Our garage, although attached to the house, sits out in front and catches the hot afternoon sun in the summer. I recently put radiant barrier foil under the rafters in our house attic and with quite a bit left over, was thinking of putting it under the garage rafters. But, the garage doesn't have soffit vents or a gable vent. I'm thinking of putting a gable vent out front with a fan behind it. For soffit vents, I'm contemplating drilling 1 or 2, 2" holes between each rafter and installing screens in them. This would give me 13-26 vents on each side of the garage, which is about 400 sq ft. Does this sound like sufficient airflow ?

If the garage is 400 square feet, you should have 1/150 of that in net free area of ventilation. 400 divided by 150 is 2.67...that's how many square feet of ventilation you should have. Half of it should be high, such as the gable vent, so it would need to have 1.33 square feet of net free area. Usually free area of a louvered vent runs 50 to 60 percent of total area, so your gable vent should have a total area of about 2.67 square feet. That is quite a bit bigger than a normal gable vent, but one that size should be available.

Then you need the rest of the area in soffit vents. A two-inch hole has a gross area of 3.14 square inches. If you had 52 of them, that would be a total gross area of 163 square inches. Let's assume screening has a free area of 70% of gross area, so your free area would be 114 square inches, or 0.79 square feet. That's about 60 percent of what you need in the soffits. If you drilled three 2-inch holes in each rafter bay, you would be close to the required amount.

I don't know what effect the presence of a fan would have on effective net free area. When it's running, of course, it won't matter, but for such an application, one shouldn't need mechanical ventilation. I would size the vents as required for gravity airflow and leave out the fan, myself.