As I understand it, quoted Uw values for windows don't have to take into account the presence of any trickle vents (do correct me if I'm wrong on that).

It seems obvious that an open trickle vent is going to compromise the U-value, but is there a way of quantifying it, or is anyone aware of any standardised way of estimating it?

Every time I see a nice new well insulated window with a trickle vent in it, I wonder just how much of an improvement it actually is on the slightly leaky single glazed window it replaced. Does the presence of the trickle vent compromise it to such an extent that it's hardly worth the cost of replacing the old window, unless you have a more thoughtful ventilation strategy?

I know that in theory trickle vents can be adjusted, but I'm sure the reality is that most of them are either always open or always closed.

Occasionally I come up against the argument that it's dangerous to change windows in older houses to double glazing because it will create condensation problems. My answer of course is that it won't if proper consideration is given to ventilation. And in cases where the budget is small, trickle vents might be the only realistic means of providing that.

I do not like trickle vents either but accept in some situations they are necessary. I do not have any in my build even though I have not done a passive house per se I have adopted the general principles of such a construction so I have a MVHR. Interestingly when the BCO called for an inspection he remarked that my installation was the first he had come across. Next time he came he said when he left me the next job he came to had installed MVHR but was not going to work properly. The builder had installed windows with trickle vents!. We live in a very windy area and have heard people state that the wind whistles through the vents and house owners have been known to squirt expanding foam into them to silence them.

At some point the building regs recognised that self-closing fire doors don't work in a domestic setting (people just wedge them open) and removed the requirement. Same needs to happen for trickle vents I think. It would be better to stipulate that all windows should have a night-latch type feature. Then it is clear to the occupants whether there's ventilation happening or not. Some people would still leave all their windows closed whilst cooking etc and have condensation problems...but they are probably the same people who would seal up trickle vents.

Are there any other jurisdictions which require trickle vents? I know they're not required in North America ...

Paul in Montreal.

My view is that there is virtually no influence on U value on a calm day, but as wind speed increases so do heat losses

Alarmingly.