Construction Design-Caulking 101-Sealant Chemistry

Part 3 of the caulking 101 series.

You’ve done the work, installed that new bathroom vanity and countertop.  You’ve got the plumbing hooked up and tried out the faucet, yep, everything works.  All that’s left is to seal the countertop to the wall, that finishing touch that completes the installation.  Is that bead of sealant simply cosmetic or is it to prevent water from flowing behind the countertop and vanity possibly causing damage?  Do you use a cheap caulk from the hardware store?  Maybe some silicone?  Which product is right for this job?

Probably the most difficult decision for any caulking job is knowing what to use when and where.  With so many choices, walking down the caulking aisle in any hardware store can make your head spin.  With this post, we are going to talk about the chemistry of sealants, just the basics, enough information so you can make an informed decision the next time you need to choose a sealant.

There are several different formulas of sealant, water based (also known as latex), solvent and synthetic rubber, butyl, polyurethane, silicone, and polyther (STPE).  Let’s get started.

Water-Based (also known as latex or acrylic)

A water-based formula is typically the least expensive of all sealants.  With that being said, there are two distinct types of water-based caulks, the less expensive “painters caulk” and the high-performance acrylic caulks designed with maximum movement and better adhesion and cohesion properties.  Both product categories cure through the evaporation of water in the product which results in a shrinkage rate of one-quarter to one-third of the sealant.  Application temperatures range from 40°F-120°F.  Because of the low odor and minimal volatile organic compounds (VOC’s), water-based sealants are a good choice for indoor applications, they are also tooled easily.  One of the best attributes, a little soapy, warm water cleans up your tools and any caulking messes.  The better acrylic water-based products work well with wood, masonry and concrete, vinyl and aluminum, but their use with polyethylene and polypropylene should be avoided.  Sashco’s Big Stretch and Clean Seal are examples of high-quality acrylic sealants.

Solvent Based & Synthetic Rubber

A solvent based sealant will use some sort of solvent, (dah) usually mineral spirits, as one of the main ingredients.  Solvent based sealants cure much like water-based products, the solvent will evaporate as it cures causing some shrinkage, cure times can take up to one week.  Synthetic rubber sealants, which are also solvent based, are very flexible and will return to their original cured shape and size when stretched.  Application temperatures are from 0° to 120° and many can be applied to wet surfaces, some formulas are designed for roof repair, even when it’s raining.  Synthetic rubbers have higher VOCs and are recommended to be used outdoor or in well-ventilated areas.  These sealants bond well to most common building products but should not be used with polystyrene, polypropylene, polyethylene, silicone and waxed surfaces. Sashco’s Lexel and Through the Roof! are synthetic rubber sealants.

When water and solvent based caulks shrink, the facing of the caulking concaves, creating an hourglass shape.


Butyls are some of the oldest “modern” sealants in use, they are a solvent-based product that, similar to the acrylic and synthetic rubber formulas, use evaporation to cure.  This evaporation results in shrinkage.  Cure rates typically take more than one week with some butyl sealants never fully curing, they remain soft and sticky throughout their service life.  Application temperatures range from 20°F to 100°F and they bond well to most common building materials.  Butyls tend to be sticky and stringy, difficult to tool and are often used in air sealing, sound proofing and sometimes in below grade applications.  Because of their solvent base, they tend to affect air quality and are best used outside and well vented areas.  Tremco Acoustical Curtainwall Sealant (also known as black death) and Manus-Bond 50-A are examples.

Manus Bond sealing polyethylene sheeting acting as a vapor retarder/air barrier to wood framing. A common detail in my very cold climate.


Polyurethane sealants are a moisture cure product, the sealant requires some moisture from either the air or substrate to start the curing process, which can take up to 7 days to fully cure.  Application temperatures range from 32°F to 100°F.  Polyurethane sealants have great adhesion properties but tend to cure harder and have less movement capabilities than other sealant chemistries.  Polyurethane can be painted but is sensitive to prolonged UV exposure.  Polyurethane can be a problem for people that have sensitivities to urethane chemicals, specifically isocyanates, PPE and applications in well vented areas is recommended.  This sealant works well with concrete and masonry products, metal, stone, vinyl and wood.  Sikaflex from Sika and Loctite’s PL Window, Door and Siding Polyurethane Sealant are two examples.


Silicone sealants have been used in the construction industry since the 1930’s.  This sealant bonds well to glass, plastics and other non-porous materials, but bonds poorly to itself.  Old silicone along with any silicone residue should be completely removed before reapplying.  Modern silicones cure with moisture, taking up to 24 hours or longer to fully cure.  Pure silicone will not accept paint.  Application temps range from 20°F to 120°F.  Titebond 100% Silicone Sealant and GE’s Supreme Silicone Kitchen & Bath Sealant are two examples.


Also called sily-terminated polyether (STPE), these sealants have been used in Japan for years, but have recently seen an increase in popularity in both Europe and North America.  Some STPE’s are also called fluid applied membranes and can be used to create a window pan flashing or a thin, tape-like seal for joints.  STPE’s cure by a reaction with moisture, either from the air or substrate to which they are applied.  In some cases, the cure time can be decreased by misting with water.  Application temps range from 0°F to 120°F.  STPE’s bond well to most building substrates.  Examples are Prosoco’s Fast Flash and Huber’s Zip System Liquid Flash.

Zip System's Liquid Flash is an example of an STPE sealant.
Zip System’s Liquid Flash is an example of a STPE sealant.

We’ve covered the major formulations of caulks and sealants.  What I’ve been really curious about is why Sashco concentrates on latex and solvent based products and has not moved into producing any of the other formulations?  This is the question I recently asked Nathan Ferraro, one of Sashco’s staff chemists.  His reply was:

“We’ve looked at a bunch of other chemistries, and it’s our opinion that anything that dries by reaction (instead of evaporation) is going to be tougher (and therefore take less movement capability) than something that dries by evaporation.  All the urethanes, STPE’s, Hybrids, etc… just don’t move well.  We prioritize durability over anything else.”

Sashco has adopted the acronym IYKYK, If You Know, You Know.  Now you know the different chemistries in the world of sealants and why Sashco makes what they make.  Hopefully that walk down the caulking isle at the hardware store won’t be quite so daunting.

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