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  • Writer's pictureAndrew Bargen

Full of Hot Air - Why attic ventilation is critical.

Mould. Ice Damming. Water Damage. Shorter Shingle Life. Higher Interior Temperatures in the Summer.

These are all potential issues caused by inadequate attic space ventilation, and unfortunately, many homes have insufficient attic ventilation. Read on to learn the ins and outs or contact Ardent to have us evaluate your home.


Why does this happen?

The main issue that results from poor attic ventilation is the inability to keep the attic air temperature close to the outside air temperature. In the winter this is caused by heat loss through your home's ceiling and attic insulation. The problem of high attic air temperature can also be compounded by snow insulating the roof or insufficient attic insulation. In the summer months, this problem is caused by the sun heating the roof covering which in turn heats the attic space by radiant heat.


Why does warm attic air cause problems?

When the air in a ventilated attic system is significantly warmer than the ambient air, several problems can occur.

  • Warmer air has a greater capacity to hold moisture. When this warm air touches a cool surface, such as the underside of a roof deck, it condenses. This condensation can compromise the roof deck as well as drip back into the living space and cause mould.

  • When the outside air temperature is slightly below freezing, warmer attics will melt snow on the roof above the conditioned space without melting the snow on the eaves. This scenario is what causes ice damming; the meltwater runs down the roof until it hits the cold eaves and refreezes into an ice dam. In severe cases, the water will start to pool behind this ice dam and potentially leak into the home.

  • During the summer months, the sun will heat the shingles considerably. Think about how hot black asphalt is on a hot summer day; common roofing shingles are also made from asphalt and reach similar temperatures. When the attic space below the shingles is underventilated the already high temperature of the shingle goes up higher. These extreme temperatures reduce shingle life. Many shingle manufacturers will not honour their warranties if there is improper ventilation.

  • Higher attic temperatures in the summer create a larger temperature differential between the home's interior temperature and the attic temperature. This differential decreases the effectiveness of the insulation and warms up your home quicker.


What is required?

The current Canadian National Building Code (NBC) requires 1 square foot of attic venting for every 300 square feet of attic floor space. This calculation assumes a balanced system with 50% intake venting and 50% exhaust venting. A 1:300 ventilation ratio is the NBC code minimum, codes in other jurisdictions and major shingle manufacturers recommend following a 1:150 rule instead.


Doing it yourself?

Although simple to install, attic ventilation can easily be installed wrong. Here are a couple of pointers.

  • Do not mix type of venting. Ie. Whirlybirds and turtle style vents, or power venting and passive venting.

  • Exhaust venting should be installed at the same height to prevent short cycling.

  • Exhaust ventilation net free area (NFA) should not exceed the intake NFA, or short cycling can occur.

  • View the National Building Code Section 9.19 for the technical requirements.


Finishing up.

Although it is not cost prohibitive to install a properly balanced attic ventilation system, many tradesmen and homebuilders neglect this vital system. Contact Ardent today if you think you have a ventilation problem and would like a consultation.

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