That dew on your fancy new windows? It's not a defect. It's proof they're working.
You've just invested in high-performance double or triple glazing. The installer has gone home, you wake up on a crisp autumn morning, and there's condensation all over the outside of your brand new windows. Panic sets in. You're already composing the email to your builder.
Put the email down. This is actually a good sign.
So why is this happening?
Condensation forms when warm, moist air meets a surface that's cooler than its dew point. You've seen it on a cold beer glass on a summer evening — same physics. On windows, it tends to show up on early autumn and spring mornings, especially after a clear night when the sky has acted like a giant heat sink and pulled warmth away from any surface exposed to it.
Here's the key thing: with a standard single-glazed window, heat bleeds through the glass all night, keeping the outer surface just warm enough that dew doesn't form. With a well-insulated double or triple-glazed unit, that heat stays inside where you want it — and the outer pane gets genuinely cold. Cold enough to collect dew.
But it looks a bit rubbish
Fair. It's not exactly the pristine view you were imagining. A couple of things to know:
It usually clears by mid-morning once the sun hits the glass or the air warms up. It tends to be more common in autumn and spring — the "sweet spot" where nights are cold but the air is still relatively humid. And it's worse on south-facing elevations that don't get direct morning sun.
If it's really bothering you, there are hydrophilic coatings that cause water to sheet rather than bead — so instead of droplets you get a thin, nearly invisible film that evaporates faster. Some high-performance glass already has this applied. It doesn't stop the condensation happening, but it makes it far less visible.
When should you actually be worried?
External condensation? Totally fine — see above.
Condensation between the panes? That's a different story. If you're seeing fogging or moisture inside the sealed unit itself, the seal has failed and the unit needs replacing. That's a warranty conversation with your supplier, not a physics lesson.
Internal condensation — on the room-facing surface? That points to high humidity inside your home and inadequate ventilation. Worth addressing, but again, not a window defect.
| Outside surface Good | Your insulation is working. Nothing to do here. |
| Between the panes Act now | Seal failure. Call your supplier — this is a warranty issue. |
| Inside surface (room side) Investigate | Humidity and ventilation issue inside the home. Worth looking into. |
The industry doesn't help here
This is one of those situations where the building industry does a genuinely terrible job of setting expectations. Salespeople will spend hours explaining U-values and solar heat gain coefficients, and then completely forget to mention that the windows might look dewy some mornings. So the homeowner assumes something is broken, the builder gets a panicked call, everyone wastes time.
All it takes is one sentence at handover: "If you see moisture on the outside of your windows on cold mornings, that's normal — it means the insulation is doing its job."
If your installer didn't tell you that, now you know.
