EcoGuide · Thermal Leaks · Updated 2026-10-03 · 1,400 words

Basement Wall Condensation: Why Fiberglass Against Concrete Gets Wet, and the Fix

The Short Answer

If you open a finished basement wall and find damp fiberglass against the concrete, with a plastic sheet over it, the wall was built in a way building scientists warn against. Building Science Corporation says that framing a wall, insulating the cavity and covering it with an interior plastic vapor barrier is common and often leads to odor, mold, decay and corrosion problems. A Department of Energy summary of the research explains why: fiberglass or cellulose in contact with a basement wall can absorb large amounts of moisture and stay chronically damp.

The fix is to put rigid foam against the concrete, keep fiberglass and wood off it, and leave out the plastic sheet so the wall can dry to the inside.

Why It Happens

Concrete below ground is cool and never fully dry. Two things then wet the insulation:

Humid air meets a cold surface. Building Science Corporation says the main problem with below-grade walls comes during the summer when warm moist air comes in contact with basement cold surfaces that are below the dew point. Natural Resources Canada says the same: condensation forms on foundation walls in summer when the air is very humid and the foundation is cool.
The plastic stops it drying. Moisture that gets into the cavity cannot leave through polyethylene, so it stays in the fiberglass.

Fiberglass does not cause the moisture. It lets air reach the cold concrete and then holds the water.

Condensation or a Leak? A Two-Day Test

Natural Resources Canada describes a simple check. Cut a sheet of plastic about 1 m (40 in.) square and tape it to the concrete wall. After a day or two, look at it:

Moisture on the room side of the plastic: humid indoor air is condensing. That is a humidity and insulation problem.
Moisture under the plastic: water is moving through the concrete from the ground.

Liquid water is different again. Building Science Corporation says that if walls are visibly wet at times or water drains onto the floor, an interior drain system should be installed before insulating and finishing. Do not insulate over a leak; fix gutters, grading and drainage first.

Step 1: Take Out the Wet Material and Dry the Wall

Remove the wet insulation and the plastic sheet. Bag them before carrying them through the house.
Run a dehumidifier with the windows closed. The EPA’s target is indoor humidity below 60 percent (ideally between 30 and 50 percent).
Let the concrete dry before anything goes back.

If there is mold: the EPA says that if the area is less than about 10 square feet, roughly three feet by three, you can usually handle it yourself; larger than that, get professional advice. Scrub hard surfaces with detergent and water and dry them completely. The EPA says using bleach is not recommended as a routine practice during mold cleanup: the mold has to be removed, not just killed. Porous materials that have gone moldy may have to be thrown away, and do not paint over it.

Step 2: Rigid Foam Against the Concrete

Every interior approach the research recommends starts the same way: a layer of rigid foam directly on the foundation wall. The Department of Energy summary lists three types that can be used, XPS, EPS (expanded polystyrene), and polyisocyanurate, and notes that closed-cell spray foam gives the best moisture control of all.

Thickness: Building Science Corporation recommends a minimum of 1 inch of XPS up to a maximum of approximately 2 inches, so the foam still lets the wall dry inward. Two inches of XPS is about R-10.
Seams: seal the joints between sheets with tape or mesh and mastic, so room air cannot reach the concrete.
How much insulation in total: ENERGY STAR’s recommendation for basement walls is R5 foam or an R13 batt in climate zone 3, R10 foam or R13 batt in zones 4A and 4B, and R15 foam or R19 batt in zones 4C to 8.

Step 3: The Stud Wall, Without the Plastic

If you want more insulation or a finished wall, frame it inside the foam.

Keep wood and batts off the concrete. Building Science Corporation: moisture-sensitive materials such as wood, paper and fiberglass batts should not be installed in direct contact with concrete. Put closed-cell sill seal under the bottom plate.
No plastic sheet. The Department of Energy summary is blunt: do not install polyethylene sheeting, vinyl or foil wallpaper, or epoxy, oil, or alkyd paint on the interior of basement walls. Any vapor retarder has to be semi-permeable so the wall can dry to the inside. Ordinary latex paint on the drywall is fine.
Cover the foam. Foam board must not be left exposed. The Building America Solution Center notes that most codes require a thermal barrier (typically ½-in. gypsum board or equivalent) over it. Building Science Corporation recommends gypsum board without paper facings below grade.

Keep It Dry Afterwards

A hygrometer tells you whether the basement stays in the EPA’s 30 to 50 percent range, and a dehumidifier handles the summer months when it does not.

We may earn a commission if you buy through these links, at no extra cost to you. Products are chosen on published specifications; we do not test them in-house (how we choose).

Govee WiFi Thermometer Hygrometer

Shows the basement’s humidity so you can see whether it stays below 60 percent.

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hOmeLabs Wi-Fi Dehumidifier

For holding humidity down in summer. Run it with windows and doors to the space closed.

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When to Call a Professional

Liquid water on the walls or floor at any time. That needs drainage work first.
Mold covering more than about 10 square feet.
Any sign that framing has rotted.
Fuel-burning appliances in the basement, before you tighten or enclose the space.

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Frequently Asked Questions

Why is the insulation in my basement wall wet?

Usually condensation. Fiberglass against concrete lets humid air reach the cold wall, and an interior plastic vapor barrier stops the moisture drying. Building Science Corporation says this common construction often leads to odor, mold and decay.

How do I tell basement condensation from a leak?

Tape a square of plastic about 40 inches across to the bare concrete for a day or two. Moisture on the room side means humid indoor air is condensing; moisture underneath means water is coming through the concrete. Liquid water running down the wall is a leak.

What is the right way to insulate a basement wall?

Rigid foam board (XPS, EPS or polyiso) directly against the concrete with sealed seams, then an optional stud wall with unfaced batts, no polyethylene sheet, and gypsum board over the foam.

Should I put a plastic vapor barrier on a basement wall?

No. Department of Energy research guidance says not to install polyethylene sheeting on the interior of basement walls, because the wall needs to dry to the inside.

Can I clean basement mold with bleach?

The EPA does not recommend bleach as a routine practice for mold cleanup. It advises scrubbing hard surfaces with detergent and water, drying completely, and fixing the moisture problem. Areas over about 10 square feet warrant professional advice.

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Home energy audit checklist — a two-page printable