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.

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?

On Aug 18, 3:09*pm, mike wrote:
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?

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?

ben wrote:
On Aug 18, 3:09 pm, mike wrote:
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?

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?

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.

Ben,

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?

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