Choose the Right Product for the Job
Expanding foam, silicone, mastic, sealant, adhesive — different names, very different uses. Answer three quick questions below, or skip straight to the decision table.
Quick Selector
1. What size is the gap or joint?
- Hairline to 12mm — Use a sealant (silicone or acrylic/mastic)
- 12mm to 50mm — Use expanding foam or a polyurethane sealant
- Over 50mm / large void — Use expanding foam (gun grade for best control)
2. Will the joint move over time?
- Yes (windows, doors, frames) — Use a flexible silicone or PU sealant
- No (fixed structural gap) — Expanding foam or rigid adhesive is suitable
3. What do you need to do to the surface afterwards?
- Paint over it — Use acrylic/mastic (decorating caulk) — silicone and foam cannot be painted
- Leave it exposed — Silicone or foam (trim flush) are both fine
- Bond two surfaces — Use a grab adhesive
Full Decision Table
| Scenario | Gap Size | Movement | Paintable | Weather Resistant | Best Choice |
|---|---|---|---|---|---|
| Door / window frame draught seal | Up to 20mm | Yes | No | Yes | Silicone or PU Sealant |
| Skirting board / coving gap | Up to 6mm | Minimal | Yes | No | Decorating Caulk (Acrylic Mastic) |
| Loft / wall cavity void | 20mm–150mm+ | No | No | No | Expanding Foam |
| External brickwork gap | Up to 25mm | Minimal | No | Yes | PU Sealant or Exterior Silicone |
| Kitchen / bathroom joint | Up to 8mm | Minimal | No | Yes (moisture) | Sanitary / Anti-mould Silicone |
| Bonding panels, boards, skirting | N/A | N/A | N/A | Depends on product | Grab Adhesive |
| Fire-stopping penetrations | Up to 25mm | Minimal | Yes | Yes | Intumescent Firestop Sealant |
Product Quick-Links
Expanding Foam
- Fischer B3 Triple X Expanding Hand Held Foam 670ml
- Fischer General Purpose Gun Foam 750ml
- Soudal Expanding Foam Gun Grade 750ml
Silicone Sealant
- Soudal Sanitary Silicone 300ml
- UniBond Anti-Mould Kitchen & Bathroom Sealant White 280ml
- Sista Sanitary Silicone 280ml
- Evo-Stik Trade Frame Silicone 280ml
- Parasilico Pro Glass Silicone Clear 310ml
Mastic / Decorating Caulk
Polyurethane Sealant / Adhesive
- Soudal 40FC Soudaflex Polyurethane Sealant Adhesive 300ml
- Soudal Fix All Crystal Clear Sealant & Adhesive
Grab Adhesive
Firestop
Browse the full Sealants & Adhesives range →
A Few Practical Tips
- Foam expands significantly — apply in layers for large gaps and leave to cure before trimming.
- Silicone cannot be painted — use acrylic/mastic if the joint needs decorating.
- Clean surfaces before applying any sealant or adhesive for best adhesion.
- Check the product datasheet for substrate compatibility, especially on plastics or treated timber.
- Gun grade foam gives more control and better yield than hand-held aerosol cans for larger jobs.