
Basic knowledge about construction foam
We often take everyday items for granted without understanding what lies behind them. Construction foam is a perfect example. Here, you’ll find the basics of PU foam. Especially in customer consultation and sales training, knowing the technical details is crucial for providing competent advice.
Construction foam is mostly air
Did you know that construction foam is 98% air? Technically, foam is gas trapped in a solid or liquid matrix. After curing, the construction foam retains its cell structure, which consists predominantly of air. Only the remaining 2% is polyurethane. The gas that propels the foam from the can creates fine air inclusions. The finer this cell structure, the better the quality and insulating performance.
More information on the chemical composition of PU foam is available here
Outstanding insulation performance with PU foam

The tiny air inclusions ensure construction foam achieves excellent insulation values. They block the flow of heat and cold, making it the ideal material for sealing window joints and other critical areas. PU foam ranks among the best insulating materials on the market. Compared to other materials, its low thermal conductivity and closed-cell structure provide outstanding insulation properties.
Shake well before use
To develop the fine cell structure with small air inclusions, it’s essential to shake the can thoroughly before use. In a new construction foam can, the propellant gas separates from the liquid polyurethane precursor. Thorough shaking – ideally upside down – mixes the gas and liquid to ensure optimal results.

Construction foam doesn’t dry – it cures!
Contrary to popular belief, single-component PU foam (1K-PU foam) doesn’t simply dry in the air; it cures through moisture. The foam requires water from the surrounding air or targeted spraying to achieve full strength. This is why manufacturers consistently recommend lightly wetting surfaces and freshly applied foam with water. This ensures the foam finds enough moisture to cure correctly.
Find more tips on how to achieve the best results with your construction foam here
Temperature and humidity affect curing
This is especially true for low temperatures! Cold air contains less moisture, delaying the foam’s curing process. Adding a little water to the surface accelerates the process.

PU foam also works as an adhesive
In addition to its insulating properties, PU foam is valued for its adhesive capabilities. Particularly in the installation of interior doors, stair treads or external thermal insulation composite systems (ETICS), its adhesive strength plays a key role.
PU foam adheres to these materials
PU foam adheres reliably to many building materials such as concrete, plaster, masonry, fibre cement and wood. It also adheres well to aluminium, pretreated metals and many plastics, provided the surfaces are clean and dry. Problematic surfaces include oily or dusty ones and specific plastics like Teflon, PE and PP.

Unbeatable for joints and cavities
With a filling volume of up to 65 litres per can, PU foam is unbeatable for filling cavities and joints. The foam is stable yet lightweight. When applying, always work from bottom to top to prevent foam from spilling.
Curious about what might happen if you ignore this tip? Check out our chapter „“12 fatal mistakes”
Best used upside down
When working with construction foam, you should hold the can upside down. Why? Unlike ordinary spray cans, a PU foam can does not have a riser tube. This would clog due to the viscous contents. When the can is turned upside down, the liquid can flow to the valve by itself and exit easily.
More information on how construction foam valves work can be found here