Over-Ovalize liner to get more insulation? Go with ¼” Heat-Fab or ½” blanket?

[formula_pilot]

You guys have been great for learning the right way to do things, I am down to a few details

I am installing a Heat-Fab rigid liner into a 6.25-6.5 x 10.5” ID clay lined flue and will ovalize it in order to insulate it. I am trying to decide what insulation method to use. The chimney is only 15’ tall, with a 20” horizontal through the wall, so I really need to optimize the install.

The Heat Fab insulation that is part of their “system” is only ¼” thick, with a stiff aluminum outer shell. If I went this route it would require less ovalizing than using a ½” blanket and should fit without too much trouble, but 1/4" seems thin to me. Do you guys think it would be better to go with thicker ½” insulation, but requires more ovalization of the liner?

Heat-Fab’s installation booklet has a disclaimer to only use their ¼” shelled insulation, but I believe that is because that it the only configuration they tested it with. The rest of the industry seems to use ½” blankets with 304SS liners and it works fine, so I am not too concerned with violating their warning. Does anyone have an idea as to how much better the draft would be if a thicker blanket was used, or would the extra ovalization negate any benifit?

Thanks
Bill

 

[begreen]

Have you looked at the pre-ovalized and insulated Simpson DuraLiner as an alternative?
http://www.northlineexpress.com/itemdesc.asp?ic=5SP-4648-OP

 

[formula_pilot]

Have you looked at the pre-ovalized and insulated Simpson DuraLiner as an alternative?
http://www.northlineexpress.com/itemdesc.asp?ic=5SP-4648-OP

Yes, I called about the Simpson oval liner, the problem is the orientation of the "T", my clay "thimble" enters the flue on the narrow side of rectangular flue, but the oval "T"s are setup with the snout on the wide side, so the Simpson liner would not work for me. I already purchased the rigid liner and ovalized a few feet of it, looks like it will work, I am just a little uncertain of using the 1/4" insulation, since every other liner for wood seems to use 1/2"

 

[begreen]

Ah, that would be a challenge. The chimney is clay lined, so the 1/4" should be ok.

How are you going to attach the 6" round snout to the narrow dimension of the heat-fab? Is there no access to the wide side of the flue?

 

[formula_pilot]

....

How are you going to attach the 6" round snout to the narrow dimension of the heat-fab? Is there no access to the wide side of the flue?

The clay pipe through the wall is 8" ID, so I planned to push the snout in through the pipe, slide the liner down through the band clamp, and use a long extension to tighten the clamp. There is just enough room to slide the verticle part of the "T" down the flue, without insulation. I will try to stuff Kaowool around the "T" it after it is in so it gets somewhat insulated. The only other access to the flue is a 6" square cleanout but it is 11 feet below in the basement, and it is also oriented on the narrow side.