How do you keep a finished basement from getting moldy?

What does mold need to grow in a basement?

Mold needs two things: water and something to eat. EPA puts the first part simply: there are many types of mold, "and none of them will grow without water or moisture." Its list of what molds grow on is just as plain: wood, paper, carpet, foods and insulation. And EPA adds that molds "can grow on virtually any substance, providing moisture is present."

A basement supplies the water for free. As I explain in Concrete 101, basement concrete wicks moisture from the soil, passes vapor, and sweats in a Minnesota summer. So when I design a basement, I control mold in two ways at once: I keep the humidity down, and I choose materials that give mold as little to eat as possible in the spots most likely to get damp.

EPA's summary is the line I design around: "The key to mold control is moisture control." You cannot sterilize a basement. EPA says there is "no practical way to eliminate all mold and mold spores in the indoor environment." What you can do is make sure the spores that land never get the water and food they need.

Which basement materials feed mold?

The common thread is cellulose: paper and wood. Those are the materials EPA lists first, and they are the ones I see in nearly every failed basement finish.

Paper-faced drywall

Standard drywall is gypsum wrapped in paper. The gypsum is fine. The paper is food. Building Science Corporation's basement research names "moisture sensitive interior wood framing and paper faced gypsum board" as the materials that should no longer touch the foundation wall. EPA notes hidden mold often grows on "the back side of dry wall," which is the side facing the concrete. That is exactly where you will not see it until it smells.

Carpet and carpet pad

Carpet over a slab is the most common source of the basement smell. EPA says carpet "can absorb moisture and serve as a place for biological pollutants to grow," and lists "the underside of carpets and pads" as a hidden mold location. EPA's guidance goes further: where there is a perpetual moisture problem, including "concrete floors with leaks or frequent condensation," do not install carpeting. BSC is equally direct: "Carpet should not be installed directly over an uninsulated concrete basement slab."

OSB, plywood and untreated wood

Oriented strand board, plywood subfloor panels and untreated studs are wood, and wood is on EPA's list. They are fine on the warm, dry side of an assembly. They are a problem when they touch concrete or sit in a cavity that gets wet and cannot dry. BSC's renovation guide calls for closed-cell sill seal between the bottom plate and the slab so framing never sits directly on concrete.

Cardboard boxes and stored paper

Cardboard is paper. EPA's own photo of basement mold is a suitcase "stored in a humid basement." A cardboard box pushed against a foundation wall or set straight on the slab is the fastest way to grow mold in an unfinished basement. Plastic bins on shelves, off the floor and a few inches off the wall, are the fix. See basement storage.

Fiberglass batts against concrete

Fiberglass itself is glass, but BSC notes batts are "sensitive to moisture" and "cannot tolerate groundwater leakage." Batts against concrete hold damp air against a cold surface and trap dust, and EPA lists insulation among the materials mold grows on. Covered with plastic, BSC says, these assemblies cannot let the wall dry and are linked to "serious problems associated with mold, decay and odors."

Which materials forgive a damp day?

No finish is mold-proof if it stays wet and dirty. But some materials give mold nothing to eat and survive getting wet. In the basements I design, these go in the places most likely to see moisture: against the concrete, at the bottom of walls, and on the floor.

• Rigid foam board and closed-cell spray foam. BSC says foam insulations "are not sensitive to moisture damage, and do not support mold growth." This is what touches the concrete in every basement I design. See basement insulation for thicknesses and fire covering.
• Paperless, fiberglass-mat-faced drywall. It replaces the paper facing with a glass mat, which removes the food source from the surface. I specify it on the lower walls in bathrooms and on walls next to the sump or mechanical room. It still has to be kept off the slab.
• PVC or composite baseboard and trim. Baseboard is the first thing a spill or a damp slab touches. A plastic or composite profile does not feed mold or swell the way MDF or finger-jointed pine can.
• Luxury vinyl plank (LVP). The plank is plastic, so it does not rot or feed mold, and it survives a spill. One caution from BSC: impermeable floor coverings like vinyl slow how a slab dries, so moisture can build up beneath them. Follow the maker's slab-moisture limits and underlayment instructions. See best LVP for basements.
• Porcelain and ceramic tile. BSC's concrete floor guidance lists ceramic tile as an option over damp slabs. It tolerates water and gives mold nothing to eat. See the tile guide.
• Area rugs instead of wall-to-wall carpet. EPA suggests area rugs "which can be taken up and washed often." You get the softness without a pad glued to cold concrete.

Basement material mold risk and better alternatives

What indoor humidity should a Minnesota basement be in summer and winter?

EPA recommends keeping indoor humidity "below 60 percent (ideally between 30 and 50 percent)." ENERGY STAR uses the same 30 to 50 percent range and notes that levels above it can encourage mold growth. Here is how I translate that across a Twin Cities year.

The winter number drops as it gets colder outside. EPA cites ASHRAE maximum indoor humidity levels by outdoor temperature: 35 percent at 20°F, 30 percent at 10°F, 25 percent at 0°F, 20 percent at -10°F, and 15 percent at -20°F. On a below-zero January night in Prior Lake, a basement held at 45 percent is well above that guidance, and the egress window glass is where the condensation shows up first.

Winter is rarely the problem. EPA notes that as temperature goes down, air holds less moisture, so cold Minnesota air is dry. The summer is the hard part. BSC chose Minneapolis as its cold-climate research site specifically for its "cold winter weather and fairly warm and humid summer months."

Measure it

EPA says humidity can be measured with an inexpensive meter. I tell every homeowner to put a digital hygrometer in the basement before the design starts and read it through a full summer. It is the cheapest piece of moisture data you can get, and it tells me whether a portable dehumidifier will keep up.

What size dehumidifier does a basement need?

ENERGY STAR sizes dehumidifiers by how damp the space is without one and how big it is. For a typical finished basement, which falls in its "small-medium room" range under 2,000 square feet, the guidance is:

ENERGY STAR says oversizing is preferable to undersizing. A few more things I build into the design:

• Buy ENERGY STAR. Certified units use about 20 percent less energy than conventional models removing the same water. Compare models by Integrated Energy Factor (IEF), measured in liters per kilowatt-hour; higher is more efficient.
• Plan the drain, not the bucket. ENERGY STAR notes many portable units have a hose fitting so water can drain to a floor drain or sump pump. I put the dehumidifier in the mechanical room or next to the sump so a short hose drains by gravity and nobody has to remember to empty it. Keep the hose away from electrical cords.
• Mind the temperature. Below about 65°F, ENERGY STAR says frost can form on the coils and the unit cycles without removing moisture. An unheated storage room in a Minnesota basement can be that cold in spring, so choose a model rated for low temperatures or one with an anti-frost sensor.
• Whole-home units. ENERGY STAR suggests considering a ducted whole-home dehumidifier in homes with central air where humidity stays above 55 percent. An HVAC professional should size it and route the drain to a floor drain or condensate pump.
• Close the windows. ENERGY STAR says to keep doors and windows closed while the unit runs. Open basement windows on a humid day undo it.

For the sump side of this, see the sump pump guide.

What causes the musty basement smell?

A musty smell is the most common sign of mold you cannot see. EPA says to suspect hidden mold when a building smells moldy but the source is not visible, and it lists the places it hides: the back side of drywall, wallpaper or paneling, and the underside of carpets and pads. EPA's standard for a finished cleanup is that "visible mold and moldy odors should not be present."

In the basements I look at, the smell almost always traces to one of these:

• Carpet and pad on a slab that sweats in summer
• Fiberglass and plastic behind finished walls
• Cardboard boxes against the foundation
• A slow leak at a water heater, laundry standpipe or ejector pit
• A slab with no insulation; BSC notes underslab insulation can reduce dampness and musty odors

Masking the smell with air fresheners or an ozone machine does not change the cause. Find the wet material and remove the water.

What should you do after a basement leak or flood?

The clock matters most. EPA says it is important to "dry water-damaged areas and items within 24-48 hours to prevent mold growth." Here is the order I follow:

• Stop the water. Fix the pipe, clear the sump, or redirect the downspout. EPA: "Fix plumbing leaks and other water problems as soon as possible."
• Check what kind of water it was. If it was sewage or other contaminated water, EPA says to call in a professional experienced with that kind of damage.
• Pull out wet porous materials. Wet carpet pad, cardboard and soaked paper-faced drywall rarely dry in time. EPA says porous materials such as ceiling tiles and carpet "may have to be thrown away if they become moldy."
• Dry hard. Run dehumidifiers and fans, and open up wet wall cavities. Keep the windows closed in humid weather so you are not adding moisture.
• Clean hard surfaces. EPA: scrub mold off hard surfaces with detergent and water, and dry completely.
• Don't paint over it. EPA says not to paint or caulk moldy surfaces; paint over mold is likely to peel.
• Know your limit. If mold covers more than about 10 square feet, roughly a 3-foot by 3-foot patch, or there was a lot of water damage, EPA points to its professional remediation guidance. EPA also says not to run the HVAC system if you suspect it is contaminated with mold.

If you have health concerns, EPA recommends consulting a health professional before starting cleanup. Check your homeowner's insurance policy before you discard damaged materials; photograph everything first.

Should you test your basement for mold?

Usually not. EPA says: "In most cases, if visible mold growth is present, sampling is unnecessary." It also notes that no EPA or other federal limits have been set for mold or mold spores, so a test cannot tell you whether your basement "passes." If you can see it or smell it, the job is the same: find the water, remove what is damaged, clean and dry.

Where testing helps is after a professional cleanup. EPA says surface sampling may help confirm an area was adequately cleaned, and that it should be done by qualified professionals.

How do you build a finished basement that resists mold?

Everything above becomes a short list of rules I apply to every layer of a basement design.

• Fix water first. EPA's renovation guidelines say to address surface water near the foundation before insulating basement walls. BSC says if walls are visibly wet or water runs onto the floor, install an interior drain before finishing.
• Foam against the concrete, sealed. BSC calls for rigid insulation that "completely wraps exposed concrete" with sealed seams, so humid room air never touches the cold wall.
• No poly against the foam. BSC: "No interior vapor barriers should be installed in order to permit inward drying." A sheet of plastic on the stud wall traps moisture between plastic and foam.
• A break at the slab. Closed-cell sill seal under every bottom plate (BSC), so wood never touches concrete.
• A gap at the floor. I hold drywall up off the slab and cover the gap with a moisture-tolerant base, so a spill or damp slab cannot wick into the paper.
• Cavities that can dry. BSC says framed cavities in front of the foam can be filled with unfaced fiberglass or damp-spray cellulose, with no plastic facing.
• Floors that tolerate concrete. No carpet directly over an uninsulated slab (BSC). EPA's renovation guidance calls for water-resistant, hard-surface flooring in bathrooms, laundry and utility rooms.
• Breathable finishes. Latex paint on walls; BSC says oil paints and vinyl wallcoverings on basement walls "typically lead to mold growth and other moisture problems."
• Humidity control built in. A dehumidifier location with a drain, a hygrometer, and bath fans vented outside. EPA lists venting moisture sources outside and using dehumidifiers among the ways to keep humidity down.

Check with your city: framing, insulation, fire covering over foam and bathroom fan venting are inspected under the Minnesota Residential Code and the Minnesota Energy Code. Some inspectors require pressure-treated bottom plates on slabs; ask before you frame.

How this plays out in a typical Twin Cities basement

Picture a 1998 walkout in Maple Grove with a poured foundation, 1,000 square feet to finish, a sump in the corner by the stairs and the furnace and water heater against the back wall. The homeowner's hygrometer read 62 percent most of last August, and there is a faint smell near the old storage shelves.

Here is how I would design it:

• Start with the smell. The cardboard boxes against the north wall go first. Plastic bins on wire shelving replace them, and I re-check the smell after a few dry weeks.
• Size the dehumidifier. 62 percent is "slightly to moderately damp" on ENERGY STAR's chart, so a unit in the 20–30 pint range or larger for this footprint, set next to the sump with a hose draining into the pit.
• Wall layers. Sealed rigid foam on the concrete, studs on sill seal, no plastic, drywall held off the slab. Paperless drywall on the lower bathroom walls and the wall shared with the mechanical room.
• Floor. LVP through the living areas with area rugs, porcelain tile in the bathroom, and PVC baseboard everywhere.
• Bath and laundry. Bath fan ducted outside. Washer on a drain pan if the laundry is moving downstairs.

The design target is simple: if the water heater fails on a Friday night, the homeowner mops up, runs the dehumidifier for a couple of days and nothing has to be torn out. For budget ranges, see basement budgets at three levels and the Minneapolis cost guide.

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