Frame Sealant: Acrylic, Silicone, or MS Polymer for UK Window and Door Perimeters

Your builder hands you a cartridge and tells you to seal around the new windows yourself to save a day on the schedule. You walk into Screwfix and pick up a tube of bathroom sealant because you recognise the brand. Six months later the seal looks grubby, has pulled away from the brickwork in two places, and water is tracking down the inside of the reveal during driven rain. Wrong product. The sanitary silicone you bought is high-modulus, formulated for static joints between tiles and a bath, not for the differential thermal movement of a window frame against brick. The fix is to scrape it all out with a Stanley knife and start again with the correct product. This page is what you should have read first.

What it is and what it's for

Frame sealant is a flexible, gun-applied jointing compound used to seal the perimeter gap between a window or door frame and the surrounding masonry. It does three jobs: weatherproofs the frame against wind-driven rain, contributes to the airtight envelope of the building, and accommodates the differential movement that happens when uPVC, aluminium, or timber frames expand and contract at different rates from brickwork.

It is not the same product as the silicone you use around a bath. The chemistry is different, the movement class is different, and the substrates it bonds to are different. A frame on a south-facing elevation can move 3-5mm across a year as the temperature swings from -5°C in February to +60°C surface temperature in August. Bathroom sealant cannot accommodate that. Frame sealant can.

There are three product families. They are not interchangeable. Choosing the right one is the most important decision on this page:

Acrylic frame sealant (also called mastic or builder's caulk). Water-based, paintable, lower movement capability. The right choice when the joint will be painted as part of the decoration scheme and movement is modest.

Low-modulus neutral-cure silicone. Solvent-cured, not paintable, higher movement capability. The right choice for unpainted external frames, especially against masonry, where the joint will see direct weather and significant thermal movement.

MS polymer (modified silane) hybrid. Newer chemistry that combines silicone flexibility with acrylic paintability and very high movement accommodation. The right choice when you need both paint and flex, or where joints are wider and movement is high.

The compliance standard you'll see on a competent product datasheet is BS EN 15651:2017. Part 1 covers facade joints (frame-to-wall sealing) and Part 2 covers glazing joints (between glass and frame). Older guidance still references BS EN ISO 11600, which used F-class for facade and G-class for glazing. The two coexist, but for new product compliance look for the EN 15651 classification mark on the cartridge or datasheet. A typical mark looks like F25LM, meaning a facade sealant rated for ±25% movement at low modulus.

Why you'll see it specified

The regulatory reason frame sealant matters is buried in Approved Document L and in the NHBC Standards.

Since the 2021 edition of Approved Document L (England), every new dwelling must achieve a maximum air permeability of 8 m³/(h·m²) at 50Pa, with a notional target of 5 m³/(h·m²). This is verified by an air pressure test (a "blower door" test) on every new build. The largest air leakage paths in a typical extension are the junctions between the window frames and the masonry opening, and between the door frames and the threshold detail. A single poorly sealed window can shift a building from passing to failing the test.

NHBC Chapter 6.7.3 requires the perimeter gap to be sealed with a product that resists water penetration and air leakage while accommodating differential movement. Minimum sealant depth is 5mm. For gaps over 6mm, a closed-cell polyethylene foam backing rod is required behind the sealant to control joint depth and provide a firm backing for tooling. The fillet should overlap 6mm onto the frame and 10mm onto the brick outer leaf where a checked rebate detail is used.

NHBC also makes a point that gets ignored on poorly built sites: sealant is supplementary to a DPC, not a substitute for one. If your bricklayer has skipped a vertical DPC at the abutment with the existing house and proposes to "just mastic it," that is non-compliant work and the inspector will fail it.

The three types compared

A few details from that table are worth pulling out because they catch people out repeatedly.

Acrylic and "permanent flexibility" are marketing language. ±12.5% movement is not a lot. On a 6mm joint that's only 0.75mm of movement either way before the sealant fails. For a stable internal reveal this is fine. For a south-facing external frame on an exposed wall, it isn't.

Acrylic without paint is vulnerable. Several recorded forum cases describe acrylic frame sealants washing off externally in heavy rain when applied to bare unpainted reveals. Exterior-grade acrylic only develops its full water resistance once it has cured fully and (preferably) been overpainted. If the elevation is going to stay unpainted, choose silicone or MS polymer.

Acid-cure silicone fails on masonry. Cheap general-purpose silicones often release acetic acid during cure (you can smell the vinegar). The acid reacts badly with the alkali in cement mortar and brickwork, and the bond fails within months. Always check the cartridge says "neutral cure" or "neutral cure low modulus" before using silicone against masonry. Bostik S41, Dow 791, Soudal Silirub LMN, and Everbuild 825 LM are all neutral-cure low-modulus products that are correctly specified for frame-to-brick.

MS polymer is the most forgiving option. It bonds to almost everything, takes paint, accommodates the most movement, and is increasingly used in trade applications. It costs less than premium silicone in many cases. The disadvantage is that the formulation varies more between brands than acrylic or silicone, so read the datasheet rather than assuming all "hybrid" products are equivalent.

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The single most common mistake

Using bath or kitchen "sanitary" silicone on a window or door frame.

This comes up in every forum thread on the topic, often as the original poster realising months later why their sealant looks bad. Bath silicone is high-modulus (Shore A around 30-40, noticeably stiffer when cured), formulated for the static joint between tiles and a bath where there is no thermal movement. It contains fungicide for bathroom use and resists water sitting on it. None of those properties matter on a window frame.

The right product is low-modulus (Shore A around 15-20, softer and more elastic). Low-modulus silicone is specifically formulated to accommodate the larger differential movement of frames against masonry without splitting or pulling away. The cartridge will say "low modulus" or "frame sealant" or carry a 25LM classification mark.

If you have already applied bath silicone to a frame and you're reading this in a panic: yes, you can remove it. Acrylic comes off like putty with a stiff scraper. Silicone needs a Stanley knife to score the bond and a sealant-removal solvent to dissolve the residue, then a fresh substrate before re-applying the correct product.

How to apply it

Frame sealing is straightforward when the surface preparation is right and slow when it isn't. Two-thirds of the time on a clean job is spent on prep and masking, not on running the bead.

The application sequence:

1. Confirm the substrate has cured. If the bricklayer pointed the reveal mortar yesterday, wait 24-48 hours before sealing. Mortar that's still chemically active will not bond properly with sealant. Touch-test: if the mortar feels cool and slightly damp, it isn't ready.

2. Clean both faces of the joint. Brush out dust, wipe with a dry cloth. Do not seal in damp surfaces; if it has rained, give it half a day in dry weather. For oily substrates (some primed timber, certain aluminium finishes) wipe with isopropyl alcohol.

3. Insert backing rod for any gap over 6mm. Closed-cell polyethylene foam rod, sized about 25% larger than the joint width so it grips when pushed in. The rod controls sealant depth and gives the bead something firm to push against during tooling. Width-to-depth ratio should be 1:1 to 2:1.

4. Mask both sides. Painter's masking tape, 2-3mm clear of the joint edge on each side. This is the difference between a clean professional finish and a smeared mess. Do not skip it because you're "just doing the back of the house."

5. Cut the nozzle to 8-12mm at a 45-degree angle. A larger gap needs a wider cut. Practise the bead on a scrap surface before going onto the frame.

6. Apply a continuous bead at 45 degrees to the joint. Push the gun into the joint, not pull it along the surface. Move at a steady pace. Stopping mid-bead leaves a join that's hard to tool out.

7. Spray the bead with soapy water (a few drops of washing-up liquid in a small spray bottle). This stops the sealant dragging during tooling.

8. Tool the bead smooth with a wet finger, a lollipop stick, or a proprietary sealant spatula. Pull in one direction along the joint. Wipe the tool on a damp rag between strokes.

9. Remove masking tape before the sealant skins. Within 10 minutes for silicone, within 15-20 minutes for acrylic. Lift the tape away from the bead, not towards it.

10. Leave undisturbed. No painting, no touching, no re-tooling. Skin in 10-30 minutes, full cure 24-72 hours depending on the product.

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The single biggest application mistake

Do not seal the bottom of the frame on the outside.

Windows and most external doors have weep paths designed into the sill. These are small openings that let any water that gets into the frame channel drain away to the outside. If you run a continuous bead of sealant along the bottom external joint, you trap moisture inside the frame. It then has nowhere to go except inwards, where it rots timber, corrodes metal, encourages mould growth, and eventually drives water through the wall.

The rule: seal the head and both jambs externally. Leave the bottom sill-to-frame gap open as a drainage path. Internally, you can seal all four sides for air tightness.

This is counter-intuitive because the visual instinct is to seal the whole perimeter. Resist it. Builders who don't know this routinely block weep holes and create future problems they don't see for two years.

Air tightness and the two-faced seal

For a Building Regulations-compliant air pressure test, frame perimeters need to be sealed on both the internal and external faces. The internal bead does most of the air tightness work. The external bead does most of the weatherproofing work. Together they form a continuous airtight envelope at the junction.

Internal sealant is usually acrylic frame sealant or paintable MS polymer because the reveal will be painted as part of decoration. External sealant is usually low-modulus silicone or MS polymer because it has to weather without paint. There is no rule that says both faces use the same product, and on a typical extension they often do not.

The principle of continuity matters. The seal must run unbroken around the head and both jambs (and along the sill internally). A 50mm gap in the bead at one corner is enough to fail an air pressure test. Inspect the work. Run a finger along the bead and look for breaks.

Cost and where to buy

A 310ml cartridge is the standard UK trade size. Some products come in 290ml or 270ml, which matters for per-millilitre comparison.

For a typical 30m² rear extension with three windows and one or two external doors, sealed on both faces, expect to use 8-12 cartridges. Total spend for the materials is £40 – £120, depending on whether you choose acrylic for internal painted reveals (cheaper) or premium silicone for the lot (more expensive).

Pricing tiers, current as of April 2026:

• Acrylic frame sealant: £3 – £7 per cartridge. Budget interior-only acrylic at the lower end (No Nonsense 480 plain). Exterior-grade acrylic at the upper end (Everbuild Window & Door External, Soudal Frame Sealant TR).
• Low-modulus neutral-cure silicone: £6 – £12 per cartridge. Soudal Trade Exterior Frame Silicone is the budget end on a 270ml tube. Bostik S41 sits in the middle. Dow 791 and UniBond Outdoor Window & Door are at the premium end of the range.
• MS polymer hybrid: £4 – £9 per cartridge. Bond It LMH25 is the budget end. Geocel The Works Pro and Soudal Fix All sit at the upper end of that range.

Where to buy: Screwfix, Toolstation, Wickes, B&Q, Travis Perkins, and Jewson all stock the major brands. Trade-grade silicone and MS polymer are easier to find at Toolstation and Screwfix than at DIY chains. For a one-off small project, Wickes own-brand acrylic is fine for internal painted reveals and saves a few pounds per cartridge.

You will also need a skeleton sealant gun unless you already have one: £5 – £20 at any builders' merchant. The lower end of that range covers a basic gun adequate for a few cartridges; if you're sealing more than 10 cartridges in one session, spend at the top of the range on a dripless model with a thumb release.

Alternatives

For some applications a wet sealant is not the best answer. Two product families to know about:

Compressible expanding tape (illbruck Compriband, ISO-BLOCO). A pre-compressed impregnated foam tape applied between the frame and the reveal during installation. After installation it expands to fill the joint and provides a weather and air seal without any wet sealant. The advantage is that the seal is installed during frame fitting, which is faster, and the foam can absorb much larger movement than any wet sealant. The disadvantage is that it is a specification-grade product that costs more upfront and is rarely used on standard DIY extensions. Used routinely on Passivhaus and high-performance commercial work.

Cavity closers are a separate but related product. They are pre-formed insulated frames installed at the reveal during wall construction, closing the cavity at every opening. They prevent thermal bridging at the reveal. They do not replace perimeter sealant; the sealant goes around the frame after the closer is installed. See the cavity-closer page for detail on specification and pricing.

For DPC junctions specifically (the vertical DPC where a new extension wall meets the existing house), a low-modulus neutral-cure silicone or specialist mastic is what NHBC requires. The job here is similar to frame sealing: bond a flexible weatherproof seal to two substrates that may move differently. Acrylic is acceptable internally if the joint will be painted; externally, use silicone.

Shelf life and storage

Sealant cartridges have a finite shelf life. Acrylic typically 12-36 months from manufacture (Everbuild exterior grade states 36 months; budget acrylic closer to 12). Silicone is 12-18 months. MS polymer 12-18 months.

Check the date code printed on the base of the cartridge before buying or using. If the cartridge feels rock-hard when squeezed by hand, the product has gone off in the tube and will not extrude properly. Throw it away. Trying to use cured-out sealant produces a stringy bead that won't tool flat and bonds poorly.

Store cartridges cool, dry, and out of direct sunlight. 5-25°C is ideal. Cartridges left in a hot van during summer cure prematurely. Cartridges stored in a damp shed for two winters absorb moisture through the nozzle and skin internally.

Once a cartridge has been opened, it is not really reusable. Some people stick a long screw or nail into the nozzle to plug it, and you can sometimes get a second use the next day, but for a job that matters, open a fresh cartridge.

Where you'll need this

Frame sealant comes up at several stages of any extension or renovation project:

• Walls and blockwork - sealing the vertical DPC at the junction where the new extension meets the existing house wall
• Windows and doors - perimeter sealing of every frame to brickwork, on both internal and external faces
• Damp proof course - weatherproofing DPC at corners and external edges
• Decoration - overpaintable acrylic finishing around frames before final paint coats

These applications appear across all extension and renovation projects, not just kitchen extensions. Any time a frame is set into masonry, frame sealant is part of the install.

Common mistakes

Using bath or kitchen sanitary silicone on a frame. The single most common error. Wrong modulus, wrong formulation, will fail within a year on an exposed elevation. Always check the cartridge says "frame sealant" or "low modulus" or carries a 25LM classification mark.

Sealing the bottom external sill gap. Blocks the weep paths. Traps water inside the frame channel. Causes timber rot, metal corrosion, and frame degradation that takes years to become visible. Seal head and jambs externally; leave the sill drainage gap open.

Acid-cure silicone on masonry. Acetic acid release during cure breaks down the bond against alkaline cement and brick. Always use neutral-cure silicone where the substrate is masonry. The cartridge will state cure type explicitly.

No backing rod in joints over 6mm. Without a foam backing rod the sealant has nothing firm to push against during tooling, the bead becomes too deep to cure properly, and the seal fails early under movement. Closed-cell PE foam rod from any sealant supplier, sized 25% over the joint width.

Leaving acrylic external reveals unpainted. Exterior acrylic needs overpainting within a reasonable timeframe to develop full water resistance. If you're not going to paint the elevation, use silicone or MS polymer instead.

Using up a cartridge that's gone hard. Cured-out sealant produces a stringy bead that won't tool flat and bonds poorly. Check the cartridge by squeezing it before opening. If it feels rock-solid, it's binned.

Skipping the masking tape. A clean professional finish is 30 seconds of masking per joint. The difference between a tidy reveal and a smeared mess is the tape, not the technique.

Painting acrylic before it has skinned. Acrylic is paintable from approximately 1 hour after skin formation. Painting before the skin has formed pulls the sealant away from the joint and produces a wrinkled finish that has to be redone.