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IronbridgeRoofing Co.

How a Roof Works

Lesson 6. Ventilation and the Attic You Never Look At

Attic ventilation is a continuous path from the soffit vents to the ridge, and breaking any part of that path costs you years of roof life.

At a Glance

  • Air in at the soffit, out at the ridge. That is the whole system
  • Insulation stuffed into the eaves is the usual blockage
  • A hot attic cooks shingles from underneath
  • Most shingle warranties require adequate ventilation

The Path

Cool outside air enters through vents in the soffit under the eaves. It rises up the underside of the roof deck, taking heat and moisture with it, and exits at the ridge. It is driven by heat rising and by wind, with no moving parts.

Both ends are required. Ridge vent with blocked soffits does almost nothing, because air cannot leave a space that nothing is entering.

The Four Things That Break It

  1. Insulation pushed into the eaves, covering the soffit vents from inside
  2. A ridge vent nailed on without the deck slot being cut underneath it
  3. Bathroom or dryer fans venting into the attic instead of through the roof or wall
  4. Mixing vent types, so a gable vent short circuits the ridge and pulls air from the wrong place
Worth knowing. Baffles are cardboard or foam channels stapled between the rafters at the eaves. They hold the insulation back and keep the air path open. They cost very little and most older houses here do not have them.

What It Costs You to Ignore

In July an unvented attic runs far hotter than the air outside. Shingles bake from both sides and age faster than designed.

In January warm damp air from the house rises, hits the cold underside of the deck and condenses. Frost forms on the nail points. The first warm day it melts and rains on your insulation. Wet insulation does not insulate, so more heat escapes, and the cycle gets worse every winter.

How to Check Yours in Ten Minutes

  1. On a hot afternoon, put your head into the attic. Very hot and still air means it is not venting
  2. Look toward the eaves. If you see insulation touching the roof deck, the vents are blocked
  3. Look for daylight along the ridge line from inside. Some is normal with a ridge vent
  4. In winter look at the nail points. Frost or rust means moisture is condensing up there
Next. Lesson 7. How to Inspect Your Roof From the Ground
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