Urethane or silicone as sealant?

Ryan Fox

My question focuses on sealing around penetrations of the slab and foundation walls.

The Pennsylvania mitigation standards states in paragraph 15.5 that sealing cracks and penetrations of the slab and foundation walls shall use "caulks, and sealants designed for such application." It goes on to recommend urethane sealants "because of their durability."

Most often I do see what looks to be urethane around the vent pipe penetration of the slab, though occasionally I see some that are sealed with silicone. I've assumed that urethanes are the way to go when sealing around the pipe through the slab, but then I did some google searching.

Apparently, silicones in general are much longer lasting than urethane sealants. Urethane is said to last around 5 years, maybe a little more, while silicone is supposed to be good for up to 20 years. Apparently the organic nature of urethanes makes them more susceptible to UV light. I also read that silicone hold up better under more extreme temperatures compared to urethane. Granted, I don't expect UV exposure is a big problem in most basements, but even so, everything I've read so far seems to point to silicone as being the more durable, longer lasting sealant. So is there a reason you wouldn't want to use silicone around penetrations of the slab and foundation walls?

Is it because of adhesion properties? Urethane sticks better to concrete than silicone does? What if you cleaned the area around the penetrations before applying silicone?

I'm just wondering what the working professionals think about this and what their knowledge and experiences tells them is the best approach.

Michael Kitto

Is urethane less permeable to radon penetration than silicone?

Bill Brodhead

Ryan. I have visited homes I sealed center floor cracks 20 years ago with urethane caulk and it is still there in great shape. Silicone often has poor adhesion to concrete and can be scuffed off with any traffic. In a basement UV light degradation should not be an issue. I have seen degradation of urethane caulk in Guam for outside sealing from the intense sunlight outside. There are many grades of silicone and I'm sure there are grades that have better adhesion to concrete but most of us will grab the lowest priced tube. Urethane is also paintable while silicone is not. Our general recommendation is use silicone for sump pit covers because you are making a seal that will latter need to be removed. Use Urethane to create a permanent seal. There maybe other products that adhere to concrete but may not have the elasticity of urethane. There are also issue with the fumes from urethane are very toxic. Maybe other more knowledgeable mitigators can better address the products that are available.

Jay Bauder

We use urethane for suction points, control joints and any other places that we want to be sealed permanently. We use 100% silicone for sump covers and any other locations where future access may be required. 100% silicone is easily pulled off a Lexan sump pit cover. It comes off like a big rubber band. If you're trying to unseal a sump lid sealed with urethane, you'd better have another cover with you, and do it before the children get home from school!

I believe the manufacturers data stated their urethane "bonds tenaciously to most substrates". I'm here to confirm that. I had a home back in the late 1980's that I had covered the basement access opening to a cross vented crawlspace. I used the "radon barrier" membrane that was black on one side and white on the other. It had a ton and a half of tensile strength per square inch. I sealed it as part of some waterproofing work because the owner complained about a musty smell coming from the crawlspace. I cut a piece of the membrane several inches bigger then the opening, caulk around the opening, and pressed the membrane flat onto the wall. Several years l later they were selling and I was called back to install a radon system. In order to install a membrane on the dirt crawlspace floor, I had to remove the seal. The urethane actually pulled the face of the concrete block off where it was attached.

I would expect both products to be impermeable to anything that doesn't degrade it. As far as fumes, I like the solvent glue the best!

Kevin M Stewart

Jay's last line suggests to me that everyone should keep in mind the following:

Everyone who uses products that may pose occupational health and safety hazards should have access to Safety Data Sheets for those products and follow the manufacturer's instructions for ventilation, use, personal protective equipment, etc.. Many solvents are neuro system toxins, and isocyanates, for example, are sensitizers (a family member experienced this). Section 8 of the SDS includes details on Health and PPE. Also keep in mind that there are generally many chemical components of products that may be referred to colloquially by a single name such as "silicone" or "urethane", so it's good to pay attention to the full ingredient list.

I'm sure that mitigation practitioners reading this thread would have recommendations regarding their hazard communication procedures and what is expected among their staff to ensure safe working practices.

Jay Bauder

Thank you Kevin. MSDS are available on the manufacturers website or you can pick them up at the store where you are purchasing the materials. I've found that putting them in plastic sheet protectors in a 3 ring binder behind the drivers seat with the first aid kit works well, and adding new products is simple. Having them on the van is handy if the homeowner has questions.

Henri Boyea

See Jay Bauder's first comment above^^. Silicone is the Post-it-Note of caulks: Sticks well to smooth, clean surfaces, and is easily removed.

Bob Wood

Good morning and Happy New Year from frigid Canada: I was having a discussion with my Dow corning sales rep and he challenged me to do some tear apart tests with his products and the caulking products that i was using. i was discussing low VOC products with him at the time. Caulk things together wood, brick, concrete, PVC to concrete, vapour barrier to vapour barrier Etc. and let sit for 30 and 60 days and then try to tear apart.
Well it became the best sales tool that he could have used. I was amazed to see that my go to poly urethane caulk literally fell out from between two pieces of 15 mil poly at 60 days, thank god we had always taped our seams as well as caulk.

I am now a die hard fan of low VOC silicone because I KNOW IT WORKS and it is cheaper (when i buy in bulk) it is also in sausage format so i have another tool that my handy homeowner does not have that's why he is paying me to do it. I recommend that you always go up a step and talk to manufacturers when you can (they do all this research) what works where and where not.

Bruce Decker BGIS

Clean and dry surfaces are also key. I have seen urethane peel off with zero effort at the cold joint and saw cuts. Simple issue of builder not following instructions.

Robert Burns

I have been using urethane for permanent crawlspace membrane sealing and silicone for sump lids since my first mitigation job in 1989. I have visited membranes sealed down over 20 years ago that are still sealed as tight as the day they were installed. I did have a job where a homeowner purchased the membrane from me but then used urethane construction adhesive instead of urethane caulk. Within days the urethane adhesive became ridged and no longer adhered to the membrane. He resealed it with polyurethane caulk and had success.

Perhaps that could explain Bob Wood's experience. Does anyone have information on the difference between polyurethane construction adhesive and polyurethane caulk (that is meant for concrete crack sealing)?

Randy Weestrand

Check out Bill Brodhead's study at wpb-radon.com/pdf/Membrane%20Adhesive.pdf
Always check your adhesive on both sides of the poly. For example: There's a very nice reinforced blue and white poly that we run into occasionally. The white side holds poly caulk well, but the blue does not. I've been told that some Rufco poly is Corona treated on one side, which allows poly caulk to adhere to it.

Robert Burns

wpb-radon.com/pdf/Membrane%20Adhesive.pdf — Randy Weestrand

Urethane or silicone as sealant?

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