Polyurethane foam: how and when to use it

Do you know how many things you can do with just one can of polyurethane (PU) foam?

Polyurethane foam spray

What polyurethane foam is used for

There are many occasions when we need to close large cracks, fill cavities or block different parts together and polyurethane foam allows us to do it quickly and permanently.

Here's a list of things you can do:

  • insulate pipes
  • fill cavities, pipes, channels
  • you can use foam for mounting electrical boxes
  • mounting fixtures (doors, windows)
  • fix insulation panels
  • you can waterproof cisterns and manholes
  • fix purification systems

Foam can be, at the same time, insulating, filling and sealing.

Obviously, among all the cans of polyurethane foam spray on the market you should try to choose the one that best suits the specific job you have to do.

The polyurethane foam is sold in convenient spray cans of various capacities and to apply it you just need to press a kind of trigger that, through a flexible tube, allows you to reach even the cavities a little deeper and then fill them with the foam itself.

The polyurethane foam, which has a light yellow color, in 2 or 3 hours passes from the state of foam to the solid state, increasing up to 3 times its initial volume and thus allowing to fill completely, through this expansion, all the cavities, even the most uneven.

Polyurethane foam is suitable not only for filling large cracks, grooves, etc., but also for securing elements. The two functions are often both solved by the use of foam: an example for all is its use for fixing wooden door jambs.

There are also special fire-retardant polyurethane foams, whose color may differ from the classic light yellow, which are also suitable for sealing fireproof doors or fixing stove pipe collars to the wall, where these are joined to the chimney.

The links below show the foams for the most common uses, which can be purchased directly online:

Loctite TITE FOAM Insulating Foam Sealant (general purpose)

3M Fire Block Foam FB-Foam, Orange, 12 fl oz, Aerosol Can

Use of polyurethane foam spray

The application of polyurethane foam, as we said, is very simple: just spray it inside the cavity that we want to fill.

Before proceeding, however, be sure to take all measures to operate safely.

When foam comes into contact with your skin, it's quite unpleasant, and for this reason, it's always a good idea to equip yourself with the right personal protective equipment (PPE).

Always wear gloves and clothing that doesn't leave your skin exposed, and always wear a pair of goggles.

Where the foam is likely to come into contact with surfaces that remain exposed, such as doors or door jambs, remember also to create a masking, with appropriate tape, of the surface at risk, because if the foam sticks then it is difficult to remove it and the risk of damaging the paintwork of the surfaces is very high.

Before spraying the foam, it is good practice to slightly wet with a sprayer the parts that will be joined and/or filled; this helps the foam to set and harden.

The polyurethane foam should then be injected into the crack through the nozzle; we should try to work continuously and without stopping and restarting as this ensures a more uniform distribution of the foam.

As soon as the foam is injected, you will notice that it tends to fill everything and also to spill out, due to its expandability (in technical jargon "self-extruding").

So, once you get the hang of it, try not to waste too much foam, but to let it escape just enough.

A gun that can be attached to the can allows you to waste less product:

Professional Spray Foam Gun 

How to shape polyurethane foam

Once the polyurethane foam has hardened you can proceed to shave off the excess material using a sharp cutter.

Much depends on the thickness of the foam, however, in some cases a long knife or even a saw with a narrow-tooth blade may be more useful.

In the solid state the foam hardens, yes, but it remains extremely workable and therefore we can cut and sand, and finally paint, without difficulty.

During cutting and sanding, dust is formed and it is best not to inhale it, so the protective device to use at this stage is a mask to protect the airways.

As already mentioned, the uses of polyurethane foam are the most varied and the hobbyist will certainly be able to add new ones.

Thanks to its insulating capacity the foam is also suitable for fixing and insulating water pipes that move inside a cavity already created or even to fix the flexible pipes of the electrical system inside the grooves of the wall; in short, an indispensable wild card that solves many DIY problems.

It is also possible to model the foam to obtain small sculptures, thanks to its easy workability, but this is definitely not the use for which the polyurethane foam was designed.

Cleaning Polyurethane Foam

Finally, we would like to remind you that cleaning the nozzles before the foam completely hardens is essential in order to reuse the product.

The best solution would be to combine the can of foam with a dispensing gun and another can that serves, instead, as a cleaner for the foam, containing a specific mixture of organic solvents for the dissolution of fresh polyurethane foam. You can find the latter product at this link:

Great Stuff PRO Dispensing Gun Cleaner, 12oz

The use of the special solvent for polyurethane foam, on visible surfaces, should be tested. The advice that can be given, in case a surface is accidentally soiled with foam, is to clean it using paper (such as Scottex), taking care not to drag the foam to avoid soiling the surface even more.

Once you have removed most of the fresh foam, spray on the solvent and, using new clean paper, remove the remaining foam.

The alternative is to not touch the foam and let it harden and then remove it very carefully. This last solution is absolutely advisable if we do not have the appropriate foam solvent on hand.