thinset mortar provide me with additional R-factors that I am not including? I'm sure they would, but I don't know if I can use them in my calculation..

Thanks for any 411.
Brian

G-rott · Dec 21, 2008

The answer to the question is "not enough to measure".

If I remember correctly masonry products have an R value around 1 per foot of thickness.

Garett

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