I draw cool air from my crawl space. The reason that I do not draw it directly from outdoors is that it gets too cold where I live to do this. The air in the crawl space stays cool year-round. In the summer it is cooler than the house air or the outside air and in the winter it is warmer.
You will want to make sure that where you live that the climate is okay for this. You will want some kind of screen so that you do not get bugs in your grow space. Most of us use passive intakes. You will probably want some kind of damper on it so that when you are not running an exhaust fan that it closes.
My grow room is in my unheated garage. Have 2 passive intakes, with an 8 inch exhaust fan. Inside room is 7' x 7' x 7', painted bright ceiling white. Use back half for growing. Have 2 300 watt LED`s. Future plans call for 2 more, when the money`s right. Live in very northern Michigan. Cold in winter, short growing season outside equals an indoor garden. Good luck with your garden.
I agree with DankHobbyist. If you are pulling cold air from outside then you will ffind it easiest to control the grow atmosphere by having a room within a room. Then pull the outside air into the outer room and heat it there so that it is at a suitable temp before going into the grow space. You will find in the winter, the cold and damp outside air, when it comes into the warmer room and is warmed itself to the 70s that the dampness in the air will diminish as the air warms, and the RH will drop a good bit depending on how damp the air is and how warm the room air is. I like to place a heat source next to my intake air for the room so that it can be warmed as it comes in. It takes some trial and error to set this up to be just right, and as the temps swing throughout the season, you will have to continually have to make adjustments to account for the changes but it gets easier after you have dialed it in.
Trail and error for sure. Ain`t it always that way ? Best way to learn IMHO. On grow #6 using my room. Is it dialed in yet ? Enough so that I can grow my own and be proud of it. Can grow year round in it, so I must be doing something right. Always room for improvement (as finances allow).