Insulation, Masonry to Framing

[djkeev]

I am building a masonry firebox.
While it will LOOK like a fireplace, it will actually have my Jotul 3 wood stove in it. No smoke shelf just a clay thimble into a clay tile flue.
Yes, I'm using clay, not a prefab chimney. Decision is made, materials are on site,

I completely understand the clearances to combustibles for both the stove itself and the masonry unit in which it sits.

As per NFPA 211 there will be a 2" air space between the house framing and the masonry unit.
I will also hold the masonry 1" air space off of the house sheathing as the chimney rises up.

Now none of us want a 2" gap letting cold Winter air blow in!

What is the proper way to seal this gap?
How do you recommend I finish the interior wall finish to the masonry?
How to finish the exterior masonry to sheathing?
Finish the siding to the masonry?
What kind of insulation....... if any?

NFPA 211 talks about using metal flashing to span this space but is silent on insulation in that air space.
Also metal flashing inside the house isn't exactly pretty.

Yes, a permit is in hand, it is simply these small finishing details that elude me.

Thanks for taking the time to help.

Dave

 

[bholler]

I am building a masonry firebox.
While it will LOOK like a fireplace, it will actually have my Jotul 3 wood stove in it. No smoke shelf just a clay thimble into a clay tile flue.
Yes, I'm using clay, not a prefab chimney. Decision is made, materials are on site,

I completely understand the clearances to combustibles for both the stove itself and the masonry unit in which it sits.

As per NFPA 211 there will be a 2" air space between the house framing and the masonry unit.
I will also hold the masonry 1" air space off of the house sheathing as the chimney rises up.

Now none of us want a 2" gap letting cold Winter air blow in!

What is the proper way to seal this gap?
How do you recommend I finish the interior wall finish to the masonry?
How to finish the exterior masonry to sheathing?
Finish the siding to the masonry?
What kind of insulation....... if any?

NFPA 211 talks about using metal flashing to span this space but is silent on insulation in that air space.
Also metal flashing inside the house isn't exactly pretty.

Yes, a permit is in hand, it is simply these small finishing details that elude me.

Thanks for taking the time to help.

Dave

You cannot insulate it but you can close off tge gap with trim on the corners. But seriously why would you choose to go with a clay liner? They are inferior to an insulated stainless liner in every way. If you want a masonry structure fine but line it right the first time with insulated stainless and you wont be redoing it in a few years. And you dont have to worry about the chimney clearances.

There is a reason very few chimneys are built with proper clearances. It is a pain to do so most masons dont.

 

[djkeev]

Thanks for your reply!
I appreciate your thoughts.
Let us focus on the masonry to the wood junction.

It is code to not have framing wood touch fireplace masonry. An air gap is required.

I can accept not insulating the air gap, it maintains the physical fire break. Super heated air cannot collect and combust the wood...... But it must be sealed so that wind doesn't blow into the house and rodents don't have an open door.

You cannot have wood framing touch the masonry, why is it ok to have a wood mantel, wood exterior sheathing or wood trim piece touch the chimney in the area framing cannot touch?

Dave

 

[bholler]

Thanks for your reply!
I appreciate your thoughts.
Let us focus on the masonry to the wood junction.

It is code to not have framing wood touch fireplace masonry. An air gap is required.

I can accept not insulating the air gap, it maintains the physical fire break. Super heated air cannot collect and combust the wood...... But it must be sealed so that wind doesn't blow into the house and rodents don't have an open door.

You cannot have wood framing touch the masonry, why is it ok to have a wood mantel, wood exterior sheathing or wood trim piece touch the chimney in the area framing cannot touch?

Dave

You do not need 2" around the firbox you just need 10" of masonry.

 

[begreen]

My BIL built his own stone chimney He included a gap between the inner masonry fireplace and the outside stonework. IIRC, the outside stonework went right up to the house structure but the inner masonry fireplace box did not. In the gap between the inner and outer he poured insulation. It works quite effectively.

 

[djkeev]

Well, I built my "fireplace", actually a wood stove recess.

For all practical purposes the look is that of a fireplace but instead of a firebox, smoke shelf, damper, etc. is a 6" thimble into an 8" square clay lined chimney.

Yes, I know, insulated stainless chimney is better....yada yada yada....... but truth is, money was an issue. Many/most of my block was free. Just had to buy lintels, mortar, clay tiles and chimney block.

All permitted and inspected by the township too.

Honestly...... It was a heck of a lot of work! If you ever decide to buikd a masonry assembly, expect to be weary at the end!

While not 100% done, it has been burning since Thanksgiving. I still need to do a stone hearth, stone firebox surround and mantle.

It is an External chimney, yes I know a major heat loss.....

Jotul F3CB in the masonry box, little natural air flow around it, another heat loss

But I wired in a very quiet fan to gently blow air from the bottom of the firebox up and over the Jotul and into the room.
The heat from the stove can force you outside!

The house is only 676 sf and the room it is in, less than 400sf. There was ZERO space to build an interior chimney or have the stove sitting in the room and vent through the ceiling!

I am also limited by zoning on % of lot covered by the house foot print. My zoning only allows 5% coverage. Yet fireplaces are "free" foot print space!
I am grandfathered in as "pre-existing non conforming" as it is.

It draws well but I only built 13' of chimney (Winter was fast approaching!), Jotul says 15' is minimum for the F3 ...... I think 2' more will change things for the better. I get an occasional puff back when I open the door.

Also I was late to the party so my wood supply is less than Ideal. Lots of old 9% moisture 2x4 from remodeling and a cord of Ash I bought..... sadly measuring 19% moisture.

From Christmas to well into January We've had one of the coldest spells in the Reading Pa area that they have seen for years. I don't anyicipate worse cold!

With a good dry wood supply next Winter, I expect even better heat and burning performance.

I plan on getting two more cords to stack and dry throughout the Summer.
It is a part time home so I should be able to get a cord ahead and have two year old wood for the following year.