best insulation for PVC drain in exterior wall?


I'm doing some siding repair right now and just pulled the lower 20" of siding off of a wall. One of the things just behind it is a PVC drain pipe someone ran inside the wall. This drain has frozen multiple times in the past.
While I have the wall open, what's the best way to insulate this pipe against further freezing? Should I use the expanding foam stuff? Or just get the best fiberglass batt I can find? What was there before was some yellow fiberglass insulation and then 1" of styrofoam and then 5/8" of T-111.
There was all kinds of rot and holes at the bottom of the T-111 (which is why I'm replacing it) so there were plenty of air holes to expose it. Presumably it will be better with new siding.
There is some expanding foam insulation between a couple other studs and I think that's protecting the laundry drain and possibly feeds too.
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Lemme guess... There was insulation all the way around the pipe which prevented warmth from the living space from reaching the pipe to keep the pipe from freezing.
Am I right?
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Great guess, but surprisingly not in this case.
There was a small bit of fiberglass insulation on the outside of the pipe (maybe 1/2" at the pipe). In front of the pipe was air and drywall. But, on the other side of the drywall is a sink cabinet (usually closed of course) and then the bathroom. This bathroom is already the coldest room in the house during the winter (it's farthest from the boiler), and can't get really cold in winter.
So, I guess it's a case where there's not much inside warmth to keep that pipe warm in the first place. But it sounds like whatever I do I should not insulate in front of the pipe, between it and the drywall?
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ben wrote:

You should insulate, but keep the pipe INSIDE the insulation.
Never heard of a drain pipe freezing before. Maybe you could run some hot water down it routinely.
As an aside, consider using HardiPlank for the lower siding. It won't rot.
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I would foam the outside wall area, fiberglass is about R 3.75", foams go from about R 5.5"-R 7.2" or twice of what fiberglass can offer, the foam will also do better at keeping out outside air, as to the whole pipe being foamed , I dont know what is best. If wall is completely enclosed even a 1/2" hole at the bottom and top of wall may give enough air movement to stop freezing. Its very hard to freeze water in a pipe that large to close it up, you must have some weather extremes and poorly thought out plumbing. Before closing it up a cheap remedy to install is Freeze Tape, its a low watt pipe heater, then you can thaw it if needed.
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Ben,

My first impression is that you have a plumbing problem. Liquids shouldn't sit in a "drain" pipe long enough to freeze, unless the pipe is clogged or is not pitched correctly for the liquid to drain off. Also, any use of the drain (running a sink, flushing a toilet, taking a shower, etc.) is going to run warmer liquids (above freezing anyway) down the drain to melt any minor ice that may be there. Running hot water should melt it even more.
Having said that, I will assume you are in a colder than average climate where freezing is a big problem. First, use a can of expanding foam to seal around the openings where the pipes pass through studs, sill plates, etc. This will keep cold air from blowing up into the wall. Then I would pack as much insulation between the pipe and the outside of the wall as possible, leaving the interior side open to the heated side of the house. A rigid styrofoam would probably provide the most insulation in the least amount of space, but fiberglass would probably be easier to install around the pipe. Once the siding is back on, caulk around any cracks, again to keep cold air from blowing in.
Oh, and you should probably clean the drains to make sure they are flowing properly. If the pipe doesn't slope down 1/4" per foot, you may want to rework the plumbing before closing up the wall.
Good luck,
Anthony
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