Traditional Hearth Modification questions

[projectanavita]

I have an 1880's house I am looking to install a wood stove in. I was told a traditional hearth, with bricks, doesn't meet the k value requirements in the manual.

So I cut into the knotty pine with the goal of building my own hearth out of aircrete since the K value is really high and even with 3/4" I would be able to achieve the requirements listed in my manual. The problem is the existing stone hearth.

(view from top with some details)

(view from basement showing arch)

I am not sure of the structural strength of the archway if I were to remove the top layer of brick.

Thoughts / options I am considering:
1) Remove the top layer of brick, then pour aicrete to level to sub floor with a mix which is stronger than the mix I plan to use to insulate the hearth. Then place a thin steel frame down into the hearth space and pour my aircrete hearth and top with tile or something decorative.

2) Remove the whole archway and span between the two main floor joists on either side of the chimney and brace out to the existing header where the brick arch ends. I would also likely add extra bracing from the spanned floor joists to the joists one out from there to make it stronger as well. From here I could add a sub floor and start from there with a 3/4" hearth or go on to the next option.

3) Do #2 and remove sub floor at exposed area. Brace basement floor joists more and put down a piece of plate steel as a new "sub floor" then pour a deeper hearth (1-3/4") with aircrete providing an extra thermal barrier.

Thoughts? Any masons / carpenters out there? This is an 1880's balloon framed house with true to dimension lumber featuring large beams / posts throughout.

 

[RSNovi]

Many stoves have a bottom heat shield and just require ember protection. Would a different stove be an option?

 

[projectanavita]

Many stoves have a bottom heat shield and just require ember protection. Would a different stove be an option?

My fireplace has odd clearances and to avoid putting in a thimble I had to go with a low flue stove (like the quadrafire I have) to get under the existing fireplace opening. The manual lays out the requirements for a .45 k value 1" board ~2.2 total R value ... with aircrete, depending on the mix,this means a 4/10ths of an inch thick pour would suffice since it has a K value of .2 ... I thinking of going to go with 1/2" aircrete and top it with tile to make it look nice.

 

[begreen]

The R value of aircrete varies inversely with its compressive strength. It appears that one needs to mix aircrete at lightest weight to achieve R= 1.82 per inch. If this is mixed on site how does one acheive a precise mix that will guarantee R value? How does one avoid cracking with a thin layer of the weakest strength over floorboards?

 

[begreen]

What stove is this for? How will it connect to the chimney?

Also, what happened to the fireplace? It looks like gray board is covering it. Is this cement board or other non-combustible surface?

As an alternate approach, consider putting 2 layers of 1/2" micore in the cutout sub-floor area with the top layer continuing over the hearth brick. Then put 1/2" cement board like Durock on top as a tile base. Yes this will raise the hearth. It can get trim framing to make it look proper.

 

[projectanavita]

The R value of aircrete varies inversely with its compressive strength. It appears that one needs to mix aircrete at lightest weight to achieve R= 1.82 per inch.

I couldn't find the source for the Domegaia chart but the R value doesn't necessarily have to mean weak concrete. There are companies out there who product 160 psi concrete with the same R value you mentioned. And there are companies who do spray in wall products which are 6lbs / sqft which have crazy R values like 6 or 7. The mix is key.

Some mixes assume a fixed amount of water to concrete then adjust foam ... some adjust foam, water, and concrete ... they tend to vary for sure. Still others will have additives to increase the operating temperature, increase strength, or increase insulation values.

I found some good documentation here in the past https://cellularconcretetechnologies.com/testing/

If this is mixed on site how does one achieve a precise mix that will guarantee R value?

Good measurements are all that is required to get repeatable results with aircrete. Since most people who make aircrete don't have professional equipment knowing the measurements by weight going into a mix is key along with the processes used to identify when through mixing has occurred.

How does one avoid cracking with a thin layer of the weakest strength over floorboards?

By spreading the load out ... I plan to lay down some sheet metal in a frame the shape of the hearth to give form to the hearth and allow it to shift seasonally if required. After it is cured I planned to put another piece of metal on top then tile over that to help distribute the forces.

At what PSI do you think would be sufficient to put a stove on if the stove is sitting on something like tile?

 

[projectanavita]

What stove is this for? How will it connect to the chimney?

Primary heating of the house. Fallback will be on the oil based boiler. Might also be used for emergency cooking surfaces if required

Via a T connector which is connected to a 6" SS liner passing through the hole where the damper once was. This connects up a lined clay chimney to the top plate which is sealed and capped. (just installed this liner last week)

Also, what happened to the fireplace? It looks like gray board is covering it. Is this cement board or other non-combustible surface?

The gray piece of sheet metal pictured was brought in to act as a very large, near perfect, shape for the hearth. It is 40x48 and I got some of it dirt cheap locally. I simply propped it up over the opening to get it out of the way.

As an alternate approach, consider putting 2 layers of 1/2" micore in the cutout sub-floor area with the top layer continuing over the hearth brick. Then put 1/2" cement board like Durock on top as a tile base. Yes this will raise the hearth. It can get trim framing to make it look proper.

I have a tight 30.5" clearance to the top of the flue to deal with since we didn't install a thimble but instead went right through the damper location with the liner. I can't put 1/2" on top of the existing hearth brick as the Quadra-fire won't fit properly and still have a flue connection which is properly slopped. https://downloads.hearthnhome.com/installManuals/250_5763.pdf ( page 9 shows diagram but the length to top of flue exiting rear seems to be larger than actually measured ~29.5 if I recall ... have it off right now to re-gasket it)

If I use micore I will need 1" of it ... and likely have to remove the brick first layer and the sub floor to maintain a 0 clearance or near 0 clearance hearth. When I was looking for stoves which would heat the house and have a low flue exhaust the quadra-fire fit the bill ... most others were undersized stoves or would have required a thicker hearth than I could provide.

 

[begreen]

I can only go by the chart that was published for AirCrete. They say that it can be made as light as 19#.

AirCrete

AirCrete is a lightweight non-toxic masonry material that is easy and inexpensive to make yourself with our Little Dragon continuous foam generator. AirCrete is a mixture of cement, water and...

www.domegaia.com

Tile will not bond to metal unless glued (a no-no for hearths).

What is the stove model?

 

[begreen]

Nevermind, that is the old Isle Royale. The Jotul F55 is the same firebox size, but requires a simple ember-protection only hearth. Flue exit height is 30". The Quad Explorer II has a flue exit height of 28.5".

 

[projectanavita]

Nevermind, that is the old Isle Royale. The Jotul F55 is the same firebox size, but requires a simple ember-protection only hearth. Flue exit height is 30". The Quad Explorer II has a flue exit height of 28.5".

I as going by what stoves I could find on CL / FB and looking up anything that looked like it had a big enough firebox. I didn't see any F55's or QE II's at the time. The IR I picked up has some issues I am fixing now but it was fairly cheap.

 

[projectanavita]

I can only go by the chart that was published for AirCrete. They say that it can be made as light as 19#.

Aircrete is a general term ... like paint is a general term. You can technically make aircrete as light as you want but you suffer from other issues. There is a company called airkrete (tm) which specializes in sprayed in place aircrete for walls which go down to 2lbs/cuft for example but it is really friable. Since it is behind drywall though it doesn't matter so much.

Tile will not adhere to metal.

Noted ... will think of something else to make it decorative and help disperse the load of the stove.