Think All Polyurethane Foams Are the Same? Think Again.

Think All Polyurethane Foams Are the Same? Think Again.

A Practical Guide to Using Polyurethane Foam the Right Way

Polyurethane (PU) mounting foams can be lifesavers in construction and renovation projects. However, if they are not used in the right place, with the right product, and with the correct technique, they can quickly turn into a serious headache. 

In this guide, we’ll walk you through the most common issues with polyurethane filling and mounting foams, along with the real reasons behind them and practical solutions.

Polyurethane foams expand as a result of a chemical reaction. While this property is highly beneficial, it can also cause serious problems if not properly controlled.

The most common issues generally stem from three main causes:

• Choosing the wrong product 
• Incorrect application technique 
• Unsuitable environmental conditions
   

Polyurethane Foam Expands Too Much (Over-Expansion)

One of the most common issues is that the polyurethane foam expands far more than expected after application, causing door frames or window frames to bend.
 

• Situation:

Polyurethane (PU) foam continues to expand as it cures and can exert significant pressure on the surfaces where it is applied.
 

• Solution:

Door frames should always be supported with spacers or wedges, and gaps should only be filled 30 –50% of their volume.

A more practical solution:
Use foams specifically developed for installing door and window frames.
Akfix 806 Door and Window Installation Foam is one of the most suitable products for this application.
 

Polyurethane Foam Not Curing Properly
Many users complain that the foam remains soft inside or does not cure at all.
 

• Situation:

PU foams cure with moisture in the air. If the environment is too dry, or if too much foam is applied into a deep cavity in one go, the inner layers cannot receive enough moisture and may remain liquid.
 

• Solution:

Lightly mist the surface with water before application and between layers. This helps the foam cure faster and creates a stronger cellular structure.


A practical solution:
For filling larger gaps, use foams designed specifically for cavity filling.
Akfix 805 General Purpose Polyurethane Foam is ideal for this purpose.
 

Cleaning and Unwanted Stains
The warning “If it gets on your hands, it won’t come off” is not a myth — it’s true.
 

• Situation:

When PU foam gets onto unwanted surfaces (flooring, marble, hands, clothes, skin, etc.), it is extremely difficult to clean.
 

• Important:

Trying to wipe uncured foam with a cloth or remove it by hand only spreads the stain and may cause it to stick to your skin.
 

• Solution:

Uncured foam can only be removed with acetone or a specialized foam cleaner spray.
Once cured, the only option is mechanical removal (scraping).

Akfix 800C Polyurethane Foam and Gun Cleaner Spray instantly removes uncured foam and is also used for cleaning professional foam guns after application. This prevents foam from hardening inside the gun so it can be reused.

Single-Use Problem
One of the biggest frustrations for DIY users is that after using only half the can, the foam hardens inside the straw.
 

• Situation:

Once a straw-type (manual) foam can is opened, air enters the valve and the internal mechanism can harden quickly, causing the remaining product to be wasted.
 

• Solution:

If your application area is small, choose smaller can sizes (for example 500 ml instead of 750 ml).

If your project is larger or will take longer, it is better to use professional gun-grade foam, because as long as the gun remains attached, the foam can stay usable for months.

If you prefer not to use a foam gun, choose products designed to prevent clogging.

Akfix 940 Safety Valve Polyurethane Foam is designed not to clog after application and can be reused multiple times.
 

Poor UV Resistance (Sunlight Damage)
Foam used on exterior surfaces often turns from orange to dark brown and begins to crumble over time.
 

• Situation: 

PU foams are not resistant to UV radiation. Prolonged exposure to sunlight causes them to degrade, lose their structure, and crumble.
 

• Solution:

After the foam cures and excess material is trimmed, it should always be covered with paint, plaster, or another protective coating.
It should never be left directly exposed to sunlight.

This applies to all polyurethane foams.

Straw or Valve Clogging
One of the most frustrating situations is when polyurethane foam does not come out even though there is still product inside the can.
 

• Situation:

Improper storage conditions, storing the can horizontally, or dried residue around the valve can block the foam outlet.
 

• Warning:

Forcing the valve or attempting to puncture the can is extremely dangerous and can cause an explosion.
 

• Solution:

Foam products must be stored according to the temperature conditions indicated on the can.
Before application, the can should be kept at room temperature for at least 12 hours.

Always store cans upright, and shake vigorously for at least 20–30 seconds before use.
 

Low Yield (Coverage Issues)
“It says 750 ml, but it filled very little space.”
 

• Situation:

The yield of a foam can depends on application technique, temperature, humidity, and application speed.
In cold environments, the foam may not expand properly and can remain inside the can.
 

• Solution:

If you need to apply foam in cold weather, place the can in lukewarm water (never directly on a heater or open flame) to bring it to room temperature.

This can increase yield by up to 25%.

Choosing the Wrong Product
Not every foam is suitable for every job.
Consumers often buy the cheapest or most readily available foam, assuming that all polyurethane foams are the same.

However, not every polyurethane foam solves every problem.

Example problems:

• Using standard mounting foam to install EPS/XPS insulation boards may push the boards out of alignment because of excessive expansion.
• Using summer-grade foam in cold weather can significantly reduce performance.
• Using foam designed for sound or thermal insulation as an adhesive may lead to long-term issues.
• Using low-expansion foam to fill large cavities can cause unnecessary material consumption and economic loss.
• Using high-expansion foam in narrow gaps may lead to deformation.

These examples could be multiplied.
 

• Solution:

Always choose the right product for the right job.

Use:

• Foams specifically designed for sound and thermal insulation
• Adhesive foams for EPS, XPS, and insulation boards
• Appropriate expansion foams depending on the gap size and application type Never assume that one foam fits every application.