Short answer: At a window, always a neutral-curing product belongs in the joint – acetic-curing silicone attacks paints, wood coatings, aluminium and galvanised components. For wooden windows, a hybrid or MS polymer sealant is the better choice than silicone: it adheres more reliably to glazed and painted surfaces and can be painted over. And the point almost everyone overlooks: with the building connection joint between frame and masonry, the rule is “tighter inside than outside”. Sealing the joint completely with silicone on the outside is a classic mistake – moisture that gets in from inside can then no longer get out.
First: which joint are we talking about?
“Sealing a window” means three completely different places, and something different applies to each. Anyone who mixes them up seals the wrong place with the wrong material.
| Joint | Where | What belongs there |
|---|---|---|
| Building connection joint | between outer frame and masonry | vapour-tight inside, driving-rain-tight and vapour-permeable outside |
| Glazing rebate and glazing beads | between pane and frame | neutral window silicone, UV-resistant |
| Window sill connection | at the sides and to the wall | silicone or hybrid, elastic |
| Sash seal | all round in the sash | rubber or foam profile, no sealant |
The most common mix-up concerns the last row: if a draught comes through the closed window, the sash seal is defective or the fitting is misadjusted. That is fixed with a new seal profile or by readjusting the contact pressure – never with silicone. A sash that has been sealed shut can no longer be opened.
The building connection joint: tighter inside than outside
This is the most important rule in window installation, and it is violated constantly.
Between frame and masonry there is a joint that is usually filled with insulating material. This insulation must not get damp. Water vapour from the room air would migrate in from inside and condense on the cold outer side – so the joint must be closed on the inside so that it is vapour-tight and on the outside so that residual moisture can escape to the outdoors.
In practice that means:
- Inside: vapour-tight connection membrane or a dense sealant. A tight joint is wanted here.
- Outside: driving-rain-tight but vapour-permeable – pre-compressed sealing tape (compression tape), a vapour-permeable membrane or a suitable sealant.
Anyone who instead closes the outer joint over its full area with silicone makes it vapour-tight. Moisture that gets in anyway – and it always does – stays in the insulation. The consequences are soaked connections, mould in the reveal and, in timber construction, a component that slowly rots.
On an existing, functioning window, an additional silicone joint on the outside is uncritical, as long as there is no moisture-sensitive build-up beneath it. For a new installation, the building connection joint has to be executed properly – the RAL installation standard describes exactly that.
Which silicone for which window type?
| Window material | Recommendation | Why |
|---|---|---|
| Wooden window, glazed or painted | Hybrid / MS polymer | adheres to coatings, can be painted over, does not attack paint |
| Wooden window, raw or oiled | Hybrid with primer | oil and resin make adhesion harder |
| Plastic window (PVC) | neutral window silicone | good adhesion to PVC, UV-resistant |
| Aluminium window, anodised | neutral silicone | acetic acid makes aluminium tarnish |
| Powder-coated frames | neutral, adhesion test | adhesion varies depending on the coating |
| Glazing rebate, glazing beads | neutral window silicone | must be permanently UV- and weather-resistant |
| Joint will be painted over | no silicone | acrylic or paintable hybrid |
Wooden windows – the special case
Wooden windows are the most frequent question on this topic, and they really are more demanding than plastic or aluminium. Three reasons:
Wood moves. It swells with moisture and shrinks in dry conditions, considerably more across the grain than along it. The joint has to absorb this movement permanently – a joint that is too narrow tears in the first winter.
The coating is the actual substrate. Silicone does not adhere to the wood, but to the paint or the stain. If that is old, chalking or flaking, the joint comes away together with the coating. Before jointing, the surface should therefore be checked and, where necessary, sanded and recoated.
Acetic-curing products are taboo. The acetic acid released attacks wood coatings, can cause discolouration and, on oak, speeds up dark stains through the tannic acid reaction. Only a neutral-curing product comes into question for wood.
All in all, a lot speaks for a hybrid or MS polymer sealant: it adheres more broadly, can be painted over – which makes the difference on a window that is repainted later – and contains no plasticisers that migrate into the wood. What defines this product group is explained in our guide MS polymer explained.
A note on timing: work on wooden windows belongs in the dry season. In autumn and winter the wood is swollen; a joint applied then is pulled apart in the dry winter air.
Outside: what sun and rain demand
A joint on the weather side lasts considerably less long than one inside. Four requirements decide:
UV resistance. Sanitary silicone does not belong outdoors – it yellows and turns brittle. Building and window silicones are designed for it.
Temperature changes. A south-facing window joint experiences a difference of over 60 kelvin between summer and winter. The joint must be correspondingly wide and must not adhere to the back of the joint as well.
Driving-rain tightness. The joint must repel water that hits it with wind pressure – so it must adhere cleanly to both flanks and have no defects.
Clean water run-off. Silicone must not form a water trap. A joint that forms a trough in which water stands does not last long. The fillet is shaped so that water runs off.
The window sill connection
The window sill is one of the leakiest places on the whole window – and one of the most frequently executed incorrectly.
Outside, the sill must have a slope of at least 5 degrees towards the front and be tied into the reveal at the sides. The connection to the outer frame is closed with an elastic sealant; aluminium sills get a connection profile for this. Important: the sill moves considerably with temperature changes – aluminium expands by several millimetres over a length of two metres. The joint must absorb this movement, so it must neither be too narrow nor adhere to the back of the joint as well.
Inside, the joint between sill and frame is less stressed, but it should be tight: condensation running down the pane collects exactly there. An open joint lets it run into the connection plane.
A common mistake is to mortar the window sill in rigidly and then joint it on the outside. The sill then cannot expand, and the joint tears again every year. The right way is a sliding bearing with an elastic connection.
Glazing rebate and glazing beads
The joint between pane and frame is a topic of its own, because here the sealant has a dual function: it seals and holds the pane in the rebate.
On modern windows this is done with glazing blocks and glazing beads, in part with an additional sealing layer. On old wooden windows, window putty took over both tasks – today it is mostly replaced by a permanently elastic sealant.
Two points are decisive here. First, the sealant must be UV-resistant: the joint lies directly against the pane and receives full sunlight. Second, it must be compatible with the edge seal of the insulating glazing – certain plasticisers can attack it, leading to a misted space between the panes. Products expressly designated for glazing take this into account.
The drainage openings in the rebate must not be closed. They lead water out of the glazing rebate to the outside. Anyone who seals them shut dams the water up in the frame – on wooden windows the direct route to rot damage.
When the window gets wet on the inside
Condensation on the pane or mould in the reveal is rarely a sealant problem – and is still often treated with silicone.
The cause is almost always building physics: warm, humid room air meets a cold surface and releases water there. With a modern window the pane is usually warmer than the surrounding reveal – so the moisture condenses at the edge, not on the glass.
What helps: airing briefly with the window wide open instead of tilting, several times a day for a few minutes. Do not put furniture and curtains directly in front of the window, so that warm air reaches the reveal. With recurring mould in the reveal, there is often a thermal bridge – for example a window that does not sit in the insulation plane after insulation work. That is solved with reveal insulation, not with a cartridge.
Silicone helps in this context at only one point: when water really does penetrate from outside through a cracked connection joint. You can tell by the moisture appearing after rain and not after cold nights.
Step by step: renewing a window joint
1. Remove the old joint
Cut it out completely, swell the residues with silicone remover and push them off. Nothing adheres to old silicone. On the frame, work with a plastic scraper so that the coating is not damaged.
2. Check and clean the substrate
Check the coating for adhesion – chalking or loose stain must be sanded and renewed. Then degrease with isopropanol. Caution with plastic frames: aggressive solvents can attack PVC; test first on a concealed spot or use a cleaner designated for it.
3. Backfill
Insert closed-cell PE backer rod, and a bond-breaker tape for shallow joints. Without backfilling, the sealant adheres to the back of the joint as well, and the joint tears at the first temperature change.
4. Mask, joint, smooth
Mask off both sides of visible edges. Cut the nozzle to the joint width, apply in a single pass, smooth immediately and pull off the masking tape straight afterwards. Shape the fillet so that water runs off to the outside.
5. Cure and protect
Protect fresh joints from rain until the skin has formed. For outdoor work, check the weather forecast – a shower one hour after jointing washes out the fillet.
The most common mistakes
Moisture in the insulation finds no way out. The damage develops unseen in the reveal.
Attacks paint, stain and aluminium. At a window, a neutral product belongs without exception.
It is not designed for UV exposure, yellows and turns brittle after one to two years.
The sash sticks fast or can no longer be closed. A draught at the closed window is fixed with a seal profile or by readjusting the fitting.
The joint adheres to a layer that itself has no hold any more. Renew the coating beforehand, otherwise both come away together.
Frequently asked questions
Which silicone for wooden windows?
A neutral-curing product, better still a hybrid or MS polymer sealant. It adheres more reliably to glazed and painted surfaces, can be painted over and contains no plasticisers that migrate into the wood. Acetic-curing silicone does not belong on wood.
Which silicone for windows outside?
A UV- and weather-resistant window or building silicone, neutral-curing – or a hybrid sealant. Sanitary silicone is unsuitable for outdoor use.
Can you close the building connection joint on the outside with silicone?
Not on a new installation. The joint must be driving-rain-tight but vapour-permeable on the outside so that residual moisture can escape – compression tapes and vapour-permeable membranes exist for this. A full-surface silicone joint makes it vapour-tight and traps moisture.
There is a draught at the closed window – does silicone help?
No. A draught at the closed sash comes from a defective sash seal or a misadjusted fitting. Both are fixed with a new seal profile or by readjusting the contact pressure.
How do I find the right seal profile?
A simple trick: press some modelling clay into the gap between sash and frame, cover it with cling film, close the window and open it again. The impression shows the actual gap width and shape – you can then choose the profile.
Silicone or acrylic at the window?
Inside, on a connection joint that will be painted over, acrylic. Outside and everywhere that movement and moisture come together, silicone or hybrid. Acrylic is not permanently water-resistant and does not belong on the weather side.
When should you joint wooden windows?
In the dry season, that is in summer. In autumn the wood is swollen – a joint applied then is pulled apart in the dry winter and tears.
May the drainage openings in the frame be closed?
Under no circumstances. The small slots on the underside of the outer frame lead water out of the glazing rebate to the outside. If they are sealed shut, the water dams up in the frame – on wooden windows that leads to rot, on plastic windows to water on the window sill.
Why is the reveal mouldy even though the window is tight?
Precisely because of that. A very tight window reduces the air exchange, the room humidity rises and condenses at the coldest point – usually the reveal. The solution is airing briefly with the window wide open and, if necessary, reveal insulation, not more sealant.
Can you paint over the window joint?
Not silicone – paint beads off or cracks. Where painting over is intended, for example on an interior connection joint, acrylic or a paintable hybrid sealant belongs.
How wide should the joint at the window be?
For movement joints in building construction, at least 10 mm applies; on a window with long frame members and strong sunlight, rather more. Joint dimensions are covered in detail in our guide How wide may a silicone joint be.
How long does a window joint last outside?
Depending on orientation, material and execution, about five to ten years, rather less on the south-west side. Like the sanitary joint, it is a maintenance joint and should be checked regularly – a crack in winter is the most expensive time to discover it.
