Tips on RadonSeal Application and Use

Do I have to Remove the Paint First?

Yes, sorry about that! You can check out our FAQs page for tips on removing paints, surface sealers, adhesives, and how to prepare the surface.

An exception: If the paint is a single layer of latex, you can seal and waterproof right through it with our Ion-Bond Armor.

Concrete - Dry Inside but Damp Outside

The concrete should be dry inside so that RadonSeal can react. Otherwise, it would just sit inside the water-filled pores and wait until the water evaporates. But meanwhile, new rain may push it out of the concrete.

Pre-dampening the surface breaks the surface tension and helps the sealer penetrate into the concrete (particularly important for tighter concrete floors).

RadonSeal is applied in successive back-to-back applications 30-45 minutes apart. Applying the successive application while the surface is still damp will push the previous application deeper into the concrete.

How Many Pails do I Need?

Coverage depends on the concrete's porosity and composition. Concretes with larger pores or more alkalis consume more sealer. Cinder blocks are very porous and have the lowest coverage.

The sprayed on film of RadonSeal should absorb within about 10 minutes (the surface still looks damp but there is no more gloss). If the concrete is unusually porous or alkaline, it will absorb very fast, letting you know that it badly needs more sealer – spray on another application.

Approximate coverage per 5-gallon pail
Concrete No. of applications sq. ft. per pail
Poured 2 (normal) 1,000
3 (very porous) 700
Blocks 3 (normal) 500
4 (cinder blocks) 400

As a "rule of thumb," one 5-gallon pail seals 1,000 sq. ft. of poured concrete in two applications. An average basement (floor 800-1,000 sq. ft.) usually needs 2 pails of RadonSeal for the floor and poured concrete walls, or 3 pails if it has block walls.

Example 1:
Basement. 10-year old poured concrete in good condition.
Dimensions: 20 x 40 ft, height of walls 7.5 ft.
Wall perimeter: (20 + 40) x 2 = 120 ft.
Wall area: 120 x 7.5 = 900 sq. ft.
Floor area: 20 x 40 = 800 sq. ft.
Total area: 800 + 900 = 1,700 sq. ft.
Pails for floor and walls:   1,700 / 1,000 = 1.7 pails
Total order: 2 pails of RadonSeal Standard
Example 2:
Basement: 30-year old with concrete block walls in fair condition.
Dimensions: 30 x 40 ft, height of walls 8 ft
Wall perimeter:   (30 + 40) x 2 = 140 ft
Wall area: 140 x 8 = 1,120 sq. ft.
Floor area: 30 x 40 = 1,200 sq. ft.
Pails for walls: 1,120 / 500 = 2.2 pails
Pails for floor: 1,200 / 1,000 = 1.2 pails
Total order: 3 x 5-gal. pails and 1 x 2.5-gal. pail of RadonSeal Plus


As cement paste cures and shrinks, it tends to create microscopic cavities next to the fibers, which may later cause vigorous moisture wicking by capillary action. Sometimes, it draws in water so vigorously that there is a layer on the surface. It badly needs sealing.

However, fibercrete often contains various additives, which may hinder the penetration and reaction of RadonSeal. Use Ion-Bond Armor (or LastiSeal Brick & Masonry Sealer), which does not depend on a chemical reaction for sealing.

Thin Concrete Floors

Concrete caps 1-1/2 or 2 inches thick are often found in very old basements and crawlspaces. Alternatively, highly reactive soils in some regions "eat away" concrete (unless previously sealed with RadonSeal) and in several years can reduce a 4-inch slab to 2 inches.

There is less concrete to seal for RadonSeal. Use Ion-Bond Armor, which does not need to penetrate as deep and forms a waterproof membrane inside the concrete.

Highly Absorbent Concretes

It is normal to find faster-absorbing areas in concrete, where deterioration and increased porosity has already started. Just spray more RadonSeal on such areas.

In some regions, particularly in the North West, cement kilns produce high-lime Portland cement by adding fly ash from kilns to the cement. The resulting concrete tends to be weaker, easily absorbs moisture, expels free lime, and is prone to dusting and chalking. It badly needs RadonSeal to bind the lime and strengthen the concrete but it seems to "drink" it. Spray on additional applications, as long as the concrete is readily absorbing the sealer. If there is lots of efflorescence on the concrete, first use our Efflorescence Cleaner to neutralize the lime on and below the surface.

Concrete Blocks

Concrete blocks (CMUs) are more difficult to seal because of larger pores and the thin walls (about 1-1/4-inch) next to hollow cores. They provide only a weak barrier against water or radon and usually start leaking as soon as the exterior waterproofing coating deteriorates.

Spray on RadonSeal Plus in three successive applications half an hour apart. If the 3rd application still absorbs fast, apply a 4th application.

Do not apply RadonSeal on actively leaking blocks or when a rainstorm is expected within a couple days. The water would push out RadonSeal from the pores before it had a chance to react. Avoid applying RadonSeal on wet or internally water-saturated blocks, because water dilutes the sealer and prevents it from reacting.

If the hollow cores are not filled, a tall water column builds up inside the blocks and the hydrostatic pressure pushes water through the thin wall of the lower blocks. Drain the accumulated water first by drilling weeping holes in the bottom blocks (3/8" masonry drill) and let dry out for a couple of days. After RadonSeal cures, fill the holes with caulk. Some contractors also use the holes to inject expandable urethane foam into the cores, in order to prevent the build-up of water in lower blocks.

Good ventilation after the application or using a fan, dehumidifier, or heater will help, particularly in damp areas, to evaporate water from the sealer so that it can react and cure before the next big rain.

If the top blocks are hollow (also a potential termite problem) rather than solid, water vapor and radon gas from the ground will be drawn through the cores into the basement. Seal the openings with expandable closed-cell foam.

Prevention is always better than cure – it is much easier to seal blocks before they start leaking. Long-term water seepage dissolves lime and salts, which shows up as efflorescence ("white powder"). This enlarges the pores in concrete and further speeds up the seepage, leaving hard-to-seal leached out blocks.

If the blocks have been leached out, have cracks or defects, or still leak after applying RadonSeal, or if you want to guarantee the result to your customers, use the additional steps below.

Cinder Blocks

Cinder blocks are more porous and contain less cement which is partially replaced by fly ash from power stations or cement kilns. (Studies show that they may release mercury and other heavy metal contaminants into the indoor air – another good reason for sealing cinder blocks.)

The same methods apply as for concrete blocks but 4 back-top-back application of RadonSeal are usually needed.

If the cinder blocks still leak after applying RadonSeal or if you just want to make sure the cinder blocks are waterproof, use the additional sealer (Ion-Bond Armor) as described below.

Additional Sealer for Porous Concrete Blocks

While RadonSeal Plus successfully seals most concrete blocks, it may not suffice for cinder blocks, more porous blocks, or leached out blocks. Such blocks may still appear wet or leak.

Allow RadonSeal to cure for at least 10 days. Clean off any purged efflorescence. Make sure the concrete is dry. Seal the blocks with our Ion-Bond Armor, which forms a waterproof membrane below the surface. This still leaves the surface paintable.

"Unsealable" Blocks

There are many different types of concrete blocks that vary widely among regions and manufacturers. Although RadonSeal can seal almost all concrete blocks, the result cannot be guaranteed across the board. RadonSeal cannot seal blocks with little Portland cement or with enlarged pores:

  • lightweight blocks or CMU's (much lighter than the 38 lbs. of standard concrete blocks), decorative split face blocks, haydite blocks, and dry-pressed blocks made with little or no Portland cement.
  • "popcorn blocks" made with a very course aggregate which have large toothpick-size holes.

You may be able to seal these "unsealable" blocks with RadonSeal Plus and the thinset method below.

Unfortunately, concrete can become unsealable after extensive water seepage – when the seeping water has carried soil through the concrete and filled the pores. The telltale sign are brown or reddish spots on the inside surface. Then, the only reliable solution is applying a waterproofing coating to the exterior of the wall.

Sealing Large Pores in Blocks

Overly porous CMUs or cinder blocks and leached out blocks need additional steps to fill and seal the pores. This method has so far proved foolproof.

Hose and scrub off the block walls couple hours after the 4 applications of RadonSeal Plus, so that there is no residue plugging up the pores on the surface.

Purchase a good-quality plain thinset mortar (without vinyl) available in the tile section of home improvement centers. We recommend Mapei "Keraset Professional Grade Dry-Set Mortar", which is inexpensive (about $20 for a 50-lb. bag), spreads easily, adheres very well, and does not shrink.

Wet the surface. Apply the mortar on the blocks with a 3/16" notched trowel just like tile adhesive. Then, using the smooth edge of the trowel and holding it at a 45° angle, push the mortar very hard into the surface. The purpose is to push the thinset into the pores. You will end up with basically no layer on the blocks and lots of droppings on the ground. You can finish it off neatly by going over the mortar joints with a round tucking tool and make it look like a brand new block wall. But if you prefer the look of a poured concrete wall, leave a smooth layer (say 1/8") on the surface.

Thinset is also porous and would soon leak. RadonSeal cannot react with it because of additives. Let it dry out for at least 4 days and then, seal it with our Ion-Bond Armor in two successive applications about 20 minutes apart. Ion-Bond penetrates through the thinset, bonds with RadonSeal inside the blocks, and permanently seals both the blocks and the mortar.

Crumbling Concrete

If the concrete blocks or poured concrete have already badly deteriorated or there are holes, a more radical step may be required. Make sure the surface is dry and apply a mixture of sand and our ElastiPoxy Crack Filler. Press it hard into the surface with a trowel.

Stone Foundations

Stone foundations usually leak through the mortar, not the stones. Scrape out and brush off any loose or soft mortar with a stainless steel brush (no rusting specs). Spray on RadonSeal Plus in 3 applications about half an hour apart. If the stone is decorative, you can apply RadonSeal only on the mortar with a sponge and wipe off the stones with a rag before it hardens.

RadonSeal also seals porous limestone, because it penetrates and reacts with free lime.

Let dry out and cure for at least 3 days before re-pointing (tuck-pointing). RadonSeal will seal the old mortar, improve the bond to the new mortar, and prevent water or efflorescence from separating the new mortar.

If there is not enough mortar left between the stones, sealing the thin layer of mortar will not be enough. You may have to parge the surface. You can install a lath (metal mesh) 3/4" away from the surface, fill with mortar, let cure for a month, and seal with Ion-Bond Armor.

Sandstone Foundations

Usually the problem is the mortar joints but some sandstone is so porous that water seeps through it, too. And there is not enough loose lime in sandstone for RadonSeal to react. First, remove any cracked or loose mortar. Re-point. Let cure for at least 2 weeks. Wait for suitable weather until everything is thoroughly dry and seal the walls with LastiSeal Brick & Masonry Sealer.

New Concrete - Curing and Sealing

RadonSeal is a great curing compound for freshly poured concrete slabs. It retards water evaporation, maintains ideal moist condition inside the concrete for hydration, and eliminates/reduces surface shrinkage cracking. It forms an integral, colloidal gel membrane, which slows down evaporation, conserves generated heat in cold weather or reduces heat absorption in higher temperatures, hinders the development of large capillaries, and results in a less porous and significantly stronger concrete.

Spray on RadonSeal as soon as the concrete can be walked on without leaving shoe marks, right after finishing, usually in 3 to 5 hours after the pour. Spray on just one application of RadonSeal in a thin continuous film at a rate of 2,000 sq. ft. per pail.

In cold weather, thermal blankets are still needed to protect outdoor concrete. Freezing the water in concrete will damage the surface just like rapid evaporation.

Other curing methods: Moist curing (misting for at least 7 days or burlap covering) is labor-intensive and may leach out lime and alkalis, causing laitance in lower spots on the surface. Plastic or paper sheets tend to leave marks, discoloration, and laitance. Chemical compounds expose workers to high levels of VOCs. Temporary wax or chemical sealers leave a surface film unsuitable for painting or tile adhesives and have to be removed first. Flooring manufacturers prohibit the use of curing compounds that form a surface membrane due to the incompatibility with their new, VOC-compliant adhesives.

This RadonSeal application also serves as a floor hardener - the surface layer will become much harder ("flint-hard"), resistant to abrasion and dusting. Compared to other floor hardeners used on industrial or warehouse floors, RadonSeal has a much higher solids content and works much deeper.

Why is my new concrete slab all white? Probably a result of "watery" concrete covered by vapor-impermeable sheets or blankets that trap all moisture. The excessive water in the surface leaches out lime and minerals, causing the white color and blotchy areas where it puddled - laitance. This can happen regardless of RadonSeal or other curing compounds and does not necessarily mean the concrete is "bad." For appearance sake or before painting, laitance can be removed with a shot-blaster, floor sander, or by acid etching.

Curing is a completely separate application from deep-sealing the concrete with RadonSeal. As the concrete slowly cures, it will still develop capillaries although much smaller. Allow new concrete to cure for at least 28 days and then, deep-seal the walls and slab with RadonSeal Standard in 2 applications. This bonds and strengthens the concrete, neutralizes alkalis, and seals it against water, vapor, and even radon gas. In case of concrete block walls, let the mortar cure for at least 21 days before applying RadonSeal Plus.

New Construction - Sealing Foundation Walls

Exterior waterproofing gradually peels and disintegrates similarly to a paint as it is attacked by the free lime from concrete. The old tar or asphalt coating is brittle and cracks as the concrete moves. It breaks down in several years. Spray-on elastomeric ("rubberized") waterproofing membranes last much longer. Elastomeric membrane sheets depend on a primer for adhesion and on a tape holding them together, which will eventually separate and let in water.

Although RadonSeal provides a tighter and permanent seal, it is usually used together with an exterior waterproofing coating. Visible exterior coating is required in most counties and it bridges holes, defects, tie-rods, and joints.

RadonSeal can be applied only after the poured concrete has cured for 28 days or 21 days for mortar in block walls. Then, the concrete has to dry out for at least 5 days before applying exterior waterproofing coating. RadonSeal neutralizes the alkali inside concrete and makes the waterproofing coating last many years longer.

Most contractors use the usual waterproofing coating on the exterior and later, apply RadonSeal on the interior of the walls and on the slab. It waterproofs and strengthens the concrete and by neutralizing lime and alkalis, helps extend the life of the exterior waterproofing.

The Permanent Vapor Barrier

RadonSeal can be used in addition to or instead of the vapor retardant barrier (plastic sheets) under the concrete slabs. It seals concrete slabs against vapor and unlike plastic sheets, does not deteriorate. Plastic barriers get punctured during construction and gradually disintegrate under the "alkali attack" of the free lime in concrete. Good-quality barriers are guaranteed for 10 or 15 years.

Contractors sometimes forget to install the "vapor barrier" required by local codes and several counties have approved RadonSeal as a substitute vapor barrier.

Complementary Moisture or Radon Reduction Steps

Sealing a basement against moisture or radon is like sealing a boat with a few holes and leaking planks. If you seal only the planks, more water will gush in through the holes. And vice versa. It has to be sealed all at once!

Seal and caulk all openings, gaps or cracks. More info on FAQs

Cover the sump pit with a rigid airtight cover made of plexiglas or wood. Transparent plastic lets you see the pump. Fasten the cover with silicone or other non-permanent caulk for its easy removal when servicing the pump. Caulk around all penetrations for water ejection pipes and electrical wiring.

Drains without U-traps let in a flow of radon-rich, moist soil gas. If not needed, cover and caulk floor drains or install floor drains with check valves. Seal the tops of open drain pipes or service pipes coming in from the ground.

Take common sense measures to keep water away from the foundation like cleaning gutters, adding downspout extensions, proper grading, a trench, drainage of window wells, no bushes or flower beds next to the foundation. More info on basement waterproofing

Testing the Results

Similarly to concrete, RadonSeal continues to react and slowly cure. The seal is about 50% effective against gases several days after application and it reaches its full strength in 60 to 90 days.

You can order radon test kits online. In addition to the lowest prices anywhere, you will get a full rebate with the purchase of RadonSeal. These are the most widely used, EPA-approved radon test kits. EPA recommends placing two test kits in tandem to minimize errors. Radon levels fluctuate widely depending on factors like barometric pressure, rain, winds, and the season. For more reliable results, use a long-term radon test kit. For more information, visit

To measure water vapor transmission through the concrete, particularly before painting or installing a floor, use our pre-weighed Vapor Transmission Test Kits. Depending on the concrete porosity and application, RadonSeal typically reduces vapor transmission rate down to the range of 3 or 4 lbs. through 1,000 sq. ft. over 24 hours. For the average basement, this means less than a gallon of moisture per day and a much drier basement.

If a qualitative indication of moisture transfer is sufficient, fasten a 2' x 2' plastic sheet or aluminum foil on the concrete with duct tape. We first insert a small object in the middle to form a tent shape, which helps droplets to form and run. After 2 days, check the underside - there should be no droplets and the concrete should not be dark due to moisture.

Concrete Driveways, Roofs, and Outdoor Concrete

Driveways and outdoor concrete like sidewalks, patios, decks, and parking areas are exposed to severe conditions: rainwater, groundwater, repeated melting and freezing, road salts or deicers, and the settling of gravel and soil. Outdoor concrete becomes porous very quickly and allows in more water.

Pores in concrete soak up rainwater from above and groundwater from below just like a sponge. When the water freezes, it forcefully expands (by 9 percent) and cracks the concrete. The freeze-thaw cycle causes pitting, spalling, and cracking. Chloride ions in salts and ice removers, particularly with ammonium, attack the concrete chemically. Water makes rebar rust, which cracks the concrete. Water erodes the sub-grade and its settling causes structural cracks.

Deep-seal all outdoor concrete with RadonSeal! It strengthens the concrete and seals it against rainwater, groundwater, and chemicals. The surface does not change and remains non-slippery. Best of all – it is permanent!

If you would like a water-repellent surface that beads, use our DryWay Water-Repellent Sealer. Since it seals below the surface, it resists UV-rays and the surface normally sheds water for over 10 years. It can be applied after permanently deep-sealing the concrete with RadonSeal.

Paints, Coatings, Adhesives, and Floor Coverings

RadonSeal does not leave a film or color when properly applied. It does not change the surface profile or friction. It leaves the pores open – the treated surface remains suitable for painting, tile adhesives, thinset, levelers, or patching compounds.

Make sure to wash and scrub the surface after the application of RadonSeal. In addition, you may also sand the floor to ensure that the surface is porous and rough.

After applying RadonSeal, let the concrete thoroughly dry for at least 5 days before painting with “breathable” paint (latex) or installing vapor-permeable floor covering like outdoor carpeting or ceramic tiles set in thinset mortar. But before installing vinyl floor tiles, linoleum, rubber carpet padding, or painting the concrete with impermeable paints like epoxy or urethane, wait at least 10 days.


Dirt floor in the crawlspace releases large amounts of moisture, molds, soil gas, and radon. It should be covered and probably vented outdoors, depending on the climate. Cover the soil with 6 mil (at least) polyethylene sheets, overlapping 12". Insert a 3" perforated PVC pipe underneath the sheets and vent it to the outside. Fasten the sheets together and to the walls with silicone caulk (very durable) or duct tape. Seal it off from an attached basement with polyethylene sheets fastened with duct tape.

Application Procedures for Specific Projects