Insulation between Firebox and Chimney

[kenj1]

I am installing a Fireplace Xtrodinair Flush insert into a Masonry fireplace. I will be running 4 foot of 6" stainless steel flex up and over the smoke shelf (after removing the Damper). The remaining flue will be 6" insulated rigid. The salesman indicated that it would be advisable to seal off the fire box from the chimney, using KAOWOOL or the like. In looking at KAOWOOL, it appears that it comes in 1" thick mats, and it could be a challenge getting it to stay in place. I have considered using fiberglass bats, but he indicates that they may melt (research indicates at 1100 f degrees). I see that mineral wool melts at 1800 F. Any thoughts on this? (possibly wrap the stainless flex in KAOWOOL, and then use fiberglass bats for the rest?

 

[MountainStoveGuy]

firbeglass is considered a combustable, why seal it at the damper? why not just block it at the top of the chimney and seal with insulation behinde the surround?

 

[kenj1]

Not a bad thought. (I wasn't even thinking that direction). His thought was that blocking it at the damper, I would get some additional radiant heat from the surround, because the airspace is warmed. The top of the chimney will be capped and sealed with the stainless plate, and High temp silicone caulk.

 

[MountainStoveGuy]

I dont think the heat in the back would be enough to worry about, and if the unit comes with a blower it will take care of what little would be back there. IMO. And, i would still install a metal plate inthe damper, just dont worry about insulation.
RYAN

 

[kenj1]

On the installation instructions, it only indicates a metal damper plate if you are dumping into the current chimney. On a fully lined, and insulated chimney, it doesn't indicate a metal block off damper plate.

 

[MountainStoveGuy]

Local installers here do both. It would help insulate the box from the chimney, so to sum this up, three places, the top of chimney, the flue damper, and the surround.
Ryan

 

[kenj1]

Thank you very much for you opinion. I will work on making a tight damper plate, and get the dead air space. Just a curiosity question, what would be your mininmum recomendation on the Gage of the metal to make this from?

 

[MountainStoveGuy]

I would use anything in the 20-26 gauage range, its a little easier to work with if you cut half circles out if two sides instead if cuttin a hole in one piece. I hope that makes sense.
Ryan

 

[Rhone]

I'm a fan of a metal block-off plate, as like Elkimmeg said you can't argue that a liner with the top sealed off AND a block-off plate is better than one having only the top sealed off. On my install, the installers sealed up the top and put in a metal block-off plate. I'm pretty sure it's required where I am. My insert doesn't have a rear heat shield, yours may not either so you and I may have more than usual heat going into the fireplace. At least with the block-off it minimizes how much is actually lost. I also had air leaks around my outer shroud that put a lot of air in behind the surround, which I'm glad for the block-off as it forced it into my living area. Lastly, the top of my chimney that's sealed with Silicone those seams broke either from a big bird that decided to use my liner as a lookout for prey, or on a real windy day my liner shifted breaking them. Thank God for the block-off. I can't imagine how inefficient my setup would've been this winter without it as the block-off is currently the only thing keeping my warm house air from sneaking to the outside 24/7 with my top seals broken like that. With an insert with a full shield, seams tight, with non-broken seams up top, and the blowers on, there isn't hardly any heat loss. But if you fail on any one of those variables, now, or in the future, you'll wish you had a block-off plate besides the safety factor it adds. Just think of a power failure, or the bearings in your blowers go and you'll be without blowers for a few weeks, or you don't have a rear heat shield, or air leaks in your outer shroud, most ugly is the top seals breaking you'll be happy with a block-off.

So, get a steel block-off plate and I recommend you purchase one online instead of making it yourself. During your liner install, I'd stuff the top 2-3 feet between your liner and flue with mineral wool insulation (good to 2200F+), and if you really want to do it right put the mineral wool insulation above your block-off plate and then put the block-off plate in. My mason also put mineral wool insulation all around the insides of my fireplace. Don't have it right against your insert though, your insert specs don't include having insulation right against it on the outside. My Mason fastened it to the insides of my fireplace instead.

 

[MountainStoveGuy]

I'm a fan of a metal block-off plate, as like Elkimmeg said you can't argue that a liner with the top sealed off AND a block-off plate is better than one having only the top sealed off. On my install, the installers sealed up the top and put in a metal block-off plate. I'm pretty sure it's required where I am and glad they did it. Also, my insert doesn't have a rear heat shield, yours may not either so I have more than usual heat going into the fireplace that would not let me recover as much into my living area without the blockoff. When my blowers were on I put my hands around my insert and felt a lot of air flow coming out seams of my insert. In my case, the outer of my insert is made of around 10 seperate pieces not sealed to each other. They had hair line seams exposed but always in the worst spots and estimate about 40% of my air flow blowing into my fireplace area. I sealed them up with stove cement and what a difference. My block-off helped me recover that heat. Lastly, the top of my chimney that's sealed with Silicone those seams broke either from a big bird that decided to use my liner as a lookout for prey, or on a real windy day my liner shifted breaking them. Thank God I have the blocl-off. I can't imagine how inefficient my setup would've been this winter had it as a backup as it's currently the only thing keeping my warm house air from sneaking to the outside 24/7 with my top seals broken like that, on top of not having a rear heat shield on my insert, and previously leaky seams. With an insert with a full shield, seams tight, with non-broken seams up top, and the blowers on, there isn't hardly any heat loss. Unfortunately if you fail on any one of those variables, now, or in the future, you'll wish you had a block-off plate besides the safety factor it adds whether that be a power failure, your blower bearings go, the top of your liner seals break, what have ya.

So, get a steel block-off plate and I recommend you purchase one online instead of making it yourself. During your liner install, I'd stuff the top 2-3 feet between your liner and flue with mineral wool insulation (good to 2200F+), and if you really want to do it right put the mineral wool insulation above your block-off plate and then put the block-off plate in. My mason also put mineral wool insulation all around the insides of my fireplace. Don't have it right against your insert though, your insert specs don't include having insulation right against it on the outside. My Mason fastened it to the insides of my fireplace instead.

perfect responce, and if you cant find any mineral wool insulation, just order from a quad dealer, there replacement sheets for there stoves. (or any manufacture for that matter who uses them to insulate the firebox.)

 

[joshuaviktor]

What do you use to seal the sheet metal to the bricks? Rutland Furnace cement ? And what do you use to get a tight seal around the flue? Furnace cement again?

 

[MountainStoveGuy]

I would use 600 degree RTV, rutland brand is red, but i would anchor the sheet metal with masonry screws and use the rtv to make it airtight. furnace cement doesnt allow much room for expansion or contraction, and i dont think it gets above 600 back there, im shure someone here will know.
Ryan

 

[Rhone]

I used furnace cement by Rutland (black and good to over 2000 degrees) to seal the air leaks in my outer shroud of my insert. For sealing gaps around a block off plate, that's a good question because it is difficult to see nor have access to the seams. You can use furnace cement or high temp silicone around the edges. The hole where the liner goes through, I recommend you carefully use mineral wool insulation and close the block-off on it. Be careful not to accidentally dent or crimp your flex liner doing it. I believe it can get too hot during a chimney fire for silicone right against your liner, furnace cement can handle it but it dries solid and easily chips and breaks apart once dry. Because of that, and chimney sweeping will cause your flex liner to move, my guess it will break off some of the furnace cement if you use it to seal the hole your liner goes through. You don't have to worry about using it around the edges, they won't move. After chimney sweeping I guess you could reapply it there, but I stuffed mineral wool insulation around the liner through the hole of the blockoff to allow for movement when sweeping. I try to minimize using mineral wool insulation to stop air flow as it only restricts not stops. But, in that spot I made an exception. My block-off plate ended up coming to 1/8" around my liner so I didn't need much stuffing.

 

[wingnut]

What about using some of the leftover foilback ceramic insulation I have to fill the gaps?