It's always advised to not exhaust into an attic since the chances of mold dramatically increases. Considering how hot+humid attics are anyways, adding more heat and moisture to that is a cocktail for something you don't want brewing above you. My .02
For the most part, I agree with this as well. It depends on what you are venting (lights, room, lights and room) it also depends on where your intake is coming from (outside hot humid air, inside cool dry air)
I went with 3 solar 1720cfm attic fans. One in each turbine vent. But I'm not venting humidity, only heat. My room is on a closed loop.
Granted, I didn't do this for my grow, it was a coincidence. I had already put them up there to blow out the heat and cool off the top floor of my house.
I'll have to get a reading on my exhaust temps going into the attic. I'd guess they are in the 90'sF.
Before I put the solar fans in (not exhausting into the attic) on a hot day my attic temps would hit 150+. Now they are about 90-100.
In a case where no one were to do anything with their attic and vent only heat into it, it wouldn't be a problem as long as the attic was well ventilated meaning some place for the attic exhaust to go.
The grow exhaust would actually help cool the attic on a hot day. If your exhaust is over 150F when it hits your attic, you have bigger problems.
Using your 18/6 or 12/12 schedule appropriately will also make a difference. Run at night if you can.
Of course all of this can be avoided completely if you can get it into the attic and then run ducting to a vent on the roof.
