How to Correct Inadequate Attic Venting to Stop Attic Condensation, Ice Dam Leaks, Attic Mold, & Roof Structure Damage
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How to Correct Inadequate Attic Venting to Stop Attic Condensation, Ice Dam Leaks, Attic Mold, & Roof Structure Damage
How to detect roof venting deficiencies, attic insulation defects, and attic condensation problems
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Here we discuss How to Correct Improper or Inadequate Attic or Under-Roof Ventilation in Buildings, part of our series of articles about ATTIC CONDENSATION CAUSE & CURE.
This article describes inspection methods and clues to detect roof venting deficiencies, insulation defects, and attic condensation problems
in buildings. It describes proper roof ventilation placement, amounts, and other details.
These recommendations are based on a survey of building science literature combined with 30 years of
building inspections, on the observation of the locations of moisture, mold, ice dams, condensation stains, and other clues in buildings,
and on the correlation of these clues with the roof venting conditions at those properties. We have measured very large changes in airflow, temperature,
and moisture before and after installing roof venting. Also see VENTILATION in BUILDINGS and INSULATION LOCATION in BUILDINGS - WHERE TO INSULATE.
How to Correct Improper or Inadequate Attic or Under-Roof Ventilation in Buildings
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.
An 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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Carson Dunlop, Associates, 120 Carlton Street Suite 407, Toronto ON M5A 4K2Toronto. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
"Unvented Roofs, Hot-
Humid Climates, and
Asphalt Roofing Shingles
Research Report - 0306
Feb-2003
Building Science Corporation", Building Science Corporation, 30 Forest Street,
Somerville, MA 02143 Quoting from the article abstract:
"When constructing unvented roofs with asphalt shingles in hot-humid climates, a vapor barrier must be
installed between the asphalt shingles and the roof deck." - Web Search 5/21/2010
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
More Information on Building Diagnostic Inspections and Repairs
More Reading about Dealing with Attic Mold, Identifying, Removing, and Preventing Mold in Attics
Be sure to review HOW TO FIND MOLD: How to Inspect Homes and Other Buildings for Mold - the Basics of How to Find Problem Mold Indoors
in our Mold Action Guide. Here are other articles that will be helpful in evaluating attic mold presence, causes, and cures:
How to Clean Moldy Wood Framing & Sheathing How to clean/seal mold from/on exposed lumber or plywood subfloor or roof sheathing indoors - some suggestions based on our field and laboratory research
Lighting, proper use of: proper aiming of a good flashlight can disclose hard to see but toxic light or white mold colonies on walls.
Stuff that is not mold but is often mistaken for it - things you may not want to test. Also, not all "black mold" is toxic - here are examples of harmless black mold.
Humidity: What indoor humidity should we maintain in order to avoid a mold problem?
Mold-Resistant Building Practices, advice from an expert on how to prevent mold after a building flood and how to prevent mold growth in buildings by selection of building materials and by anti-mold construction details.
InspectAPedia® Home & Site Map - Building & Environmental Inspection, Testing, Diagnosis, Repair, & Problem Prevention Advice: In-depth research & advice on diagnosing, testing, correcting, & preventing building defects & indoor environmental hazards. Unbiased information, no conflicts of interest.
Building & Indoor Air Quality Environmental Inspection, Testing, & Diagnosis Experts providing on-Site IAQ, Gas, Air Testing, Mold Investigation, Sick Building Diagnosis, Lab Services, & Remediation Plan Preparation - indoor air quality testing, problem source determination, supporting lab work, written remediation plan addressing removal of environmental and other hazards and prevention of their recurrence. Also see MOLD CLEANUP COMPANIES
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