Sealants
PU sealant or silicone? 5 criteria for the right choice
Both sealants stay flexible and keep water out. The differences show in paintability, substrate, sun, movement and wet areas.
Short answer
Choose PU sealant when the joint will be painted, sits on concrete, stone, metal or timber, or carries foot or vehicle traffic. Choose silicone for glass and ceramics, for wet rooms such as bathrooms and kitchens, and for constant sun or high temperatures. Whichever you choose, the joint only lasts with the right geometry and a backer rod.
Choose PU sealant
- Joints that will be painted
- Concrete, stone, metal and timber joints
- Floors with foot or vehicle traffic
Choose silicone
- Glass and glazing
- Bathrooms, kitchens and wet rooms
- Constant sun and high temperatures
1Will the joint be painted?
This is the quickest way to decide. Most PU sealants can be painted once cured. Cured silicone, on the other hand, has a very low surface energy: paint cannot wet the surface and does not hold.
If a joint on a facade, an interior wall or a window frame must be painted the same colour as the wall, a paintable sealant, meaning PU sealant, is the right choice.
Try a small area before painting. Hard paint films can crack over time on an elastic sealant that moves; flexible paints are a better match.
2What surface will it bond to?
PU sealant bonds strongly to building materials such as concrete, stone, brick, metal and timber, which is why it is common in floor, facade and window joints. Silicone stands out on smooth, non-porous surfaces such as glass, ceramics and enamel.
On porous or dusty surfaces, check the product’s technical data sheet (TDS) to see whether a primer is needed.
3How much sun and heat is there?
Silicone is inherently resistant to UV radiation and high temperatures, which is why it is preferred for glazing joints that face the sun for years. PU sealant can yellow or chalk on the surface after long exposure to the sun. This mostly affects appearance; if the joint is painted, the paint film protects the sealant as well.
4How much does the joint move and wear?
Building sealants are classified to EN 15651 in Europe and to ISO 11600 internationally. In a class such as 25LM, the number is the movement capacity, ±25% of the joint width, and the letters give the modulus: LM low modulus (soft), HM high modulus (stiffer).
A low-modulus sealant puts less stress on the joint edges as it moves, so it suits facade and expansion joints. On floors with foot or vehicle traffic, the tear and abrasion resistance of PU sealant matters most. For floor joints, see our industrial floor guide.
5Is it a wet room that needs hygiene?
In permanently damp areas such as bathrooms, kitchens and showers, sanitary silicones with anti-mould additives are used; they are classified to EN 15651-3. In these areas a sanitary silicone is the better choice than PU sealant.
Comparison table
| Property | PU sealant | Silicone |
|---|---|---|
| Paintability | Mostly yes | No |
| Adhesion to concrete and metal | Strong | Depends on the surface |
| Abrasion and tear resistance | High | Medium |
| Long-term UV resistance | Depends on the formula | High |
| High temperatures | Medium | High |
| Glass and wet rooms | Depends on the application | Suitable |
Which one is right for me?
Answer three questions to see our suggestion.
Application tip: joint geometry
Whichever sealant you choose, it will fail early if the joint is not prepared correctly. As a general rule the sealant depth is about half the joint width (2:1). In narrow joints the depth stays close to the width and does not go below 6 mm; check the product’s TDS for exact figures. A backer rod sets the depth and prevents three-sided adhesion.
- Sealant
- Backer rod
- Concrete
The sealant bonds only to the two sides, with the backer rod at the base, so it stretches like an hourglass and takes the movement easily. W:D ≈ 2:1.
Frequently asked questions
Can silicone be painted over?
Usually not. The surface of cured silicone stops paint from holding. For joints that will be painted, use PU sealant or products their maker states are paintable.
Can PU sealant be used in a bathroom?
Sanitary silicones with anti-mould additives suit permanently damp areas that need hygiene better. PU sealant is the choice for building joints such as floors, facades and window frames.
What does 25LM mean?
It is the class of a sealant that can take movement of ±25% of the joint width and has a low (soft) modulus. EN 15651 and ISO 11600 use this classification.
Why is a backer rod needed?
It sets the depth of the joint and stops the sealant from bonding to the base. A sealant bonded on three sides cannot follow movement and fails early.