A chap was showing me his van which had marine roof hatches. I asked him where his ventilation was and got a blank look. I have permanent venting skylights, but to some folk ventilation does not seem important. What do you do?
My VW T4 was a ex mobility Transporter and had windows all around and all fixed - ventilation was achieved by cracking open the cab windows
My VW T5 was the Shuttle model - so had windows all around but the two in the sliding doors were openers. But as they slid open, not useful in the rain so again it was the cab windows getting cracked open....
My VW LT was a ex-minbus - so yet again windows all around and this time just one window had a small top section that slides open.
But for THIS build, and from what I learned from living with the previous VW builds, one of the very first things I bought before I even started the conversion was a Maxxair Deluxe Fan and I can't recommend it enough. With the cover down it has a small amount of ventilation possible; The cover will raise and the fan can be used in the rain and when driving; The fan will adjust its speed automatically depending on difference between actual and target temperature you set and will even turn itself on and off as required (this time of year I see it shut down and lid drops in the evening and then start itself up come morning time).
An expensive bit of kit but worth the outlay and would be an automatic buy for any future build almost certainly.
Permanent ventilation is vitally important if for no other reason than it stops condensation.
My 'Millie' has (cheap) MPK rooflights and they are OKish apart from I can't find any way other than taking them fully apart to clean their insides, but with these fitted, even when they are closed they allow a more than adequate amount of ventilation.
ps.if anyone knows how to clean the insides of MPK rooflights then please let me know as the last thing I want to do is to take them fully apart.
Sylvia was made in a Factory many moons ago, she has a fibreglass shell. (Talbot Talisman) There are marine like rooflights that are sealed but also there is ventilation all round the windows like the sort you get in double glazed window systems
I open the vents when parked though Sylvia seems to have airflow in general and does not suffer from any bad condensation problems
My midi heki has permanent ventilation built in but the rear maxi heki I always leave on the first catch opening which leaves a gap of about 13mm at the rear of the light. When parked up for short periods or when in van down time I open the toilet sky light fully. Phil