Unveiling Caulk Types: The Complete Guide

What are the different types of caulk, and which one do you need?

Caulks fall into four broad families — silicone, latex, expandable and butyl-rubber — and they are not interchangeable. Each is good at something the others are not, and picking on price rather than on type is the most common reason a bead fails within a year.

The short version: silicone for anything wet or exposed, latex for interior work that will be painted, expandable foam for large voids, and a weather-resistant product for outdoor joints in metal, concrete and timber. The rest of this page explains why, and what the practical consequences are.

Silicone caulk

Silicone is incredibly durable and flexible, and can also be water repellent, which makes it the best choice for windows and bathrooms. Silicone caulks withstand temperature changes with direct sunlight or water, and they are often used to seal gaps between two different materials.

That last point is the one worth dwelling on. Two different materials expand at different rates, so the gap between them changes size daily. A silicone bead accommodates that. Selleys silicone data sheets publish this as joint movement capability — typically ±25% across the range, and ±50% for Brick & Concrete Silicone, which also gives 960% elongation at break.

Acetic and neutral cure — the split within silicone

Silicone caulks divide again, by how they cure, and this decides which substrates they suit.

• Acetic cure silicones release acetic acid as they set — the vinegar smell you notice for the first day. They bond well to glass, glazed ceramics and porcelain, which is what most shower recesses are made of.
• Neutral cure silicones cure by reacting with atmospheric moisture without releasing that acid. They are what you need where the joint touches a mirror edge, natural or engineered stone, or a metal the acid could affect.

Both are 100% silicone; both are flexible and waterproof. The difference is entirely about what is on either side of the joint.

Where silicone falls down

It cannot be painted. Across the Selleys range, silicone sealants are not paintable, and their data sheets require painting to be completed 7 days before the sealant is applied. If the joint has to be painted, silicone is the wrong family — no matter how good it would otherwise be for the location.

Silicones also generally exclude permanent water immersion, aquariums, food and drinking water contact, and polyethylene, polypropylene and PTFE plastics. Check the pack's limitations before buying, not after.

See the wet area silicone range for bathrooms and kitchens, and Roof & Gutter Silicone for roofing, gutters and flashings.

Latex caulk

Latex caulk is a great choice for use on drywall, timber or masonry. Latex caulks are easy to use, easy to apply and easy to clean up — water clean-up rather than solvent. A significant benefit is that latex caulk can be sanded once cured, which silicone cannot.

The trade-off is durability. Latex caulks are not as durable as silicone, and their durability may be affected by moisture.

In the Selleys range the equivalent products are the water-based No More Gaps fillers, and their published figures show exactly what this family is for:

Product	Technology	Skin time	Paint over time	Joint movement
No More Gaps Interior Fast Dry	Water based	6 minutes	Gloss 10 min, semi gloss 0 min, low sheen 10 min, flat 100 min, ceiling 120 min	Up to ±10%
No More Gaps Bathroom & Kitchen	Water based	20 minutes	60 min (gloss, sheen, low sheen), 180 min (flat, ceiling)	±25%
No More Gaps Exterior	Water based	Skin before painting, approx. 30 min	Approx. 30 min; protect from water for 24 hours	—

Indicative data only, not intended for the preparation of specifications.

Two practical notes carried on all three data sheets. To avoid paint cracking, apply to gaps no larger than 5 mm deep and 10 mm wide, at temperatures above 10 °C — above 20 °C for Interior Fast Dry — and avoid applying paint too thickly. And allow 24 to 48 hours before painting with oil-based paints, with longer drying times needed under flat and ceiling paints because those are highly filled and do not flex as much when the sealant dries.

Expandable caulk

Just as the name implies, this type expands when sprayed onto the material. It is often referred to as expanding foam. It is great for larger cracks and gaps, and its sealing properties can also help with soundproofing a room.

The Selleys glossary describes it as a foam usually contained in an aerosol, that expands and hardens once sprayed from the container — useful for filling awkward gaps, cracks and cavities, with uses from insulation to adhesion to waterproofing depending on the type. It also carries a warning worth repeating: be careful, because due to its expanding nature it is easy to use too much.

That is the defining characteristic of this family. Expanding foam keeps growing after you stop spraying, and over-filling a cavity can bow a door frame or push a lining out. Fill roughly a third of the void and let it find its own volume.

See Selleys Pro-Trade Expanding Foam.

Butyl-rubber caulk

Butyl-rubber caulk is the type described as perfect for outdoor use. It has been designed to withstand high temperatures and to be water-tight, and it works well with most materials found outdoors — aluminium, metal, concrete, timber, sidings and gutters.

For those same exterior applications, the Selleys answer is a neutral cure silicone. Roof & Gutter Silicone is UV and weather resistant with a service temperature range of −60 °C to +150 °C, and its substrate list covers aluminium, steel and stainless steel, mild and galvanised steel, powder-coated steel, PVC, timber and plywood, fibre reinforced cement, bricks, concrete and masonry. Brick & Concrete Silicone covers wall, patio, floor, fencing and expansion joints with a −40 °C to 140 °C range.

One caution that applies here: Roof & Gutter Silicone's limitations specifically say to avoid contact with materials containing high levels of plasticisers, such as rubber and bituminous substrates. Mixing sealant families on the same joint is not a good idea.

Colours and finishes

The Selleys range of caulks and sealants comes in a variety of colours, including neutrals and products that dry clear. The range of colours available lets you match the other materials you are working with, and there are caulk products that can be painted to colour-match your space. A clear or neutral white sealant suits most applications.

Some concrete examples from the product data sheets: No Mould is supplied transparent or white; Wet Area Silicone in matt white; Brick & Concrete Silicone in grey; and Roof & Gutter Silicone in a Colorbond colour-matched range including Basalt, Black, Classic Cream, Cottage Green, Deep Ocean, Dune, Galvanized Grey, Grey, Ironstone, Manor Red, Monument, Paperbark, Shale Grey, Surfmist, White and Woodland Grey, plus translucent.

Drying and curing times

Caulks and sealants can typically be dry to touch within half an hour, but they require time to cure and set to ensure they are fully waterproof. Fast-drying products can set in less than 24 hours, depending on the humidity of your space.

The advantage of a fast-drying caulk is the ability to paint the seal earlier, which shortens the whole job. No More Gaps Interior Fast Dry, for instance, is paintable in about 10 minutes with gloss and low sheen paints.

Each product has different drying and curing times — read the labels for exact figures. That advice matters most with silicones, where dry-to-touch and cured are separated by days rather than hours: silicone sealants generally cure at around 2 mm per 24 hours, working inwards from the skin.

Choosing between them — a quick guide

• Shower, bath, basin, splashback: silicone, from the wet area range. Not paintable.
• Skirting boards, cornices, architraves, window and door frames indoors: a water-based gap filler. Paintable.
• Damp interior areas that also need painting: No More Gaps Bathroom & Kitchen — but note its own data sheet directs you to a wet area silicone for gaps around baths, sinks and showers where a waterproof seal is needed.
• Weatherboards, eaves, exterior windows and doors: No More Gaps Exterior, which is UV resistant and paintable.
• Roofing, gutters, flashings, metal: a neutral cure exterior silicone.
• Large voids and cavities: expanding foam, used sparingly.
• Masonry and concrete expansion joints: Brick & Concrete Silicone, rated ±50% movement.

Safety: one of these four families is in a different class entirely

This page compares silicone, latex, expandable and butyl-rubber caulks on performance. They are not comparable on hazard, and that belongs in the choice.

The silicones and the water-based No More Gaps fillers are mostly Warning-level products: eye irritation, and skin sensitisation (H317) on several of them. No More Gaps Interior Multipurpose is not classified as hazardous at all.

The expandable family is the outlier. Selleys Pro-Trade Expanding Foam is an isocyanate-based polyurethane, signal word Danger, Dangerous Goods Class 2.1 flammable gas, carrying H222 (extremely flammable aerosol), H229 (pressurised container, may burst if heated), H334 (may cause allergy or asthma symptoms or breathing difficulties if inhaled), H351 (suspected of causing cancer — suspected, not established), H372 (causes damage to organs through prolonged or repeated exposure), H362 (may cause harm to breast-fed children), plus H332, H335, H315, H317 and H319.

H334 is the one that should decide the choice. Respiratory sensitisation is not irritation that passes. Once your immune system has learned the response it is generally permanent, and afterwards much smaller exposures set it off. Symptoms can also appear hours after the work, so feeling fine while you spray tells you nothing. Foam's whole purpose — filling cavities, voids and awkward gaps — puts it in enclosed spaces with no air movement, which is the worst case for an inhalation sensitiser. The product asks for active ventilation, not an open door, and says not to work in a confined space. A subfloor or a roof space is a confined space.

H222 compounds it. An extremely flammable aerosol discharged into a cavity, near a downlight, a flue or a heater, is a different proposition from a caulk squeezed out of a cartridge. And H229 means the can must not be left in a hot car, a roof space or in the sun.

Where the job allows it, choose down. This page's own limit is the deciding line: up to 10 mm wide and 5 mm deep, a water-based filler does the work, and the interior multipurpose version carries no classification. Reach for foam when the void is genuinely too large for a caulk — not because it is quicker.

Poisons Information Centre — 131 126 in Australia, 0800 764 766 in New Zealand.

Common questions about types of caulk

Which caulk is best for a bathroom?

Silicone. It is durable, flexible and water repellent, which makes it the best choice for windows and bathrooms.

Which type can I paint?

Latex-type, water-based products. Silicone sealants cannot be painted, and painting must be completed 7 days before they are applied.

Which type can be sanded?

Latex caulk can be sanded once cured. Silicone cannot.

Why is latex less durable?

Its durability may be affected by moisture, which is why it is a drywall, timber and masonry product rather than a wet area one.

What is expandable caulk for?

Larger cracks and gaps, and it can help with soundproofing. Use it sparingly — it is easy to use too much.

What should I use outdoors?

A product designed to withstand high temperatures and stay water-tight on aluminium, metal, concrete, timber, sidings and gutters. In the Selleys range that means a neutral cure exterior silicone, or an exterior-grade water-based filler where the joint will be painted.

How long does caulk take to dry?

Typically dry to touch within half an hour, with fast-drying products setting in under 24 hours depending on humidity. Full cure is a separate and much longer figure — check the label.

Does humidity affect drying?

Yes. Setting times depend on the humidity of the space, and moisture-curing silicones actually need atmospheric moisture to cure at all.

Can I use a clear sealant everywhere?

A clear or neutral white sealant suits most applications, but check the product's substrate limitations rather than choosing on colour alone.

Where can caulk be applied?

Anywhere two materials join or there are small gaps. See where can you caulk?