Blowing Hot Air

Hot air rises, cold air falls.

June 05, 2000

Hot air rises, cold air falls.

This simple fact of physics has a dramatic impact on roof and attic installations. During the winter, warm air rising from a home can dramatically effect an icy or snowy roof, possibly causing damage if the attic isn't vented properly. And often, this damage won't become noticeable until spring and summer rain starts to fall.

"Ventilation related leaks start inside the house in the attic," says John Morris, a roof expert with Globe Building Materials. "Warm air rises on the inside of the house into the attic. The warm attic air then increases the temperature of the roof's underside." As snow and ice melt on the roof's exterior, water runs down the framing and drips onto the attic floor. Eventually, this water will leak through into the home.

Proper attic ventilation includes system to force excess hot air outside through passages where water won't be able to enter.

Using the roof's natural, peaked shape, ventilation can be installed that allows cool air to come into the attic through intake vents at the eaves. Rising warm air is then forced out of the roof's peak, through ridges that won't allow water to leak back inside. The rest of the roof remains cool enough to prevent snow and ice from melting.

In the summer, the same condition can also cause different types of damage. Heat can cause floorboards and roof shingles to warp, or possibly damage shingles. Unbearably hot attic conditions are almost a sure sign of improper ventilation.

Often, homeowners underestimate the amount of ventilation needed for adequate airflow in an attic. In addition, few homeowners often neglect installation of proper air intake that will allow the hot air to move freely.

To inspect a home for improper ventilation, look for the following signs:

  • Moisture building up in the attic when air isn't moving. This situation can also compact insulation, preventing it from doing its job.
  • Rotting roof decks signify collecting water in the attic.
  • Using more energy than normal to cool your home is also a sign that your attic may be too warm.

"Proper ventilation will go a long way toward eliminating leaks that are so hard to understand or fix," says Morris. "Not only does proper ventilation ensure your shingle warranty, maintain the energy efficiency of your home and conform to building codes, but it can also help eliminate leaks."

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