Where to Use Gap Filler: A Comprehensive Guide

Where can you use gap filler around the home?

Gap filler — caulk, in the trade term — does more than tidy up a line. It prevents airflow, water leakage and the intrusion of dust and insects, which is why it matters to thermal comfort and moisture protection as much as to appearance. Filling the gaps around a house is one of the cheapest things you can do for both.

The short answer to where: gap fillers and putties can be applied internally and externally, provided you select a product appropriate for the intended use. That proviso is the whole game. An interior filler in an exterior joint will fail on UV and weather; a rigid filler in a moving joint will crack; a general filler around a bath will not keep water out.

How gap filler is applied

Gap fillers are easy to use and apply with a caulking gun, which forces the filler into the crack. For larger gaps, a spatula or a larger nozzle opening lets you work more material in.

The important word there is into. A bead laid on the surface of a joint has almost no depth of bond and will peel away at the first movement; one pressed into the gap grips both faces and can flex with them. When you are gunning, work forwards and push the product in as you go.

Where to use it, room by room

Between walls and skirting boards

Best filled with a flexible or semi-rigid filler that dries faster, such as the Selleys interior range — No More Gaps Interior Multipurpose or No More Gaps Interior Fast Dry. Both handle a maximum gap of 10mm wide by 5mm deep and are paintable — Fast Dry allows semi-gloss immediately and gloss at 10 minutes, against 60 minutes for the Multipurpose version.

See how to fill gaps between skirting boards and walls for the step-by-step.

Cornices and the wall-to-ceiling join

Same products, same reasoning: this is a joint between two elements that move independently, so it needs flexibility rather than hardness. See how to install cornices.

Around windows

Internally, an interior flexible filler. Externally, a product formulated for a weatherproof seal — No More Gaps Exterior, which is weather, mould and UV resistant with ±25% movement capability (ISO 9046). Gaps around window frames are where a surprising amount of a home's draught comes from. See how to fill gaps around windows.

Holes in timber and nail heads

Wood putties are made for this. Selleys Woodfilling Putty and Plastic Wood fill holes in timber and take a finish, which a wall filler will not do convincingly.

Bathrooms, kitchens and high-humidity areas

Water-resistant fillers belong here. No More Gaps Bathroom & Kitchen is flexible, mould resistant and paintable, with ±25% joint movement, and is intended for cracks near windows, cabinets, skirting boards and walls in these rooms.

Where it does not go: around bathtubs, showers and sinks. Those joints need a waterproof silicone seal — Selleys Wet Area Silicone or Selleys No Mould. Nor is it for outdoor use or constant moisture exposure.

Timber floors

No More Gaps Timber Floors, which contains wood fibres for a timber-look finish and can be stained. Not for gaps larger than 3mm wide in trafficable joints, and not between tongue and groove boards — the Exterior product covers that case.

Brick, cement and pathways

No More Gaps Bricks & Mortar gives a mortar-like finish for repairs in brick and cement, pathways, and between brick and the roof. Not for replacing mortar in construction, not for moving cracks, and not for areas of frequent water immersion.

Large voids

Where a caulk would simply vanish into the cavity, use a foam. No More Big Gaps and Pro Trade Expanding Foam expand to fill and cure into a semi-rigid seal.

How to use a caulking gun

1. Get to know the gun. It has a barrel or cradle holding the cartridge, a plunger rod, a large trigger to extrude product, and a small trigger to release the plunger rod when you want the cartridge out.
2. Prepare the cartridge. Cut the tip off the cartridge — secateurs work very well and are safer than a knife. Cut the nozzle to the width of the joint or the bead size you want, at a 45 degree angle. Insert the cartridge into the gun with the plunger rod pulled all the way out.
3. Apply the product. If it is your first time, practise on cardboard to get a feel for the flow. Then apply into the joint, gunning forwards and pushing the product in as you go. Small blobs where you release and re-pull the trigger will smooth out when you tool the bead.
4. Finish the joint. Soapy water often works well for smoothing, though the product manufacturer will recommend the best finishing method for their own sealants and fillers.

See how to use a caulking gun for more detail.

Limitations to respect

• Maximum gap. The interior No More Gaps products and No More Gaps Exterior all state 10mm wide by 5mm deep. Beyond that, shrinkage during cure becomes visible and paint over the top can crack — most noticeably with low binder flat and ceiling paints.
• Application temperature. No More Gaps Exterior is not to be stored or applied below 10°C; Bricks & Mortar and Timber Floors have a 5°C floor.
• Water immersion. None of the No More Gaps fillers is intended for frequent water immersion. Where that applies, use a silicone sealant.
• Moving cracks in masonry. Bricks & Mortar is explicitly not for moving cracks. A crack that is still opening in brickwork is a symptom worth diagnosing before you hide it.

Safety: what changes when the gap gets big

Room by room, almost everything on this page is a mild product. The exception is the last category — large voids — and the difference is worth understanding before you buy a can.

The flexible fillers are the easy end. No More Gaps Interior Multipurpose is not classified as hazardous at all: no signal word, no hazard statements. No More Gaps Exterior and Bathroom & Kitchen are signal word Warning with one classification each — H317, may cause an allergic skin reaction.

H317 is worth a sentence because of the finishing step above. Smoothing a bead with soapy water is usually done with a bare finger, and sensitisation is not irritation that washes off: it is the immune system learning to react, generally permanently, built up by repeated casual contact rather than one big exposure. Run a whole house of skirtings and window reveals that way, job after job, and that is the pattern. A gloved finger or a tool costs nothing.

The foams are a different product class. No More Big Gaps and Pro Trade Expanding Foam are isocyanate-based polyurethanes carrying the signal word Danger and the longest hazard lists in the range: 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, Category 2), 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 matters for where this page sends you. A void large enough to need foam is, by definition, a cavity — behind a wall lining, under a floor, in a roof space, around a pipe or duct penetration. You fill it kneeling or lying down with your head at the opening, in the least ventilated part of the house. Respiratory sensitisation is permanent once acquired, and afterwards much smaller exposures trigger it. Ventilate the space, work in stages with breaks outside it, and wear eye protection and gloves. If someone on the job is pregnant or breastfeeding, H362 means they should not be the one running the can.

H222 has its own consequence for cavity work: a fresh fill in a ceiling or wall may be next to a downlight, a flue or a heater duct. Fill away from those, and do not use a heat gun or solder in the space while the foam is curing.

The lower-hazard route. The maximum gap for the No More Gaps range is 10 mm wide by 5 mm deep. Inside that, use the flexible filler and none of the above applies. The foam is for what a caulk would genuinely vanish into, not for a gap you could have caulked.

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

Common questions about where to use gap filler

Can I use the same gap filler inside and outside?

No. Exterior work needs a product formulated for a weatherproof seal, with weather, mould and UV resistance. Interior products do not have that.

Can I use gap filler around a bath or shower?

No. No More Gaps Bathroom & Kitchen is not recommended around bathtubs, showers and sinks. Use a wet area silicone sealant, which forms a waterproof seal.

How do I get filler into a deep crack?

Apply with a caulking gun, pushing the product in as you gun forwards. For larger gaps, use a spatula or open the nozzle wider. Do not exceed the product's stated maximum depth — for the No More Gaps range that is 5mm.

Which filler for a gap between the wall and the ceiling?

A flexible interior filler. That joint moves, and a rigid filler will crack along it.

Can I fill a gap in a pathway?

Yes — No More Gaps Bricks & Mortar covers pathways as well as brick and cement repairs, provided the crack is not still moving and the area is not subject to frequent water immersion.

What do I smooth the bead with?

Soapy water works well on most fillers. Check the pack, since the manufacturer will specify the best finishing method for that product.

What is the best tool for cutting the nozzle?

Secateurs — they cut cleanly at the angle you want and are safer than a knife.

Will filling gaps actually make a difference to draughts?

Caulk prevents airflow, water leakage and the intrusion of dust and insects, which is why it is relevant to thermal insulation and comfort as well as appearance. See how to prevent draughts in your home.