If your home isn't properly ventilated, water vapor can condense and collect on insulation and rafters. This will reduce the effectiveness of the insulation and could damage the house.

Tips on ventilation

Usually one square foot of vent is needed for every 300 square feet of attic area. If you do not have a vapor barrier in this area, the amount of ventilation could be doubled.

• Note that screens and louvers on vents reduce venting capacity, so openings must be increased proportionately. Use a combination of vents, half located low on the roof and half higher up, so air can flow in one and out the other. For example, use soffit and ridge vents.
• Install baffle boards as necessary to prevent insulation from blocking air flow.
• Keep vent screens clean.
• Natural ventilation is usually sufficient but using electrically powered vents can be an option. However, power vents require additional attic ventilators and may interfere with proper furnace and fireplace venting. Choosing the correct size power vent is important.