Adding Mylar

I have a cab about 5 X 3 X 6 and added mylar over my first grow white paint. The temp is running about 86 using my conversion MH, which I want to use until I flower (400w); I'm venting with a 4" inline duct fan (drawing through a homemade carbon filter). Considering the heat, should I use my HPS throughout? How much cooler does an HPS generally run? Enough to make a difference?

I'd just as soon not get a new fan. If I do, can I reduce a 6" inline duct to fit my $5 flange?

Also, what's the easiest method to up the humidity?

Thanks.

timeout said:

Also, what's the easiest method to up the humidity?

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Get yourself a humidifier that doesn't have a shutoff and it can stay on constantly.

This is the one I use:

So I have about 77 cubic feet. If I get a 4" can filter and a growbright fan (HTG Supply), will it both cool and clean?

It's rated at 170cfm.