Even in Zone 5, heat buildup is a problem to deal with.

In Zone 8, you seriously need pro-active measures whether you plan to keep plants in the GH in Summer or not.

I'm redesigning my hoophouse to allow for pop-top vents.

Depending on what style of GH you're thinking of it should be fairly easy to design in roof vents.

The single most efficient way to control temp in a greenhouse is to allow for convective air flow.

Put your air intakes at the ground level and pop-up exhaust vents in the roof and you'll get a natural air flow as warmed air rises to the roof.

It will cut down heat buildup without using a lot of electricity.

The second way to dissipate heat is to put in a swamp cooler. It's easy to do, you need some mister nozzles, a water filter and a water timer.

Although I haven't found an ideal water timer yet. The inexpensive models are usually limited somehow.

The ball-valve style timers are not optimal for cycling on and off all day... the best diaphram model I found, however, is limited to only three hours per day.

I just built my very first greenhouse about 8 months ago. It's not totally finished but functional. The best decision I made was to put in a full length ridge cap which can be opened by thermally activated levers. I am quite surprised by the amount of cool ground level air it can suck up. On days when it is in the high 20's (Celcius), it actually feels cooler inside the greenhouse.

For small GH ventilation, like the HFGH, see my post above on "beating the heat with a HFGH".

Thanks for the input. I'll put the roof vents back into the plan.