George E. Cawthon wrote:

First thing you do is cover that dirt floor with 6
mil plastic. If you are going to be down there,
then throw some board on top of the plastic where
you wall.

Second, make sure the water supply pipe is buried
below the freeze line and that it is well
insulated where it comes into the crawlspace. If
it comes out of the ground, then construct a 2
square foot box (no bottom or top around the pipe
and fill to a 1 foot depth of insulation.

I will look next time I'm up.

That said, if you kept the set at 15 C it should
never have frozen.

Well, I kept the "upstairs" at 15 C. I am not sure how well the heater
travels down to the crawlspace (hence my thought of heating the
crawlspace).

You say the cinder block is
well insulated? how?

It is insulated with the rigid foam insulation. I haven't measured it,
but I may do that. There is an access door (from outside), that wasn't
sealed properly last year when I had freezing problem. I believe I
have it insulated now (with the rigid foam and some Fiberglass pink).

You should have at least 3"
of insulation and it should run from the floor
above to the bottom of the wall. Note that a lot
of people insulate to the top of the concrete wall
and then fail to insulate the wood space from the
top of the concrete to the sub floor above. (you
con lose a lot of heat if that 8" or 10" space is
not insulated.

Another thing I will check when I get back up there!

The best way to insulate in your case is to
insulate just the walls and not insulate the
bottom of the floor. At some temperature probably
-5 C or so you need some heat in the crawl space.
and let that heat filter up to protect the pipes
above the crawlspace.

That's what I was thinking (still need an answer whether there is a
heater I can leave down there unattended). As the outside temperature
will likely get to minus 20 or 30 C (as it did last year), I was
looking for help.

Ransley suggests draining, which is a good option.

As I said to the other helpful posters, I am trying to avoid that, if I
can.

Regards (and thanks),

Gideon