FOAM EXPANDS. DO NOT FILL THE JOINT TO THE TOP.
It looks simple: shake, apply, wait, trim. Most mistakes happen precisely between those four steps.
PU foam expands after application. A good result does not come from filling the joint as much as possible, but from dosing the right amount and letting the foam develop the volume itself.
CLEAN THE SUBSTRATE
The foam has to bond to the building material, not to the dust on it. Before application remove:
- dust
- grease
- loose particles
- plaster residues
- ice and frost
- other contamination
On masonry, concrete and other dusty substrates use a brush or a vacuum.
ImportantPU mounting foam adheres well to many building materials but not equally to all surfaces. Materials such as PE, PP and PTFE usually give very low adhesion. On an unfamiliar substrate, test first.MIST LIGHTLY
Standard one-component PU foam uses moisture from the air and the substrate to cure properly. On dry mineral substrates light misting before application helps with:
- more even curing
- a better cell structure
- better foam development
- faster cure
HowUse a sprayer and apply a fine mist. Do not soak the wall — the surface should be slightly damp, not covered in water. When applying several layers, mist lightly between them as well.ImportantThis applies mainly to standard one-component mounting foams. Two-component foams and some specialised products have different requirements — always check the data sheet.SHAKE THE CANISTER PROPERLY
One of the most underrated steps. The canister holds different components that must be mixed properly before use. Shake it vigorously at least about 20 times, or as the product instructions state. During longer work, shake it again periodically.
Insufficiently shaken foam can give:
- an uneven structure
- larger voids
- poor yield
- unstable expansion
- material left in the canister
CHECK THE TEMPERATURE
Temperature strongly affects how PU foam behaves. A canister that is too cold gives weaker, harder-to-control application. Store and condition the product within the range stated in the data sheet.
Do not do thisNever warm the canister with a torch, on a radiator, with a hot air gun or in boiling or very hot water. It is a pressurised aerosol container. Work only within the temperature range set by the manufacturer.HOLD THE CANISTER CORRECTLY
Most standard hand-held and gun foams are used with the valve pointing down, so the content feeds correctly to the valve and the product can be used efficiently.
With gun foam set the flow with the gun screw and start low. Adding more foam is easier than repairing an overfilled joint.
DO NOT FILL THE WHOLE VOLUME
PU foam keeps expanding after application. Apply only part of the final volume and let the material expand. Some manufacturers recommend about one third of the cavity, others about half or more.
There is no universal ruleDo not assume hand foam means one third and gun foam means one half. Gun foams often allow more precise dosing and many have lower post-expansion, but actual behaviour depends on the formulation. Check the post-expansion figure and the application instructions in the data sheet.On siteIf you are unsure how much a given foam expands, start with less. You can always add after it has expanded.WORK BOTTOM UP IN VERTICAL JOINTS
In a vertical joint start at the bottom and move the applicator upwards. You get better control over the quantity and over filling the cavity.
Do not move the applicator too fast. Aim for a continuous, even bead of foam, not separate lumps.
FILL LARGE CAVITIES IN LAYERS
Do not try to fill a large, deep cavity in one go. One-component PU foam cures with moisture, and in too large a mass the core can cure far more slowly.
For large cavitiesapply a layer → mist lightly → apply the next layer. Follow the times and method specified for the product.This gives more controlled expansion and a more even foam structure.
DO NOT OVERFILL AROUND FRAMES
One of the most expensive installation mistakes. Too much foam between wall and frame can exert pressure as it expands. The result can be:
- a bowed frame
- an altered gap around the leaf
- difficult closing
- a door that starts to bind
Before foaming, the frame must be levelled, correctly positioned, fixed and braced where the installation system requires it. Wedges and spacers are not removed immediately after foaming — wait for the cure required by the installation system and the product data.
Very importantPU mounting foam must not be treated as a substitute for the mechanical fixing of a window or door. Where the system requires anchors, screws or other fasteners, they have to be used. The foam primarily fills, insulates and seals the installation joint.TRIM ONLY WHEN THE FOAM IS READY
Skin formation, cutting time and full cure are different properties. The foam can feel hard outside while the core is not cured enough. Cutting too early damages its structure.
Wait for the cutting time stated in the data sheet of the specific foam. It depends on:
- temperature
- humidity
- joint size
- the amount of foam
- the formulation
Then trim the excess with a sharp knife.
FRESH AND CURED FOAM CLEAN DIFFERENTLY
Fresh PU foam is cleaned with a suitable PU foam cleaner while still uncured. Keep the cleaner to hand before you start.
Once cured, an ordinary cleaner no longer dissolves it effectively. Then it comes off mechanically — knife, scraper or another suitable tool.
STRUKTORR ruleFoam on the frame gets cleaned now, not tomorrow.DO NOT LEAVE FOAM EXPOSED TO THE SUN
Cured PU foam has good thermal and acoustic properties but is not made to stay permanently exposed to UV. Sunlight gradually degrades the surface — the foam yellows, darkens, becomes friable and loses its surface structure.
After curing and trimming, protect it with a suitable finishing layer. Depending on the detail this can be:
- render
- filler
- paint
- a suitable sealant
- installation tape
- a profile
- another part of the façade or window system
RememberPU foam is not a finishing façade layer.IN WINDOWS THE FOAM IS ONLY PART OF THE DETAIL
When installing a window the foam mainly fills the space between frame and wall and provides thermal and acoustic insulation. On its own it does not cover every function of the installation joint.
A properly designed connection also has to address:
- water ingress from outside
- airtightness on the inside
- water vapour
- UV protection
- movement between frame and structure
In professional window installation the foam is therefore part of the whole installation system, not the only material in the joint.
- UNSHAKEN CANISTERYou get an uneven structure and unpredictable behaviour. Fix: shake well before use and periodically during work.
- TOO MUCH FOAMThe joint overflows, consumption rises and sensitive elements can deform. Fix: leave room for expansion.
- FOAM ON DUSTThe foam can come away with the dust in one piece. Fix: clean the substrate first.
- TOO MUCH WATERMisting is not flooding. Fix: use only a fine water mist.
- LARGE CAVITY FILLED IN ONE GOThe outside cures while the core stays poorly developed. Fix: work in layers.
- CUTTING TOO EARLYYou destroy a structure that has not cured yet. Fix: observe the cutting time from the data sheet.
- FOAM LEFT EXPOSED ON THE FAÇADEUV gradually degrades it. Fix: protect it with a suitable finishing layer after installation.
STRUKTORR PRODUCTS FOR THIS JOB
- STRUKTORR Pro Mounting Foam 300H
- STRUKTORR Pro Mounting Foam 800H Expert
- STRUKTORR Pro Mounting Foam 800G Expert
Work in a well-ventilated space and follow the safety information on the packaging and in the product safety data sheet.
HAND-HELD OR GUN FOAM?
Expansion, however, depends on the specific foam, not only on whether it is hand-held or gun-grade.
HOW MUCH FOAM DO YOU NEED?
The approximate volume of a joint is width × depth × length. For example 3 cm × 3 cm × 100 cm = 900 cm³, that is 0.9 litres of final volume per linear metre.
That does not mean applying 0.9 litres of fresh foam — PU foam gains volume after application. The declared yield of a canister is also measured under specific laboratory conditions and should not be taken as the real yield in every installation joint.