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Closed-Cell Attic Insulation and Condensation at Chimney Penetration

homeowner007 | Posted in General Questions on
What I first thought was leaky flashing now appears to be something else entirely. After a recent windy rain, we discovered a small amount of water in the attic at the chimney. I thought it must be wind-driven rain. But today I had a roofer come take a look and his hypothesis follows.
As background, the attic roof deck has been sprayed with closed-cell to bring the attic into the building envelope. Within the attic, there is a gap between the chimney and the roof framing members. I believe that per code that should be 2″, the gap at our framing is more like 1/2″ – but I digress. There isn’t any insulation in this gap.
The roofer thinks that the water is condensation – forming from attic air meeting the cold air at the chimney penetration. I suppose this could be happening both inside and outside – eg there does not have to be a leak in the roof for us to experience water in the attic.
The said, is the answer simply to break out my foam gun and hit this gap to seal it up? or should I stuff it with mineral wool first and then some foam, or caulk, or?? Could any of this be coming from warm flu air hitting the cold and then dripping back down the chimney?
Here are some photos of the flashing. You can see some water/ice on the flashing – dripping down from condensation between the brick chimney and the copper flashing? I also included some photos from inside the attic looking up at the gap and some thermal imaging of the same
We’re in the NYC suburbs – it’s 18 degrees out tonight.
Thanks again for any guidance or insights.

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  1. charlie_sullivan | | #1

    The good news is that your flashing looks well done. It's hard to find people to do good copper flashing, so congratulations on that.

    The bad news is that solving the thermal bridging that a brick chimney creates and maintaining safety and code compliance are tricky, and you may need to live with some condensation there. The part I think is fairly straightforward is to air seal at the bottom of the gap: Use some sheet metal to air seal between the chimney and the rafters, to keep moist air from getting up there, along with high-temperature caulk. It would be great if you could also stuff the gap with foam, but that would be a fire hazard in contact and would be against code. Paradoxically, rockwool in there is probably against code, even thought might actually be safer than having wood that close. Specifically rated high-temperature insulation would probably be better in practice than an air gap or rockwool, but whether it meets code is confusing. Here's a thread with more discussion of that:

    What is the chimney used for? A fireplace? A stove? A furnace? How is it lined? The level of fire risk depends in part on how it is used.

    1. homeowner007 | | #2

      thank you! today it is for only an oil fired furance, which will be decommissioned soon. It may stay dormant for a while, although eventually, we may line it for a wood-burning stove in the basement. my main concern is water damage to the roof deck; which seems very real at this moment.

    2. Expert Member
      BILL WICHERS | | #3

      Mineral wool is commonly used as a fire stop material commercially, it's common to stuff it around things and then cover it with fire caulk or putty. The fire stop "pillows" are usually filled with a block of mineral wool too. Flamability wise, mineral wool isn't a problem -- it won't do anything in a fire until well after the rest of the structure is ash on the ground.

      The issue is will your building department people be OK with it since the gap is no longer "free air". I think it's worth a call to them on that one. Tell them you want to stuff that gap with mineral wool and see if they think that would be OK. My guess is they'll think it's fine. I would call the material "mineral wool" for this purpose (because that's what we call it in the commerical world when using it as part of fire blocks), and it's the generic term for the material that "rockwool" is made of.


      1. homeowner007 | | #4

        Awesome, Bill. I’m on it!

        The thermal imaging has turned up a host of other spots that could eventually become condensation spots - a great example is on the sistered rafters we added to supoort the solar panels. Had i seen that comming i would have added something between them before the fasteners were installed. better late than never, I guess.

  2. Etc90 | | #5

    One thing to consider is sealing the chimney with a masonry sealer.

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