Do I need a dehumidifier?

I'm in Durban which has pretty high humidity and I'm starting a Grow in an outbuilding with a1000w MH/HPS setup.. There's is about a 1m*1m gap where wind can blow in.. Will I need to close the gap up and get a dehumidifier or not?

Yes, both

Thanks I'm on it... Think a 22l dehumidifier will be sufficient for about 20 to 30plants?

Not sure man, probably not, I have a 16l/day and it does My 7 ok wouldn't expect much more than 20 on mine tho, wouldn't cope

Alryt and how big inline duct fan do I need going off my 1000w light .. 4" enough just to cool the light?