Humidity, Sump Pumps, and Radon

Dry air

Mold and condensation follow humidity. EPA’s advice: keep indoor humidity below 60 percent, ideally 30 to 50 percent, and check it with a $10-50 humidity meter. Condensation on walls, windows, or pipes is the sign it’s too high ¹. Vent the dryer outdoors, insulate cold water pipes that sweat, and use a dehumidifier when needed ¹.

Sizing a dehumidifier

Dehumidifiers are rated in pints per day. ENERGY STAR’s guide for spaces under 2,000 square feet ²:

Bigger is better than too small. Run the hose to a floor drain or sump so you don’t empty buckets. Basements are often below 65°F, where coils can frost; look for a unit rated for low temperatures ².

A dehumidifier treats the air, not the cause: fix gutters and grading first (lesson 1).

Sump pumps

If you have a sump pump, it has to work exactly when storms knock out power. Check it, the dehumidifier, and any water alarms regularly; a battery back-up keeps a sump running in a storm, and its battery should be checked twice a year. Water alarms near the sump, floor drains, and laundry give early warning ³.

Test for radon

Radon is a radioactive gas from uranium breaking down in the ground; it seeps in through foundations ⁴. It’s the second leading cause of lung cancer, and smokers’ risk is about 10 times higher ⁵. About 1 in 10 Wisconsin homes has high radon ⁴, but in Dane County roughly half of homes tested over three years came back high ⁵. An old fieldstone foundation with cracks and gaps is no protection, and the only way to know is to test ⁵.

How to test ⁵:

• Put the kit in the lowest lived-in level of the house, windows closed, for 2 to 4 days. A basement counts if you spend at least 7 hours a week there.
• Kits cost $10-30 ⁴; Public Health Madison & Dane County sells short-term kits for $10 and long-term ones for $15 ⁵.
• Read the result in picocuries per liter: under 4, retest every two years and after air sealing, heating changes, or foundation work; 4 to 7, do a long-term test; 8 or more, do a second short-term test right away and fix the house if the average is over 4 ⁵.

Fixing it

A mitigation system uses a fan to pull air from the soil under the house and vent it through a pipe above the roof edge, while cracks and openings in the foundation are sealed ^{6 4}. Common types ⁶:

• Sub-slab suction: a pit dug under the basement floor.
• Drain tile suction: through existing drain tile, with the sump covered.
• Sub-membrane: for dirt floors and crawl spaces, a plastic sheet sealed over the dirt and up the walls, with the pipe pulling from beneath it. That’s the method for an old basement with a dirt floor or a dirt corner.

The fan belongs in an unconditioned space like the attic or outside, never in or below living space; a U-tube gauge on the pipe shows it’s running; and the exhaust must be at least 10 feet up and 10 feet from windows and doors. Systems should bring radon below 4 pCi/L, and good ones below 2. Expect roughly $1,500 to $3,000. Retest at least 24 hours after it starts ⁶. Wisconsin DHS lists qualified contractors ⁵.

Try it

Buy a short-term radon kit and a cheap humidity meter on the same trip. Set both in the basement for the same few days. One tells you whether the air is safe to breathe, the other whether it’s dry enough to keep mold away, and together they tell you what, if anything, the basement needs next.