Quality properties of window foam
As specified in the Energy Saving Ordinance, new buildings should have an airtight building envelope. There are a lot of details that need to be taken into account when fitting windows. Not only should the window itself meet all defined energy ratings, the joint between the window frame and the wall must also remain consistently tight for decades; this is ensured by combining various sealing materials. One of the main products used to insulate and fit window joints is PU foam. Today, about 80 percent of all windows and doors in Germany are fitted and insulated with PU foam. What distinguishes a good window foam?
Highly flexible construction foams are suitable for use when fitting windows. Why? Because the size and position of the window frames change when subject to wind or high temperature fluctuations. In the case of south-facing windows, this can even be up to 100 degrees Celsius. The material heats up in the summer and cools down again in the winter, and the frame expands and shrinks accordingly.
For instance, the frame material of aluminium windows can change by up to one millimetre per running metre under various climatic conditions. Changes of up to two millimetres can also be expected in PVC windows. Also, the foam always connects two different materials that have varying expansion behaviours, which is why a certain amount of joint movement can always be expected. The window foam needs to be able to cope with all these factors without tearing.
Flexibility is important
Flexible PU foams can usually be identified by their product name: for instance, when variations of the terms ‘flexible’ or ‘elastic’ are used. If the product name on the can gives no indication, take a look at the product data sheet. The flexibility of the foam is stated as an elongation-at-break percentage and/or as the compression force. Conventional foams usually lie at around 10 percent; the elastic foams between 25 and 45 percent.
Fine-pore window foam
One visible quality feature of window foam is the consistency with which it leaves the can. Ideally, it should look like whipped cream. Oily or viscous foam are indications of an inferior product. It is also important that the PU foam adheres well to the surface immediately and does not run into the joint. Specialists refer to this behaviour as staying put and/or slipping. Only foam that adheres well ensures even insulation and distribution.
Another quality feature becomes apparent when the foam hardens: when the excess foam residues are cut off with a knife, the foam structure becomes visible. The finer the pores and the more even they appear, the better the thermal insulating properties. Ideally, hardened window foam should resemble a foam mattress.
Selection of the right PU foam is an important factor for professional window fitting. Window fitting also includes other important steps that cannot be illustrated here. The RAL Window Fitting Guide itself is 300 pages long. For more information, please refer to e.g. the window and façade association.