Help! My new double-glazed windows fog-up on the outside.
Can that be that right?
It can come as a bit of a shock to find on a fine autumn morning, that your nice new double-glazed windows are misted up, on the outside, where you can't get at it.  What's going on?

I recall my own dismay when I installed a new office window over two decades ago. I had used a new-fangled soft-coat Low-E glass with superb insulation values, and I was extremely peeved when I found that on some days I had go downstairs and open the door to see who was in the car park.

The problem arises because modern windows are now so well insulated.

With so little heat from the room passing through the glazing to keep the outer leaf of glass above the local dew-point, on many cold and damp mornings the water vapour present in the outside air may well choose to condense on the cold outer pane of glass, with the effects you can see in the pictures on the right.  If nothing else, this shows how effective the heat-retention is. Keep that expensive heat inside, where you want it.

What's the answer?
There is none (although I suppose a set of magnetic window wipers could work well).
The new Building Regulations demand these high insulation values. And don't we all welcome them?  You might consider reversing the sealed unit in just one window to slightly reduce its insulation value.  The small additional heat loss through the window will tend to reduce the incidence of this condensation, and give you at least one window you can see through when the others are fogged.  But it's not every day, and it generally evaporates as the sun comes up, so best think of the comfort you've now got and the money you're saving on the gas bill, and put up with it.
the greenest option
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Last update Mar 2017