What is Caulk? Learn How to Seal Gaps and Cracks

What is caulk?

Caulk is an elastic substance that can be moulded into crevices to seal or fill the gap between two surfaces. Every building and home has gaps and cracks in it, allowing airflow to penetrate. Caulk is the sealing agent that stops this.

It is used in bathrooms, kitchens, floors, roofs, windows and ceilings — anywhere two edges join. The classic examples are the joint between tiles and a benchtop in a bathroom, and the joint between wood and metal in a window.

Caulk plays a crucial role in maintaining the integrity of your living spaces by preventing airflow, water leakage, and the intrusion of dust or insects. Sealing unwanted cracks or holes in your home is a straightforward way to improve its warmth and security.

What caulking is for

The purpose of caulking is threefold:

• Thermal insulation. Gaps let conditioned air out and outside air in. Closing them is one of the cheapest efficiency improvements available in an existing house.
• Stopping water leakage or penetration. This is the load-bearing one in a wet area, where water that gets behind a surface causes damage you cannot see until it is expensive.
• Closing gaps where dust, air or insects may enter.

Those three purposes explain why the product range is as varied as it is. A joint that only needs to stop draughts has different requirements from one that has to stay waterproof under daily wetting, and buying the wrong one is the usual reason a bead fails.

Why "elastic" is the key word

The definition describes caulk as an elastic substance, and that elasticity is what separates it from a filler.

The two surfaces on either side of a joint almost never move together. Timber swells and shrinks with humidity; metal expands and contracts with temperature; a building settles. The gap between them is therefore not a fixed dimension — it is a gap that changes size, every day, in a small way.

A rigid material in that gap cracks. An elastic one stretches and recovers. This is why sealant technical data sheets publish a figure called joint movement capability: most Selleys silicones are rated at ±25%, meaning the bead can accommodate the joint widening or narrowing by a quarter of its width. Brick & Concrete Silicone is rated ±50%, with 960% elongation at break and 89% elastic recovery, because masonry joints move more than most.

The same idea appears in the building term expansion joint — a gap deliberately left to allow expansion to occur in a structure, mainly in masonry walls, pathways and driveways. Those are always filled with a flexible sealant, never a rigid one. There is more of this vocabulary in the glossary.

How caulk gets into the gap

A caulking gun is the most common tool used to apply caulking sealants to the material that requires it. The product comes in a cartridge, the gun's plunger pushes it out through a nozzle you cut to the width of the joint, and you gun forwards along the joint pushing the product in as you go.

That last part — pushing it in — is the technique that matters. A bead laid across the surface of a gap has almost no depth of bond and peels off. A bead pressed into the gap grips both faces and can flex with the joint. See how to use a caulking gun for the full method.

Caulk, sealant and gap filler

These words overlap, and the product families behind them differ in ways that matter:

• Silicone sealants are flexible, waterproof and durable, and suit anything wet or exposed. They are generally not paintable — across the Selleys range, painting must be completed 7 days before the sealant is applied.
• Water-based gap fillers are paintable, easy to clean up with water, and suit interior joints around skirting boards, cornices and door frames. They are less durable in wet conditions.
• Expanding foam fills large, awkward voids and cavities, and hardens once sprayed. It is easy to use too much, because it keeps expanding after you stop.

The full breakdown is at types of caulk.

Dry and cured are not the same thing

This is the single most useful thing to understand about caulk before you use it.

Caulks and sealants can typically be dry to touch within about half an hour. That is a surface skin, not strength. Silicone sealants cure from the outside inwards at roughly 2 mm per 24 hours, so a bead pushed properly into a 6 mm joint is still curing three days after it looked finished.

Two consequences follow. First, tool the bead early — most Selleys silicones want smoothing within 5 to 10 minutes of application, before a skin forms, because smoothing a skinned bead tears it. Second, respect the cure time before loading the joint. Some products are engineered to shortcut this: Selleys No Mould is waterproof and shower ready one hour after application, though full cure is still at least 48 hours and the bead must not be touched until then.

Where caulk is worth the effort

Caulking is most often found between windows, sideboards and wet spaces like kitchens and bathrooms. Damp areas require a sealant that is mould resistant and watertight, such as a silicone-based caulk.

Beyond that, exterior windows and doors are worth sealing to keep out draughts, water and insects, and caulk can support the installation of wood panelling or fill holes and cracks in drywall. For a room-by-room view, see where can you caulk?

Common questions about caulk

What is caulk made to do?

Seal or fill the gap between two surfaces, to provide thermal insulation, stop water leakage or penetration, and close gaps where dust, air or insects may enter.

Is caulk the same as sealant?

The terms are used interchangeably for the same class of material. Selleys' own range is described as caulk and sealant products.

What is the difference between caulk and a gap filler?

Broadly, silicone caulks are flexible and waterproof but not paintable; water-based gap fillers are paintable and easy to clean up but less durable where there is moisture.

Can caulk be painted?

Water-based fillers can, after their stated paint-over time. Silicone sealants cannot — painting has to be completed 7 days before the sealant is applied.

What tool do I need?

A caulking gun is the most common tool for applying caulking sealants.

How big a gap can caulk fill?

It depends on the product. Water-based No More Gaps fillers specify gaps no larger than 5 mm deep and 10 mm wide to avoid paint cracking. Larger voids call for expanding foam, and deep joints may need a backer rod behind the sealant.

How long does caulk last?

It depends on the product and the conditions. Several Selleys silicones carry a 25 year guarantee against cracking, crumbling or peeling away when used as directed for private use within 12 months of purchase.

Does caulking really save on energy bills?

Sealing gaps stops airflow, which is what thermal insulation depends on. Draughts, moisture and insects can enter through gaps small enough that you would not notice them.

Can I caulk over old caulk?

No. New sealant may not adhere well to old cured sealant, and where mould is involved an established colony can penetrate a new seal even if the new product contains mould inhibitors. Remove the old bead first.

Which caulk should I use in a shower?

A wet area silicone — see the wet area silicone range.

Where can I read more?

Types of caulk, where can you caulk?, and how to use a caulking gun.