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How to insulate chimney-roof clearance?

DaveTimberFrame's picture

I am looking for ideas on how to insulate, and control condensation in the space between my Chimney and foam core roof panel.

I currently have a 2”clearance between my foam core roof panel and my stone fireplace. The roof sheathing is ¾” clear of the chimney. The chimney opening is in the center of the house, and exits the roof about 3’ below the ridge. The house is an oak timber frame with structural insulated panels on both the walls and roof. It is tight. Also, I operate a woodstove in the fireplace throughout the winter. Finally, I run Panasonic through-wall exhaust fans in the winter season to remove humidity and achieve a healthy level of air change. I live in Northern New Jersey.

Over the past two years I have gently placed (not stuffed) fiberglass insulation in the void between the chimney and foam core panel. I have not trimmed out the 2” opening, thus the insulation is exposed to the room inside. The result is that humidity seems to migrate up through the insulation, condense and freeze at the top near the copper flashing…then on a warm day, when the sun strikes the chimney/flashing…. the condensate drips back down through the insulation into the house.

(post #112299, reply #1 of 8)

You mention a stone fireplace.

could you describe exactly the makeup of the chimney flue?



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(post #112299, reply #2 of 8)


The chimney is approx 23 feet from the first floor to where it goes through the roof. The house is an open configuration, the chimney is open on all four sides, it touches nothing until it goes through the roof.

Make up of the chimney flue: The chimney has two tile lined flues. One services a potential wood stove (thimble) in the basement, and is temporarily sealed off at both ends as it is not used. The main flue services the first floor fireplace. The construction is tile flue, concrete block, finished with 4" limestone stone. The wood stove is a Hearthstone Sherborne. It sits in the fireplace, the fireplace throat is blocked securely with sheet metal and packed with fiberglass. The stove is connected with a stainless steel liner all the way to the top.

During the last heating season, I measured the temperature in the 2" area between the foam panels and the chimney and found the max temp to be 84 degrees (I slipped the probe up between the insulation and stone).

I considered using closed cell Handifoam (expands 3:1). I was glad I tested it on the ground, because I found it is impossible to spray that material "upside down" in to an overhead cavity.  I would have had a mess all over the place if I had tried to apply that material in my chimney opening. Bad idea.

I wish I could send a picture. Hope this info can help you analyze my problem

(post #112299, reply #3 of 8)

FG will, as you've discovered, pass air and moisture, and you'll get condensation. Particularly at the top of a space as you've described. But, you can't bring flammable materials in contact with the chimney. (I'd consider losing the FG and replacing with the green acoustical/fire resistant insulation that you'll have to buy at an insulation supplier. HD doesn't carry it, and it's far itchier than FG. FG will burn, or at least the binder resins will. Toss some on an open fire and watch.)

The critical issue is air sealing. You need a solid air barrier at the ceiling plane. You can use sheet metal (galvanized stuff intended to for ducts works just dandy), or perhaps Hardi-Backer. Either way, you're in for some scribing. Copper would work, too. You could use fire caulk to seal any little holes.

Andy Engel

Senior editor, Fine Woodworking magazine

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(post #112299, reply #5 of 8)

still a bit of info missing for what I am thinking.

I read an article a few years ago now that the newest code ( you would need to check this with your local AHJ before doing this) has a couple of exceptions to the 2" to combustables rule.

even the old codes had that rule for framing only and did allow trim to come in contact with the chimney.

But the new exception is that when a full 12" thickness of masonry exists between the fire chamber or flue channel, combustables may contact the masonry.

So as you describe the chimney section, I would hope that you mean -
1" flue
1" air space
8" CMU block
4" stone face
14" of separation, which is effective as you have semi proven to yourself with the thermometer. But you are testing this at normal operating temperatures. The greater purpose is for that one time in this chimney's life that it WILL experience a chimney fire, with temperatuires ranging above 2000°F. Chimney fires have been known to melt mortar. Flames can then escape to contact structural lumber, and the rest is future history potential...

extreme caution is still warranted, see. I have known two chimney fires that had FG smoldering red and smoking when stuffed the way you describe. not on my house, thankfully.

so while you can possibly be technically permitted to contact the chimney, do it carefully and thoughtfully. I have used wood trim and a fireproof caulk ( years ago so no memory of name) successfully. I also once made a mix of vermiculite and mortaor to grout a space such as yours, buit I don't know whether you can work that in upside down now or not. I didn't know that the fire-resistant spray foam mentioned above was available, but that sounds like a go to me. Foam from spary cans can be applied with longer tubing also for difficult to reach spaces.

I don't suppose that you can get t



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(post #112299, reply #6 of 8)

Piffin, Andy Engle, & JonE,

Thank all of you for taking the time to think my problem through. Your suggestions were well thought through, and I will certainly use your advise in my "final solution".

Piffin, I'll check in with my building inspector to see if he is up to speed on the "exception" you referred to.

Andy, the tip on fire retardant insulation clearly is a good bet, especially combined with a good air seal as you problem is an almost impossible tight/constrained area to work in. The alignment of the Ridge Beam, 2 Rafters, and Girt make it difficult to execute the "seal", which is the most critical aspect of the project.

Jon E., Question: do you have any contact on Touch 'n Seal?    That foam, if it can practically be applied "upside down", may work. Also, let all of us know what your solution is.

Thx, Dave 


(post #112299, reply #7 of 8)

They have a web site.  Also, Do-It-Best stores carry their line, so if you have one in the area....



(post #112299, reply #8 of 8)

"I considered using closed cell Handifoam (expands 3:1). I was glad I tested it on the ground, because I found it is impossible to spray that material "upside down" in to an overhead cavity. I would have had a mess all over the place if I had tried to apply that material in my chimney opening. Bad idea."

The stuff is flamable. Good thing you didn't do it.

However, if the void is fairly small it can be done (spraying it up I mean). The only problem is that you end up wearing a lot of it. If you get substantial quantities on your hand do not use solvents to get it off. All that happens is that it dissolves enough to go down a few more layers of skin.... Then you get to wear it for a few weeks.

Try misting whatever you are spraying it on with bit of water first. The stuff sticks better. (I am not really recommending putting it against your chimney though).

Now, just see what happens when the stuff drops down onto your face.

If my house wasn't a 100 years old... there would go a really fine hobby.
If my house wasn't a 100 years old... there would go a really fine hobby.

(post #112299, reply #4 of 8)

Touch 'n Seal makes a gun foam that contains a fire retardant and should work for your application.

Also, try calling your panel supplier and see what they recommend for use in that application.  I imagine that most SIP-TF homes have a chimney of some sort and they can advise you on the proper sealing technique.

I'll be following this thread.  I have to put a chimney up through my roof, exact same scenario.  I'll be interested to know what you finally do.