How to ventilate a roof cavity below and around a skylight
How to Correct Inadequate Attic Venting to Stop Attic Condensation, Ice Dam Leaks, Attic Mold, & Roof Structure Damage
Questions & answers about how to get roof ventilation working around skylights.
This article explains How to Correct Improper or Inadequate Attic or Under-Roof Ventilation at skylights, This article on the design of proper venting around roof skylights is part of our series of articles about ATTIC CONDENSATION CAUSE & CURE.
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Roof Ventilation Details at Skylights to Provide an Air Path - Eaves to Ridge
This article series 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.
Localized hot spots such as skylights can
also lead to ice dams below, due to blocked ventilation as
well as melt water from skylight heat loss.
Notching the
rafters on either side of the skylight will help maintain airflow
above the skylight (Figure 2-59 at left).
If icing on your skylight window interior surface in winter is still a problem, add an interior storm window
to reduce heat loss through the glass in cold weather.
As a backup to prevent leaks at skylights, during skylight installation and even though modern skylights are usually provided with a factory-built flashing and counterflashing, it is always a good idea to seal the skylight
curb and surrounding roof area with a bituminous membrane
(see Figure 2-5 at left).
Cathedral Ceiling Ventilation Alongside of Skylights
Cathedral ceilings require the same continuous air barriers,
and balanced soffit and ridge vents, as attics.
Both air
sealing and ventilation are more critical, however, since
any trapped moisture in the roof cavity will remain longer
and potentially cause greater damage than in an open attic.
Also, since there is little or no communication from bay to
bay, an effective ventilation system must reach every bay
(Figure 2-57 at left).
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com.
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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
Thanks to reader - Mike Martino for discussing attic ventilation retrofit ideas - August 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.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
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