TL;DR
- Condensation is a surface temperature problem, not a humidity problem. At 20°C and 60% relative humidity the dew point is about 12°C, so anything in the room colder than that gets wet, and the frame is usually the coldest thing there.
- A plain aluminium frame conducts heat well enough that its indoor face can drop below that line and grow frost, even with good double glazing sitting in it.
- A thermal break is a strip of low conductivity plastic inside the profile that stops the cold outer half touching the warm inner half. Adding one takes an aluminium frame from roughly 2.0 down to about 1.0 Btu/hr·sq ft·°F. Half the heat loss, for a component you never see.
- If a quote does not name the system and say it is thermally broken, assume it is not. The two versions look identical once they are painted.
Short version: the water is not coming from the glass or from a failed seal. It is condensing on the coldest strip of metal in the room, and whether that strip exists was decided before anything was manufactured.
New windows, good glass, and every morning in January the bottom of the frame is beaded with water and the sill underneath is damp. Nothing has failed. What has happened is that the frame is running colder than the air in the room can tolerate, and the room is depositing its moisture on the coldest surface available. Whether the frame runs that cold comes down to one component buried inside the profile.
What a thermal break actually is
It is a strip of low conductivity plastic, usually reinforced polyamide, that sits inside an aluminium profile and physically separates the outer half of the frame from the inner half.
Aluminium is an excellent conductor. That is why it is used for saucepans and heat sinks, and it is a serious inconvenience in a window frame, where you want the two sides to stay at different temperatures. In a frame without a break, the outside face and the inside face are the same continuous piece of metal, so on a cold morning the inner face sits somewhere close to the outdoor temperature. The break interrupts that path. The outer half gets cold, the inner half stays near room temperature, and the plastic in between carries almost nothing across.
The word break is doing a lot of work in the name. It is not insulation packed into a cavity. It is a deliberate cut through the metal, bridged by something that conducts badly on purpose.
Why the water lands on the frame and not the glass
Because the frame is colder, and condensation goes to whatever is coldest first.
Air holds a certain amount of water vapour, and the temperature at which it can no longer hold it is the dew point. In an ordinary heated room at 20°C with relative humidity around 60%, the dew point sits near 12°C. Any surface below that will collect water. A modern sealed glazing unit keeps its inner pane well above that line, which is exactly what the gas fill and the coating are for. An unbroken aluminium frame will not, and in a cold snap it can go well below freezing, at which point the beads become a rim of ice.
This is also why the problem is worst in kitchens, bathrooms and bedrooms, and worst overnight. Those rooms carry the most moisture, and the small hours are when the outdoor temperature bottoms out. People reach for a dehumidifier, which helps a little by moving the dew point down, but the underlying issue is a cold strip of metal that should not have been cold.
How to tell whether the frame you are being sold has one
You cannot tell by looking at a finished window, which is the whole problem. Once a profile is powder coated, a broken and an unbroken frame are indistinguishable from either side.
Three things settle it. Ask for the system name and check the manufacturer’s own literature for the phrase thermal break or the equivalent in the local language. Ask for the quoted whole window U-value rather than the glass only figure, since the whole window number is where a bad frame shows up and the glass only number hides it. Then ask to see the cross section drawing for the profile, which is the fastest check of the three because the break is visible in the drawing as a distinct strip between two metal halves.
Any fabricator who genuinely works with thermally broken systems has those drawings and will produce one without being chased, whether that is a workshop like AdKey Aluminium Tanger on the north Moroccan coast or the joinery firm three streets from you. A supplier who cannot find the drawing is telling you something useful.
What the break is actually worth
Roughly half the frame’s heat loss, which is a large return on a component that costs very little.
The National Fenestration Rating Council, which runs the energy rating scheme most window labelling is based on, is blunt about the underlying problem: the biggest disadvantage of aluminium as a frame material is its high thermal conductance, and a simple aluminium frame can easily get cold enough to condense moisture or frost on its inside surfaces. The same source puts the improvement from standard thermal break technology at a drop in frame U-factor from about 2.0 to about 1.0 Btu/hr·sq ft·°F.
Two points worth holding onto from that. The first is that the fix is old, well understood and standard on any serious system, so a supplier offering unbroken profiles for a heated building in a cool climate is cutting a corner rather than offering a variant. The second is that the calculation changes in hot climates, where solar gain through the glass matters more than conduction through the frame, and the argument for a break becomes about air conditioning rather than about heating.
Why this is worth more than comfort
Because water that appears every morning goes somewhere.
Repeated condensation on frames and sills feeds the plaster, the reveal and whatever sits against the wall below, and persistent indoor dampness is not a cosmetic issue. The World Health Organization’s indoor air quality guidelines on dampness and mould connect persistent damp indoors to respiratory symptoms and asthma, and recommend preventing or fixing it rather than managing the symptoms. A frame that wets itself for four months of the year is a small, slow version of exactly that.
If you already have unbroken frames
Replacing the profile is the real fix and it is not cheap, so it is worth being honest about what the cheaper measures do.
- Ventilation and extraction lower the room’s humidity, which lowers the dew point and buys you a few degrees of margin. This genuinely helps and it is the first thing to do.
- Heating the room more evenly, especially overnight, raises the frame’s surface temperature. Rooms that go cold and are then heated hard in the morning condense worst.
- Secondary glazing or an internal panel adds a warmer surface in front of the cold one. It works, and it looks like what it is.
- Wiping the frames daily is not a solution, but it does stop the water reaching the plaster while you decide.
Winter is also when the rest of the house shows its weaknesses, and most of that is seasonal maintenance you can plan for rather than react to.
The thing to take from all of this is that the component doing the damage is invisible, cheap, and specified before anything is manufactured. Nobody stands in a showroom and asks about a strip of polyamide. It is, all the same, the difference between a window that sits quietly in the wall for thirty years and one that hands you a wet sill every winter morning and never explains why.
