There are many varying opinions on the effectiveness of attic ventilation and the purpose attic ventilation serves. Many experts disagree with the conventional wisdom that venting will extend the life expectancy of shingles or that ventilation will keep an attic cooler in summer and drier in winter. Dr Roof has included several articles from different sources discussing ventilation to help you understand the role ventilation plays in your home.
Attic ventilation is considered to be an important part of roofing. Proper ventilation is required by the building code, as well as by the shingle manufacturers for shingle warranties to be valid. In theory proper attic ventilation extends the life of a roof and reduces problems by minimizing the temperature differential between the attic and the outside air. The attic temperature should be close to the outside air temperature. In theory proper ventilation will assist in the removal of heat from the attic keeping it cool in the summer and drier in the winter.
Attic ventilation consists of two components; soffit vents for intake and roof vents for exhaust. As warm air rises it creates a slight suction at the intake vents. This fresh air replaces the warm attic air which is exhausted through the roof vents thereby keeping the attic cooler and drier. For this to work the soffit vents must have clear air flow and cannot be blocked. In some older homes this is difficult or impossible to achieve due to construction of the home.
How much attic ventilation do I need? Building codes and shingle manufacturers generally require 1 square foot of attic ventilation for every 300 square feet of attic floor space or 1/300. Approximately 50% should be intake or soffit vents and 50% should be exhaust or roof vents.
A third component to proper ventilation is mechanical venting, such as fan vents in bathrooms and kitchens which exhaust outside. This type of venting is critical in controlling the relative humidity in the home and reducing the amount of moisture available to escape into the attic. It’s this moisture along with heat loss that creates frost build up in the attic and helps create ice dams.
There is much debate as to the effectiveness of attic ventilation as a whole and especially in the winter months. Contrary to popular belief ventilation will not solve attic moisture problems. CMHC states that adding vents in certain homes may actually contribute to the problem by drawing additional warm moist air into the attic space compounding the problem. Condensation problems in the attic are caused by heat and vapour escaping from the heated living space into the cold attic space due to ineffective or non existent vapour barriers. This warm moist air then forms frost on the underside of the decking before it has an opportunity to vent out of the attic space. The only solution to condensation problems is to reduce the relative humidity in your home and reduce or eliminate the heat loss into the attic by improving the vapour barrier and sealing any areas of heat loss.
Dr Roof encourages you to read the other articles under the Ventilation tab to further assist you in understanding ventilation and to help you make an educated decision when it comes time for you to shingle your home.
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