PU foam questions and answers

PU foam sealant is an expanding polyurethane compound. A standard one-component can holds a prepolymer and a propellant. Once it leaves the can, the compound expands, reacts with moisture from the air and from the substrate, and then cures into a rigid cellular structure.

One-component foam cures with moisture and suits the vast majority of installation work. So-called 1.5-component foam contains a separate catalyst that is mixed into the base before use: it cures quickly, but once activated the can has to be used up within about 15 minutes.

True two-component polyurethane systems are mixed by dedicated equipment at the moment of application and belong to a different class of products.

It is used to fill and insulate gaps when installing windows and doors, to seal gaps and cracks, to insulate pipe and cable penetrations, and to insulate balconies and other building structures. Standard PU foam sealant is not a waterproofing material on its own: it can absorb moisture and always needs to be protected from UV.

DIY foam comes with a plastic straw applicator, professional foam with a valve for a foam gun. The gun meters the output more precisely, speeds up the work and usually gives a higher yield from the can. Professional foam normally has noticeably lower post-expansion and a wider application temperature range.

For a small one-off job without special tools, a DIY can with a straw is convenient. For regular installation work, precise metering, critical joints and work at low temperature, professional foam and a gun are the rational choice. When window and door frames are installed, it is especially important to choose a product with low post-expansion.

No. Professional foam is ahead in control, yield, post-expansion and often in temperature range. For the same mass of product, DIY foam usually builds a denser structure, and the higher density can give good thermal insulation performance. The choice depends on the particular job, not only on the name of the grade.

It is the volume of cured foam that can be obtained from one can. It depends on the mass of the active product, the formulation, the density of the foam, the gun and the application conditions. The stated yield is measured under standard conditions — about +23 °C and 50% relative humidity; in freezing or dry weather the actual result can be considerably lower.

Initial expansion is the increase in volume immediately after the product leaves the nozzle. As the pressure drops sharply, the gases dissolved in the prepolymer expand and form the foam. This is why the can has to be shaken thoroughly: without mixing the components, both the cell structure and the yield will be poorer.

This is the further increase in volume after application, while the foam has not fully cured yet. In professional products it is usually much lower than in DIY ones: the published reference figures are 15–60% for professional foam and 200–300% for DIY foam.

If no room is left for expansion, excess foam can press on the frame or another structure and distort it. Before a large job with an unfamiliar product it is worth making a test application.

No. Post-expansion shows how much the volume will increase, while curing pressure shows how hard the foam presses on the structure. The two are related but not proportional: a product with a larger increase in volume does not necessarily build the highest pressure.

The manufacturer prints the cutting time on the can; under standard conditions it is usually a matter of a few tens of minutes. Temperature, humidity and joint thickness affect the result considerably, so before trimming the whole joint, make a test cut and check that the core has already cured.

Under standard conditions full curing takes roughly 12 hours. In cold or dry air the process can stretch over several days. The thicker the joint and the harder it is for moisture to reach its middle, the higher the risk of an uncured core.

One-component foam needs moisture to cure completely. Mist the surface lightly before application and the filled joint after it; when you work in layers, mist every layer. The surface should be damp, but not wet. In freezing weather, mist small areas and foam them immediately, so that the water has no time to turn into ice.

Moisture penetrates the foam effectively to about 3 cm from the edge, so a single bead should not be more than 6 cm in diameter. A large opening is filled in layers: the next layer is applied once a skin has formed on the previous one, and the surface is lightly misted again.

The main difference is the permitted range of ambient and can temperature. Summer products typically work from about +5 to +35 °C, while dedicated winter products can be used at a considerably lower temperature. The exact limits are always printed on the particular can.

No. If the instructions allow warming, the can is held in warm water without a sharp change of temperature. Open flame, a heat gun and other sources of intense heat must not be used: the can is pressurized and may burst if it is overheated.

Cans are stored upright, valve up, normally at a temperature of +5 to +25 °C. The application temperature range and the storage conditions are not the same thing: even winter foam must not be kept in freezing conditions permanently. If the product has frozen, it should be thawed slowly, avoiding rapid heating.

• Wear protective gloves and follow the instructions on the can.
• Shake the can thoroughly before use.
• Mist the substrate lightly; fill a vertical joint from the bottom up.
• Allow for post-expansion and do not overfill the opening.
• After application, mist the joint lightly once more and let it cure completely.

A dense protective skin forms on the surface of cured foam and reduces water absorption. Cutting opens the cell structure, so the joint becomes more vulnerable. It is better to meter the output in advance with expansion in mind, and if trimming is needed anyway, to cover the surface with a finishing material.

Yes. UV radiation destroys polyurethane foam, so a finished joint must not be left exposed for long. Once it is fully cured, it is protected with plaster, sealant, tape, trim or another material provided for by the design of the joint.

Fire-rated foam is used in assemblies with higher fire safety requirements; it is often colored red or pink for identification. The fire resistance rating shows how long the joint keeps its integrity and its insulating capacity under fire. Results can only be compared for identical joint designs: homogeneous ones or combined ones, for example with basalt wool.

Foam adhesive is also made on a polyurethane foam base, but its formulation is designed primarily for bonding. It has relatively low initial expansion and post-expansion and higher bonding strength. Such products are used to install thermal insulation boards, construction blocks, drywall, wood-based materials and other substrates — within the scope of the particular product.