Chimney Air Gap

Old 07-09-15, 11:52 PM
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Chimney Air Gap

Hi All,

Thank-you in advance for any responses.

I have a 1 1/2 storey house, built in the 1940s, and I am having the slanted upstairs ceiling spray-foamed.

My question is this: My understanding is that code expects an air gap around the chimney. Are there any exceptions to this rule?

I ask because my chimney has a metal flue running inside of it to vent the main-floor gas fireplace. The chimney itself is not used, and I have been told would need to be torn down and rebuilt if I ever wanted to go back to a wood-burning fireplace - there is no liner in the chimney. The chimney does not get hot when the gas fireplace is used. Therefore, it makes sense to me to have the spray foam guys seal around the chimney, but I do not want to violate code and certainly do not want to do anything unsafe.


And if I do need an airgap, what is the minimal distance required?

Lastly, the drywall I tore down from the entire slanted ceiling was fire-rated. I believe this was overkill?

Just trying to do my homework before the sprayfoam guys show up.
Old 07-10-15, 04:41 AM
Join Date: Oct 2008
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Although I have never heard of a chimney fire from a gas appliance, judging that you don't feel the chimney being warm is not the total issue. As in wood, a chimney fire can exceed 2,000 degrees. I don't claim to be a pro on fire codes, but they are there for a reason.

You say "My understanding is that code expects an air gap around the chimney". My understanding is they want a 2" space to combustibles, but it may be an air space. Roxul is very dense and would limit air leakage, but the final say belongs to your local code official. And the air leakage should already be blocked by metal flashing and fire rated caulk.

Foaming the bottom of the roof deck introduces several concerns.
1. Will the final r-value meet your code.
2. Are they using open cell or closed cell foam
3. Will there still be any attic spaces, sides or top, that will need venting?
4. How new are your shingles? Leaks will be difficult to detect after the foam.

Others will comment on sealing around that chimney.

Old 07-10-15, 06:23 AM
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Bud, thank-you for the reply.

To answer your questions:

1. The final R value of the spray foam (true 2x4 cavity plus shiplap) will be about R 30. R 44 is expected today. I don't want to drop the ceiling any lower than current height.

2. Closed cell.

3. No venting needed - I'm going with a "hot" attic. I had the sloped roof of the knee wall attics sprayfoamed when I bought the house (5 years ago), and the attic space at the apex of the roof is filled to the top with blow in. I was nervous about this work, but local inspector, roofer, and insulation company all gave it their blessing. 5 years later, I have no known problems. The only area that I have not upgraded is the sloped ceiling, hence this post.

4. I had new asphalt shingles and a layer of 1/2 inch plywood put on the roof 2 years ago. I agree that leaks will be hard to detect...

Bud, this is a little off topic, but you answered a post I made a few months ago about spray foaming the side attic rim joist. Thank-you for your input. The small town I live in now has a spray foam company, and they were happy to come out to do the small job - I gather that there isn't much work for them right now. They costed the job with the same formula they would have used for a larger job. The side attics are now cooler than the upstairs living space. This is a notable difference to what was there before. And was that knee-wall space ever hot when I first moved in! So again, thank-you for your input - I acted on the advice I received.
Old 07-10-15, 06:32 AM
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If you are spraying the rafter cavities themselves you REALLY should run baffles from the vented eaves to a vented ridge. I have seen a number of asphalt roofs fail prematurely when ventilation is not supplied.

At a minimum I HIGHLY suggest you contact the shingle manufacturer to ask if omitting the baffles will alter their warrranty. Do not call the roofing contractor to ask, because they may give you misinformation.

I know for a fact that Owens Corning has had similar problems with SIP roofs, which is not much different than you are trying to accomplish.

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