Caulk glossary: 67 terms from acetoxy to width-to-depth

A

Acetoxy cure: A silicone cure chemistry that releases acetic acid as it reacts with moisture in the air, hence the vinegar smell. It cures quickly and bonds well to glass and tile, but the acid can corrode some metals and mark natural stone and mirror backings. Compare neutral cure.See also RTV.
Acoustic sealant (non-hardening sealant): A sealant that stays soft and flexible permanently, used to seal air gaps in soundproofed walls and ceilings: drywall perimeters, plates, and electrical boxes. It's meant to be hidden inside the assembly, not left exposed.Acoustic caulk on the types page · OSI SC-175 Acoustical
Acrylic latex (painter's caulk): The most common caulk on the shelf: a water-based acrylic that's easy to gun, cleans up with water, and takes paint well. Best indoors on trim and drywall. It shrinks somewhat as the water evaporates and doesn't belong in joints that sit in water.Acrylic latex on the types page · Latex
Adhesion: How well a caulk sticks to the surfaces on either side of the joint. Adhesion depends on the product, the substrate, and above all on prep: clean, dry, dust-free, and free of old caulk. When a bead peels cleanly away from one side, that's an adhesive failure.Compare cohesion.
ASTM C920: The standard specification for elastomeric joint sealants in the United States. It sorts sealants by type (single- or multi-component), grade (non-sag for vertical joints or pourable for horizontal ones), class (how much joint movement they handle, such as 25, 35, or 50 percent), and use (for example traffic or immersion). Seeing C920 and a class number on a tube is a quick sign of an exterior-grade sealant.Sikaflex-1a is one example of a C920-rated sealant.

B

Backer rod: A round foam rope pressed into a joint before caulking. It controls the depth of the bead, saves caulk, and keeps the sealant from sticking to the back of the joint so it can stretch freely. Choose a diameter slightly larger than the gap so it stays put. Closed-cell rod is the common choice outdoors.Backer rod and joint design · Three-sided adhesion · Bond breaker
Bead: A continuous line of caulk laid into a joint, and the unit of pride around here. A good bead is continuous, consistent in size, pressed into both sides of the joint, and tooled once.How to lay a perfect bead · Fillet bead
Bond breaker (bond-breaker tape): Tape or another material placed at the back of a joint that's too shallow for backer rod. Sealant doesn't stick to it, which prevents three-sided adhesion. Polyethylene tape is common.
Butyl rubber: A sticky, stringy synthetic rubber sealant that grips metal and masonry and stays pliable for a long time. Used on gutters, flashing, and metal roofing. Messy to apply and slow to cure.Butyl on the types page · DAP Butyl-Flex

C

Cartridge (tube): The familiar cylinder that loads into a caulking gun. The standard size is 10.1 fluid ounces (about 300 milliliters), with larger 28-ounce cartridges for big jobs and small squeeze tubes for touch-ups. Sizes vary by brand and region.How many cartridges do I need?
Caulk: As a noun, a flexible material used to fill and seal joints. As a verb, the act of filling them. The word goes back through Old North French to the Latin calcare, to tread or press, and the trade goes back to shipwrights driving fiber into the seams of wooden hulls.A sticky history of caulk · Oakum
Caulking gun (caulk gun): The hand tool that pushes caulk out of a cartridge. Each trigger pull advances a plunger rod. Smooth-rod guns give a steadier flow than older ratchet-rod guns, and a dripless mechanism stops the ooze when you let go.Choosing and using a caulk gun · Thrust ratio
Cohesion: How well a caulk holds together within itself. When a bead tears down the middle while both edges stay stuck, that's a cohesive failure, often a sign the joint moved more than the product can handle or the bead was too thin.Compare adhesion.
Construction adhesive: A thick glue in a caulk-style cartridge, made for bonding materials together (subfloors, paneling, trim) rather than sealing joints. Some hybrid products do both, but most construction adhesives are poor sealants and vice versa.MS polymer
Control joint: A groove cut or formed into concrete so the slab cracks along a straight, planned line instead of randomly. Sealing control joints with a flexible sealant over backer rod keeps water and debris out.Garage guide · Expansion joint
Cure time: How long a caulk takes to reach its full strength and flexibility, as opposed to how long until it's dry to the touch. Most caulks cure from the surface inward, so thick beads take longer, and cold or dry air can slow moisture-curing products. Tubes often list several times: skin or tack-free, paint-ready, water-ready, and full cure.Temperature, humidity and cure time

D

Dripless gun: A caulking gun that automatically backs off pressure on the plunger when you release the trigger, so caulk stops flowing instead of oozing out of the nozzle.
Durometer (Shore A hardness): A measure of how hard a cured rubbery material is, reported on the Shore A scale for sealants. Lower numbers are softer and generally stretchier; data sheets list it for comparison.

E

Efflorescence: A white, powdery mineral deposit on concrete, brick, or block, left behind when water carrying dissolved salts evaporates at the surface. It's a sign water is moving through the masonry. Caulk won't bond well over it; brush it off and address the water.Basement guide
Elastomer: A material that can stretch substantially and return to its original shape, like rubber. Elastomeric sealants (silicone, polyurethane, hybrids, and elastomeric latex) are what you want in joints that move.Elastomeric latex · Joint movement
Elastomeric latex: A water-based caulk formulated to stretch and recover far more than ordinary acrylic latex, while staying paintable and easy to clean up. Popular for siding, trim, and cracks that keep reopening.Elastomeric on the types page · Sashco Big Stretch
Expansion joint (isolation joint): A gap built into a structure (between slabs, or where a slab meets a wall) so materials can expand and contract without cracking. These joints move, so they need a high-movement sealant, usually over backer rod.Control joint

F

Fillet bead: A triangular bead laid into an inside corner, such as where a tub meets the wall. Tooled slightly concave, it sheds water and bonds to both surfaces.Hourglass profile
Firestop sealant (fire-rated sealant): A sealant tested to help keep fire and smoke from passing through openings in fire-rated walls and floors, such as pipe and cable penetrations. It counts only when installed as part of a listed, tested system that meets local code. Many are intumescent.3M Fire Barrier CP 25WB+
Flashing: Thin metal or membrane pieces that direct water away from joints in a roof or wall: around chimneys, vents, windows, and roof valleys. Sealant helps flashing do its job, but it isn't a substitute for flashing.Roof & gutters guide

G

Glazing putty (glazing compound): A traditional compound, often linseed-oil based, that holds glass panes in wooden window sashes. It stays workable for a long time, needs to skin over before it's painted, and is a different product from caulk.Caulk vs. sealant vs. grout vs. putty
Grout: The cement- or epoxy-based material that fills the spaces between tiles and cures hard. Grout belongs in the field of tile; caulk belongs at inside corners and where tile meets another surface, because those joints move.Caulk vs. grout · Sanded caulk

H

Hourglass profile: The ideal cross-section for a sealant joint: thicker where it bonds to each side and thinner in the middle, so it can stretch without tearing. You get it by tooling the bead over backer rod.Backer rod and joint design · Width-to-depth ratio
Hybrid sealant: Informal name for sealants that combine traits of silicone and polyurethane, most often MS polymer products.
Hydraulic cement: A fast-setting cement that hardens even with water present, used to plug active leaks in concrete and masonry. It's rigid, so it isn't a joint sealant for anything that moves.Hydraulic cement on the types page · Quikrete Hydraulic Water-Stop Cement

I

Intumescent: Describes a material that swells when heated. Intumescent firestop sealants expand in a fire to close the gap around pipes and cables.
Isocyanate: A family of reactive chemicals used in some polyurethane sealants and foams. Uncured isocyanates can irritate skin and airways and can cause respiratory sensitization, so ventilate, wear gloves, and follow the safety data sheet. Cured product is considered inert. MS polymer sealants are isocyanate-free.SDS

J

Joint: The gap between two surfaces that a caulk or sealant fills. Joints are defined by their width, depth, and how much they move, and all three should drive your choice of product.Joint movement
Joint movement (movement capability): How much a joint opens and closes, and how much a sealant can handle, expressed as a plus-or-minus percentage of the joint's width. A sealant rated for 25 percent movement in a half-inch joint can handle roughly an eighth of an inch of stretch or squeeze. Exterior joints, siding, and slabs move the most.ASTM C920 · Elastomer

L

Latex: In caulk, latex means a water-based emulsion, usually acrylic. It generally doesn't refer to natural rubber latex, though anyone with a latex allergy should check the product's safety data sheet.Acrylic latex

M

Mildewcide: An additive that slows mold and mildew growth on the surface of a cured caulk, common in kitchen and bath formulas. It helps but doesn't make caulk immune, especially if water gets behind the bead. Mildewcide products are not for aquariums or food-contact surfaces.Why shower caulk turns black
Mineral spirits: A petroleum solvent used to clean up uncured solvent-based sealants, polyurethanes, and silicone smears. Use it with ventilation and away from flames, and test it on finishes first.
MS polymer (silyl-modified polymer): A moisture-curing hybrid sealant chemistry that's paintable, flexible, waterproof, low-shrink, and free of isocyanates, and that bonds to a wide range of materials. It costs more than traditional caulk.MS polymer on the types page · Soudal Fix ALL

N

Neutral cure: Silicone cure chemistries (such as oxime or alkoxy) that don't release acetic acid. Choose neutral cure for metals, natural stone, mirrors, and other surfaces an acetoxy silicone could damage.
Non-sag (gun grade): A sealant thick enough to stay put in a vertical or overhead joint. The opposite of self-leveling.

O

Oakum: Loose fiber, traditionally hemp or jute soaked in tar, that shipwrights drove into the seams of wooden hulls with a caulking iron and mallet. Plumbers once used it to pack cast-iron pipe joints. The ancestor of every tube on the shelf.A sticky history of caulk
Open time (working time): How long you have to tool a bead before it starts to skin. Heat, sun, and low humidity usually shorten it; work in shorter runs when it's hot.Cure time

P

Paintable: The caulk accepts paint once it reaches the time the label specifies. Acrylic latex, most polyurethanes, and MS polymers are paintable; 100% silicone is not. "Paintable" doesn't mean every paint on every schedule, so test first.Why silicone refuses paint
Plumber's putty: A soft, non-hardening putty used to seal sink drains, faucet bases, and strainers. It isn't caulk and shouldn't be used as a sealant elsewhere. Standard putty can stain some natural stone; stain-free versions exist.Caulk vs. putty
Polysulfide: One of the earliest high-performance synthetic sealant chemistries, still used today in some marine, aerospace, and insulating-glass work. You rarely see it in a homeowner's cart.A sticky history of caulk
Polyurethane: A tough, flexible, paintable sealant that bonds tenaciously to concrete, masonry, and wood. The workhorse for exterior and concrete joints. Slower to cure, messier to clean up, and some formulas contain isocyanates.Polyurethane on the types page · Sikaflex-1a
Primer (sealant primer): A liquid brushed onto a joint's sides before sealing to improve adhesion on porous or tricky substrates. Many sealants don't need one for typical home jobs; the product data sheet says when they do. Not the same as paint primer.

R

RTV (room-temperature vulcanizing): Silicone that cures at room temperature by reacting with moisture in the air, instead of needing heat. Nearly every silicone caulk on the shelf is RTV silicone, cured by either acetoxy or neutral chemistry.100% silicone

S

Sanded caulk: Caulk with fine sand mixed in to match the look and texture of sanded grout. It's made for tile joints where you want the caulk to disappear into the grout lines.Polyblend Sanded Tile Caulk
SDS (safety data sheet): The manufacturer's document describing a product's hazards, safe handling, protective equipment, and first aid. Every caulk has one, usually on the maker's website. Read it for anything with a strong smell.
Self-leveling (pourable): A sealant runny enough to flow flat on its own, made for horizontal joints in driveways, sidewalks, and garage floors. It will run straight out of a vertical joint. The opposite of non-sag.Sikaflex Self-Leveling
Shelf life: How long an unopened tube stays usable, often about a year or more for common caulks. Look for a date or batch code on the tube, and store it cool, dry, and above freezing.
Shrinkage: The loss of volume as water or solvent evaporates from a caulk during cure. Water-based caulks shrink noticeably and can leave a concave bead in deep joints; 100% silicone and MS polymers shrink very little. Backer rod reduces the problem by keeping beads shallow.
Silicone: A silicon-based rubber sealant that's highly flexible, waterproof, and resistant to UV and temperature swings, and that can't be painted. The standard for tubs, showers, sinks, and glass.100% silicone on the types page · GE Silicone 2 Kitchen & Bath
Siliconized acrylic (siliconized latex): Acrylic latex with added silicone for better adhesion, flexibility, and water resistance. Still paintable, still water cleanup, still not a replacement for 100% silicone in joints that sit in water.Siliconized acrylic on the types page · DAP Alex Plus
Skinning (skin time): When the surface of a fresh bead forms a thin film. After a caulk skins, tooling it drags and wrinkles the surface, so tool before it does. Skin time is often listed as minutes on the tube.Open time · Tack-free time
Spray foam (expanding foam): Polyurethane foam that expands to fill large gaps and holes, such as around pipes and window rough openings. It isn't caulk. Low-expansion versions are made for windows and doors, because regular foam can push frames out of square.Windows & doors guide
Substrate: The surface a caulk bonds to: wood, vinyl, aluminum, tile, concrete, glass. Tubes list the substrates they're made for, and a caulk that loves glass may hate polyethylene.Adhesion · Primer

T

Tack-free time: How long until a fresh bead no longer feels sticky to a light touch. It's usually a little after skinning and long before full cure.
Three-sided adhesion: When a sealant sticks to the back of a joint as well as both sides. Bonded on three sides, the bead can't stretch freely and tends to tear as the joint moves. Prevent it with backer rod or bond-breaker tape.Backer rod and joint design
Thrust ratio: A caulking gun's mechanical advantage: how much force reaches the plunger for each unit of squeeze on the trigger. Around 10:1 is plenty for latex; thick polyurethanes, adhesives, and cold-weather work go more easily with higher ratios, such as 18:1 or more.Choosing and using a caulk gun
Tooling: Pressing and smoothing a fresh bead with a finger, tool, or spoon. Tooling forces the caulk against both sides of the joint for better adhesion, shapes the profile, and leaves a clean surface. Do it once, before the bead skins.How to lay a perfect bead
Tripolymer (terpolymer): A solvent-based rubber sealant made from a blend of three polymers, known for gripping metal, vinyl, and weathered or damp surfaces. Sticky, stringy, strong-smelling, and very effective on gutters and siding.Geocel 2300 Tripolymer

U

UV resistance: How well a cured sealant holds up to sunlight without chalking, cracking, discoloring, or losing flexibility. It matters for every exterior bead. Silicone is especially UV-stable; many other sealants rely on paint to shield them.

V

VOC (volatile organic compounds): Carbon-based chemicals that evaporate from products as they cure and contribute to indoor air pollution and smog. VOC limits for caulks and sealants vary by region, and low-VOC products are widely available. Ventilate regardless.Reading the label

W

Weep hole: A small, intentional opening at the bottom of brick veneer or a window frame that lets water drain out. Never caulk a weep hole; you'd be trapping water inside the wall.Exterior guide
Width-to-depth ratio: The proportion of a sealant joint's width to the depth of the bead. A common rule of thumb for moving joints is about two to one (a half-inch-wide joint gets a bead about a quarter inch deep), within the minimum and maximum depths on the data sheet. Backer rod sets the depth.Hourglass profile