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Home » RadonSeal Uses, Limitations, and Application

RadonSeal Uses, Limitations, and Application

Concrete Must Be Dry Inside!

The concrete must be dry inside so that RadonSeal© Concrete Sealer can react with the concrete. Otherwise, it would just sit inside the water-filled pores and wait until the water evaporates. But new rain or water pressure may push it out of the concrete.

Dampening the surface with water before applying RadonSeal breaks the surface tension and helps the sealer penetrate into the concrete (particularly important for tighter concrete floors).

RadonSeal Penetrating Concrete Sealer is applied in successive back-to-back applications on damp surface about 30-45 minutes apart. Applying the successive application while the surface is still damp will push the previous application deeper into the concrete. Otherwise, the successive applications may just dry on the surface which is not helpful in sealing the concrete.

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.

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

RadonSeal Standard or Plus?

RadonSeal Standard is designed for poured concrete in good condition – indoor poured concrete less than 20 years old or outdoor poured concrete less than 5 years old. Do not use RadonSeal Plus on tight poured concrete because it would not penetrate as well as RadonSeal Standard.

RadonSeal Plus carries more minerals to seal larger pores in concrete. Use it on very porous poured concrete (indoor over 20 years old or outdoor over 5 years old) or on concrete that has already seeped water. Designed for concrete blocks and cinder blocks.

How Many Pails Do I Need?

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

The sprayed-on layer 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 RadonSeal very fast, letting you know that it badly needs more sealer – spray on another application!

Approximate coverage per 5-gallon pail
ConcreteNo. of applicationssq. ft. per pail
Poured2 (normal)1,000
3 (very porous)700
Blocks3 (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 concrete 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

The RadonSeal and Ion-Bond Armor Combination

If the concrete or blocks have already badly deteriorated due to extensive water seepage, heavy efflorescence, or other issues, RadonSeal may not work well enough. It may not suffice for very porous, cracked, deteriorated, or leached out concrete or blocks, particularly cinder blocks. Such blocks may still appear wet or leak.

In more challenging situations or for the tightest possible seal against water, vapor, and radon gas seal the concrete again with Ion-Bond Armor. It is also a penetrating sealer but of different chemistry. It bonds inside the concrete with RadonSeal and forms a waterproof membrane below the surface. This still leaves the surface paintable.

First, allow RadonSeal to cure and dry out for at least 10 days. Clean off any purged efflorescence. Make sure the concrete is thoroughly dry and then, spray on Ion-Bond Armor. It does not form a film and leaves the surface paintable.

The RadonSeal Plus and Ion-Bond Armor combination is recommended for cinder blocks and concrete blocks.

Concrete Blocks

Concrete blocks (CMUs) are more difficult to seal because of larger pores and the thin walls (only 1-1/4-inch) next to the hollow cores. They provide only a weak barrier against water or radon and usually start leaking as soon as the exterior waterproofing coating starts cracking or peeling. The hollow cores will fill up with water or radon and soon, will seep through the unsealed interior wall.

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 has a chance to react. Do not apply 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 with mortar, 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 water column first by drilling weeping holes in the bottom blocks (3/8" masonry drill) and let it dry out for a couple of days. After RadonSeal fill the holes with caulk.

Some contractors also use the weeping holes to inject expandable urethane foam into the cores to prevent the build-up of water in lower blocks.

Good ventilation after the application of RadonSeal 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 open 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 still leak after applying RadonSeal Plus, seal them again with Ion-Bond Armor. The combination of RadonSeal Plus and Ion-Bond Armor (see above) provides the best seal in concrete blocks, particularly if they are older, very porous, or when they had already leaked. More information at Waterproofing Concrete Block Foundations

Cinder Blocks

Cinder blocks are more porous than common concrete blocks 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.) Cinder blocks are harder to seal.

The same methods apply as for concrete blocks but four back-top-back application of RadonSeal Plus are normally needed.

The combination of RadonSeal Plus and Ion-Bond Armor (see above) is recommended. More information at Waterproofing Cinder Block Walls

Unsealable Blocks or Concrete?

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 huge 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 are made with little or no Portland cement. Some large-box stores sell lightweight blocks to unwitting customers. When used in basements or as exterior walls, water will just flow in. Seal and waterproof these blocks with our LastiSeal Brick & Masonry Sealer which does not depend on the chemical composition of the substrate.

Popcorn blocks” are made with a very course aggregate and have large toothpick-size holes. You may be able to seal these “unsealable” blocks in basements with RadonSeal Plus and the thinset method

Poured concrete or blocks in basements may become unsealable after extensive water seepage when the seeping water has carried soil through the concrete and filled the pores. Penetrating sealers cannot penetrate! 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.

How to Seal Blocks with Large Pores

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.

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.

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

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 parging (say 1/8″) on the surface.

Thinset is 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 thinset.

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 (avoid 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 dries.

RadonSeal will strengthen and waterproof the old mortar, and prevent water or efflorescence from pushing off the new mortar. Let dry out and cure for at least 3 days before re-pointing (tuck-pointing).

If there is not enough mortar left between the stones in old stone foundations, sealing the thin layer of remaining mortar will not be enough. You may have to parge the wall. You can install a lath (metal mesh) 3/4″ away from the surface, fill with mortar, let cure for a month. Seal it with Ion-Bond Armor.

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

Sandstone Foundations

Usually the problem is the leaking mortar joints but some sandstone is so porous that water seeps through it, too. There may not be enough loose lime in the sandstone to react with RadonSeal.

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. It penetrates deep into the pores and cures as a plastic. It’s like “injecting epoxy” into the pores.

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. By purging minerals and dirt from the concrete, it actually opens up the pores in the surface, making it 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 smooth floors to ensure that the surface is porous and rough enough for the paint.

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.

The alternative concrete prep for painting is LastiSeal Concrete & Brick Sealer which is a great primer for epoxies and other paints, except for latex or silanes/siloxanes. It also is a deep-penetrating concrete sealer that permanently waterproofs concrete.


As cement paste cures and shrinks, it tends to create microscopic cavities next to the fibers, which may later cause vigorous wicking of water by capillary action. In extreme cases, it draws in water so vigorously that there is a layer on the surface. It badly needs sealing. Fibercrete often contains various additives against clumping of the fibers, which may hinder the penetration and reaction of RadonSeal.

Use Ion-Bond Armor or LastiSeal Brick & Concrete Sealer that do not depend on the chemistry of the substrate.

Stamped Concrete

RadonSeal is not recommended for use on stamped concrete surfaces due to the varying amounts of release agents used in the stamping process, tight surfaces (common for stamped concrete), and the use of clear membranes (acrylic) which are often applied to protect the color.

Thin Concrete Floors

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

There is not enough concrete to seal for RadonSeal. Use Ion-Bond Armor, which does not need to penetrate as deep and forms a waterproof membrane below the surface of 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 of RadonSeal Plus, 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.

Exterior Sealing of New Foundation Walls

Exterior waterproofing eventually peels and disintegrates similarly to a paint because it is attacked by “saponification” due the free lime from the concrete. The cheap 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 before applying an exterior waterproofing coating. Visible exterior coating is required in most counties and it bridges holes, defects, tie-rods, and the footing joints.

RadonSeal can be applied only after the poured concrete has cured for 28 days, or 21 days for mortar in block walls. Afterwards, 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, also 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 slab. It seals concrete slabs against vapor and gases, and unlike plastic sheets, it does not deteriorate. Plastic sheets get punctured during construction and gradually disintegrate under the “alkali attack” of the free lime in concrete. Good-quality barriers are guaranteed to last 10 or 15 years. Should you install a new plastic barrier in 15 years?

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


Dirt floor in the crawlspace releases large amounts of moisture, molds, soil gas, and radon. It should be covered and depending on the climate, probably vented outdoors. 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.

Seal the concrete floor and block walls with RadonSeal.

Take Complementary Moisture and 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 in 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. Learn more on How Radon Gas Sneaks into Homes

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 below the concrete 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!

RadonSeal is deep inside the concrete and does not “bead” water on its surface. If you would like a “beading” surface, let RadonSeal cure for at least 5 days and seal it again with LastiSeal Brick & Concrete Sealer.

If you don’t need a deep-penetrating sealer and 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 up to 10 years. It also can be applied after permanently deep-sealing the concrete with RadonSeal.

Curing and Sealing New Concrete

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

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 Standard in a thin continuous film at a rate of 2,000 sq. ft. per 5-gallon pail.

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

The standard Seal & Cure sealers used by builders are based on acrylics or wax. 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.

Other curing methods: The traditional moist-curing using burlap and misting for at least 7 days is labor-intensive. Misting may leach out lime and alkalis, causing laitance in lower spots on the surface. Covering new concrete with plastic or paper sheets usually leaves marks, discoloration, and laitance. Chemical curing compounds expose workers to high levels of VOCs.

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, i.e. laitance. This 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.

RadonSeal 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.

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 floors 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.