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It's 1 (one) degree F outside and I discovered some very small (less than 2 inch) cracks/holes in the caulk between my fireplace surround (wall) and the hearth that extends out from the wall (see pic below for a close up of the largest crack). This is letting in some insanely cold air, and I'd like to stop it.

We don't use the fireplace so I technically don't have to use fire-rated caulk, but I'm worried that any kind of caulk won't properly cure when the temperature is so low (the temp of the fireplace and hearth measured with an IR thermometer is in the 35-40 degree F range).

I'd like to temporarily seal the gaps (at most, there are 3-4, with the largest being the 2" gap pictured) crack and then deal with this permanently when it warms up in the spring (considering installing a pellet stove insert).

What's a good temporary fix to stop the air infiltration?

I was thinking about using HVAC foil tape but not confident this actually stops the air from coming in.

Worth mentioning this is in a kids' playroom, so prefer to avoid anything toxic since we can't easily ventilate in the cold.

And yes...I am learning my lesson about inspecting this stuff when it's still warm enough outside to fix it!

Close-up of largest crack in fireplace caulk

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    If you're considering foil tape then I assume that appearance is not at the top of the requirements list. However, as you want a temporary solution, perhaps masking tape would be better. – Andrew Morton Jan 7 '18 at 16:45
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    Or use foam rope, and press it into the crack until the temps are high enough to do a proper repair. Don't assume the next homeowner won't use the fireplace. So temp fix until warm enough, then proper fix. – Jeff Cates Feb 1 '18 at 6:40

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