Concrete Basement Floors
A basement concrete floor can take paint, epoxy or a film sealer only if it passes a moisture test, usually 3 lb per 1,000 sq ft per 24 hours or 75 to 85 percent relative humidity, depending on the product. A slab that fails gets a breathable penetrating sealer, or a moisture mitigation membrane before any coating.
Every floor type, basement or not, is compared in the concrete floor guide.
How to test a basement floor for moisture
Start with a plastic sheet overnight, then get a number from a calcium chloride or relative humidity test before you buy a coating.
- Plastic sheet test, ASTM D4263. Tape an 18 inch square of poly down on all four edges and leave it at least 16 hours. Moisture under the sheet or darker concrete means it fails; the test gives no number.
- Calcium chloride test, ASTM F1869. A dish under a dome for 60 to 72 hours gives pounds of vapor per 1,000 sq ft per 24 hours. It measures only near the surface, says Wagner Meters.
- Relative humidity probe, ASTM F2170. Drill to 40 percent of the slab depth, since a basement slab dries from the top only, and read after 24 hours. Use three holes for the first 1,000 sq ft and one per extra 1,000, per Wagner's F2170 guidelines.
Test under living conditions. F2170 wants the space at service temperature and humidity for 48 hours before testing, so run the heat and dehumidifier first.
What moisture reading passes
The product's data sheet sets the limit; ASTM only sets the test method, as Wagner notes. Sherwin-Williams wants 3 lb or less on a calcium chloride test, and relative humidity limits run from 75 to 96 percent.
Where each limit comes from
| Test | Typical limit | Applies to |
|---|---|---|
| Calcium chloride (F1869) | 3 lb per 1,000 sq ft per 24 hr | Sherwin-Williams resin floors; UFGS 09 67 23.13 |
| In-situ RH (F2170) | 75 percent | Common benchmark (Wagner); UFGS resinous flooring default |
| In-situ RH | 85 percent | Sikafloor-218 DF epoxy |
| In-situ RH | 96 percent | Sikafloor-1620 moisture-control epoxy primer |
New concrete needs at least 28 days of curing before an epoxy, per Sherwin-Williams, and Sika wants the slab at least 5°F above the dew point when you coat.
Which finishes work on a basement floor
On a damp slab, use a breathable sealer or put a mitigation membrane down first. Epoxies "trap moisture and can peel or blister if applied to damp concrete," says Prosoco. Paint, epoxy and urethane need a passing test.
Penetrating sealer
No film
Silane or siloxane soaks in and lets vapor through. See the sealing guide.
Water-based acrylic
Breathable
Solvent-based acrylic isn't, so check the label.
Polyurethane
Dense film
Traps vapor, so coat only a slab that passes.
Floor paint
Needs a pass
A film that lifts on a wet slab. See the painting guide.
Epoxy
Blisters if damp
Test first, every time. See the epoxy guide.
Mitigation membrane
For failed slabs
Goes down first so a coating can follow; a moisture-control primer does the same job.
A membrane has to meet ASTM F3010: no more than 0.1 perms and a pull-off strength of at least 200 psi, per CTLGroup's summary of the standard.
Every finish is compared side by side in the finishes guide.
What it costs to finish a basement floor
About $3 to $12 a square foot, from stain to epoxy, on Concrete Network's rough figures.
Stain
$3 to $6
Per sq ft, single color.
Polish
$3 to $5
Per sq ft, basic finish.
Epoxy or polyaspartic
$8 to $12
Per sq ft, after a passing test.
Stamped overlay
$8 to $20
Per sq ft, needs a sound slab.
Carpet runs $2 to $8 and vinyl plank $3 to $10 for comparison. To pour a new slab, get the volume from the concrete floor calculator and the full price in concrete foundation cost.
What the white powder on a basement floor means
It's efflorescence: salts carried up by water and left behind when it evaporates, per BRANZ. If it comes back after cleaning, the slab is still moving water and isn't ready for paint or epoxy.
Clean it off early. Calcium hydroxide in the deposit reacts with carbon dioxide and turns into calcium carbonate, which water no longer dissolves and is harder to remove.
Pouring a new basement slab: where the vapor barrier goes
Directly under the concrete, on a crushed stone base. Building Science Corporation warns that sand placed over the poly becomes a wet layer that can only dry up through the slab, into your finish.
A new slab holds a lot of mix water, so wait at least 28 days and test before coating. Choosing the sheet and insulation is covered in under-slab insulation and the vapor barrier.
Frequently asked questions
How do I know if my basement floor is too damp to finish?
Run the plastic sheet test first; if it stays dry, get a number from a calcium chloride or relative humidity probe test and compare it with the product's limit on its data sheet.
What moisture reading is acceptable before coating a basement floor?
Whatever the product data sheet says, since ASTM sets no limit. Sherwin-Williams wants 3 lb per 1,000 sq ft per 24 hours or less, Sikafloor-218 DF epoxy wants 85 percent relative humidity or less, and 75 percent is a common flooring benchmark.
Can you epoxy a basement floor?
Yes, once the slab passes a moisture test, because epoxy traps vapor and blisters on a damp slab. If it fails, a membrane meeting ASTM F3010 goes down first. The epoxy guide covers application.
What is the white powder on my basement floor?
Efflorescence. Scrub it off as soon as it appears; if it returns, the slab is still wet, so hold off on paint or epoxy.
How much does it cost to finish a basement concrete floor?
Roughly $3 to $12 a square foot for stain, polish or epoxy, and up to $20 for a stamped overlay, on Concrete Network's figures. Carpet and vinyl plank run $2 to $10 for comparison.