Improving insert efficiency with insulation?

[Jon1270]

I am jealous. Specifically, I am jealous of those members who can apparently keep their homes at tropical temperatures with their woodstoves, because so far I can't do so. No doubt that's partly because this is my first year and my wood supply is less than ideal, but I suspect it's also due to the fact that I have an insert rather than a freestanding stove, and the chimney is on an outside wall. Even when the stovetop temperature is cruising over 600F (IR thermometer), it takes a long time to heat up the room, let alone the rest of the house. I suspect a fair bit of heat is making it past the air jacket and escaping into the fireplace behind the surround.

The fireplace behind the insert is an early (1950's) Heatilator-style prefab steel box. The Heatilator (actual brand unknown) had its own air jacket, which was connected to some convection vents to circulate heat into the room. The orignal smoke shelf rusted into oblivion before I installed the insert. It rusted so badly that it basically doesn't exist anymore. The back and sides of the original firebox are solid, and I did install a 22g block-off plate in place of the original damper. But what this means is that my insert is surrounded by a second air jacket which is open, thorough the nonexistent smoke shelf and the space between the stainless chimney liner and the (quite large) original terracotta tile liner, to the cold, cold sky. And that's where I think a large fraction of my heat is going.

So anyhow, I have a couple of ideas about how to improve this situation, and I'd like some feedback.

1. Lay a bat of some high-temperature insulation directly on top of the insert, held back a couple of inches from the chimney connector, to reduce heat loss through the top of the insert.
2. Insulate the fireplace interior by adding a second layer of sheet metal, with some sort of small spacers to create an insulating air gap.
3. Finally -- and now that I write all this down this solution is starting to seem like a fairly clear winner -- dump a bunch of pour-in insulation down the chimney (*around* the liner ). Not enough to fill the chimney (it's already an insulated liner) but enough to at least partially fill in the Heatilator's air jacket and the original smoke chamber, cutting off the path to the sky.

What say you?

 

[PLAYS WITH FIRE]

I think that insulating the old fireplace firebox will help you a ton. Even if it's a small layer behind metal like you said. Insulating the top may not be so important or safe! I would ask the mfg about that, I personally don't see why not but it is best to be safe. But if you don't have a block off and insulation stuffed up in the smoke chamber, that will cause a major heat loss! I don't think insulating the pipe more, unless you have draft problem, will help with heat loss.
My insert for the moment does not have the top surround on and that makes quite a difference,so maybe this is an idea?

 

[mellow]

For sure insulate that fire box, check my signature for a link on my install.

 

[Jon1270]

Thanks for pointing that out, mellow. I see I'm not the first to recognize this problem.

Interestingly, the old Heatilator is now starting to look like an asset instead of a liability. It provides a nice, fat cavity between firebox and brick, where it will be easy to stuff insulation. Only downside is that I'll have to pull the insert out to do it properly; this may have to wait until springtime.

 

[ddahlgren]

Why not talk to the people who made the insert and get their advice about what to do and if insulate what kind. If you have no idea of what the skin temp of the insert is how do you know what to insulate with. Why not fix the top if open to the sky with some cement board or steel then cement board. If you can see the sky from the back of the insert you are just heating the outside air.

 

[Jon1270]

Just to clarify, there is already a block-off plate in place. What I meant was that there's only one layer of sheet metal between the insert and the open chimney, not only at the block-off plate but at the back and sides too. There's no path for heat to escape without first going through metal, but sheet steel isn't much of an insulator.

 

[PLAYS WITH FIRE]

Roxul above the plate and anywhere else you can stuff it is your best bet! If it is easy, pull off the top surround and see how much heat escapes. I am going to do similar to what you want to do but I will not be putting my surrounds back on. I want to take my time and make it presentable so it don't look like crap!

 

[danham]

I've had very good results with Roxul above and behind my insert in an outside chimney. I installed it mid-season last year and didn't have to pull the insert out very much -- not necessary to disconnect the pipe or anything.

-dan

 

[Dune]

Nothing wrong with pouring some insulation down the flue either. An insulated liner is not insulated all that well.

 

[Jon1270]

I suspect I may have to do most or all of the job with pourable-type insulation rather than Roxul. The Heatilator's air jacket is only 2" or so wide at the sides and at the bottom of the back of the firebox, and access is difficult at best. Even if I were to completely pull the insert and remove the block-off plate, I can't think of any way to stuff batts of a springy material in there without creating voids and overpacked areas. What I really need is a sort of blow-in material.

 

[mellow]

Why not talk to the people who made the insert and get their advice about what to do and if insulate what kind.

Osburn = SBI

SBI has OK'd Roxul insulation to be used with their inserts in fireplaces. PM FyreBug, he works for SBI if you have any questions.

 

[WoodpileOCD]

Did you plug the gap at the top of the chimney with Roxul as well or just seal the cap on with the liner in it. I stuffed as much Roxul down from the top as I could to stop any of the warmed air in the chimney from escaping out the top. No idea how effective it is but the theory made me feel better.