What is Selleys No More Big Gaps, and where should you use it?

Selleys No More Big Gaps is an expanding polyurethane foam that cures into a semi-rigid closed cell foam. It is formulated to fill large or irregular holes and cavities — around pipes, behind skirting boards and architraves, and around window and door frames — where a caulking gun and a bead of gap filler would be the wrong tool entirely. In doing so it blocks draughts, noise, pests and birds, and it provides excellent insulation.

The one thing to understand before you press the trigger: this product expands 2.5 to 3 times its initial extruded volume. That is the feature and the hazard in the same number. Fill about half the space and let expansion do the rest; overfill and the foam will force apart whatever it is between as it grows.

Technical specification

Property	Typical result
Technology	Polyurethane
Colour	Cream coloured firm foam
Expansion	2.5 to 3 times initial extruded volume
Tack free	Within 15 minutes of application
Cure time	A 3 to 5cm deep application is fully cured in approximately 5 hours
Shrinkage	None
Yield	Approximately 25 times can volume in normal conditions — a 750mL can yields at least 19 litres, a 390mL can at least 10 litres
Adhesion	Adheres to most building materials such as timber, brick, concrete, fibre cement, metals, PVC, most plastics and weatherboard
Durability	Interior and exterior (exterior applications must be protected)
Packaging	390mL and 750mL aerosol cans
Shelf life once opened	Use within 4 weeks

Indicative data only, not intended for the preparation of specifications. Yield varies according to conditions, size and confinement of the application.

What the expansion and cure figures mean in practice

Expansion of 2.5 to 3 times is why the filling instruction is "about half". If you fill a 40mm cavity to the brim, you are committing to somewhere between 100mm and 120mm of foam trying to occupy 40mm of space. In a window reveal that is enough to bow a jamb. Half full is the number to work to, and Selleys adds the detail that shallow applications expand more than deep applications — so a wide, shallow gap needs a lighter run than your instinct suggests.

Tack free in 15 minutes, fully cured in about 5 hours for a 3 to 5cm deep application. Those two figures do different jobs. Tack free means the surface has stopped being sticky; it does not mean the foam has finished. Cutting or trimming belongs after the full cure, not after the fifteen minutes.

Depth is the variable that changes everything. For confined or large spaces, the manufacturer's method is to fill in layers 4–5cm deep, letting each layer cure before applying the next, and to spray the surface of the cured foam with water before re-applying. That is slower than one continuous fill, but a deep single pour has a cured skin over an uncured core, and that core can stay soft indefinitely.

The yield of approximately 25 times can volume is the figure to size your purchase against — at least 19 litres of cured foam from a 750mL can, at least 10 litres from a 390mL can, in normal conditions. Once you open a can, use it within four weeks.

Why moisture matters

This foam cures with moisture, and two of the manufacturer's instructions follow directly from that. For maximum expansion and faster curing, moisten the surfaces by spraying with water before you apply. And after application, spray the expanding foam lightly with water to assist curing.

The same chemistry produces one of the stated limitations: the product will not cure in a sealed space without moisture present. If you inject foam into a completely enclosed void with no moisture, it can simply stay uncured.

Where to use it

• Around pipes
• Behind skirting boards and architraves
• Around door and window cavities
• Large irregular shaped gaps, hollows and cavities in buildings

The purpose in all of these is the same: blocking draughts, noise, pests and birds, while insulating the cavity.

How to apply it

Store the can upright and read the can label before use. Use in a well-ventilated area, and wear protective glasses and the protective gloves provided in the cap. Avoid breathing vapour — the product is difficult to remove from skin and clothing.

1. Prepare the surfaces. Ensure they are clean and free of dust, oil and grease. For maximum expansion and faster curing, moisten them by spraying with water.
2. Shake the can vigorously for 60 seconds.
3. Screw the applicator nozzle onto the can valve. Do not over tighten.
4. Turn the can upside down and apply gentle pressure on the trigger to extrude foam. Vary the pressure to control the flow rate.
5. Fill about half of the space. Do not overfill — the product will continue to expand. For confined or large spaces, work in layers 4–5cm deep, allowing each layer to cure and spraying the cured surface with water before applying the next.
6. Keep shaking the can from time to time during extended use.
7. Spray the expanding foam lightly with water to assist curing.
8. Clean up any overspray and spillage immediately while still wet, using a dry cloth. Acetone or nail polish remover will assist.
9. Wait. The product is tack free within 15 minutes and fully cured in approximately 5 hours.
10. Once cured, cut back or trim with a sharp knife, then sand or paint.

If you are using it to block rats or mice, install small mesh bird wire before applying the foam, so that the wire is embedded inside the cured product.

Limitations — where No More Big Gaps should not be used

These are the manufacturer's stated limits, and several of them are the kind that only become obvious after the job has failed.

• Not approved as a fire blocking filler. Do not use it to fill a penetration that has a fire rating to maintain.
• Not suitable for the manufacture of flotation aids or life saving devices.
• Will not adhere to some surfaces, for example polyethylene, polypropylene, Teflon and silicone.
• Will not cure in a sealed space without moisture present.
• Will not block rats and mice where access holes are in current use.
• Do not apply at temperatures below 5°C.
• Exterior applications must be protected with paint, sealant, filler or another opaque material.
• The page also states it is not suitable for underwater or continuously damp conditions, and is not load-bearing or structural.

The exterior point deserves emphasis. The product is durable and rated for interior and exterior use, but only when exterior work is covered by an opaque coating. Bare foam left exposed outdoors is not what the rating describes; the page notes that prolonged UV exposure may cause surface discolouration, and recommends painting if required.

Clean up, re-use and storage

Clean up overspray and spillage immediately with a dry cloth while it is still wet; acetone or nail polish remover will assist. Once cured, the product can only be removed by cutting, sanding, grinding or scraping away.

To re-use a part can, remove the nozzle from the can valve after use and rinse the valve and nozzle immediately with nail polish remover or acetone, so foam does not cure inside them. Remove any cured product from the nozzle before re-using. Once opened, No More Big Gaps should be used within 4 weeks.

Storage carries a genuine warning: this is a pressurised container that may burst if heated, and an extremely flammable aerosol. Store it locked up and upright in a cool, dry, well-ventilated place, out of the sun and away from corrosive materials such as water and chemicals. A rusted or damaged can may explode on impact. Do not expose it to temperatures exceeding 50°C. To dispose of unwanted product, spray it onto newspaper or into garbage bags, allow it to cure, and dispose of the remaining contents and container in accordance with local, regional, national and international regulations.

Hazard classification

Classified as hazardous under Safe Work Australia GHS 7, with the signal word Danger.

This is an isocyanate-based polyurethane foam, and it carries the longest and most serious hazard list of any product on this site. It is worth reading before use rather than after.

• H222: extremely flammable aerosol. H229: pressurised container, may burst if heated.
• H334: may cause allergy or asthma symptoms or breathing difficulties if inhaled (respiratory sensitisation, Category 1).
• H351: suspected of causing cancer (carcinogenicity, Category 2).
• H372: causes damage to organs through prolonged or repeated exposure.
• H362: may cause harm to breast-fed children.
• H332: harmful if inhaled. H335: may cause respiratory irritation.
• H315: causes skin irritation. H317: may cause an allergic skin reaction. H319: causes serious eye irritation.

What is in it

Chemical entity	CAS number	Proportion (w/w)
Diphenylmethane diisocyanate (MDI) isomers and homologues	9016-87-9	30–60%
Alkanes, C14-17, chloro-	85535-85-9	10–30%
Isobutane	75-28-5	10–30%
Dimethyl ether	115-10-6	1–10%
Propane	74-98-6	1–10%
Non-hazardous ingredients	—	Balance

MDI — diphenylmethane diisocyanate — is 30 to 60% of the can and is the source of the respiratory sensitisation, carcinogenicity and organ-damage classifications. The isobutane, propane and dimethyl ether are the propellant, and are why it is also an extremely flammable aerosol.

H334 is the one to understand. "May cause allergy or asthma symptoms or breathing difficulties if inhaled" is respiratory sensitisation, and it works like skin sensitisation but in the airways. The immune system can learn to react to isocyanate vapour, and once that happens the reaction is generally permanent, can be triggered by concentrations far below the one that caused it, and gets worse with each subsequent exposure. Occupational asthma from isocyanates ends careers in spray-foam and paint trades. There is no safe re-exposure once sensitised.

The exposure happens while the foam is expanding and curing, not afterwards. Fully cured foam is inert. The risk window is from the moment you press the trigger until the foam has fully cured, and it is concentrated in exactly the places this product gets used — wall cavities, roof spaces, subfloors, window reveals. Small, enclosed, unventilated.

What to do about it: ventilate the space actively, wear an organic vapour respirator rather than a dust mask, wear gloves and eye protection, and keep other people — particularly children and anyone with asthma — out of the area until it has cured and the space has been aired.

H362 deserves separate mention because it is unusual on a hardware product: it means the substance can be passed to an infant through breast milk. Anyone breastfeeding should not use this product.

Do not pierce or burn the can even when empty, keep it out of sunlight and below 50 °C, and do not smoke or work near an ignition source while dispensing.

Protection

• Respiratory protection — an organic vapour respirator where the product is sprayed, dispensed or mixed in quantity; a dust mask for sanding cured material.
• Gloves — nitrile rubber; essential where a sensitisation classification applies.
• Eye protection — chemical goggles or safety glasses.
• Ventilation — active ventilation, not an open door. Do not work in a confined space.

First aid

Exposure	What to do
Inhalation	Move the person to fresh air at once and keep them at rest. If breathing difficulty, wheeze or tightness develops, seek medical attention immediately and say the product contains isocyanates or solvents as applicable — symptoms can be delayed by several hours.
Eyes	Rinse cautiously with water for several minutes, removing contact lenses if present and easy to do, then keep rinsing. If irritation persists, get medical advice.
Skin	Wash with soap and water. Remove contaminated clothing and wash before reuse. If a rash or itching develops, including hours later, get medical advice — this may indicate sensitisation.
Swallowed	Do not induce vomiting. Rinse the mouth and seek medical advice immediately.

If poisoning occurs, contact a doctor or the Poisons Information Centre — 131 126 in Australia, 0800 764 766 in New Zealand — with the container to hand. Selleys emergency line: 1800 220 770 in Australia, 0800 220 770 in New Zealand.

Common questions about Selleys No More Big Gaps

How much should I put in the gap?

About half. The foam expands 2.5 to 3 times its extruded volume, so filling the space completely will force it apart as it grows.

How long before I can cut it back?

It is tack free within 15 minutes, but a 3 to 5cm deep application is fully cured in approximately 5 hours. Cut back, sand or paint after it has cured, not after it has gone tack free.

Can I fill a very deep cavity in one go?

No. Fill confined or large spaces in layers 4–5cm deep, allowing each layer to cure and spraying the cured surface with water before applying the next.

Why does Selleys tell me to spray it with water?

The foam needs moisture to cure. Moistening surfaces beforehand gives maximum expansion and faster curing, and spraying the applied foam assists curing. In a sealed space with no moisture present, the product will not cure at all.

Can I use it around a fire-rated penetration?

No. The product is not approved as a fire blocking filler.

Will it keep rats and mice out?

It blocks pests as part of sealing a cavity, but it will not block rats and mice where access holes are in current use. If you are blocking a rodent entry, install small mesh bird wire first so the wire ends up embedded inside the cured foam.

Can I use it outside?

Yes, but exterior applications must be protected with paint, sealant, filler or another opaque material. Left bare, the surface may discolour with prolonged UV exposure.

What will it stick to?

Most building materials, including timber, brick, concrete, fibre cement, metals, PVC, most plastics and weatherboard. It will not adhere to polyethylene, polypropylene, Teflon or silicone.

How much does a can do?

In normal conditions the yield is approximately 25 times the can volume — at least 19 litres from a 750mL can and at least 10 litres from a 390mL can. Yield varies with conditions, size and how confined the application is.

Can I save a part-used can?

Yes, if you clear the nozzle. Remove the nozzle from the valve and rinse both immediately with acetone or nail polish remover. Use the can within 4 weeks of opening.

How do I get it off my hands or a bench?

Wipe it up immediately while wet with a dry cloth; acetone or nail polish remover will assist. Once cured it can only be removed by cutting, sanding, grinding or scraping. This is why the gloves in the cap are worth using — the product is difficult to remove from skin and clothing.

What is the coldest I can apply it at?

Do not apply at temperatures below 5°C.

For gaps small enough for a caulking gun, the flexible fillers in the No More Gaps range are the better fit — see No More Gaps Interior Multipurpose for general interior work or No More Gaps Exterior outdoors.