Dealing with Heat in Your Grow Room Area
The first and most important thing to consider before all others in the summer is the temperature.
The best advice I can give you in dealing with the out of control summer heat is to get an air conditioner, add CO2, and seal your area up. If you can control the temperature in a sealed up growroom, with CO2 added, you've achieved a "state-of-the-art" indoor gardening atmosphere. Then, when the summer heat is over and it cools down, you can go back to your "winter setup" with exhaust fans on a thermostat or other atmosphere control device.
If you have to run air conditioning, then consider a portable air conditioner that has two duct's attached to it. These are available at mainstream hardware stores for around $400-$600. This should be a sealed up air conditioner, and will draw outside air in through one 4" duct, then exhaust hot air through the other. This type of air conditioner usually won't draw air from your room through it's exhaust duct, which will help to conserve CO2 in your room. (You may want to check with the manufacturer of the air conditioner you're interested in to confirm that this is the case with that particular model prior to making the purchase.) Portable air conditioners with only one exhaust duct, will draw air from your room, and exhaust it outside, with your CO2.
If you've got to run exhaust 100% of the time that lights are on in order to maintain a preferred temperature, then CO2 may have to be sacrificed.
Although you can add CO2 to the area where the fresh air is coming from, and just accept having a higher CO2 cost for the summer because it's being exhausted out at a higher rate.
If you sealed the area up (except for venting your lights separately), and added air conditioning so you wouldn't have to exhaust out of the area, then you could run your CO2 in the area without wasting it. Unfortunately, this is probably the best answer I can give you for dealing with summer heat.
Even if you run air conditioning, IF you're using CO2, you can run into temps into the low 90's without stressing the plants too much.
Without CO2, you've got a cap of 85 degrees before the plants get seriously stressed.
If there's nothing else you can do, and your temp is going to go over 85 degrees without the CO2, then consider adding a silicate product to help with the plants ability to deal with the heat.
We've stock several quality brands:
Any of these silicates, when added to your nutrient program, will help with heat stress with or without CO2 being added to the room. It beefs up the cell walls in the plant tissue, effectively cutting down on transpiration, and making them more resistant to the heat.