I am afraid you are very much mistaken there @DiyNutJob . ALL sealed units have dessicant . Also very very few sealed units are sealed in wood with putty these days. No manufacturer of sealed units will warranty a unit puttied in. One , due to Linseed oil degrading seals and two, putty cracks over time and let's water in , and a lot of wooden older frames had no 'drainage ' ( as upvc does ) and thus this lead to those with cracked putty ' sitting ' in water
double glazing installed into timber these days always has a 5-6mm gap and the sash is drained and vented.
the beading may be fitted with rubber seals, butyl tape or low mod silicone, but the air gap behind is maintained.