I'm Too Old for the Draft
January 18, 2010 12:49 PM   Subscribe

Is it safe to insulate around the crawlspace exit point for a hot water heater vent pipe?

We have a one-story house with crawlspace. The tankless hot water heater exhausts to a standard aluminum vent pipe that runs underneath the house to an exit hole in the foundation, bends upward after it exits the crawlspace, and runs up the side of the house. The hole in the foundation is a couple of inches larger in diameter than the pipe, leaving a gap for drafts.

The crawlspace has more than enough ventilation, so I would like to put a small amount of insulation around the vent pipe to seal the gap. I think (but am not sure) that the pipe is primarily a vent for combustion byproduct rather than heat, so I should be able to just buy a small piece of insulation to wrap around the pipe and fit into the gap between the pipe and the concrete, but will doing this cause any problems and/or is it safe to do?
posted by Gingercat to Home & Garden (4 answers total)
Feel the vent pipe where the pipe exits the house and see if it is hot to the touch. If not hot (when heater is working) then you should be fine to insulate that gap around the pipe. You could also consider using that "Great Stuff" foam insulation in a can. The can says safe to 240ยบ and I doubt your pipe gets hot at all.
posted by JayRwv at 1:05 PM on January 18, 2010

I would worry about the Great Stuff, over the long run. It does not stand up to long term sun exposure. I suggest that you flash around the pipe, instead, with aluminum. Perhaps you can stuff the area behind the flashing with fiberglass, too. It may be a little tricky making a circular flashing. Can you buy something like that?
posted by Midnight Skulker at 2:50 PM on January 18, 2010

Here is an easy solution (assuming we are talking about 4 inch vent pipe):


Slip the elbow off of the pipe, place this shield on the pipe up close to the house and hold it in place with a 4 inch pipe clamp. Replace the elbow. Go inside and fill the gap with Great Stuff. The shield will provide a form to keep the foam from oozing out all over the outside and protect it from the sun's harmful rays.
posted by Old Geezer at 3:23 PM on January 18, 2010

Safe, yes, as long as your insulation material is not flammable. The flue contains possibly-hot combustion products, so no foam-in-can. Some Al flashing should be fine.

I think what you're doing here is not so much insulating anything as filling an air-gap. You won't make an appreciable difference to how much heat is list from the flue.
posted by polyglot at 12:28 AM on January 19, 2010

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