Double Wall Rigid Liner to Flex--Insulate the Flex? and How Long on the Rigid?

[Dirtgrain]

My chimney is 17' 8" from the top of the clay-lined flue to bottom, where it shifts over a foot or so to where the damper is. The clay liner is 10 3/16" straight across from one side to another, although the clay flue sections don't seem lined up very well. I'm thinking of using 16' of double wall rigid liner (6" diameter) and then attaching flexible liner to the bottom to weave through the area where the damper is (I'm going to cut out the damper and install a block-off plate with insulation).

Should I insulate the flexible liner that I will attach to the bottom of the rigid double-wall insulated liner?

Should I get another one-foot section of rigid to make it 17' of rigid? I understand this is a petty question, but I'm wondering if it would make a difference.

 

[CarbonNeutral]

I stuffed rock wool up into the smoke shelf area, then closed off with a seal-off plate.

 

[Hogwildz]

I used same set up. Works great.
I have 5' pc of flex to get through the smoke shelf, damper ares and down to stove.
The double wall insulated rigid up to the top, and out the top about 16" inches.
I did not insulate the 5' of flex. And have no problems at all.
As far as the extra foot, it can't hurt if ya end up needing it to get out the top.
But its an added expense you may not need. I have been thinking of taking the last 2' pc I have going out the top and checking to see if 1' will get me out the top, and have less pipe showing.
At 27', I have more than enough draft.

 

[oconnor]

If you need the insulation to meet a code requirement, then there is a theoretical area where your chimney ceases to be a chimney, and instead becomes part of the fireplace. That is where the insulation requirement would stop. Assuming your existing chimney was built correctly, that should be where the first clay tile starts. In my situation, I had concerns about proximity to combustibles outside the chimney structure very close to the fireplace, so I insulated the entire liner from where it passed thru the damper until the very top.

As for the block off plate, if you insulate the entire run of the liner, then I see no benefit in adding insulation above the steel block off plate from an R value perspective. In the end, even insulated, the liner will be putting way more heat into the space inside the brick chimney that can be transmitted from the insert thru a much smaller metal plate at the damper. If anything, I would guess heat is moving from behind the block off plate into the room, and not vice versa. Insulation liker rockwool may make for a more airtight block off, but even that should be done with high heat sealants.

Good questions.

 

[Dirtgrain]

Thanks for the replies all. Last night, I read the installation PDF for Simpson liners (it didn't occur to me to do this when I posted this thread). It matches up with what you all have written. Simpson does suggest that one could insulate the flex liner for improved draft and for better safety, but it is not required--from the bottom of the bottom clay liner to the firebox (and it may be required from the bottom of the bottom clay flue tile up).