What does window foam need to be able to do?

Mr Jehl, what is the function of building foam when it comes to window installation? What can it do, and what not?

In short: PU foam insulates! It ensures that the joint between the window frame and the wall is characterised by good and continuous thermal insulation. In doing so, in winter the heat stays inside the house and the cold outside. In summer, of course, the reverse is true. PU foam is very well suited for insulating joints thanks to its excellent insulation properties.

And what about fastening – is PU foam effective when it comes to this, too?

It is helpful. Anyone who has ever installed a window can confirm that a window insulated with PU foam fits more snugly in the recess. The foam sticks to the edges of the joint and thus connects the window frame with the wall. This is a welcome side-effect, but you cannot take this into account statically. In order to fix a modern window securely and permanently in place, you cannot do without suitable mechanical fasteners such as screws, dowels and the like. This also applies to all other insulation materials. By the way, when we use the term “permanent”, we mean the typical service life expectation for windows, on average 40 years.

The question remains as to the tightness of the window joint ...

The same applies here as for the fastening. Of course, a joint that has been filled with PU foam is tighter than a joint that has not yet been insulated. If the foam is well bonded to the window frame and the wall, i.e. both joint flanks are glued together, this also has a very positive effect on the joint sound insulation. But this alone is not enough to permanently seal a joint according to the rules of technology.

Why?

The joint must be airtight all around on the warm side, i.e. the room side, in order to prevent the air from flowing through the joint and thus to stop both heat loss and draught. The special feature is that the window connection joint is always a movement joint. You, therefore, need sealing products that can accommodate all the expected movements of the joint and still retain their sealing function. Further steps are, therefore, necessary to ensure that the joint is permanently airtight. Tradesmen often resort to joint sealing tapes, joint sealing foils or sprayable sealants. However, there are also other suitable materials.

And from the outside – what steps are necessary?

From the outside, we need a so-called driving rain-proof connection, which prevents water from penetrating the joint during heavy rain and wind. As a rule, the joint is also sealed here with specially suitable, movement-absorbing sealing products (see above). But be careful: the waterproofing products inside and outside should be matched to each other in such a way that any moisture that may occur in the joint can be wicked away to the outside. In addition, there are measures with which rainwater can be drained off in a controlled manner, for example, window sills. What exactly needs to be done here depends to a large extent on the elemental forces to be expected for the window in question.

No matter how many work steps are required to install a window in accordance with RAL, is PU foam still used for joint insulation?

Oh yes, I assume that around 80 per cent of the windows installed in Germany are insulated with PU foam.

Why do window fitters still reach for installation foam so often?

To install a window according to the rules of technology means to fix, insulate and seal (see e.g. VOB/C ATV DIN 18355, Joinery, Section 3.5, Installation), it always requires professional and careful processing and execution in addition to a suitable choice of materials. As previously mentioned, these are, of course, the great insulating properties of PU foam, i.e. its low thermal conductivity. Furthermore, it is very efficient and economical to process. The ift investigated this particular aspect in a study in 2005. Compared to insulating materials that are stuffed into the joint, joint insulation with PU foam is at least twice as fast. In unfavourable installation situations, it can even be up to five times faster with PU foam. For example, when you are dealing with uneven, difficult to access joints in an old building.

And what's with the “miracle foams” that are reported on again and again?

It is true that a few suppliers advertise that their foam can cover all the necessary functions on its own. I can only say from my own experience that no such foam has ever been successfully tested in this respect at the ift Rosenheim. We have been developing test methods for several years. However, to date, no foam has been presented to our institute that has passed all the necessary tests.

About Wolfgang Jehl

Dipl.-Ing. (FH) Wolfgang Jehl, Product Manager for assembly, windows and external finishes at ift Rosenheim. As an expert, he is in charge of preparing the RAL guidelines for the planning and execution of the installation of windows and front doors for new buildings and renovations.