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Inadequate Attic

How to Correct Improper or Inadequate Attic or Under-Roof Ventilation in Buildings

this simple sketch shows the optimium attic vent design with inflow at eaves and outflow at ridge

Home inspectors and building owners should be especially concerned about insulation placed directly under the roof sheathing such as between the rafters. This can trap heat and moisture and damage sub-roofing as well as roofing shingles.

Also, cathedral ceilings without vents in the soffit and up at the ridge or without adequate air path between the roof insulation and the underside of the roof sheathing can lead to major condensation problems, rot, insect damage, and severe structural damage in just a few years.

Do look for those dark rafter lines and don’t underestimate the damage that can exist.

The best “fix” for cathedral ceilings and un vented roof cavities is to assure that there is an air path into the attic up the under side of the roof along the building eaves.

If insulation blocks this opening, it is corrected simply by installing soffit vent baffles at the house eaves between every rafter pair (cardboard or Styrofoam pieces made for that purpose and sold at most lumber yards) and adding vents if not in place.

Un-Vented “Hot Roof” Designs Risk Costly Hidden Damage

Two approaches for insulating cathedral ceilings and flat roofs (C) Carson Dunlop Illustrated HomeAn alternative un-vented “hot roof” design (the right-hand cathedral ceiling structure shown at the right in Carson Dunlop‘s sketch) is touted by some building experts who argue that under-roof venting is not needed whatsoever.

Roof surface temperature affects the life of the roof covering. Studies in Florida confirmed that asphalt roof shingle surface temperature varies more as a function of shingle color than as a function of whether or not the under-roof cavity is ventilated.

Mark Cramer informs us that an insulated roof is only 7 deg. F. hotter than an un-insulated roof. By this view roof life may not be reduced significantly by an un-vented design. But here are some concerns that the “hot roof” design leaves incompletely addressed:

  • Hidden leak damage: If insulation is placed between the roof rafters, especially using some water resistant foam insulation products, leaks into the roof cavity from outside are likely to cause significant and costly structural damage (and possibly mold infection) before the leak is discovered.
  • Higher heating/cooling costs: A sloped, insulated roof has a larger radiating surface area than an insulated attic floor, increasing building cooling or heating costs. The conversion of the attic space to a “conditioned” space (by placing insulation between roof rafters) is a bonus in some buildings if the space is to be used, but a cost in buildings where it is not to be occupied.
  • Lack of moisture damage resistance: A building with an interior moisture problem that results in moisture movement up through the building to the roof underside is more likely to suffer mold and rot problems if that moisture is trapped in an un-vented attic or roof cavity. Good building management includes identification and correction of leaks or other sources of un-wanted moisture. But good building design takes into account “real world” conditions that actually occur in the field, not just optimal conditions that describe what people should do.

Leaks and moisture problems are common on buildings in most climates and these conditions are likely to occur over the life of a building. Good building design resists water damage. Readers who have hot roof designs or hard-to-vent roof cavities should also see Un-Vented Roof Solutions.

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