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I've got a large hole behind my bathroom sink on the floor. Is there a easy but serious way to plug that hole, from which comes tremendous amounts of cold air?

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Is there basement under there or crawlspace or slab? If possible, it'd be nice to prevent that space from getting cold, in addition to just plugging the hole. As is, you're losing heat thru your floor. – Wayfaring Stranger Feb 13 '14 at 15:03

Get a can of expandable foam from your hardware store. If they have it get the one for extra large gaps. Also pick up a tub of ready mixed tile grout. If the hole is really deep the foam may fall through before it had time to expand so you may need to stuff some newspapers into the hole to give the foam some thing to rest on until it had time to set up. Get as thick a layer of foam in there as possible for insulation.

After it has set, trim it off half an inch below floor level and trowel in the grout mixture level with the floor.

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I'd use a fireblock foam, something like GREAT STUFF™ Fireblock Insulating Foam Sealant.

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Not only will it stop drafts, but it will also help prevent the spread of fire through unblocked passages.

NOTE: There are other fireblock foams available, I simply used Great Stuff as an example because it is a well known brand.

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