A below-grade slab — a basement floor, a lower-level garage, any concrete poured below the surrounding ground — sits on soil that never dries out. Ground moisture rises through the concrete as vapor, condenses at the surface, and shows up as dampness, sweating, a musty smell, or flooring that fails from underneath. The fix is not a film on top of the floor. Bone Dry Original, Bone Dry Plus, and Bone Dry Pro are permanent penetrating sealers applied from the interior side of the slab: they soak into the pores, react with the free lime in the concrete, and form a vapor barrier inside the slab itself. Nothing to peel, nothing to recoat, no excavation, and the concrete looks unchanged.
How do you seal a below-grade concrete slab against moisture?
Why below-grade slabs sweat — and why it never stops on its own
Concrete looks solid, but it is laced with capillary pores — the path moisture vapor rides from the ground to the surface. On an unsealed below-grade slab the supply is unlimited, because the soil under and around the floor stays damp year-round. A missing, torn, or aging under-slab vapor barrier makes it worse. Dehumidifiers treat the air, not the source. Paints and topical sealers sit on the surface and eventually lift, because the vapor keeps pushing from below.
That vapor is what ruins whatever goes on top. Buckling wood, curling vinyl, peeling carpet, and bubbling paint are four different failures with one shared cause: moisture rising through unprotected concrete. It emulsifies and weakens the water-, latex-, and acrylic-based adhesives under flooring, feeds mold and mildew beneath non-permeable coverings, and wicks into cardboard stored on the floor until it loses its strength. Seal the slab once and the failure chain never starts.
First, confirm it is the slab: the 48-hour plastic-sheet test
Tape a 2×2-foot square of plastic sheeting to a dry patch of the floor and leave it for 48 hours, then lift it. Beads of moisture underneath mean vapor is coming up through the concrete — a faulty or missing vapor barrier — and the slab needs sealing. If it is bone dry under the plastic, the moisture is coming from the room’s humidity, and ventilation is the fix. Two days, zero cost, and you know exactly which problem you have.
Which Bone Dry product for a below-grade slab?
| Your situation | Use | Why it fits |
|---|---|---|
| Existing basement or below-grade floor going under flooring | Bone Dry Original | Permanent penetrating seal and densifier; about 1,100 sq ft per 5-gallon pail; the concrete looks unchanged |
| Musty basement, prior mold, or a below-grade space where hygiene matters | Bone Dry Plus | The same permanent seal with antimicrobial protection built into the pores — prevents mold, mildew, and odor-causing bacteria in the slab |
| Commercial or institutional below-grade slab — new pour or existing, warranty required | Bone Dry Pro System | Formulated for slab on grade, below grade, and slab on deck; sprayed as soon as the slab can be walked on; cure and seal in one pass; 25-year warranty against moisture-related and bonding failures |
| New below-grade pour where you can coordinate with the batch plant | Bone Dry Pro Admix | Waterproofing dosed into the mix — every cubic inch is sealed from day zero; no spray crew and no onsite moisture testing |
| The block walls around the slab weep too | Bone Dry Block Wall system | CMU needs the two-step Primer plus Original or Plus system — sealer alone cannot react with block |
Not sure where your project lands? See the full Bone Dry product selector.
When a penetrating sealer is not enough — or not yet
- Liquid water coming through cracks, joints, or the wall-floor cove. A penetrating sealer closes the pores against moisture vapor. A running leak needs the crack, joint, or exterior drainage problem corrected first — then seal the slab.
- Hard-troweled or burnished slab. A sealer cannot treat what it cannot enter. Open the pores with Etch-A-Crete — a garden sprayer, a broom, and a mop, no acid and no grinding — then seal.
- Painted or coated floor. Prior coatings need mechanical profiling or a cementitious skim coat before the sealer goes on.
- Old slab with black mastic or cutback adhesive. After chemical abatement, use Bone Dry Pro Encapsulator instead of a penetrating sealer.
- Block walls. Sealer alone will not stop a weeping CMU wall; it takes the Block Wall Primer and sealer system.
How to apply Bone Dry on an existing below-grade slab
- Clean. Sweep, and remove oil or grease with Bone Dry Concrete Degreaser. The surface must be clean and porous.
- Open the pores if needed. Burnished or hard-troweled concrete gets one coat of Etch-A-Crete, agitated with a broom and mopped clean.
- Spray. Any common garden sprayer with a wide fan tip; soak the whole slab. If what you have sprayed disappears into the concrete within 15 minutes, the slab wants more — reapply.
- Mop. As the seal forms it pushes leaching alkali and lime to the surface. Flush or mop the floor with water 24 hours after application to clear it.
- Floor it. With the Quick Dry option — premixed in the pail, no change to how you apply — flooring can go down in as little as 6 hours.
Coverage: about 1,100 sq ft per 5-gallon pail, depending on the porosity of the slab. Safety: Bone Dry sealers are slightly corrosive — wear goggles or a face shield and rubber or plastic gloves, and use a NIOSH-approved respirator in confined areas. A basement counts as a confined area: ventilate while you work. See the SDS for details.
What can go on top afterward
Bone Dry leaves the surface profile unchanged and ready for adhesives. It works under adhered systems — hardwood, vinyl, tile, broadloom carpet, carpet tile, sheet goods, luxury vinyl, VCT — and floating systems such as engineered hardwood, laminate, and luxury vinyl plank. Resinous coatings (epoxy, polyurethane, polyaspartic, polyurea, PMMA) go over Bone Dry Plus after shot blasting, and floor, wall, acrylic, epoxy, and masonry paints bond normally. Third-party ASTM E-96 and ASTM F-1869 test results are available on request, and the Bone Dry Pro System carries a 25-year warranty against moisture-related and bonding failures.
Sealing before flooring? Send us the square footage and we will size the order — or call 262-694-9748.
Yes. Bone Dry Original, Plus, and Pro are applied to the top of the slab, from inside the building, with a garden sprayer. The sealer penetrates the concrete and reacts with the free lime to form a permanent vapor barrier within the slab, so there is no excavation, no membrane, and no film on the surface.
Use the plastic-sheet test. Tape a 2×2-foot square of plastic sheeting to a dry patch of the floor for 48 hours. Beads of moisture underneath mean vapor is rising through the concrete and the slab needs sealing. A dry patch means the moisture is coming from the room’s humidity, and ventilation is the fix.
Both provide the same permanent penetrating seal at the same coverage. Choose Plus if the basement smells musty, has had mold or mildew, or will be finished as living space where air quality matters — its antimicrobial technology prevents mold, mildew, and odor-causing bacteria in the pores of the slab and keeps working for the life of the seal. Original is right for a standard slab without a growth problem.
With the Quick Dry option, which ships premixed in the pail, flooring can go down in as little as 6 hours. Otherwise, flush or mop the surface with water 24 hours after application to remove the alkali and lime the seal brings up, then install the flooring.
If the walls are CMU block and they weep, sealing the floor alone will not solve the room. Block does not carry enough alkali for a Bone Dry sealer to react with, so the walls need the Bone Dry Block Wall system: two coats of Primer, then Original or Plus applied while the primer is still damp. Poured concrete walls take Original or Plus directly.
Both. On an existing slab the surface has to be clean and porous — degrease oil stains, etch a burnished finish, and profile or skim-coat old paint or coatings — then apply as normal. New pours can be sealed with Bone Dry Pro as soon as the slab can be walked on, or waterproofed in the mix with Pro Admix.
No. The Bone Dry sealers penetrate and react inside the slab rather than forming a film, so the concrete looks unchanged and there is nothing on the surface to peel, yellow, or wear through.
