I've got a spot by a fireplace that lets cold air in. Unfortunately its right where the metal of the fireplace meets the wooden framing/molding of the fireplace on the inside. This seems like something I should be able to take care of. Its a rather sizable gap (maybe 3/4 of an inch). I didn't think I could caulk such a large gap. I wanted to use some kind of spray insulation such as "Great Stuff", but the label indicates that its flamable (not to mention highly toxic and needs to be covered).

What should I do to cover this gap and prevent air flow in a way that's safe and effective?

With regards to sealing outside

Unfortunately I can't seal the outside without great expense. My home energy auditor has said that its likely that the area surrounding the fireplace is not insulated. He has told me that it would be expensive to gain access to the gap between each stud and blow in insulation.

He suggested sealing the crack on the inside as the most cost effective way to improve the comfort in this room. When he came and used his IR camera, the spot on the floor next to this gap was in the 30's in temperature.

There's a gap between black slate(?) of fireplace and white wooden fireplace frame.

  • I hold on my first suggestions, I do wonder however if the black metal pieces I am seeing in the picture are part of a removable fireplace screen set or perhaps an insert? Getting a look behind them would be priceless. Jan 15, 2011 at 18:23
  • they might not be part of the fireplace unit directly. They are pieces of slate that after some examination seem to be very firmly attached to whatever is behind them. I wouldn't want to take it out unless absolutely necessary.
    – Doug T.
    Jan 17, 2011 at 16:27
  • I edited my original answer based on your comments. Take a look. Should not be a difficult fix. Good luck Jan 20, 2011 at 10:39

7 Answers 7


I assume your gap is on the indoors side?

Regardless, if air is filtering through into the living space, there must be a gap outdoors ( external chimney) as well as indoors.

I recommend you check out the caulking along the chimney to the exterior siding as well as addressing the interior gap. Since this gap inside is directly adjacent to wood trim, I also have to assume that excessive heat is not a factor. Is the metal you are referring to an insert trim of some type? A pic would be a big help here.

If the assumption are right and the gap is 3/4 inch wide, I would use a caulking backer in the gap before using a good grade painter's or acrylic caulk as a topper. The caulking backer is a round foam solid tube that can be cut to length, inserted into the gap to seal it and hold the caulk. After the caulk has cured, it can be painted to match the trim. If for some reason you are concerned about high heat in this area, there is a fire rated caulk made especially for gaps in fire or common walls. Readily available at any box or hardware store from a number of manufactures.

Ok after reading your comments, and learning that the black vertical sides are slate, I would still do similar to above. Perhaps after injecting larger amounts of fire rated caulk as deep into void as possible, then use the solid, tube style caulking backer to fill the majority of the larger, deeper gaps, followed by using some paint-able silicon/acrylic mix caulk.

Before you apply the finish caulk, run a nice straight line of painter's tape up the front of the slate even with the edge of the wood trim. When you fill over the foam backer with the caulk, use a small putty knife to draw the caulk smooth and flat. Immediately remove the tape while caulk is fresh. This should result in a nice crisp straight line. Let the caulk cure for 24 hours. If there is some shrinking or cracks, no problem, simply repeat the process with new tape and another coat of caulk.

Alternately, you could also use a setting type drywall mud, such as durabond 30 or 45 to fill the deeper gaps etc. Then again use a second coat to create a smooth straight extension of the wood trim to the slate.

Either method will result in a neat, easy to paint fill that should stop your air infiltration problem and make the finish look nice.

  • See my edited question for a picture and comment on sealing from outside.
    – Doug T.
    Jan 15, 2011 at 17:08

You can buy concrete in a caulk tube. I actually just finished doing this same thing to my fireplace that I remodeled, here's the product I used for the smaller cracks that I wasn't able to do with my bulk batch of concrete:


After seeing the photo of the gap in question a high heat silicon sealant would give a better matching finish:


  • Is this intended for indoor use or exterior use?
    – Doug T.
    Jan 14, 2011 at 23:08
  • that stuff also doesn't appear to be fire rated. Thanks though.
    – Doug T.
    Jan 15, 2011 at 17:10
  • 4
    It can be used indoors or outdoors. After seeing the picture though this is not what you want to use. You'll want to use something like this: amazon.com/Meeco-High-Temperature-Silicone-Sealant/dp/… It will give it a better looking finish too.
    – XOPJ
    Jan 17, 2011 at 13:51
  • It might be better to update your answer with extra info (in this case with regards to high-temp silicone) instead of adding a separate comment, as it will make it easier for others to read the full answer. Just a suggestion...
    – Pedro
    Jan 19, 2011 at 23:28

I would use: GREAT STUFF™ 16 Oz. Fireblock Insulating Foam Sealant


  • Foam is tack-free in about 5-10 minutes and cures in eight hours.
  • Cured foam can be trimmed, shaped, sanded, painted or stained. To trim, use a sharp knife or serrated blade.


  • Can will seal itself shut if allowed to sit unused for over two hours and may not be restarted. Expect one-time use.
  • 2
    another con, the warnings about the level of toxicity of this product will make your hair stand on edge.
    – Doug T.
    Jan 24, 2011 at 1:38
  • 1
    Other cons: great stuff is sticky, impossible to clean.
    – Bryce
    Jun 19, 2012 at 17:55
  • 1
    Fireblock has the same flammability as the regular stuff. Both cans say that it will combust at the same temperature, I think its 240F. Read the comment from DOW on this Home Depot page: homedepot.com/p/…. It specifically says that even the fireblock stuff "should not be used around heaters, furnaces, fireplaces, or recessed lighting fixtures where it contacts heat conducting surfaces. Cured foam is combustible and may present a fire hazard if exposed to flame or temperatures above 240F."
    – Ian
    Mar 15, 2017 at 13:37

The reason for the gap between the wood and the stove pipe is to prevent overheated pipe from causing a flash fire in the wood.

Even though you probably have double-wall pipe through the wall (which is what allows for the wood surrounding it) you still want something between the wood and the pipe that can act as an insulator against much higher temps in case of a chimney fire.

Look for high temp stove and gasket cement. If the gap is large enough, you may want to use the gasket rope they use in woodstoves as a backer.

Before you do anything: Check the building codes in your town and ask your insurance company! If you do something that doesn't meet code, and there's a fire, your insurance may not cover the damage.


Fire-rated silicone sealant available at Lowes, Home Depot, etc.

Lowes: http://www.lowes.com/pd_219973-68-18806_4294858104_?productId=3025055

Home Depot: http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&productId=100678630&langId=-1&catalogId=10053

I'm sure you could find it elsewhere as well... check around!


I've used fire rated silicone around my flue in the past and it has worked a treat to seal between the flue and the roof.


There is a product called smoke tite that is used from chimney companeys, its a compressed air spay system that sprays high heat cement in all the proper gaps and areas needed to be sealed, my company is baltimore based and we use this treatment all the time for sealing throaghts in the chimney, hope this can help ur issues

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