Basics of
PU insulating foams

PU insulating foams

In window installation, 1-component insulating foams (such as TEROSON EF 537) have proven their worth for many years, which professionals use with foam guns. This makes it easier to control the foam dosage and foam cans that have been started can be used again without any problems even after a few days.
A major advantage of joint insulation with PU foam compared to 3-level sealing tapes, for example, is that even window connection joints that do not run evenly and parallel are optimally sealed against heat and sound. Especially in the area of window renovation, particularly in old buildings, uneven joints are a very common occurrence.


The advantage of PU foam over mineral wool is that it is much more even, faster and easier to process.

Connection joints can also be insulated with mineral insulating materials, but optimum joint insulation is only possible from a joint width of more than 15 mm. Below this, it is very difficult to optimally fill a joint with mineral insulating materials over the entire depth of the window profile. Window foams do not absorb moisture and do not collapse even in large joints after curing. They also stabilize the mecha­nical window fastening and make installation considerably more secure. After curing, the TEROSON EF 537 1-component gun­foam exhibits long-term flexibility, which is able to absorb long-term movements of the building element, e.g. due to wind loads.

Advantages and disadvantages of
1-component PU foam at a glance:


  • Very efficient
  • Simple processing
  • Best sound and insulation effect
  • No complicated corner formation/stuffing
  • Can be processed at low temperatures
  • Almost independent of joint dimensions

  • Dangerous goods
  • Frame soiling with inaccurate foaming
  • Cleaning of the PU foam gun necessary
  • Moisture necessary

Window insulation with
PU insulating foam in detail

When processing 1-component PU assembly foams, make sure that the foam is shaken well before processing (077) so that the different components of the formulation, e.g. prepolymer and blowing agent, mix properly before processing. The blowing agent settles very quickly in the can due to its low density. Shaking is sometimes underestimated by processors, but has a major influence on the expansion and the quality of the foam in terms of heat and sound insulation. For a 1-component insulating foam to cure, it needs moisture. On normal days, the air humidity is sufficient for the foam to build up its optimum structure. On days with low humidity (e.g. in winter) or when filling very wide/deep window connection joints, it is necessary to add additional moisture to the foam (see “Foaming in winter”, page 103).
The foam absorbs the moisture from the outside (078) and the chemical reaction begins. The 1-component insulation foam first forms a skin on the outside. The moisture slowly migrates to the inside of the foam. In the case of very large window connection joints or when filling large cavities, it may therefore be advisable to fill the joints twice. If the foam receives sufficient moisture during curing, a fine cell structure is usually formed (depending on the formulation); if insufficient moisture is absorbed during curing, an uneven cell structure with larger cells is formed. For large vertical window connection joints (wider than 4 cm), the joints should be foamed from bottom to top (079 I 080) so that the foam has better adhesion to the element and the building structure and does not fall out of the joint when fresh.
Bild eines Fensterausschnittes mit Glattstrich zur Darstellung der Anwendung von Dichtfolien.
077 | Shake PU foam before processing
Bild eines Fensterausschnittes mit Glattstrich zur Darstellung der Anwendung von Dichtfolien.
078 | Curing process of 1-component PU foam
Bild einer Verankerung des Blendrahmens zur Darstellung der Anwendung von Dichtfolien.
079 | Foaming of horizontal joints
Bild eines Andrucks mit einem PE-Spachtel zur Darstellung der Anwendung von Dichtfolien.
080 | Foam from bottom to top for large vertical joints

Replacing PU foam cans in detail

Ideally, the old PU foam can should be twisted off over a waste bucket so that any residual foam can be collected (081). This can happen, for example, if the cans are not stored upright and the rubber seals in the valves are brittle. In the case of PU foam guns matched by the manufacturer with the appropriate valve system on the PU foam cartridge, only as much foam escapes as is still present in the valve stem. The valve should close immediately when it is turned off and only a small amount of foam should escape from the valve. The new can should be turned onto the gun with the valve facing upwards immediately after twisting off the old can (082).
As the thread depth of the PU foam guns is not standardized, premature foam leakage can be avoided by opening the valve early if the valve is pointing upwards. However, old foam residues on the threaded basket must be removed before screwing on the new foam can so that the thread of the foam can can be screwed completely into the threaded basket and the valve inside the can opens fully. This ensures the flow rate of the foam as tested by the manufacturer. When the foam cans are changed, a little moisture always gets inside the foam gun. To avoid a chemical reaction in the guns, it has proven useful to spray out some foam and the trapped moisture directly after changing the foam can.

Foaming in winter

During the cold season, the processing of PU installation foams can be problematic in some cases. Many PU foam manufacturers specify a minimum processing temperature of +5 °C for their products. The product TEROSON EF 537 is also suitable for processing in the cold season. The foam can be processed down to an ambient temperature of -10 °C. On winter days with low humidity, it is advisable to add additional moisture to the foam. In winter, it is essential to foam the window connection joint first before spraying moisture onto the foam with a spray bottle (083). When spraying the window connection joint before foaming, a layer of ice immediately forms on the masonry, which leads to a loss of adhesion of the 1-component insulation foam to the building.


In order to have approximately the same flow rate of the foam as in summer, it is necessary that the can temperature is not colder than 0 °C.

083 | In winter, moisture should only be added to the foam after the joint has been foamed out

Cleaning PU foam guns

It is only necessary to clean the guns if the flow rate of the foam decreases significantly during the foaming of the window joints. After the empty PU foam can has been unscrewed, the PU cleaner can be screwed onto the gun (084). The remaining foam should then be sprayed out of the gun until the cleaner comes out of the gun. The cleaner can be left on the gun overnight so that the cleaner can dissolve the hardened foam residue in the gun. The next morning, the foam residue should be sprayed out together with the cleaner. The cleaner can then be turned off and a full PU foam can opened. The flow rate should then be restored.

PU foam cans are disposed of in Germany via the PDR system.


PU foam cans should be stored upright in accordance with the labeling on the cartons (085). This ensures the long-term function of the foam valve. During transportation, PU foam cans must be transported in a suitable box on the loading area in accordance with the German Dangerous Goods Transportation Act (GGBefG). Carrying the cans in the rear of the vehicle is not permitted.

085 | Symbol for upright positioning

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