Water gathering along the bottom edge of the glass and pooling on the sill is one of the most annoying things to find on a cold morning. If you have new upvc windows with condensation around the bottom, it usually has a simple cause, though occasionally it points to something worth checking. Let us walk through why it happens there and what to do about it.
Why water collects at the bottom
The bottom of the glass is the coldest part of the window. Warm air rises and cool air sinks, so the chilliest layer of room air sits low, right against the base of the pane. When that damp air meets the cold glass, it condenses, and gravity does the rest, running the water down to gather along the bottom edge and on the sill.
That is why you see it there rather than in the middle. It is the same condensation you get anywhere on the glass, just collecting at the lowest point.
The usual cause: indoor humidity
In the vast majority of cases, pooling at the bottom of new windows is down to moist indoor air, not a fault. New uPVC windows seal tightly, so the moisture your home produces from cooking, showers, drying washing and breathing overnight cannot leak away like it did with old draughty frames. It builds up and settles on the coldest surface, which is the base of the glass.
The fix is ventilation and humidity control:
- Open the trickle vents along the top of the frame and leave them open
- Air the house for ten to fifteen minutes each morning
- Run extractor fans when cooking and showering
- Dry washing outdoors rather than on radiators
- Consider a small dehumidifier in a persistently damp room
Most homes see the pooling drop right off within a week or two of doing this. Our uPVC windows in York are made to seal tightly, and that tight seal is exactly why household moisture shows up on the glass until you ventilate.
Check the drainage before you worry
uPVC window frames have small drainage slots along the bottom of the outer frame, sometimes hidden behind little flaps. These let rainwater that gets into the frame escape outside. If they get blocked with dirt, leaves or old sealant, water can back up and sit in the frame.
Have a look along the outside bottom edge of the frame. If you spot the slots, check they are clear, and gently poke out any debris. Clear drainage slots are a quick, free thing to rule out.
When it might be more than condensation
Most bottom-of-window water is condensation, but a few signs suggest it is worth a professional look:
- Water appears even in dry weather with the heating off, which hints at a leak rather than condensation
- One window pools badly while the rest stay clear despite the same ventilation
- The frame or sill feels damp underneath, or you notice staining or swelling below the window
- Water seems to come from outside in, especially during driving rain
These can point to a seal that has not bedded in, a gap in the external silicone, or a drainage fault. On a newly fitted window, that should be put right by whoever installed it. Our window and door repairs in York team can inspect the seal, the drainage and the fit, and tell you honestly what is going on.
Look out for misting between the panes
One more thing to separate out. If the moisture is trapped between the two panes of glass, inside the sealed unit, that is a failed seal, not condensation you can wipe. On a new window that is unusual and should be replaced under guarantee. You will know it is this because wiping the glass does nothing, since the water is sealed inside.
What to do first
Start with the free checks. Open and keep the trickle vents open, air the house daily, use your fans, and make sure the outside drainage slots are clear. Give it a fortnight. If one window still pools while the others are fine, or you see water in dry weather or misting inside the unit, call your installer. For a proper look at your windows, request a free quote and we will come and check.
FAQs
Why does condensation gather at the bottom of my windows?
The base of the glass is the coldest part, and cool damp air sinks and settles there. Moisture condenses and runs down to pool along the bottom edge and sill.
Is water at the bottom of new windows a leak?
Usually it is condensation from humid indoor air, not a leak. If water appears in dry weather with the heating off, or the sill feels damp underneath, it is worth having the seal and drainage checked.
What are the little slots at the bottom of my window frame?
They are drainage slots that let rainwater escape from inside the frame to the outside. If they clog with dirt or debris, water can back up, so keeping them clear is worth doing.
Can I stop condensation pooling on my sills?
Yes, mostly by lowering indoor humidity: open trickle vents, air the house daily, use extractor fans and dry washing outdoors. Wiping the sill also stops water soaking in and causing mould.
When should I call someone about it?
Call your installer if one window pools badly while others are clear, if water appears in dry weather, if the sill feels damp underneath, or if you see misting sealed between the panes.
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