"As a bit of a trial last night in two bedrooms that are occupied we cracked the windows ever so slightly"

Small openings can shift large amounts of air over time. 

Cleanaire is another brand of HRV.

"eg i think its branz that has it somewhere, a wooden window with single glazing has higher insulation value than a non-thermally broken aluminum double glazing. "

In the real world it's going to depend on the specifics. A large solid aluminium frame with high performing double glazing is going to perform better than if it was wooden with standard single glazing. With smaller or other frame-intensive windows an uninsulated frame wipes out most possible gains from better glass so the wood would perform better. Standard single glazing is only R0.16. 

networkn:

 

I have a question. Would it be feasible/good or bad idea, to simply double/triple glaze SOME windows, in each of the badly affected rooms? Retrofitting our entire house is a massive expense, but if we could dramaticall improve things, by say replacing 1/3 would that be an idea to consider? 

 

Condensation on the window sills is not a problem per se unless it's spilling and causing the wood to rot. Some better quality non thermally broken aluminium had collection channels to prevent that from happening. People associate condensation on the frames as "the" problem" with humidity but you can have humidity problems without visible condensation and you can have sill condensation without relatively humidity being too high. So it's best to have an objective measurement of what the relative humidity is in your house unless you can just tell it's far too high. 

Retrofitting glass without changing the solid aluminium frames won't stop the condensation on the frames, and there is no point in putting triple glazing in frames without a thermal break. Also, solving the thermal problems in a house won't solve a humidity problem if there is not sufficient removal processes such as from rangehoods, shower fans, ventilation / dehumidification. 

"the common problem, especially for landlords is people put their hands next the to the vents and "thats cold" and shut off the ventilation and then wonder why the house is now moldy."

People refusing to run the extractor fans and rangehood because they don't see the benefit or think it'll sting them with electricity costs is a big problem for landlords. There are units that turn on automatically when needed, possibly worth the investment.