Proper Attic Ventilation: A drafty attic is actually a good thing

Charlene asks,

We have a Cape Cod-style home with attic vents on each end of the attic, along with a ridge vent. The attic is insulated. These vents are very helpful in the hot summer. However, in the winter the upstairs is very cold. I want to cover the attic vents in the winter when the air doesn’t need to flow through like in the summer. Your thoughts?

Our Answer

While a drafty house is a bad thing, drafty attics are actually a good thing when it comes to proper ventilation! Here’s why: Attic insulation works best when it is dry. In the winter, attic insulation can get damp from humidity that works its way up from the heated spaces below. If you allow just two percent moisture to get into fiberglass insulation, the R-Value (resistance to heat loss) goes down by a whopping 33 percent! Having plenty of attic ventilation means that the moisture can dry out and the insulation remains effective.
For the most effective attic ventilation, I recommend continuous soffit and ridge vents. Air will enter the attic at the ridge, run up under the roof sheathing when it carts heat away in summer and moisture away in winter, and exit at the ridge. This 24/7 ventilation solution is far more effective than any other type of mechanical or passive attic ventilation solution.
A Cape Cod home with a partially finished second floor is particularly difficult to insulate. However, I can offer one trick of the trade. Make sure that the joists (first floor ceiling joists/attic floor joists) between the floors are sealed at the ends so that cold air which gets into the attic crawlspaces does not pass through between the first and second floors. To access these areas, open the doors to the attic crawlspace at the eaves and look down. These are the open areas of the joists which must be blocked. Use foam insulation cut to fit in between each open joists to stop air flow across the first floor ceiling, and you’ll achieve proper attic ventilation.