Hello again Sbrus,
You can find answers to all of your questions in my articles under the “Attic” category and by following one additional links below.
Sealing any and all of the air leaks between the attic and your home’s conditioned space is extremely important and you should do it regardless of any attic ventilation improvement attempts – it will help in keeping the attic’s temperature down and save you money on heating and cooling - check "Mold in attic"
The window and the fan (assuming they are located on the opposite sides of the attic) might work fine but for the fan to remove any possible moisture buildup you’d need to replace its thermostat with a humidistat / thermostat combo. Just put “attic fan humidistat” into the Google’s search bar, it’s about $40 - $50, or get a new fan with such controller for about $150.
The power attic vents are rated in CFM’s (air movement in cubic feet per minute). The higher the number, the more exchanges of air per minute it can handle. To find out the power vent size that will accommodate your attic you need to know the square footage of the attic’s floor and multiply it by 0.7.
1000 sq feet attic x 0.7 = 700 CFM - you’d need to get a power attic vent with higher (but closest) or equal CFM’s.
Of course for the attic power vent to operated and circulate attic’s air efficiently you need to have properly sized air intake; just use this simple formula:
CFM of the power vent / 300 = Air intake net free area in feet x 144 = air intake net free area in square inches
700 CFM / 300 = 2.33 2.33 x 144 = 335.52 sq inches
This would be probably the easiest / less labor consuming way of providing attic ventilation; however, you may still find it insufficient. I’ve seen several attics with similar installation and yet, still suffering from moisture and mold presence. I’m assuming (impossible to determine the absolute cause during just a few minutes spent in the attic during a home inspection) that the main reason behind the problem is the power vent’s humidistat sensor located on a gable wall not being able to provide correct moisture level readings.
Maybe, just maybe by installing the power vent near the ridge / center section of the attic and balancing the required intake area between the two gables would solve this problem. Also, I had a chance to observe that gable vents weren’t always capable of removing condensation from the areas along the lower parts of the attic, where the roof meets the attic’s floor.
One of the reasons behind that problem could be insufficient attic floor insulation and lack of vapor barrier.
Soffit intakes and a ridge vent in a regular, undivided attic / under a common roof shape is probably the most effective type of ventilation. If there’s no passage / air flow between the rafter bays I would drill a hole (or more) in each one of them, install a ridge vent, and eliminate / block gable vents.
Again, you have to properly calculate and balance intakes and exhaust, minimum isn’t usually enough with freezing winters, more air flow is better to keep your attic as cold as possible and to remove any unwanted vapor containing air - check "How many attic vents your roof needs
No attic ventilation in a climate with temperatures dropping sometimes below freezing in most cases leads to condensation and mold growth. However, I have to confess that I’ve seen miracle attics with no ventilation, no floor insulation, and no moisture / mold (no explanation
There will always be some temperature differential between the home attic’s environment and the ambient temperature on the exterior. Even if you try to perfectly insulate everything; floor, penetrations, chimney, plumbing vents, entrance, etc., you may still get some frost on the roof decking surface which will eventually change to vapor and… condensate instead of being discharge to exterior.