Is it a gable roof, or hip roof? Gable is the most common. Second is what is the walls made of brick, plywood with siding? Easiest method would be to cut in some vents if you have a gable roof.

The code requirement where I live is as follows.

The total net free ventilating area

shall not be less than 1 to 150 of the area of the space ventilated

except that the total area is permitted to be reduced to 1 to 300,

provided at least 50 percent and not more than 80 percent of the

required ventilating area is provided by ventilators located in

the upper portion of the space to be ventilated at least 3 feet (914

mm) above eave or cornice vents with the balance of the re-

quired ventilation provided by eave or cornice vents. As an al-

ternative, the net free cross-ventilation area may be reduced to

1 to 300when a vapor barrier having a transmission rate not ex-

ceeding 1 perm (57.4 mg/s · m2 · Pa) is installed on the warm

side of the ceiling.

Be sure to add headers / bracing as needed if you have to cut studs to get the vent in.