What to Do When Condensation Damages Window Frames in Winter

The call usually comes in the second week of January. A premises manager describing water running off the glass onto the sill every morning, black spots on the rubber seals, and paint starting to lift. Here is the short answer: where the water sits tells you which of three problems you have. On the room-side face of the glass, it is humidity and ventilation. Between the panes, it is a failed sealed unit. On the frame itself, it is usually a thermal bridge.

Three problems, three different fixes, and one of them is free.

That free one is the weep holes, the small drainage slots on the outside of a uPVC or aluminium frame. They block, water sits in the frame chamber, and the frame is destroyed from the inside while everyone is busy wiping the glass. We will come to that. It's the same principle behind every commercial cleaning contract we run: fix the cause, not just the symptom you can see.

Quick answer: Where the water sits tells you the problem. Condensation on the room-side face of the glass is a humidity and ventilation issue. Misting between the panes means the sealed unit has failed and must be replaced. Water on the frame itself usually means a thermal bridge. In every case, check that the frame's weep holes are clear, because blocked drainage is what actually damages frames.

Where is the water, exactly?

Before anyone buys a dehumidifier or rings a glazier, stand at the window on a cold morning and look at where the water actually is. This single observation sorts the problem in about ten seconds, and it is the first thing we do on a winter survey.

The physics behind all of it is the same. Air holds moisture, and warm air holds more of it than cold air. Cool that air down and it eventually reaches its dew point, the temperature at which it can no longer hold what it is carrying. It then deposits the difference on whatever surface is at or below that temperature. In a heated Dublin office in January, the coldest surface in the room is almost always the glass edge, the frame or the window reveal. So that is where the water lands first, every time, before any other surface in the building shows a thing.

Three controls change the outcome: lower the humidity, raise the surface temperature, or move the air. Nothing else really matters.

On the room-side face of the glass

You can touch it and wipe it away. It comes back tomorrow.

This is straightforward surface condensation, and it is a humidity and ventilation problem, not a glazing fault. The room is producing moisture faster than the building can remove it, and the glass is simply the coldest place for that moisture to collect.

It does not mean your windows are broken. It does not mean the building is damp in the structural sense. It does mean that if you keep wiping and nothing else changes, the water will keep arriving until it works its way into the frame joints and the sill. Fixable, and we cover the fixes further down.

Between the panes

You can see the misting but you cannot touch it. A cloth does nothing. Often there is a faint tide mark or a permanent haze on the glass even on a dry afternoon.

The perimeter seal of the sealed unit has failed. Moist air is getting into the cavity between the panes and the desiccant inside the spacer bar, the material whose whole job is absorbing that stray moisture, is saturated and finished.

Be very clear about this one. No amount of ventilation, heating, cleaning or specialist product will fix it. The unit has to be replaced by a glazier. If anyone offers to clean it, or to drill and dry it as a permanent repair, keep your money. We have walked into buildings where a manager was being invoiced monthly for "extra attention" to windows that were physically incapable of being cleaned clear. If you want the fuller explanation of condensation inside double glazing, we have written that up separately.

On the frame itself

The glass may be relatively clear while the frame is running wet, or beaded with droplets along its whole length. Sometimes the plaster or paint immediately around the frame is stained and soft.

That pattern points to a thermal bridge, also called cold bridging: a path that conducts cold straight through the building fabric to the inside surface. When the frame is colder than the glass, the frame is almost certainly aluminium without a thermal break, which is very common in Dublin office blocks put up between the 1970s and the 1990s.

It can also show up on a metal window board, an exposed lintel, or a steel structural element at a reveal. The frame is not faulty as such. It is doing exactly what a lump of uninsulated metal does in a maritime winter.

And what if it is not condensation at all?

Two other things get mistaken for it, and both need a different trade.

Interstitial condensation happens inside the build-up of the wall or the frame construction rather than on a visible surface, where warm moist air passes into the structure and hits its dew point somewhere in the middle of it. You do not see it directly. You see the consequences: blistering paint on a reveal, a soft patch of plaster, corrosion appearing on ironmongery for no obvious reason.

A genuine water ingress leak is the third. The giveaway is timing. Condensation appears on cold mornings and is worst after a cold night. Ingress appears during and after driven rain, usually from one direction, and often stops in dry weather regardless of how cold it is. Track it for a fortnight and note the weather each time. That log tells a building contractor more than any guess will.

Why Dublin buildings get this worse than they should

Ireland's winter is not cold enough to be dry. That is the whole problem in one sentence.

A mild, damp maritime climate keeps outdoor relative humidity high for weeks at a time, often well above 85%, while heating runs indoors and internal moisture climbs. In a genuinely cold continental winter, the outside air is dry, so ventilating actually removes moisture quickly. Here, you are frequently ventilating with air that is already close to saturated, just at a lower temperature. It still helps, because that air warms up indoors and its capacity to carry moisture rises, but it works more slowly than people expect.

Layer the building stock on top of that. Georgian and Victorian conversions around Dublin 2, Dublin 8 and Rathmines carry single glazing, solid uninsulated walls, deep reveals and original timber. They will always be the first to run with water. Then the 1970s to 1990s stock in Sandyford, Leopardstown, Park West and parts of Clondalkin, full of non thermally broken aluminium that behaves like a cold radiator all winter.

Fit-outs make it worse without anyone noticing. We regularly find trickle vents painted shut, taped over, or blocked behind a new blind box. We find floors subdivided into meeting rooms with the original extract and supply left exactly as it was, so half the plan now has no air movement at all. Part F of the Building Regulations governs ventilation provision and Part L governs energy performance, and a fit-out that tightened the envelope without re-balancing ventilation has created the condensation directly.

Hybrid working is the newest cause. A building that sits cold and unoccupied from Friday evening to Monday morning cools right through, then takes a heavy moisture load from a full Monday and Tuesday. Surfaces are still cold. The water goes straight onto the glass and frames. We see the sharpest damage in buildings with the busiest Tuesdays and the emptiest Fridays.

Gyms, food premises and showrooms sit in their own category. A Dublin gym with showers, a studio full of people and a laundry produces a moisture load an office cannot compare with, and the frame damage shows up in one or two winters rather than five.

The damage, by frame type

Condensation absolutely can damage window frames, and it does it differently depending on what the frame is made of. Knowing your material tells you what to watch for.

Timber is the one with a hard deadline. Repeated wetting at the bottom rail and along the glazing bead breaks down the paint or stain film. Once that film cracks, water gets into the end grain at the joints, sits there through a wet Irish winter, and rot follows. The control is the decoration cycle, not the cleaning. Exterior timber in this climate typically needs recoating every three to five years depending on exposure, sooner on a south-west elevation taking the weather off the Irish Sea and the prevailing wind.

uPVC does not rot, which lulls people into ignoring it. The gaskets, drainage slots and hardware are what suffer. Black spotting on the EPDM seals is mould growing in the moisture that sits permanently against the rubber. Here is the warning that matters: bleach and abrasive cream cleaners wreck both the gasket and the frame surface. Bleach hardens EPDM and it starts to shrink and leak. Abrasives take the smooth outer skin off uPVC, and the dulled surface then holds dirt permanently. Use a pH-neutral cleaner and a soft cloth. We have a fuller guide to cleaning uPVC window frames without damaging them.

Aluminium without a thermal break conducts cold straight through the section. It runs with water all winter, and that water then attacks the plaster, the paint, the window board and the sill below. Retrofitting a thermal break into an existing section is generally not possible. The realistic controls are humidity, air movement, background heat, and secondary glazing where the budget allows.

The weep holes nobody checks

This is the part almost nobody knows, and it is the single most useful thing in this article.

uPVC and aluminium windows are not designed to be watertight inside the frame. They are designed to drain. Water that gets past the outer gasket, whether from rain or from condensation running down the glass into the rebate, collects in a chamber inside the frame and leaves through small slots on the external face of the outer frame, usually along the bottom, often behind a small hinged cover. Those are the weep holes, or drainage slots.

They block. Constantly. With dust, moss, spiders and insect debris, wind-blown grit, and very often with paint or silicone applied by someone who did not know what they were covering. In Dublin city centre we find them clogged with a hard black crust of traffic soot and grime within a couple of years.

When they block, the water has nowhere to go. It sits in the frame chamber all winter, corrodes the hinges, screws and locking gear, saturates any timber sub-frame behind, and eventually finds its way past the internal seal and out onto your sill and plaster. Managers then assume it is a leak in the wall. It is not. It is their own drainage backing up.

Clearing them takes seconds. A soft brush, a wooden or plastic probe and a rinse. Never a screwdriver or a metal wire, which will damage the drainage path or the frame section. Ground floor and safely reachable windows only. Anything above ground level is a job for a contractor with the right access equipment, and nobody should be on a ladder at an upper storey to poke at a drainage slot.

Here is what bothers us: this is almost never written into a cleaning specification. A window cleaning contract says "clean glass" and stops there. Frames, gaskets, sills and drainage are the parts that actually fail, and they cost almost nothing to maintain if someone is simply asked to look. It's exactly the gap we close in our own commercial window cleaning service.

Seasonal weep hole and frame check

• Locate the drainage slots on the external face of the outer frame, low down, sometimes behind a small flap
• Clear debris with a soft brush and a plastic or wooden probe only
• Pour a small amount of water into the frame rebate and confirm it drains out within seconds
• Check the external gasket for shrinkage, gaps at the corners or hardening
• Check the internal gasket for black spotting and clean with a pH-neutral product
• Check the sill and reveal below for staining or salt marks
• Note any window where water does not drain, and pass it to a glazier
• Ground floor and safely accessible openings only, everything else goes to a contractor with proper access

Reducing the moisture in the first place

You cannot make the frame warmer overnight. You can lower the moisture in the room today, and that is where the fastest results are.

Ventilation. Open the trickle vents and leave them open through the heating season, which is exactly when people close them. Check that toilet and kitchen extract fans are actually running and not just humming, and that the ducts and grilles are not blocked with lint. Where a fit-out subdivided a floor, get the mechanical ventilation re-balanced by a competent contractor. Regulation 6 of the Safety, Health and Welfare at Work (General Application) Regulations 2007 (S.I. 299/2007) requires sufficient fresh or purified air in enclosed workplaces, and Regulation 7 covers reasonable room temperature. The Health and Safety Authority publishes plain-language guidance on both. The duty sits on the employer occupying the workplace, not on the cleaning contractor and not on the landlord.

Purge ventilation first thing is the cheapest win available. Ten minutes of windows wide open at eight in the morning dumps the overnight moisture load before the heating has to fight it.

Heating. Consistent background heat beats intermittent blasts every time. Surface temperature is the variable that decides whether condensation forms, and a surface that has been cold since Friday will not warm up in twenty minutes on Monday. Buildings on a hybrid pattern should hold a low background temperature over the weekend rather than switching off completely. SEAI publishes practical guidance on ventilation and heat loss in existing buildings that is worth reading before making changes.

Humidity. Buy a hygrometer for each floor. They cost a few euro and they are the cheapest diagnostic tool in the building. Aim for roughly 40% to 60% relative humidity in a heated commercial space. Above 60% through a cold week and you will get condensation somewhere. In gym changing areas, laundries and shower blocks, a dehumidifier running during and after peak use pays for itself in avoided redecoration.

Housekeeping. Do not stack boxes or files on top of the radiator under a window, because that radiator is what keeps the glass and frame above dew point. Keep blinds off the glass overnight where you can, since a closed blind traps a cold, humid layer against the pane and makes that spot the wettest in the room. Move desks and storage a few centimetres off external walls so air can move behind them.

If the moisture is also showing up as musty air rather than visible water, that is a related but separate diagnosis, and we cover it in our piece on why an office smells stale despite daily cleaning.

The winter maintenance schedule to add to your cleaning contract

We were asked to look at an office block in Sandyford a few winters ago. Late 1980s building, aluminium frames with no thermal break, three floors, and every north-facing reveal had lifted plaster from the sill up to about knee height. The manager had been told it was a roof leak and had already paid for two investigations that found nothing.

We walked it on a cold Tuesday morning at half seven, before the heating had properly caught up. The frames were running with water and the glass was comparatively clear, which told us thermal bridge straight away. Then we opened the weep hole covers on the ground floor. Every one of them was packed solid, mostly with old silicone from a re-sealing job and years of grit. Water was standing in the frame chambers.

Nothing exotic was needed. Weep holes cleared and put on a seasonal check, frames and sills dried down after cold mornings on the two worst elevations, gaskets cleaned with a pH-neutral product instead of the bleach spray that had been used previously, hygrometers on each floor, and the weekend heating set to hold a low background temperature instead of switching off entirely on Friday. The following winter the reveals stayed dry. The plaster still had to be made good, but it only had to be done once.

That routine became the schedule below. October to March, month by month.

The split matters. Drying down, gasket cleaning, weep hole clearing, sill inspection and reporting are all cleaning scope, and they belong in a written specification with a frequency against each line. Fan balancing, heating settings, glazing replacement and redecoration are not. Any contractor who tells you they will fix the sealed units is describing work for a glazier, not a cleaner.

Two related cleaning tasks worth writing into the same schedule while you are at it: how you handle marked sills, covered in our note on cleaning window sills, and the frequency and method for the glass itself, which we set out in our guide to commercial window cleaning and in the detail piece on getting glass genuinely streak-free. One caution on home remedies in winter: vinegar as a window cleaner has its uses on glass, but keep acidic solutions off aluminium and off gaskets.

When to stop cleaning and start replacing

There is a point where maintenance is just delaying a bill, and we would rather say so than keep invoicing.

Replace, do not clean, when:

The misting is between the panes. The sealed unit has failed. Nothing else applies.

Timber has soft end grain. Press the bottom rail and the joints firmly with your thumb. If it gives, the rot is already in and paint will only hide it.

Non thermally broken aluminium sits in a space that cannot be ventilated or heated properly, such as an internal meeting room or a basement gym. You will manage it, but you will never win.

Water has been tracking internally for more than a season. Once the sill, the reveal and the plaster behind are wet, you are into building fabric, not glazing. If mould has followed, our guide to mould on office walls in Dublin covers what to do next.

Be realistic about the buildings that cannot be fixed. Single glazing in a Georgian conversion in Dublin 2 will produce condensation on a cold morning no matter how well you manage it. The honest goal there is control and damage limitation, not elimination.

Then there is the lease. Whether the glazing and frames are landlord repair or tenant responsibility depends entirely on the wording of your specific lease and the schedule of condition, and the answer is genuinely different building to building. Read the repairing clause before you commission anything, and take your own legal advice on your particular lease and the works you are considering.

Frequently asked questions

How do I stop condensation on office windows in winter? Lower the humidity, raise the surface temperature and move the air. Open trickle vents, purge ventilate for ten minutes each morning, hold consistent background heat rather than switching the system off overnight and at weekends, and keep relative humidity between roughly 40% and 60%. Clear the frame drainage slots too, so water that does form drains away instead of sitting in the frame.

What does condensation between double glazing mean? The perimeter seal of the sealed unit has failed and the desiccant inside the spacer bar is saturated with moisture. Air and water vapour now move freely into the cavity. Cleaning cannot reach it and ventilation cannot dry it. The unit needs replacing by a glazier. Do not pay anyone to clean glass they physically cannot get between.

Can condensation rot window frames? Yes, particularly timber. Repeated wetting at the bottom rail breaks down the paint film, water enters the end grain, and rot develops from the joints outwards. uPVC and aluminium do not rot, but the gaskets, drainage slots, hinges and locking gear corrode or degrade, and water backing up in the frame chamber damages plaster and sills internally.

What humidity level prevents condensation? Roughly 40% to 60% relative humidity in a heated commercial space is the working target. The exact threshold depends on surface temperature, because condensation forms whenever a surface falls to the dew point of the air touching it. A cold single glazed pane can run wet at 55%, while a warm thermally broken frame stays dry at 60%. Measure with a hygrometer per floor.

What are weep holes and why do they matter? Weep holes are small drainage slots on the external face of a uPVC or aluminium frame that let water escape from inside the frame chamber. They block with dirt, moss, insect debris, paint and silicone. When they block, water sits inside the frame, corrodes the hardware and eventually tracks internally onto sills and plaster. Clearing them takes seconds and prevents the most expensive damage.

Is condensation damage the landlord's responsibility in a commercial lease? It depends on the lease. Full repairing and insuring leases commonly place glazing and frame repair on the tenant, while some leases keep the structure and external envelope with the landlord. The schedule of condition attached at the start of the term usually matters as much as the repairing clause. Check the wording and take your own legal advice on your specific lease.

Get the frames looked after, not just the glass

Most window cleaning contracts in Dublin stop at the glass. That is why the frames are the part that fails.

Premier's commercial window cleaning covers frames, gaskets, sills and weep holes, not just the glass, which is what stops a winter of condensation turning into a repair bill. We do a free site survey across Dublin and hand you a written, room-by-room scope before any quote, including a winter maintenance schedule you can hold us to. If a scheduled clean is not up to standard, we re-do it at no charge.

We are a family-run team based at Mount Argus Mill in Dublin 6W, working across the IFSC, Grand Canal Dock, Ballsbridge, Dundrum, Blanchardstown, Swords, Blackrock and the wider commuter belt, including our wider office cleaning contracts, with a 5.0 rating from over 100 Google reviews.

If you are heading into another winter of wiping the same sills, book the survey in October rather than January. Ring Catalin directly on 086 083 6141 or email enquiries@premiercontractcleaning.ie.

Catalin Fatul

Catalin Fatul is the founder and expert behind Premier Contract Cleaning, dedicated to providing top-notch cleaning solutions and tips. With a passion for cleanliness and a commitment to quality, Catalin brings years of experience in the cleaning industry to help readers maintain pristine spaces. Whether it's offering the latest cleaning hacks or recommending the best products, Catalin's mission is to make cleaning efficient, effective, and enjoyable.