Why did my expanding foam not cure, or bow the frame?

Four failure modes with four different causes — an uncured sticky core, a bowed frame, crumbling in daylight, and foam that expanded far more than expected.

Difficulty: intermediate
Time: 25 min
Read: 5 min
Safety: warning

Short answer

A sticky core means it had no moisture to cure with, so mist the surfaces with water and build up in layers rather than one mass. A bowed frame means the gap was overfilled and the packers were removed too early. Crumbling means it was left exposed to sunlight. All of them come from the same misunderstanding: foam expands two to three times and cures from the outside in.

Expanding foam is a moisture-cured polyurethane. It reacts with water — from the air, from the surfaces, from a mist bottle — and produces gas, which is the expansion. Once you know that, all four common failures become obvious: no water means no cure, too much material means too much gas, and the reaction happens fastest where the moisture is, which is the outside.

Safety

Expanding foam contains isocyanates, which are respiratory and skin sensitisers, and sensitisation is permanent. Wear nitrile gloves and eye protection every time and ventilate properly — foam in an eye is a hospital matter. Standard foam has no fire performance whatever and must never be used in a fire-rated opening, which requires a certified fire-rated product installed to its own tested detail.

How expanding foam fails, in section

Step by step

Identify which failure you have.Hard outside, sticky or dark and soft in the middle is an uncured core. A frame that no longer closes evenly is over-expansion. Orange-brown crumbling powder is ultraviolet degradation. Foam bursting out of the joint hours later is simply overfilling.
For an uncured core, add moisture and reduce the section.Foam cures by reacting with water. A thick lump in a dry, cold, enclosed void has almost none available and can stay tacky indefinitely. Mist both surfaces with water before dispensing, and build a deep void up in layers with a few minutes between them rather than one mass.
Cut out an uncured core rather than leaving it.It will not cure later. Uncured foam is sticky, has no strength, holds moisture and continues to release isocyanate. Cut back to sound foam, mist, and rebuild in layers.
For a bowed frame, understand the force involved.The expansion is powerful enough to bend a uPVC or timber jamb inwards, and it does it slowly over the hours after application. Once cured it stays bowed. Low-expansion frame foam exists specifically for this, and packers must be left in place until the foam has fully cured.
Fill about a third of the void, not the whole of it.This is the single discipline that prevents most foam problems. It expands into the rest. Filling the gap means foam everywhere, a bowed frame, and a lot of trimming.
Shake the can hard before every use.A full minute. The components separate in the can and unshaken foam cures poorly and expands unpredictably, which is a genuine cause of both the sticky-core and the over-expansion failures.
Never leave foam exposed outdoors.Ultraviolet breaks the polymer down within months into a friable powder that absorbs water. Any foam visible outside must be trimmed back and covered with sealant, render, a trim or paint.
Do not disturb it while it rises.Poking, smoothing or compressing partly cured foam collapses the cell structure, which ruins both the insulation value and the strength and leaves voids.
Remember what foam is not.It is not structural, so nothing should be supported on it. It is not a fire stop unless it is a certified fire-rated product used to its tested detail. And it is not a substitute for a proper mechanical fixing on a frame.

Tips

Gun-grade cans give far better control than the straw type, can be paused and restarted over days, and dispense a much finer bead. For anything more than one small gap they are worth the applicator.
A gun left with foam in it is ruined. Clean it with the gun cleaner immediately after use, and leave a partly used can attached rather than removing it.
Cured foam does not come off skin. It has to wear off over several days. Gloves are the only prevention.
Foam is a good draught seal and a poor insulator compared with proper insulation. Do not fill a large void with it in the belief that it is doing an insulation job.
Never use foam to fill around a boiler flue, in a fire-rated opening, or against a hot flue pipe. Those need certified products and specific details.

If it doesn't work

Hard skin, sticky brown centre weeks later

Cause: No moisture available in a thick enclosed section — Fix: Cut the uncured material out, mist both faces with water, and refill in two or three thinner layers with a few minutes between each.

The window or door will not close properly since foaming

Cause: Over-expansion bowing the frame — Fix: Cut the foam out along the bowed section, release the frame, re-pack it straight, and refill with low-expansion frame foam filling only a third of the gap. Leave the packers in until fully cured.

The foam outside has gone crumbly and orange

Cause: Ultraviolet degradation — Fix: Rake out the degraded material, refoam if needed, trim back below the surface, and cover with sealant, render or a trim. Foam must never be left exposed outdoors.

It kept expanding for hours and burst out of the joint

Cause: Too much dispensed, or an unshaken can — Fix: Trim back once fully cured with a serrated knife. Next time shake for a full minute and fill about a third of the void.

The foam has come away from one surface

Cause: A dusty, greasy or wet surface — Fix: Foam bonds like an adhesive and needs a clean surface. Brush the joint out, mist rather than soak it, and refill.

Questions people ask

Why is my expanding foam still sticky?

It had nothing to react with. Foam cures with moisture, so a thick section in a dry enclosed void stays soft in the middle. Mist the surfaces before dispensing and build up in layers.

How much should I dispense into a gap?

About a third of the volume. It expands two to three times, and filling the gap is what causes almost every foam problem.

Can I use expanding foam around a window frame?

Yes, with low-expansion frame foam, filling about a third of the gap and leaving the packers in until it has cured. Standard foam can bow the frame enough that the sash will not close.

Does expanding foam need to be covered?

Outdoors, always — ultraviolet destroys it within months. Indoors it is mainly cosmetic, though trimming and covering it keeps it clean and stops it being knocked.