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"has any one exrtacted in to a loft/attic before if so what happened"
Exrtacted? I looked it up in a dictionary so I wouldn't look too stupid if it was a word.
Extracted air to the attic? Grown in the attic?
I grow in my attic and duct the filtered air from my garden (high in oxygen) to my heating system in the winter and outside in the summer. Keeps things cool in summer and warmer in winter and the low pressure in the attic sucks the air we breath from the house up to the attic - more CO2 for the plants.
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As for the lights - go with the 600 Watt and suck the air out of the ceiling of your grow room, it'll keep things cool enough in winter, summer might be hotter. You've got 2'x4'= 8'sq x 50 watts/square foot = 400 watt minimum, but you have 4' to travel and a 400 watt will be too weak on the edges.
The two 250s would cover but you get so little penetration your production will be lessened. Think about taking two measuring cups with 250 ml of water in each - each are held 250 mm (~10") above sand and the water poured in - the depth of the hole the water makes in the sand is the penetration through your canopy. There will be two relatively shallow holes.
Then take 600 ml of water and pour it into sand from 600 mm (~24") - the depth is the penetration that you will get through your canopy. A much deeper hole.
Not exact analogies but that's the general idea - you're not just looking to cover a 2 dimensional area but a 3 dimensional rectangular prism. I believe the term for light intensity is radiant flux.
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