"Dehumidifiers for the basement."
Column #855 07/16/11
On The Level
Q. Do you recommend a de-humidifier for my finished basement? If you do recommend a de-humidifier please suggest a size, make and model. The house was built in 1991. Our average basement temperature runs 68 degrees winter and summer, the average humidity is 50% in winter and 60% in the summer.
The finished square feet is 660 and unfinished is 200. It has carpeting, carpet padding, dry wall, a suspended ceiling and insulation between the first floor joists. We painted the block walls with UGL sealer and used basement insulation in the 2X4 wall construction that breathes. We left a gap in between the block walls and the interior walls and the suspended ceiling to let it breathe.
The basement door is always open to the first floor. The only source of humidity is a covered sump that is active when it rains and when the heat pump A/C condensate drains into it. We also have a full bath with an exhaust vent. No water leaks or moisture. The HVAC guys installed a fresh air return and tapped 4 vents into the duct system, backed up with a couple of baseboard heaters. It is too cold to leave the A/C duct vents open in the summer, and we don’t want to run the baseboard heaters to lower the humidity in the summer. What do you think?
A. You’ve done everything more than right. Looking past what you’ve told me I can tell you that since you have a heat pump system when you bought the house the basement wasn’t finished. That’s why insulation was between the first floor joists. The code reads unfinished basement space as non-conditioned and wants it separated from conditioned space above via insulation and the door to the basement from upstairs had weatherstripping on it.
What you’ve done in the finished space sounds first class. The fact that your HVAC service providers cut supply vents and an air return for the finished space is right on the top of the list of smart things to do for humidity control for that space. You say it’s too cold down there in summer so you close off the air vents. The air return vent is still pulling air into the system so it will pull air from wherever it can. It’s good that you leave the door to upstairs open because if you close it the air return will pull air and it will try to suck air from around windows and door plus to a small degree your sump pit. And that will increase humidity levels, especially with the hot, humid air we've had outside these past couple of weeks.
Would that you had had a supply duct cut into the unfinished space as well because that would have done lots to control the overall humidity levels down there. When I’m inspecting a house with an unfinished basement with the air-handler located in it and the ductwork running up under the joists distributing the conditioned air to the rooms above I’ll sometimes see one or two registers placed into the supply runs but they’ll be closed off. I recommend that at least one be opened for humidity control. It works.
Generally I don’t recommend dehumidifiers if they can be avoided for a couple of reasons. Remember a dehumidifier is little more than an air conditioner whose rear end is not sticking out of a window. They have a tendency to elevate the temperature of the room they are in and we know that warm air can hold more moisture than cold air. That’s why operating your strip heaters in the basement in summer would do little in helping dehumidification.
Check the windows and doors to the exterior from the basement for weatherstripping and tighten them up. Consider sealing your sump pit. I dehumidified the basement of a 1955 Rancher by sticking a cheap window air unit in the window and letting it run. The basement became cool and dry.
If you decide that you do want to buy a dehumidifier get the cheapest one you can. They start around $240. from what I see and they are rated in pints per day of moisture removal. The smaller ones cover about a thousand square feet of space so placing it in the unfinished section would be the best place for it as your entire basement is less than a thousand square feet. Some fancier models have all sorts of bells and whistles that might look fun to play with but if you’re looking for just plain dehumidification no need for fancy. Remember they use electricity so they can be pricey to run over long periods.
If you get one place it near the sump and drain it into the pit. You don’t want to be running down in the basement to empty the reservoir or worrying about it. But my recommendation is to try other things first.
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