We are in the midst of building an ICF house with a crawlspace with cement slab floor and ICF in the above grade walls. We are in Western Canada and winter has arrived. The house has the windows ddoors and tin roof installed, but our water line comes up in the crawlspace floor,and we have four sump pits 2 of which fill each day, I manually pump daily, so we have to keep the crawlspace heated with a space heater, the roof has not been insulated yet as the electric and plumbing still has to go in. On Nov 20 the temp dropped to -32 degrees celcius the popped up to plus 11 celcius a couple days later, this resulted in condensation in the crawlspace and now I am noticing mold on the underside of the subfloor. What can I do to stop the moisture build up on the subfloor and joists

Until the house is closed in and the interior is maintained as a lived in space you will have the issue again.

To prevent mold get some heat in the crawlspace, dry everything out and spray KILZ on the joists and subfloor.

Thanks, for your reply,
I do have some heat in the 2000 sq ft crawlspace, but maybe I need more air flow.

Heat + a little airflow + cover the water to minimize evaporation.

Is this ground water or the result of rain/snow? How will you be finishing the crawlspace when the house is closed in? From your description of the temperature you might be close by in the Calgary area??

When you say that the "waterline" comes up in the crawlspace floor, are you referring to the actual ground water level or the pipe that carries the municipal water service? Why are there 4 sump pits down there?

I'm a little suspicious of the crawlspace heating strategy. That might work to drive off water from condensation, but it sounds like you have way more water than that. Wouldn't the heat with the water down there just make the mold all the happier?

If you have that much water in the crawlspace, I would recommend vapor barrier (providing there is not interior vapor barrier) over the joists and seal out the moisture from there.

Rigid foil faced ISO will give you the permeability ratings to keep the moisture out of the joist bays and make sure the ribbon boards are spray foamed and the assembly can dry to the interior.

Air barrier details would have to be really spot on and very tight.

Please read about THE BARRIER www.TheGreenBuildingProductsStore.com great to use in crawlspaces for moisture issues. Good luck

Seal it up as best you can as recommended above. Then get a humidistat-controlled fan to force-feed air through there; as much as necessary to dry it out.