My house in southern Ontario, Canada, has a cold cellar under the front porch. The house also has a sump pump, located in the cold cellar.

The builder insulated the perimeter basement walls, including the wall separating the cold cellar from the main area of the basement, leaving the porch un-insulated and outside the warm air envelope of the house, and included two 4" vents in the cold cellar.

The original/previous owner insulated the exterior walls (down to the floor) and ceiling of the cellar and blocked up the vents, effectively making the cellar part of the warm air envelope of the house. Part of the problem of doing this is the humidity and odor introduced into the "cellar" space by the sump.

I have chosen to re-open one of the vents, and keep the (insulated) cellar door closed. The concern I have is temperature inside the cellar because of the sump (don't want it to freeze). With outside temperatures having briefly dipped to -15C, I found the cellar temperature drop to about 4C.

Can I expect the cellar and sump to remain above freezing throughout what could be a colder than average winter (can expect periods below -25C)? Will the insulated walls prevent the cellar from drawing enough ground heat to stay above freezing? In other words, should I pull off the insulation below the frost line (about 4ft below grade)?

  • This is an OLD question which has been bumped up - I would simply comment that the SUMP (and likely the FLOOR) is presumably in "full contact" with the deepest soil you have available, so if it freezes under current conditions, it's simply too much ventilation to overcome whatever "heat from the ground" you actually have available. I don't see removing insulation from the walls (if so, ONLY below frost depth) as likely to help much at all, .vs. the exposed floor, and the presumably deeper sump.
    – Ecnerwal
    Apr 29 '19 at 3:16

Its difficult for me to change my thinking about using the ground to “warm up” your cellar. The benefit of subterranean houses is having the ENTIRE house below grade and taking advantage of the stable ground temperature.

Adding air vents into the space defeats the subterranean ground affect. However, the vent does solve the “musty smell” issue.

Having just a portion of the space below grade (insulated or not) the room temperature will be affected MOSTLY by the air temperature, not the ground temperature.

The wall insulation usually helps in the transfer of heat out of your cellar. Likewise, I guess it could prevent the transfer of heat into your cellar...but minimally.

Because heat rises and air vents are located at the ceiling level, I think the wall insulation probably has little effect.

  • My concern was about the potential for the sump to freeze given that there is supposed to be ventilation there and there was originally no insulation to impede heat flow between the space and surrounding soil - perhaps allowing the space to get colder than it might otherwise. If the air gets a little colder, I would only be concerned if it means the sump could get cold enough to freeze.
    – Anthony X
    Dec 17 '17 at 14:41
  • Yes, I’m sure it will freeze, with or without the insulation. I think the insulation has little or no affect on the cellar’s temperature. Where I live, we wrap hose bibbs, pipes, etc. when it gets down to freezing. For you, you’ll need to install a little heat in there when it gets down to where you’re concerned...-15C ??
    – Lee Sam
    Dec 17 '17 at 15:00

I think you are safe with the conditions you explain, however if you remain concerned, (you are the one who lives there. I do not live in your climate), I would start by restricting the size of the air vents. Your goal is to do no more than to keep the air fresh without dropping the temperature below a safe level. You could install a freeze control alarm for the area. Keep in mind that if the sump is buried in the ground it is a good deal warmer down there, (I would say at least 5*) than the room temperature is. When it is all said and done, it is your house and your concerns. My comments are simply that, comments; from someone who has been in and around this type of business all my working life, 40+ years. P.S. Can you do anything to mitigate the smell. (I remember hearing it said once: In a multitude of counsel there is wisdom.)

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