Slab Edge Issue

Slab edge 1

Slab edge, 3″ of XPS covered by the vapor barrier.

Slab edge 2

Slab edge, in front of the south facing windows.
The edge is double width here due to a concrete ledge.

I was determined that insulating the slab would take precedence over esthetics. Despite Alberto’s misgivings, I was convinced I could solve this slab edge problem by covering the edge of the extruded polyurethane insulation in some manner after the floor was laid. I didn’t really think about the opening to the soils that was created all around the edge. The green advice that I read said that minimizing this insulation at the edge of the slab was short-sighted. The edge is one of the most leaky areas of the house. But they didn’t have a good solution for covering the area after the slab was poured. So besides staining and sealing the floor, I now have to seal the edge of the slab in a way that will exclude radon gas and hopefully be reasonably esthetic too.

The product I found after much searching online was Sanitread. They make a three part system for waterproofing and radon sealing a basement. However they have a rubberized caulking product for filling holes and cracks in foundation walls called LRB (Liquid Rubber Base) and a thickening agent called TAV.

This product is considered low in VOC. (83 g/l) but still requires good ventilation during application. It is a quartz silicate material that reacts with water to form a runny liquid, (like Elmer’s glue) that can be thickened with TAV to provide a good caulk consistency. The manufacturer claims the material remains elastic and creates a permanent bond to concrete. This is an expensive product though, according to the crack filling calculator, I would need 13 gallons of LRB and enough thickener to reach the correct viscosity to fill 265 perimeter feet about 3″ wide. If I could get by with 10 gallons and 5 gallons of thickener, the cost of the material would be about $1000 plus shipping. But I may have to coat the edge with the Permaflex liquid too. I have ordered a sample pack that will answer some of my questions about how to use the product and whether it will fill in the edge.

Other companies that offer radon sealing caulks seem to limit their products to narrow crack solutions. These typically come in a tube to be used in a caulking gun and are not meant to fill in a wide flat area like the slab edge.

Emecole makes a radon caulk for narrow cracks. Some of their products are availlable to the do it yourself types directly from their website.

Foundation Armor makes a two component joint filler that is not specifically formulated to exclude radon.

Titebond makes a similar radon sealant. It is low VOC and sold on green building supply sites.

Smalley and Company has a location in Denver and they sell a large variety of caulks and sealants, some of which may be less expensive but do the same job as Sanitred. But their list of products is long and either each product site would have to be investigated or the experts at Smalley and Company contacted to figure out which might fill this area and seal out radon.

There are also some spray on insulation products that might work on this area. A product called Ecoseal is available only to insulation professionals and sounds like it would be thin enough to just cover the edge of the slab. It is produced by Knaupf insulation. It is made to seal all joints at studs, rafters and joists before putting in batt insulation but it is also rated to block radon and is elastomeric so it will stay flexible. I filled out the form to order a sample so I hope I can try it out. There are two installers in the Denver area that have websites to find out more; Big  Al’s Insulation and RG Insulation

Update: Sanitred works! The 8 oz. of Liquid Base and 4 oz. of thickener yielded about 18″ or .3 sq. ft. of coverage. But its VOC is over 50 and the cost estimate after trying the sample is $1200 with shipping. It is definitely a reasonable consistency to spread over the 3″ gap and sticks well to both the cement slab and the wall. The substance creates a bond with these two sides and then spans the rest with an impermeable barrier. It can be covered with their Permaflex paint that would cost another $120 with shipping.

Sanitred Liquid Rubber Base

Sanitred Liquid Rubber Base-expensive, and not a very attractive option

I read in the EPA design documents for radon control that any polyurethane caulk is considered capable of excluding radon. An air barrier is essentially a radon barrier. So I began to investigate less expensive options. There are several acrylic and poly based roof coatings that I considered, but during that search Eternabond Roofing Tape was an option. I have experience with Eternabond on our RV and I realized that it would also span the area between the two concrete sides, has adequate stickiness to place over the PVC and foam, and will provide an air and water proof barrier over the gap. The Webseal tape is meant to be painted or coated over so I chose that in 4″ and 6″ rolls. There are approximately 180 lineal ft. of wall that has the 3″ or 6″ gap, the rest of the 265 ft. perimeter is interior walls that can be caulked with gunned polyurethane, so for less than $200 I purchased enough tape to cover the gap. My supplier for Eternabond was Big Rock Roofing Supply. UPDATE: No longer in business–check Amazon instead.

I spent quite a bit of time researching what to coat the tape with. (Although not at Smalley and Company). I decided to look for an elastomeric coating that is low or no VOC and I prefer a color that would blend in with the stained floor. I would have liked to use Rustoleum’s Sierra 2 part epoxy paint, but it did not come in brown and I could not find a kit of just one gallon of both parts online, at least not in the tile red color that appeared to be closest to brown.  Home Depot and Lowes do not carry this version of  Rustoleum’s epoxy paint in the Denver, Colorado area. So I decided to use a one part Duckback acrylic deck paint that is <50 VOC and had decent reviews. A gallon should provide at least two coats to cover the 50 square ft. or so of perimeter.

Here were the best candidates I found for the perimeter solution:

Square Ft. of Perimeter 50
Perimeter Treatment Amount in oz/ft Coverage in sq. ft. Total Total x Coverage Gal Cost VOC
Sanigard Sample 8 0.3 50 166.9 1333.3 10.4 $1200 Medium
Through the Roof Caulk 32 3 50 16.7 533.3 4.2 $268 High 190
Eternabond Webseal Roof Tape 50 50 lin. ft. 150 Plus remainder of 6″ roll on hand $276 No
Covered by:
Sierra Epoxy Paint 256 100 50 0.50 128.00 1.00 $150 No
Duckback Paint 128 100 50 0.50 64.00 0.50 $59 Low 50
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