Fireplace insert: Insulate the exterior, or not?

[pdqdl]

I am re-habilitating my fireplace insert, a Quadrafire 4100. It's been a great heat supplement to the house, but has really sucked since the stovepipe up the chimney went bad. Yeah, I know. I should have fixed it a long time ago. Still, apart from the fact that it was hell to get started, it burned clean and added heat to the house. I also need to replace the firebrick and backer board & blanket on the inside.

Rignt now I have it pulled out, with plans to install new, stainless steel double-wall stovepipe to the top of the chimney. Something like Menards' 6" pipe.

I also intend to modify the stove to take the makeup air into the firebox from the holes I intend to punch through the fireplace walls. I really don't like the fact that I am burning wood with warm "inside" air, instead of taking it from the outside.

Now that you got the picture, here's the question: This fireplace insert is basically a steel box inside of a steel box. The heater blower circulates warm air into the house from the space between the two boxes. The outside shell, however, is sitting in a damned cold fireplace with cold air coming in from above, and theoretically, from the holes I intend to make in the rear. This means that a significant amount of heat will be shed to the outside, when I'd rather keep it inside the house.

I wish to insulate the outside of the fireplace insert with some inch thick kaowool ceramic fiber blanket, something like this stuff:
https://brickwoodovens.com/products...ilns-ovens-pizza-ovens-furnaces-forges-stoves. I haven't decided whether or not to attempt attaching it physically with metal screws and galvanized roofing washers, or perhaps some other scheme that hasn't yet occurred to me.

I figure all the heat I can keep in the house will be an improvement, especially since that insert lets tons of cold air into the house when isn't burning. Your comments and advice are welcome!

 

[Bill G]

Pictures are needed.

 

[pdqdl]

Really? It's the back side of metal box, inside of a conventional brick fireplace. I guess I could, but I didn't think that mattered.

At this point, I don't even have the stove out of the fireplace all the way. Getting the flexpipe detatched has been extra difficult. I had to heat the dinky 1/4' sheet metal screws cherry red before I could get them out. Then I had to let it cool off before proceeding, now I'm completely cold to the thought of going any further tonight.

I'll put up some pics when I get it out all the way. OK?

 

[Del_]

I see no need for photos for this theoretical discussion.

Is the mass of the fireplace in the living space? If it's not, your idea sounds sound.
If it is in the living space, have you considered sealing the area between the double walled stainless chimney and the walls of the chimney it's in? I'm assuming there is an existing masonry chimney. This is what I've done with mine. My chimney is double wall Metalbestos chimney and I've used unfaced fiberglass insulation stuffed tightly between the liner and the masonry chimney for about 8 feet.

There is very little evidence that an outside air supply is of any benefit and I have never used on.

There is very good insulation for flex pipe and it would be significantly less expensive.

In my first house(1978 to 1993) I had a 7 inch heavy duty stainless steel liner fabricated(22ft long) and rested this liner on a 1/4" steel plate through which it passed. I then poured vermiculite insulation from the top and fill the space between the liner and the chimney. I used a RiteWay Model 37(which if still own) and it used a 7" chimney. It was way cheaper than 22ft of double wall chimney plus it fit well in the 12 x 12 flue tile. Worked great.

The Outdoor Air Myth Exposed​

https://www.woodheat.org/the-outdoor-air-myth-exposed.html