Expert: Daniel Humphrey - 1/15/2009

Question
I live in Toronto,  I have an 8' x 8' cantina (cold room) in the basement directly under my front porch.  The room is very cold.  The house is only 2 years old.  I am getting heavy water condensation, mostly on the concrete ceiling.  This is in the form of big heavy water droplets that form on the ceiling and drip on the floor, and/or freeze.  There is no window in the room but I drilled a 3" hole through the foundation wall to the outside for airflow. In this room I have a subpump buried in the floor.  What's causing the water and whats the fix?  I know other people that practically have the same room set up, but no water.  Do I need a de-humidifier in the room?  I know somehow cold air from the outside is meeting warm air inside causing the condensation, but where is the warm air coming from.  The door to the room is always closed.  As you would imagine this only happens in the winter.

Answer
Dear Mario,

Of course you know that moisture condenses when it touches a surface colder than dew point. That's key in figuring out what to do. In essence, your cold room is a big dehumidifier.

For condensation problems, you have two potential solutions, any one of which would work alone:

1. Warm up the condensing surface.
2. Reduce moisture in the air.


In your case, the concrete ceiling is colder than the air inside the room. This happens because the earth remains above freezing, while the outside air gets colder than that. You can either warm up the concrete above dew point, or insulate the interior air from it. The porch itself is probably really cold. One solution would be to install heated tile backer on your porch, tile it, and keep it warmer than the air in your cold room. This would have the advantage of melting any snow and ice on the porch. The other approach would be to allow the concrete to be cold and insulate the underside with a layer of extruded polystyrene panels glued on, or painted on urethane; but you must prevent the moisture from ever touching the concrete. And the third option would be to heat the concrete from below with a radiant heat source; just keeping the door open might not be enough and would let in a lot of moisture.

If you want to reduce the moisture in the air, you can either ventilate it or dehumidify the room, as you mentioned. Or you may be able to stop the source of moisture. The moisture is coming from three places: the slab, the basement, and the walls. Concrete sucks moisture out of the ground and puts it into the air. For this reason, plastic is laid down before a slab is poured, and concrete walls are sealed with a waterproof layer. You may have these features, or may not; adding them later is almost impossible. Enough moisture can come through a wooden door and crack around the door to do a lot of what you are seeing. Painting the door might help, but sealing the door is a must.

I'm interested to hear that the water is freezing on the floor. Usually below-grade slabs remain above freezing unless they are in permafrost areas. Outside ventilation is probably making it cold enough to freeze, but that outside air is drier than any other vent source. A heat recovery ventilator could work pretty well there, bringing dry air in and recovering some of the outgoing warmth with the heat exchanger.

The potential solution with the lowest cost would be the foam insulation on the ceiling. It would be worth a try and allow you to keep the outside air vent and other features pretty much the same.

Good luck with that. Winter is half over!

Daniel