Both caulks and sealants are intended to fill gaps, but they perform differently. Learn about the differences between the two, and discover their individual properties, applications, and chemistries.
Caulk is an old boat-building term; sealant originated in home building. Some manufacturers use caulk as an all-purpose term and sealant to describe their high-performance products. Most often, the terms are used interchangeably.
The main difference between a caulk and a sealant is elasticity1. Sealants are made from flexible materials, making them ideal for areas prone to expansion and contraction1. They are also somewhat waterproof, while not all caulks are. Some industry experts do not differentiate between the two; but they do distinguish between adhesives and sealants.
Caulks are fairly rigid when dry, and are intended for use in areas with minimal expansion and contraction1. As stated, they are not necessarily waterproof, so they are a better fit for use indoors.
They are not intended for areas exposed to high moisture levels, such as around kitchen sinks and in bathrooms. In building construction, a material that has the adhesive and cohesive properties to form a seal (ASTM C 717-07a) is known as a sealant.
One- and two-part systems
There are one-part and two-part systems. One part cures with the interaction of the environment (moisture) or liberation of a component. A two-part system involves a physical mixing immediately before application and a chemical cure between the 2 constituents.
Applications and chemistries
Water-based caulks are the easiest with which to work because they apply without difficulty, are paintable, have little odor and clean up with water. They effectively fill gaps in baseboard and trim, as well as interior window and door frames.
Since they are water-based, their application is dependent upon the weather with respect to temperature for interior use. For latex caulks, the ideal curing conditions are above 40° F (4° C). For exterior use, surface temperature is critical at application and during the full cure cycle to ensure optimal performance.
Vinyl latex caulks
Within this category, there are sub-types of caulks with specific characteristics. Vinyl latex caulk has a useful life of around five years and is most suitable for small cracks in baseboards and small gaps around windows. Vinyl latex is non-flammable and paintable but is not very flexible. It hardens over time due to its vinyl chemistry, which lightly cross-links. It can also yellow somewhat over time.
Acrylic caulks are paintable and can be cleaned with water. However, their limited elasticity makes them susceptible to cracking in areas that experience large temperature swings. They are more durable than vinyl acrylic caulks.
Acrylic caulk adheres to most surfaces and is best used on wood and masonry. It can be painted shortly after application and is available in pigments as well, allowing it to match many surfaces. It remains effective for 10 to 15 years, however, it is not recommended for an area that is subject to excessive water collection such as tubs or sinks. Acrylic caulk is flexible and maintains that flexibility over time.
Tub and tile caulk
Tub and tile caulk is a specialty performance caulk. It has an added mildewcide to protect against mildew growth in kitchens and bathrooms where moisture can be excessive. Many formulations are a combination of an adhesive and a sealant. Other products utilize Ag+1 ions for their antimicrobial properties.
Siliconized acrylic caulk
Siliconized acrylic caulk is a type of sealant that combines silicone with acrylic latex formulas for improved water resistance. This medium-performance, water-based caulk can withstand greater movement than acrylic latex. It can be used interiorly or exteriorly with good adhesion, even to glass and ceramic tile. This caulk also comes in a variety of colors as well as clear formulas.
Like the former water-based sealants, it applies easily (best applied in temperatures above 40° F), is non-flammable, paintable, mildew-resistant and cleans with water. It can endure moderate temperature changes and has a total life expectancy of about 25-35 years.
The flexibility of silicone sealants allows them to keep a watertight seal, even in areas subject to wide temperature swings. They are not paintable, and over-applications must be cleaned with a solvent or scraped off after setting. Strong fumes may result when silicone sealants are freshly applied because of the liberation of acetic acid (acetoxy) or amines (amine and aminoxy), depending on the chemistry.
They are good for use around bathtubs and sinks because they are water resistant with excellent adhesion to smooth surfaces such as metal, glass and tile. They also resist mold and mildew growth. Silicone sealants do not generally adhere to masonry and do not adhere well to wood. However, they remain flexible after curing and are not affected by UV sunlight.
The following is a typical silicone sealant formulation, provided by Gelest2, based on many of their raw materials:
|Polydimethylsiloxane, OH terminated 50,000cps||65.9||Polymer|
|Polydimethylsiloxane, trimethylterminated, 1000cps||20||Plasticizer|
|150 sq.mfg surface area fumed silica||8||Filler|
Contrary to some beliefs, most sealants have reasonably good resistance to UV sunlight in either clear or pigmented products. However, there can be a misconception that clear sealants resist UV poorly. In actuality, the UV radiation mostly passes through and degrades the interface of the sealant and substrate, which can result in sealant adhesion failure.
Polyurethane foam is a sealant used for a variety of jobs, most often around electrical outputs, pipe penetrations and in large voids or openings where the elements can breach a structure. It expands to fill gaps, holes and voids, and is good for insulation purposes.
Polyurethane foam also comes in different expansion rate formulas. It is easy to apply, cures quickly, is paintable and offers good adhesion. It is not overly resistant to UV and should be protected by painting.
Butyl rubber sealants
Butyl rubber sealants are solvent-based with a life expectancy of only two to ten years due to their inherent chemistry. However, they are a good choice for sealing against water in lap joints, such as gutters and for metals and masonry, as well as outside for chimneys.
They are the best waterproofing sealant for below-grade applications, such as foundations.
Synthetic rubber is a relatively new caulk category. It is perhaps the most flexible product on the market. Synthetic rubber cures clear and is ideal for exterior joints that typically expand and contract. It can be applied in adverse weather conditions (wet and cold) and stretches and recovers easily without breaking.
It’s also great for use on roofs, wood siding and joints that frequently show movement.
Modified silicone polymer sealants
Modified silicone polymer sealants deliver excellent performance on vinyl, fiber cement, aluminum and wood siding. They combine the best characteristics of polyurethane, silicone and water-based products, offering permanent flexibility.
They also can be applied in wet weather, low temperature applications and around exterior windows, doors and vents. Modified silicone can be painted with latex paint. They are one of the more expensive choices of sealants.
Some of the critical and required performance testing includes the following:
- ASTM C834 and C920 compliant sealants
- ASTM C794 Adhesion-in-Peel
- ASTM C719 Cyclic Movement
- ASTM C793 Low Temperature Flexibility
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Additional information can also be found at https://www.adhesives.org/home.
- Merritt, Cam. What is the Difference Between Caulk & Sealant https://www.hunker.com/12255727/what-is-the-difference-between-caulk-sealant
- Flacket, Dave. One Part Silicone Sealants, https://www.gelest.com/wp-content/uploads/18One.pdf
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15 Responses to “Discover the flexibility of caulks and sealants”
Great article on caulks and sealants. No wonder I never know which one to use for a project. Do you have a recommendation to seal a 0.75 inch gap around the decking of a pool and the pool itself?
If it is above the water’s surface, a silicone caulk may be employed, but you must ensure that both surfaces are adequately dried before application. What is your concern?
I am decidedly jealous of your qualifications and at the age of 70 i am too old to go to university and start learning all you already know.
However, i wonder if you would be so kind as to share a tidbit of wisdom with me regarding the flexibility of pure acrylic polymer?
i am formulating a transparent coating for waterproofing but when applied thicker than1 to 2mm the formulation cracks. It appears there is noone who can solve this challenge?
Could you recommend an additive to stop this cracking?
Many thanks in advance.
Thank you for the kind words. I’ve moved around a lot in the industry and had the opportunity to learn a lot of different things.
You actually have provided insufficient information to answer succinctly. Unless it is 100% solids, any caulk will shrink somewhat, and in your case it being clear does not help. Normally, pigments and fillers increase solids and decrease shrinkage. Normally, even a clear caulk might contain micro talc that allows bridging as it cures. You also didn’t mention the glass transition or your temperature, so you may need to add coalescent or plasticizer depending on the chemistry. I hope that helps somewhat
What caulk do you recommend for filling in gap between window and wood trim? It is exposed to extreme weather – heat and cold. Previous 3 caulks failed within 6 months or less. Should I try synthetic rubber? Do I need to remove the old caulking?
Kevin, unless you’re only spending a dollar on caulk, it shouldn’t fail in six months. You don’t say how wide the gap is, how weathered the wwod is, and what the window is attached to on the other side. Ideally, a siliconized caulk that claims 25 years probably will last that long. But, you must remove the old caulk, make sure that you’re not trying to fill a gap wider than is recommended, and wash down removed old caulk.
I am imagining the worst case scenario, which is trying to caulk over weathered caulk, and trying seal a gap 1″ or more, binding wood which is not sound, to a window frame that is also not sound. Some people treat caulk as something you put down and don’t worry about anything else. If you follow the directions, and follow the advice, it will last more than six months. If you don’t intend painting it, you can use 100% silicone caulk, which may run you five dollars to six dollars a tube. But again you have to follow the directions and make sure all the surfaces are sound.
We need to reseql a glass solarium room for leaks. Old caulk or sealant is still preseent. What is the best method to reseal after haing pressure washed down the previous sealant surface.
You’ve supplied insufficient information. What type of caulk or sealant is present now? I can’t imagine pressure washing a solarium inside a building so that has me a little confused. Assuming that it is not silicone or siliconized, you could use something to patch what is there. I’m not sure what you mean by reseal. Theoretically, you should remove all of the caulk or sealant and then replace the sealant that was there with a quality silicone. The warranty on those are a minimum of 10 years.
Great Article..!! Thanks for sharing such great information about caulks and sealants. You have shared almost all the thing which is useful for every homeowner who is thinking about it. You have mentioned all the types of sealants are very useful to fill gaps and cracks as well as seal joints and seams. We like all the points that you have explained. We recently have done a post about types of sealants (Here is the link: https://gtsealants.co.uk/blog/sealant-guide-types/ )
Sorry for the link but We thought it might be helpful.
I am installing 12 inch wide solid oak treads on framing for an interior residential stairway. I am using 8 3-inch screws per tread but to remove any possible squeaking, I would like to add a sealant where the tread sits on the framing. The bottom of the oak tread has a coat of polyurethane. The underlying wood framing is unfinished.
Because the treads will move with changes in seasonal moisture, I am looking for a sealant with very good elasticity. And because I am installing 9 treads at once, I would like to have an hour before the sealant grabs.
My thinking was to use a silicone but possibly Lexel (which I believe is a synthetic rubber) would be better bc of elasticiity. Your thoughts?
Thanks very much.
Thanks for the link. We have a limited amount of space and number of words that we use, and we are educating and not marketing. Still, your link is useful.
You know that you probably will get creaking from the board themselves, and not necessarily wood on wood. The expansion and contraction of wood itself will cause it to shrink and expand and possibly crown. Still, the thoughts you had on the types of sealants to use and for the reasons that you have explained are sound. There are also elastomeric paints that aren’t as viscous as a sealant that are as flexible and much less expensive. Your thinking is solid so I would just go with the sealants.
Thank you for sharing this information with us, it’s a fantastic post. 3M marine adhesive sealant is also a nice quality in my opinion. This 3M product will never let you down if you’re looking for a fast-curing, permanently bonding marine sealant.
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