Foam or caulk for that gap?
It depends on whether you are filling a void, finishing a joint or fixing one material to another. Here is a quick way to narrow it down.
| Job | Gap and movement | Surface and exposure | Finish | Consider |
|---|---|---|---|---|
| Fill a deep void around a pipe or frame | Large, irregular gap; not a visible moving joint | Check the foam suits the substrates and location; protect cured foam where required | Trim after curing and cover or finish as specified | Expanding foam |
| Close a small internal gap at skirting or trim | Narrow joint with limited movement | Dry, interior surfaces | Needs painting | Paintable decorating caulk (often called acrylic mastic) |
| Seal a bathroom or kitchen joint | Narrow joint that may move | Wet or splash-prone area; check compatibility with the surfaces | Usually left unpainted | Sanitary silicone sealant |
| Seal an external frame joint | Joint may move | Check the sealant is rated for exterior weather and the frame material | Choose a compatible colour or paintable product if needed | Suitable exterior silicone or flexible sealant |
| Fix a board or trim in place | Bonding rather than filling a void | Check both substrates and whether the location is damp or outside | Follow product instructions | Grab adhesive |
Rule of thumb: foam fills larger voids, caulk makes a paintable interior finish, silicone seals flexible or wet-area joints, and adhesive bonds materials. “Mastic” is a broad trade term, so check the actual product type rather than relying on the name alone.
For the fuller selector and links to the range, see Which Foam, Sealant or Adhesive Do I Need?.
Before applying, clean the surface and check the product label for gap limits, substrate compatibility, curing time and suitability for moisture or outdoor use. For fire-rated openings, use a specified firestop system rather than general-purpose foam or sealant.