I've been told - No insulation in Chimney: Opinions Please !

[jpbellis]

First my installation details...

I have a 20 foot high chimney in the the center of a ranch style house. It is a clay flue inside a cement block chimney and no combustibles within 6 inches of the internal flue. I'm putting a mid size stove directly into a fireplace. I have a Flex king pro smooth wall 2 layer liner ready to install. The liner will JUST fit down my flue so it can not have any insulation wrap. Here is where the "experts" stories go separate ways.

1. I was told by the company I bought the liner from that I can simply stuff some insulation in the top of the chimney and then stuff some into the bottom in the smoke box and ensure everything is nicely sealed.

2. I always want multiple opinions on something I am not an expert for, so I made a call to a very large and highly respected stove company here in CT. These guys are are known to be top notch. I called to ask them if I would use rock wool instead of the more expensive ceramic blanket insulation, since reading that rock wool will work just fine and is very safe to use. Their response threw me off the tracks... it was a definitive "there was NO reason to stuff insulation in the top and bottom". not required and completely unnecessary That the homes configuration doesn't need it at all and they would never install it.

3. I was a bit taken back at the above response, so I called another large and reputable shop across state and they gave the same answer! In fact they went so far as to say that the insulation was actually a bad idea as opposed to just being not necessary. They said it will develop and collect moisture and cause corrosion, even with a stainless sleeve.

I simply want to do the right thing, but I don't want to put in stuff just forsake of installing it. I will have a tightly sealed cap and a steel plate inside the flue to seal off the chimney. So my question to the folks here that have lot of experience is, "what do you think?"

John

 

[bholler]

http://www.rumford.com/code/clearances.html

Read this and look at the other liner insulation thread that is up now. It absolutly is not a bad idea and in most cases it really is required.

 

[fossil]

I can't understand how it could hurt anything. You want to install a block-off plate in the top of the firebox anyway, and some stuffed insulation above that will help seal it and prevent the unnecessary loss of stove heat up the flue around your liner. More insulation stuffed around the top of the liner before the support plate and cap are installed will just "seal the deal" so to speak. With no significant air exchange in that little space around your liner, there's little chance of any significant moisture accumulation. I guess I'm telling you to "stuff it". Rick

 

[begreen]

If the chimney is totally within the home envelope and properly isolated then unlined should be ok. Most of the heat transferred to the chimney is going to stay indoor and slowly be released by the masonry. No harm in stuffing it though. If the last 3 ft is exterior, stuffing can keep the flue gases warmer as they exit the house. That might keep build up a bit lower.

 

[jpbellis]

If the chimney is totally within the home envelope and properly isolated then unlined should be ok. Most of the heat transferred to the chimney is going to stay indoor and slowly be released by the masonry. No harm in stuffing it though. If the last 3 ft is exterior, stuffing can keep the flue gases warmer as they exit the house. That might keep build up a bit lower.

Very helpful. The outside portion of the Chimney is about 40". Do you see any issue with Roxul rockwool?

 

[bholler]

do you have 2" space between the outside of the chimney and any combustibles? if not by code it needs to be insulated

 

[jpbellis]

do you have 2" space between the outside of the chimney and any combustibles? if not by code it needs to be insulated

2" space outside of the chimney or outside the flue? If you mean the chimney, please help me understand how there could be a 2" gap between the chimney and structure?

 

[branchburner]

The only possible reason I could see not to stuff rock wool around the top 3 feet of the liner is that, technically, it is not an application specifically and formally approved in the written specs for the liner. So while using that insulation might improve the performance and safety of the setup, it does not fall under the officially established modes of insulating a UL-listed liner. This formality mainly applies to zero-clearance applications (UL1777). So, for example, instructions might read: "liner will lose UL-listing if not insulated with ProFoil wrap (UL-listed) or TherMix (UL-listed)."

I could be wrong, but I think this is why the stove companies will not recommend that you proceed this way. Technically, the liner specs say EITHER install with no insulation OR with the specified insulation (wrap or TherMix)... there is no leeway in the instructions for a modified, homemade insulation application of rock wool, loose perlite, etc. (Loose perlite is what I used, as well as some rock wool). The CYA position of an installer is quite logically to go strictly by the book, and not do anything that doesn't conform with written specs or instructions, even if the non-conforming application makes perfect sense from a practical standpoint.

 

[Charles1981]

This can be a major point of contention on these boards.

I have a chimney completely dead center to my house. I was told insulation wasn't necessary, however the installer did stuff roxul at the top and bottom of the chimney to help maintain temperature in the dead space around the liner.

I haven't had any significant problems. I don't think creosote is a big problem but it is a 27 foot chimney and I get about 40-60oz (measure by large folgers coffee can) of creosote every 3-4 months of burning (light, flakey...not glossy/shiney stuff).

Maybe it would work better if it was insulated entirely? Maybe not. I haven't bothered to think about giving it a go just yet really.

If the chimney was on the exterior of the house I would definitely want it insulated however.

If the fit is tight I know there are pour in insulations you can use (i think pearlite is something people use).

 

[begreen]

Lots of variables here. The dryness of the wood, the temp the stove is run at, the length of the run, exterior exposure, and average flue temps all affect creosote build up.

All things are not equal, but in comparison I have a 20 ft interior all metal flue system. About 7+ ft are exterior. I gotten about 12 oz of sote for every 8 cords burned so far. However that is in a milder climate so our exterior portion is warmer and I try to be sure the flue gases stay above 250F coming out of the stack.

 

[ctswf]

im totally new to this and just started reading before im about to get an insert.

when reading about insulation at the top and bottom it did make me worry about condensation, or what if a small leak appeared in the liner. that dead space has nowhere to escape.

that being said I have no experience with either method yet, and I suppose sealing off the block off plate and cap plate make the same dead space anyway.

would it be a terible idea to insulate the block off plate and leave a small air gap near the cap plate for air to escape if it ever needs to? and use an insulated liner?