I've got a retrofit project in central MA with a ~6' wide exterior brick chimney/fireplace with ~10' of exposure below the roof line, extending out about 18" from the clapboard siding.  All told it adds up to about 100 square feet of ~R3 masonry (averaged).

I'd like to slap 3" of iso on the outside held in place with furring through-screwed with Tapcons to the masonry, and hang either fiber-cement or cedar clabboards, or  even a panelized thin-brick (if there's something with a reasonable match to be found, TBD) on the furring.

The brick is pretty hard stuff, circa 1915, and it's been re-pointed at some time in the past 20 years with a hard mortar- it's structurally in excellent shape in side and out. There's another ~12' or so above the roof line to clear the peak of this 1-1/2 hip-roofed house.  Insulating the below the roof line is intended primarily to lower the heat load rather than improving the draft, since  there's already an insulated stainless liner inside the old terra-cotta for dealing with drafting issues.

Any code/other issues with this approach? (I've never done one of these.)  Is it better (or required) to go with rock wool?  (High-density rock wool panels seem like a good fit, but are more expensive and WAY heavier than iso.)

oh my god, are you asking a question?

I'm not under IRC and I don't even know what year IRC is in enforcement, but check 2003 IRC R1001.15 - chimney clearances. Basically says a properly lined chimney can have combustibles attached to exterior surface. Hope this helps.

http://www2.iccsafe.org/states/seattle/seattle_residential/PDFs_residential/Chapter%2010.pdf

Thanks!

MA code is currently based on IRC2009- I'll have to read & measure carefully, and craft a plan A & plan B before pulling the permit.

In a quick read, the wall thickness at the back of the original 1915 firebox may be an issue, and I'm not sure if the fact that it currently has a woodburning insert stove relieves any of it, since arguably that insert could be removed at anytime, converting it back to an open fireplace.

There's a second flue (now abandoned) that used to serve a hot air furnace, but they've since gone hydronic with a side-vented boiler. I'll probably want cap that off at the top too. It's currently cemented in at the bottom, but open at the top, with visible degradation of the terra cotta liner at the top, probably from freeze/thaw spalling. They never intend to re-use that flue.

I'll probably want cap that off at the top too.

I filled mine with a domed concrete cap and then covered with tar.

Posted By jonr on 18 Oct 2012 02:57 PM

I'll probably want cap that off at the top too.

I filled mine with a domed concrete cap and then covered with tar.

The liner on that flue extends ~3" above the already substantial domed cap for the whole 2' x 6' beast- I'm figuring I'll glue a reasonably sized piece of fiber-cement to the liner with RTV with enough lip to be cut/pried free later if need be and call it a day.  It also has a wide stainless steel cap/cage that spans both flues.

I’m thoroughly intrigued as to why you would want to do this. I can see wanting to insulate (isolate) the chimney from the main part of the house, but I would think it rarely necessary to insulate a chimney. I would think the only advantages of an insulated chimney would be to a) perhaps reduce heat-up time for a chimney that is not regularly used or, b) improve draft on a chimney that has very marginal draft.

 

Also, wherever possible, I like my chimneys exposed, so that I can keep an eye on their response to environmental conditions (acid etching of mortar, fractures due to whatever structural/thermal stresses are occurring, poor masonry, etc.)

 

In re-reading, I note “Insulating the below the roof line is intended primarily to lower the heat load rather than improving the draft, since there's already an insulated stainless liner inside the old terra-cotta for dealing with drafting issues.” That takes care of the draft issue.

 

It seems to me that the major improvement would result from properly sealing the fireplace to prevent air leakage (primarily) and possibly conductive heat transfer (a very poor third). How this should be done would depend upon intended frequency of use. Anyway, you have my interest.

PMH

I have urethane foam going on all brick surfaces of my house, just because we ordered some from Germany for another purpose and there was a imperial/metric conversion problem. The stuff came as about 2.5lb foam and we need 20lb foam. It will go over the chimney anyway, then get siding or solar panels on it. I don't think the chimney temps will be an issue as I measured the brick surface temp and they were way below any critical foam temps.

Posted By philipmhowe on 25 Oct 2012 11:33 PM

I’m thoroughly intrigued as to why you would want to do this. I can see wanting to insulate (isolate) the chimney from the main part of the house, but I would think it rarely necessary to insulate a chimney. I would think the only advantages of an insulated chimney would be to a) perhaps reduce heat-up time for a chimney that is not regularly used or, b) improve draft on a chimney that has very marginal draft.

 

Also, wherever possible, I like my chimneys exposed, so that I can keep an eye on their response to environmental conditions (acid etching of mortar, fractures due to whatever structural/thermal stresses are occurring, poor masonry, etc.)

 

In re-reading, I note “Insulating the below the roof line is intended primarily to lower the heat load rather than improving the draft, since there's already an insulated stainless liner inside the old terra-cotta for dealing with drafting issues.” That takes care of the draft issue.

 

It seems to me that the major improvement would result from properly sealing the fireplace to prevent air leakage (primarily) and possibly conductive heat transfer (a very poor third). How this should be done would depend upon intended frequency of use. Anyway, you have my interest.

PMH

Lemme 'splain ya:

The fireplace and brick is already pretty tight, without huge passive air leaks. (Compliments, to whomever installed that flue liner & stove!) Air leakage isn't the issue, conducted heat is:

When the wood stove isn't burning and they're using the boiler system to heat the place,  at the outside design temp of 5F with a 70F room temp the 100 square feet of ~R2 of masonry would be conduction (100 x (70-5)/2 = ) 3250 BTU/hr out of the house, which turns out to be roughly 10% of the whole-house heat load of this particular house at that design condition.

When the wood stove is cookin' along and brick in the firebox is running more than 120F (usually would be at LEAST that when trying to heat the whole house on a cold day) the heat loss would be about doubled, reducing the effective efficiency of the wood burner as a space heater.  Insulating the chimney with R15 continuous insulation will reduce those losses to about 12-15% of their current levels.

This chimney was re-pointed about 15-20 years ago (as reported by homeowner- the exact date uncertain) with a hard mortar, and it's a hard brick, all in GREAT shape. The house has ~2' overhangs, and the chimney penetrates the overhang- the lower portion of the chimney has much less rain-washing than the portion above the roof line.  Buttoning the chimney up behind siding and insulation means it would see even less moisture transfer and lower thermal stresses than it has seen in it's nearly 100 year lifespan to date.  If the inspector insists on a lime-mortar parge for further protection it wouldn't be a big deal, but it doesn't really seem called for, given how solid it is despite it's age. It's supported by a poured concrete foundation, and shows no signs of efflorescence anywhere. 

Moving it inside the thermal & pressure boundary of the house wouldn't be any different than chimneys running in chases up the centers of other houses. Since insulating it will make it warmer & drier on average, I see no reason why it wouldn't last another 100 years or more, just like any brick chimney built into a chase indoors.

Posted By MikeSolar on 26 Oct 2012 08:09 AM
I have urethane foam going on all brick surfaces of my house, just because we ordered some from Germany for another purpose and there was a imperial/metric conversion problem. The stuff came as about 2.5lb foam and we need 20lb foam. It will go over the chimney anyway, then get siding or solar panels on it. I don't think the chimney temps will be an issue as I measured the brick surface temp and they were way below any critical foam temps.

The brick temps of the chimney will rise considerably once insulated, since they can no longer shed heat, no?

Posted By Dana1 on 26 Oct 2012 04:54 PM

Posted By MikeSolar on 26 Oct 2012 08:09 AM
I have urethane foam going on all brick surfaces of my house, just because we ordered some from Germany for another purpose and there was a imperial/metric conversion problem. The stuff came as about 2.5lb foam and we need 20lb foam. It will go over the chimney anyway, then get siding or solar panels on it. I don't think the chimney temps will be an issue as I measured the brick surface temp and they were way below any critical foam temps.

The brick temps of the chimney will rise considerably once insulated, since they can no longer shed heat, no?

True, they will rise, how much depends on the efficiency of the stove. Remind me if there is a SS liner because, I have poured vermiculite down around flues before (not mine yet) to insulate the liner from the brick and it helped a lot.

My stove is a british 82% beastie which I will put a back boiler on, but haven't yet. The temps I measured were with a SS liner, one layer of brick to outside and a full hot burn. I hit about 35C max.