Insulate stone chimney - need thoughts/ideas

Eventually I'd like to build a rocket mass heater, but I'll probably be doing that in our final house when we build ourselves. Currently I'm living in a home I bought to make into a rental, but we're living here for now. The fireplace/chimney is a pretty nice focal point inside and outside the house. It looks original to the 1940's house and is built of round fieldstone gathered from the property itself. I love the look of it.. but since it is on the outside wall it just seriously leaks heat out.

I'm planning on putting in an insert to make it much more efficient at heating the house, but I fear that I'll be wasting so much heat as it conducts out through all the stonework, instead of letting me use that stone as thermal mass working for the house.

My thoughts have been to put Styrofoam board (2") against the stone on the outside, use spray foam or some sort of vermiculte to dump in between all the gaps (the stones are not really even at all) Then do a stone facade on the outside of the foam board (I'd have to make sure to drill in wires/supports that stick out through the foam to help hold up the facade i think).

I hate to cover up the original stone on the outside (would still be visible on the inside) but I'm not sure how else to make this fireplace work for us instead of against us.
Any thoughts would be appreciated.

I don't know how big this chimney is, but if it is small, and gets relatively hot, I would make sure you don't have foam insulation against it. Maybe unfaced fiberglass first.

The chimney is very large and could never get hot enough on the outside to be a danger to foam board, but I understand the concern. Here are pictures from when I bought the house.

Not 2 inches, 4 inches. And then you put stucco over the styrofoam. At least that's how they are retrofitting old Communist build buildings in Poland that they want to give a new lease on life to.

Once you have insulation around the chimney, then it acts as heat storage to re-radiate it back inside the house instead of losing it to the outside. And if you like stonework, then you can put some new stonework on the outside of your insulation.

Insulation will trap heat inside, and there's a slight chance you might discover some kind of "ash cleanout" or "air intake" that leaks enough heat to be dangerous.
I'd take a poke around Buckley Rumford's site (www.rumford.com) for specifics on fireplace and chimney detailing.

I'd be inclined to use Roxul (rock wool), it's commonly used as wall insulation in Canada, and handles higher temps than our fiberglass.
But I'd use fiberglass before I'd use plastic foams.
Just my own preferences. I hate adding accelerants (highly flammable stuff) to the building when something else can do the job, especially when I know they offgas too.

We've had some bad experiences in natural building with stuccos cracking and leaking moisture into trapped cavities. This can rot wood, diminish insulation values, and lead to higher heat transmission and mold problems.
You should be able to detail things so the stucco is less likely to crack, and/or so that any moisture that does leak in can find its way out if it starts pooling below.
Watch the flashing at connections to roof, house, etc to divert any moisture there too.

Another option is to use a breathable siding or wall finish like the rest of the house. If the temps are safe for foam, they will be more than safe for siding. Siding and wood panel tend to breathe a bit better than stucco.

None of the above are deal-breakers - just things to look into so you know how you're going to handle them from the get-go.

-Erica

one other thought if you really like the stonework:
- what about building a little sunroom in that corner and capturing the heat so you can vent it back into the house?

Thank you for the ideas! I like the idea of using the Roxul and adding a good bit of insulation, then figuring out the way to cover it. I think if I go with too much the chimney will really look gigantic against the house, unless I cover it up with siding like the rest of the house (Planning on upgrading the siding someday).

I like the sunroom idea, will be a little difficult but it would help recover a lot of the heat as you mentioned. Also thinking that a small greenhouse on that side would do the same, the house is not set up well to get any passive solar currently.

With the Roxul ideas I'm thinking I could cover it up and seal it well, and if I want to sell or rent I can take it back off if the chimney stonework is really a good selling point. Though I'd rather leave it on and cover with more stonework or something very nice looking. I like that with the Roxul (or fiberglass) it wouldn't Have to be permanent, but probably would.

I appreciate knowing that others (ie. Poland) have the same thoughts for old chimneys.