I have seen above assertion many times by parties explaining the merits of ventilating coops. I keep a good number of birds in pens that are separated from surroundings only by 2' x 4' welded wire. Last three mornings I have had birds in those pens nearly covered in frost. Ventilation in those pens can not be beat by any coop unless forced air ventilation were used. I am still an advocate of ventilating coops but frost formation alone is not an indication of poor ventilation.