In article , David Hearn
writes
On 11/02/2011 15:42, wrote:

There's a formula for the vent minimum area based, based on floor area
of the room, but can't recall the % right now.

Is that the case for window replacement (if the existing windows do not
have any permanent background ventilation), or just for new
builds/extensions/conversions etc?

I'm afraid the need for background ventilation is re-assessed on window
replacement and it makes no difference if the old ones had it or not.

Be aware though that you are not required to use window trickle vents if
you have other means of background ventilation. Window installers
normally wont be aware of this option so may need some persuasion to
comply with your wishes.
--
fred
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