Straw foams vs. gun foams: a comprehensive guide to PU foams

One-component polyurethane (PU) construction foams are widely used in building and insulation applications. These foams come in many sizes and forms, and sometimes it might be difficult to decide which is the right product for the task at hand. Our goal is to ease your decision-making by comparing the advantages of PU straw foams and gun foams.

Both gun and straw foams can be used for insulation, filling, sealing and installation work. While both foams can be used for similar applications, the properties of the two products differ significantly.

Straw foams:

• When to use straw foams? Straw foams, often referred to as manual foams, are ideal for small, one-time projects or DIY applications. They are generally more user-friendly than gun foams, as no special skills or equipment are needed. Straw foams are ideal for filling large gaps, sealing cracks, and insulating small areas.
• Ease of use: They are very easy to use and clean. No special equipment or additional investment is required: just shake the can, attach the straw and apply the foam. Once you finish the task, simply dispose of the can and straw.
• Post-expansion: The main difference between gun and straw foams is that straw foams have higher post-expansion. This means that straw foams are excellent for filling large joints and big openings. Because of this high post-expansion, a straw-foam joint must not be filled to more than 50%: you need to leave enough room for the foam to expand. This high post-expansion also means it should not be used to seal pressure-sensitive joints that might warp when the foam expands.
• Bead control and precision: With straw foams, pressure is released when the foam extrudes from the can. This means that the user has less control over the bead size, or the amount of foam dispensed, than with gun foam.
• Foam structure and yield: Straw foams usually have a more solid foam structure than gun foams, and because straw foam has a higher density, its yield will be lower compared to gun foam.

Gun Foams:

• When to use gun foams? Gun foams are often preferred by professionals working on large projects where accuracy and efficiency are needed. Gun foam’s precise dosing and lower post-expansion makes it an ideal choice for installing windows and doors, filling penetrations and sealing connecting joints. However, compared to straw foams, gun foams require additional investment in the form of a specialised foam gun, such as the Penosil FoamGun S1, and a cleaner.

• Ease of use: The foam gun allows the user to apply the foam more precisely, making it easier to achieve an even foam coverage. The foam gun can be reused as long as it is taken care of properly: after each use, it must be cleaned with a foam cleaner, such as Penosil Foam Cleaner 929, to prevent the foam curing inside the gun.

• Post-expansion: Generally, gun foams have lower post-expansion than straw foams, making them more suitable for delicate work, such as installing windows and doors, and filling narrow and pressure-sensitive joints. When using a gun foam, the joint should be filled to around 60-70% with foam.
• Bead control and precision: The foam gun allows better control over foam output and bead size. With gun foams, pressure is released at the gun’s nozzle, resulting in a more consistent foam structure. Because the dosing is precise, there will be less wastage and danger of overfilling the joint.
• Foam structure and yield: Gun foams typically have better and more elastic cell structure than straw foams. They also provide better value for money when a large quantity of foam is needed, because each can has a higher yield.

Best Practices for PU foams:

To achieve optimal results, we recommend following these tips when using PU foam:

• Shake it! Always shake the foam can thoroughly before use to properly mix the PU foam components. If the components are not mixed enough, the foam structure will not be correct. We recommend shaking the can at least 20 times before attaching a straw or gun. If the can is screwed onto the gun without shaking it first, some gas will come out, which has an impact on the foam yield and some of the foam may remain in the can.
• Temperature Matters: Before each use, make sure that the foam can is at room temperature (around 20°C). Even if it’s cold outside, if the foam inside the can is at room temperature, it will have a better structure and yield.
• Humidity is key: One-component PU foam expands and cures by reacting with the moisture in the air. For both straw and gun foams, a previously moistened surface ensures a better foam structure and much faster curing time. Moistening is especially important for deep gaps and dry porous substrates. To moisten the surface, we recommend lightly misting the surface with a spray bottle: the surface should not be dripping with water, but just slightly humid. If you see water droplets, dry them off with a cloth before applying foam: PU foam does not adhere to wet surfaces.

In summary, choosing between straw and gun foam largely depends on the size and requirements of your project. Straw foams are convenient for small, DIY tasks, while gun foams offer greater control and precision for larger, professional projects. When used in small quantities, straw foams, such as Penosil Universal Strawfoam 265, are more user-friendly and economical. When you start a large-scale construction or renovation project, gun foams, such as Penosil All Purpose Gunfoam 172, offer a more versatile and efficient solution.