Sealant and Caulk Types: Silicone, MS Polymer, Acrylic and Fire-Rated — Selection Guide

Summary

Selecting the wrong sealant is one of the most common and costly mistakes in the finishing stages of a construction project. A silicone sealant in the wrong application will fail to adhere, allow mould growth, or be impossible to paint over. An acrylic caulk in a wet area will dissolve or crack within months. Fire-rated sealant used in the wrong compartment may not provide the rated protection.

The range of sealant products available in UK builders' merchants has expanded significantly in recent years. Modified silicone (MS polymer) and hybrid sealants have become the dominant all-purpose choice for many trades because they combine the flexibility of silicone with the paintability of acrylic — but they too have limitations that must be understood.

This guide covers the five main sealant chemistries used in UK construction — silicone, MS polymer, acrylic, polyurethane, and intumescent fire-rated — plus specialist acoustic and structural sealants. The selection table at the end provides a quick reference by substrate and application.

Key Facts

• Silicone — sanitary grade — contains fungicide to resist mould; use in bathrooms, showers, and kitchens; colour-matched ranges available; not paintable
• Silicone — neutral cure — no acetic acid off-gassing; suitable for metals (aluminium, stainless steel), natural stone (granite, marble), and mirror glass; NOT acetoxy type
• Silicone — acetoxy (acetic acid cure) — the most common and cheapest type; smells of vinegar when applied; NOT suitable for metals (causes corrosion), natural stone (stains), or polycarbonate
• MS polymer sealant — hybrid modified silicone; paintable; excellent adhesion to most substrates; available in sanitary grade; significantly more expensive than acetoxy silicone
• Acrylic caulk — water-based; paintable; quick-drying; flexible up to approximately 20%; not suitable for wet areas, external applications, or movement joints; for internal dry joints only
• Polyurethane sealant — structural grade; excellent bond strength; slow cure (3–7 days); paintable; suitable for underground, road joints, and structural bonding; solvent odour
• Intumescent fire sealant — expands on heat exposure to seal the void; rated to BS EN 1366-4; resistance periods of 30–240 minutes depending on product; must be applied to the required depth
• Acoustic sealant — low-modulus, flexible; non-hardening; applied behind partition boards to maintain acoustic separation at junctions
• BS EN ISO 11600 classification — defines sealants by type (F=facade, G=glazing), movement class (7.5/12.5/20/25%), and optional P designation (low modulus, flexible)
• Movement accommodation — higher number = more movement capability; a 25-class sealant accommodates ±25% of the joint width
• Minimum joint width for silicone/MS polymer — typically 3–5mm; optimal 5–10mm; do not use sealant to fill gaps wider than 20–25mm (use foam backer rod + sealant)
• Backer rod — closed-cell polyethylene foam rod installed in deep joints to limit sealant depth; prevents three-sided adhesion and improves flexibility
• Removal — silicone removed with Stanley blade + silicone remover solvent; MS polymer removed similarly; acrylic removed wet with damp cloth; polyurethane removed mechanically

Quick Reference Table: Sealant Selection by Application

Detailed Guidance

Silicone Sealants

Silicone sealants are the most widely used type in construction. They are chemically inert once cured, extremely flexible (capable of accommodating movement over 25% of joint width), and have excellent resistance to water, UV, and temperature extremes (-50°C to +200°C).

Acetoxy cure (most common): Cures by reaction with atmospheric moisture, releasing acetic acid (vinegar smell during cure). Very commonly available and cheap. Suitable for most non-metallic, non-porous substrates.

Do NOT use on:

• Aluminium and other non-ferrous metals (acetic acid causes corrosion)
• Natural stone including granite, marble, and limestone (acid staining)
• Mirror glass (the silver backing is destroyed by acetic acid)
• Polycarbonate (crazing risk)
• Copper (corrosion risk)

Neutral cure silicone: Cures without releasing acetic acid — uses oxime or alkoxy chemistry instead. No smell during cure. Suitable for metals, natural stone, and sensitive substrates. Typically more expensive than acetoxy. This is the correct choice for any situation where acetoxy is inappropriate.

Sanitary grade silicone: Contains a fungicidal additive that inhibits the growth of mould and mildew at the sealant surface. This is the correct type for all wet room applications — baths, showers, sinks, and behind splashbacks. Not all sanitary silicones are neutral cure — check the data sheet if using on natural stone or metal.

Paintability: Standard silicone sealants are not paintable. Emulsion and oil-based paints will not adhere to cured silicone. If the sealant joint will be painted over, use MS polymer or acrylic instead.

MS Polymer Sealants

MS polymer (modified silicone or modified siloxane) sealants are hybrid products that bridge the gap between silicone and acrylic. They are based on silane-terminated polyether or polyurethane chemistry and offer:

• Paintability — can be overpainted with emulsion and oil-based paints after skin-over (typically 1–2 hours)
• Adhesion — excellent bond to most substrates without primer (concrete, masonry, timber, UPVC, painted surfaces)
• Flexibility — typically 25% movement class — comparable to silicone
• No solvents — water-clean during application; low VOC
• Sanitary grade available — MS polymer sanitary sealants combine the paintability and adhesion of MS polymer with the fungicidal protection of sanitary silicone

Limitations: more expensive than acetoxy silicone (typically 2–3× the price per cartridge). Some MS polymers have a longer full cure time (5–7 days vs 24 hours for acetoxy). Not appropriate for glazing (too adhesive and may cause glass stress).

MS polymer has become the preferred all-purpose sealant for many finishing trades — joiners, decorators, and general builders — particularly for internal and external joint sealing that will be painted.

Acrylic Caulk

Acrylic (water-based) caulk is a low-cost filler and sealant for internal dry applications. It is water-based, easy to apply and tool, and can be overpainted immediately or after a short skin-over.

Properties:

• Flexible up to approximately 15–20% movement
• Water-soluble when wet (clean up with damp cloth)
• Quick-drying — paintable in 30–60 minutes
• Very low cost

Limitations:

• Not water-resistant once cured — will dissolve or soften in persistently wet conditions
• Not suitable for external use — UV degrades acrylic; water ingress fails the joint
• Movement class typically 7.5–12.5% — not appropriate for structural movement joints

Correct applications: Internal decorating joints — skirting boards to plaster, coving to ceiling, window boards to plaster, door architraves to plaster. These joints need a small gap-filler that will accept paint and flex slightly with building movement.

Incorrect applications: Bathrooms, external windows, movement joints, any substrate subject to water exposure.

Polyurethane Sealant

Polyurethane (PU) sealants are used where high strength, durability, and paintability are required simultaneously, or where the sealant will be exposed to load-bearing conditions.

Properties:

• Excellent adhesion to concrete, masonry, metal, timber
• Paintable
• High movement accommodation (typically 25% class)
• Suitable for underground use, road joints, flooring joints
• Good chemical resistance

Limitations:

• Slow cure (3–7 days full cure)
• Solvent-based — strong odour; ventilation required during application
• Must be tooled quickly — skin-over begins within 10–20 minutes
• Higher cost than silicone

Applications: Expansion joints in concrete floors, joints in car parks, below-DPC sealing, road and airfield joint sealing, structural bonding applications.

Intumescent Fire-Rated Sealant

Fire-rated intumescent sealants are required wherever a pipe, cable, or duct passes through a fire compartment wall or floor. They expand significantly when exposed to heat (typically 800–1000°C) to seal the void and prevent fire spread.

Performance standards: Products are tested to BS EN 1366-4 (fire resistance of linear joint seals). Performance is expressed in minutes (30, 60, 90, 120, 180, or 240 minutes). The required fire rating must match or exceed the fire compartmentation requirement of the building.

Application requirements:

• The sealant must be applied to the manufacturer's specified minimum depth — typically 30–50mm depending on the rating required; read the technical data sheet
• The void width must be within the tested/assessed range — intumescent products have tested maximum joint widths
• Where pipes are metal (steel, copper), the sealant expands around the pipe to fill the residual gap as the pipe melts or deforms
• Where pipes are plastic (UPVC, ABS), intumescent collars or sleeves are usually required in addition to — or instead of — sealant

Third-party assessment: All fire-rated sealants used in Building Regulations-compliant installations should have a current ETA (European Technical Assessment) or national assessment document (UKCA post-2021). Verify the product's stated performance is appropriate for the specific installation before use.

Common makes: Hilti CP 606 Flexifoam, Sika Firesil, Tremco Spectrem 4 Firestop.

Acoustic Sealant

Acoustic sealant is a permanently flexible, non-hardening sealant used at junctions in acoustic separation systems (party walls, acoustic floating floors, acoustic ceilings). Its purpose is to prevent "flanking transmission" — sound travelling around the edges of the partition through the junction with the floor, ceiling, or adjacent walls.

Key properties:

• Non-hardening — remains permanently flexible
• Applied as a continuous bead behind the plasterboard at every junction
• Typically used with acoustic mineral wool quilt within the partition

Application: Applied to the floor, ceiling, and junction wall before the plasterboard is fixed, so the board face bears down on the sealant bead, creating an air-tight acoustic seal. The sealant must be continuous — any gap breaks the acoustic isolation.

BS EN ISO 11600 Classification

BS EN ISO 11600 is the European (and adopted UK) standard for building and construction sealants. The classification system works as follows:

• F — Facade sealants (external joints in walls and cladding)
• G — Glazing sealants (contact with glass)
• 7.5 / 12.5 / 20 / 25 — movement accommodation class (% of joint width the sealant can accommodate)
• P — low modulus (flexible) sealant designation; P-class sealants exert less stress on the substrate as they move

Example: A sealant classified 25 HM F is a facade sealant with 25% movement class and high modulus. A 25 LM G P sealant is a low-modulus glazing sealant suitable for structural glazing where the sealant must remain flexible without stressing the glass.

For most residential and light commercial work, the classification is referenced on the product data sheet — the key information to check is the movement class and whether it is F or G type.

Application Technique

Regardless of sealant type, application technique significantly affects performance:

1. Prepare the substrate — remove all dust, grease, and old sealant; clean with IPA (isopropyl alcohol) wipe for silicone and MS polymer
2. Apply masking tape — for clean finish lines on visible joints; apply tape tight to the joint edge
3. Cut the nozzle at 45° — cut to approximately 1–2mm larger than the joint width
4. Apply a continuous bead — no stops and starts; fill the joint in one pass
5. Tool the sealant — within 5 minutes of application (sooner for fast-skinning products), tool with a wet finger or profiling tool to seat the sealant against both sides of the joint and create the correct profile (concave for internal joints, flat for movement joints)
6. Remove masking tape immediately — remove before the sealant skins; pulling tape off cured sealant tears the bead
7. Three-sided adhesion — avoid; sealant should bond to two opposing faces only; use backer rod in deep joints to prevent bonding to the back of the joint

Removal Methods

Frequently Asked Questions

Can I apply silicone over old silicone?

Not successfully. Silicone does not bond to cured silicone. Any residual silicone on the substrate must be completely removed before the new sealant will adhere. Even a thin film will prevent proper adhesion. Use a silicone remover product and a Stanley blade, and wipe down with IPA before applying fresh sealant.

My silicone has gone black with mould — can I clean it?

Mould on silicone can be treated with dilute bleach solution (10% bleach, 90% water) applied with a cotton pad and left for 30–60 minutes. This removes surface mould but will not reverse mould growth into the sealant body. If the sealant is heavily moulded, it should be removed and replaced with fresh sanitary silicone. Anti-mould primers are available that can extend the life of replacement sealant.

What is the difference between a sealant and an adhesive?

Sealants are primarily designed to fill and seal joints — they accommodate movement and prevent water, air, or fire penetration. Adhesives are designed to bond two surfaces together structurally. Many modern MS polymer and hybrid products are dual-purpose (labelled "adhesive and sealant") and can perform both functions to varying degrees, but a dedicated structural adhesive (such as a construction adhesive) will have significantly higher bond strength than even a premium sealant.

How long should sealant last before needing replacement?

Well-applied silicone in a dry internal joint: 10–20 years. Sanitary silicone in a wet room (bath/shower): 5–10 years depending on cleaning products used (some bleach-based cleaners degrade silicone). External silicone joints subject to UV and movement: 10–15 years. Acrylic caulk in dry internal decorating joints: 3–7 years before cracking or peeling.

Regulations & Standards

• BS EN ISO 11600:2003 — Building construction — jointing products — classification and requirements for sealants

• BS EN 1366-4 — Fire resistance tests for service installations — linear joint seals; standard for fire-rated sealant performance

• BS 8000-16 — Workmanship on building sites: code of practice for sealing joints in buildings using sealants

• UKCA marking — UK Conformity Assessment; fire-rated sealants require UKCA-marked products or equivalent assessment [verify current requirements]

• COSHH Regulations 2002 — solvent-based sealants (PU) require adequate ventilation; read SDS before use

• BSI — BS EN ISO 11600 — Sealant classification and movement requirements

• Sika UK — Sealant Technical Guides — Product selection guides and application data

• Tremco Illbruck — Sealant Selection — Technical guidance on fire-rated and weatherseal products

• NHBC Standards Chapter 7.1 — Weatherproofing — Joint sealing requirements for new-build weatherproofing

• BBA Certificates — Fire-rated Products — Assessment certificates for fire-rated sealants

• HSE — COSHH Guidance — Control of substances guidance for sealant application

• adhesive selector — Adhesive selection guide by substrate

• bath installation — Sealing a bath or shower enclosure

• fire stopping — Fire compartmentation and stopping requirements

• expansion materials — Movement joints and expansion materials