Once the walls go up, the foundation is hidden. Water, radon and low ductwork all cost less to deal with while the basement is still bare. Here's what to check, in the order we check it.
Water · Radon · Ceiling height · What stays reachable · Insulation · Checklist
1. Water
On August 20, 2018, the west side of Madison got 11.63 inches of rain, Middleton 11.03 and Cross Plains more than 14, most of it in a few hours. (NWS) (NWS storm reports) Dane County Emergency Management counted nearly 2,000 damaged homes, and fewer than 4% of them were in a mapped floodplain. (Dane County EM) A basement doesn't have to be in a flood zone to take water.
Look for signs first
Water stains on the walls or floor, a white crust on block walls (efflorescence), a floor that's damp after rain, a musty smell, or a sump pump that runs often. If water gets in now, it will get in behind new drywall.
Fix the outside before the inside
Madison's order is: grading, then gutters and downspouts, then a drainage and sump system if you still need one. (City of Madison) An inch of rain puts about 1,250 gallons on the roof of a 2,000-square-foot house. Grading and downspouts decide where it goes.
- Ground should fall at least 5% away from the foundation for the first 10 feet. (Building America)
- Downspouts should discharge at least 10 feet from the foundation, or to a storm drain.
Sump pumps in Madison
- A new sump pump needs a building permit. (City of Madison)
- Discharge onto lawn that slopes away from the house, onto the driveway, or into the storm sewer, never into the sanitary sewer or a floor drain, and not straight onto a neighbor's yard or the street. (City of Madison)
- Connecting to the storm sewer takes a right-of-way permit and a contractor the city has prequalified. For pumps that run several times a day, the city prefers that connection: less algae in summer and less ice in winter. (City of Madison)
Sewer backups
A backwater valve lets water leave the house but closes when the city sewer backs up. Madison recommends one for most homes and has offered to reimburse 75% of the lowest of three quotes, up to $1,500, for 1- and 2-unit homes without one; check the program page to see if it's open. (City of Madison) Valves can stick open from lint or debris, so look at yours twice a year. (City of Madison) Plan the finished walls so you can still get to it.
Humidity
Madison suggests keeping a basement at 40 to 50% humidity, with a dehumidifier if needed; at 60% it will likely smell musty. (City of Madison) A humidity meter costs $10 to $50. (EPA) Run one for a few weeks of summer before you finish.
Check your lot
- FEMA Flood Map Service Center: the official flood maps, by address. (City of Madison)
- Madison Watershed Study Flood Risk Map: where a 6.66-inch, 24-hour storm would flood. It's not an official FEMA map and can't be used for insurance.
- Madison Historical Street Flooding Map: where streets have flooded before.
Standard homeowner's insurance usually doesn't cover flooding. Flood insurance through the National Flood Insurance Program is available outside mapped floodplains too. (Dane County EM)
2. Radon
Radon is a radioactive gas that comes up through the soil. About 1 in 10 Wisconsin homes has high levels, meaning above 4 picocuries per liter. (Wisconsin DHS) In Dane County, about half the homes tested in the past five years came back high, though that counts only homes whose owners chose to test. (Public Health Madison & Dane County) The EPA puts all of Dane County in Zone 1, its highest-potential group. (UW–Madison EHS)
The EPA says to test before turning an unfinished basement into living space, and again after. (EPA)
How to test
- Start with a short-term test (2 to 4 days) on the lowest level someone will use at least 7 hours a week. Winter is a good time, with windows shut. (Wisconsin DHS)
- If it reads 4 pCi/L or higher, test again. If the two average above 4, the state recommends a mitigation system.
- Kits cost $10 in person at Public Health Madison & Dane County (2300 S. Park St., Suite 2010, or 2705 E. Washington Ave.), or $10 to $30 elsewhere. Questions: the radon information center at 608-243-0392. (PHMDC)
If it's high
The usual fix is sub-slab depressurization: a pipe through the floor slab and a small fan that vents the gas outside. The state puts the average installed cost at $1,000 to $2,000, and a working system can bring levels below 2 pCi/L. Sealing cracks alone doesn't do much. (Wisconsin DHS) Wisconsin doesn't license radon contractors, so look for one certified by NRPP or NRSB. (Wisconsin DHS) Do it before you frame: the pipe has to run from the slab to the outside, and that's easy in an open basement and hard in a finished one.
The state building code doesn't require radon-resistant construction, so a passing inspection says nothing about radon.
3. Ceiling height and what's overhead
Finished rooms need 7 feet of ceiling, with beams and other projections allowed to drop 8 inches below that. (SPS 321.06) Ducts, the main drain and beams set how low the ceiling ends up. Measure to the lowest of them, not to the joists, and decide where soffits go before you plan rooms around them.
4. What has to stay reachable
- Main water shutoff and meter
- Main drain cleanout
- Electrical panel, with clear space in front
- Furnace and water heater, with room to replace them
- Sump pit and pump
- Backwater valve covers (required in Sun Prairie)
5. Insulation that won't trap water
- Put foam board or closed-cell spray foam (an inch or more) against the concrete. It keeps damp indoor air off the cold wall. (Building America)
- No plastic sheeting on basement walls. Wisconsin's energy code bans it on below-grade walls and on stud walls in front of them. (SPS 322.38(4))
- Dane County needs R-15 of continuous insulation, R-19 in the stud cavity, or R-13 plus R-5, top of wall to floor. (SPS 322.31)
- Keep drywall at least 1/2 inch off the floor. (Building America)
The short version
- Watch the basement through a few heavy rains, and fix grading and downspouts first
- Check where the sump discharges and whether you have a backwater valve
- Run a humidity meter for a few weeks
- Test for radon, and mitigate before framing if it's high
- Measure to the lowest duct, beam or pipe
- List what has to stay reachable
- Plan an egress window for any bedroom
- Check your town's permit and inspection rules
Frequently asked questions
Should I test for radon before finishing my basement?
Yes. The EPA says to test before turning an unfinished basement into living space, and again after the work. About 1 in 10 Wisconsin homes has high radon, and if you need a mitigation system, the pipe is much easier to run before the walls go up.
Can I finish a basement that gets water?
Once the water is dealt with. Madison's advice is to fix the outside first (grading, gutters, downspouts) and add drainage and a sump only if that isn't enough. Don't frame over a wall that's still getting wet.
Where can my sump pump drain in Madison?
Onto lawn that slopes away from the house, onto the driveway, or into the storm sewer, never into the sanitary sewer or a floor drain. A storm sewer connection needs a right-of-way permit and a contractor the city has prequalified, and a new sump needs a building permit.
Do I need a backwater valve?
Madison recommends one for most homes but doesn't require it. It stops a backed-up sewer main from pushing sewage into your basement. If you have one, keep it reachable after you finish: Sun Prairie requires both access covers to stay accessible.
My house isn't in a flood zone. Can the basement still flood?
Yes. Dane County Emergency Management found that fewer than 4% of the nearly 2,000 homes damaged in the August 2018 flood were in a mapped floodplain. Standard homeowner's insurance usually doesn't cover flooding, but flood insurance is available outside floodplains too.
When we finish your basement
We check the water, the height and what has to stay reachable at the walkthrough, before we price anything, and we won't frame over a wall that's still wet. See how a basement job runs with us.