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This article series describes roof venting problems and solutions: ice dams, attic condensation, attic mold, and 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 many 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, and frequent literature review and professional discussion. We have also measured changes in airflow, temperature, and moisture before and after installing roof venting.

Readers should see our detailed articles at ROOF VENTILATION SPECIFICATIONS.

© Copyright 2012 InspectAPedia.com, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use page top links to major topics or use links at the left of each page to navigate within topics and documents at this website. Green links show where you are in a document series or at this website.

Attic Condensation and Roof Leaks as a Source of Building Mold - Diagnosis and Cure

Sources of Attic Mold: Roof leaks or, alternatively, high levels of attic moisture due to a combination of inadequate attic (soffit intake and ridge outlet) ventilation combine with building moisture sources (such as a chronic or even a single-event wet basement, plumbing leaks, or a leaky roof from roof failure or from ice dams) are likely to cause excessive moisture or actual wet conditions in an attic.

High attic moisture levels or actual wet attic conditions invite extensive mold growth. [Also see DEW POINT TABLE - CONDENSATION POINT GUIDE.]

Visible frost may appear on attic roof surfaces if the building is located in a freezing climate and high levels of moisture are trapped in a poorly vented attic or roof cavity (photo, left).

Visible mold may appear on wood surfaces in an attic such as on rafters or roof sheathing. Hidden mold may be present and may be even more of a problem if it forms in insulation or in the ducts and air handler of an air conditioning or heating/air conditioning system.

Typical building air convection currents tend to move air up and out from lower to upper building levels, so one would not think that much mold would move down from an attic into the living area. But important exceptions to this can quickly move problem mold from an attic into a living area.

Conditions that Cause Air Movement Upwards into an Attic or Roof Cavity Space

As home inspection expert Roger Hankey has pointed out,

"... attic bypasses are one of the primary causes of ice dams. You effectively discuss insulation and ventilation (at INSULATION INSPECTION & IMPROVEMENT and at VENTILATION in buildings) but if attic bypasses remain in an insulated and ventilated attic, then the result can be frost and moisture damage to the roof sheathing, and/or spot ice dams." - R. Hankey 01/28/2008.

Because warm air rises up through buildings by natural convection, tending to displace heavier cold air, warm building air leaks and forces its way into roof spaces primarily through small openings leaking from heated space into the roof cavity or attic space. The pressure difference between a warm interior ceiling and a cooler attic or cathedral ceiling space needs only to be slight for air to move from warm to cool spaces in a building.

Remarkably, the current of rising air in a two story or higher building can be quite adequate to even draw cool, moist, or possibly moldy air from a building's crawl space or basement too.

Details about air bypass leaks themselves can be read at AIR BYPASS LEAKS.

Readers who want to understand the significance of air bypass leaks and how this problem is corrected should see ENERGY SAVINGS RETROFIT CASE STUDY and ENERGY SAVINGS RETROFIT LEAK SEALING GUIDE.

Readers who want to find and fix un-wanted air bypass leaks should also see AIR LEAK DETECTION TOOLS and AIR LEAK MINIMIZATION as well as AIR SEALING STRATEGIES.

Conditions moving air and potentially moldy air downwards from an attic or roof space include

• Mold growth in heating and air conditioning HVAC ducts or air handlers found in an attic
• Mold on any attic surface or in attic insulation if it is a species producing airborne spores (moldy attic photo at left) and if the building uses a whole house ventilating fan, especially if there is inadequate exit venting for the fan operation. This condition pressurizes the attic and moves mold down through various openings into the floors below.
• Mold on building surfaces in an attic or attic knee wall space which opens onto or has a knee wall common with an upper floor living space such as a bedroom.
• Ventilation fan or exhaust fan use causing downwards movement of air from upper building levels
• Air conditioning use causing down-currents of air in buildings from an upper floor: heavier, cool air flows downwards in the structure and draws attic air into the upper floor from attic or roof cavity air bypass leak points (such as at light fixtures, stairs, attic hatch).

Building Exteriors Leaks and Mold vs Attic Ventilation & Moisture Troubles

No mold cleanup project will be successful unless you correct the conditions that caused mold growth in the first place. An expert inspection and report should find and suggest remedies for site and building exterior conditions that produce mold or for building areas that serve as a mold reservoir or as amplifiers for allergens, mold, mildew, excessive pollen or pet dander.

The basic steps: find all unwanted moisture sources, correct appropriate building, site, landscaping, & construction details. 90% of the wet basements and crawl spaces I see are caused by bad or missing roof gutters and downspouts.

An IAQ investigator who has training and experience in building science, mycology (mold science), and IAQ, or in some cases an experienced ASHI-Certified home inspector or sick building investigator who is who has a similar in-depth understanding of construction failures can be helpful at this step.

How to Prevent Ice Dam Formations (and Ice Dam Leaks) on Roofs

As explained in Best Practices Guide to Residential Construction, chapter on BEST ROOFING PRACTICES:

Ice dams form when heat leaking into attics or roof cavities from the building below, or from attic ductwork, melts the bottom layer of snow on the roof.

The melt water runs down the length of the roof to the eaves, where it refreezes, forming a dam and icicles.

In the worst cases, liquid water pools behind the dam and flows under the shingles and into the building (Figure 2-54 shown at left).

Research has indicated that the ice-dam risk is greatest when temperatures range between 15°F and 20°F— when it is warm enough for snow to melt but cold enough for it to refreeze at the eaves.

Also, the greater the depth of snow on the roof, the greater the risk of ice dams due to the insulating value of the snow itself.

Cold Roofs Prevent Ice Dams

Ventilation helps prevent ice dams by keeping the roof surface cold enough to limit uneven melting. Tests conducted in 1996 at the U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory (CRREL), showed that the traditional 1:150 ventilation rule was sufficient to prevent ice dams on roofs with R-25 or greater ceiling insulation.

The 1:300 rule proved adequate for roofs with R-38 or greater insulation. Since most standard eave and ridge vents sold today meet the higher ventilation rates, most new homes are protected as long as there are no large heat leaks into the attic, or tricky sections of the roof with inadequate ventilation.

-- Adapted with permission from Best Practices Guide to Residential Construction.

Questions & Answers regarding this article

Questions & answers about how to diagnose and fix attic moisture, condensation, and mold sources

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ATTIC LEAKS, CONDENSATION & ATTIC MOLD
  Air Bypass Leaks, Thermal Tracking
  Blocked Soffit Intake Vents
  CATHEDRAL CEILING INSULATION
  CATHEDRAL CEILING VENTILATION
  Comparing Two Houses
  Heat Tapes: Use on Roofs for Ice
  HEAT TAPES & CABLES on Roofs for Ice Dams
  HOT ROOF DESIGNS: Un-Vented Roof Solutions
  HOUSEWRAP AIR & VAPOR BARRIERS
  HUMIDITY LEVEL TARGET
  ICE DAM PREVENTION
  ICE DAM CURE: Comparing Two Houses
  Inspect Attics for Moisture or Mold
  Inspect Attics for Blocked Soffit Intake Vents
  Inspect Basements for Moisture or Mold
  Cold Pour Joint Leaks
  Form Tie Leaks
  Wall Crack Leaks
  Floor-Wall Joint Leaks
  Inspect Building Exterior - Roof Venting
  Inspect the Ridge Vent System from the Attic
  Inspect the Soffit Vent System from the Attic
  ROOF VENTILATION SPECIFICATIONS
  Roof Venting: Intake - Outlet Area Ratios
  Roof Venting: Proper Locations
  Roof Venting: Both Ridge & Eaves Venting Needed
  Roof Venting: Eaves Intake if no Overhang
  Roof Venting: Soffit Intake Vent-Continuous
  Roof Venting: Un-Vented Hot Roof Solutions
  ROOF VENTING ENERGY SAVING DETAILS
  ROOF VENTING NEEDED?
  SKYLIGHT VENTILATION DETAILS
  Soffit Ventilation

ATTIC VENTILATION

• Alan Carson Carson Dunlop Associates, Toronto, Ontario. Mr. Carson is a home inspection professional, educator, researcher, writer, and a principal of Carson Dunlop Associates, a Toronto home inspection and education firm. Mr. Carson is a past president of ASHI, the American Society of Home Inspectors Some great illustrations of the proper under-roof ventilation pathways are offered by Carson Dunlop.
• 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. (727) 595-4211 mark@BestTampaInspector.com 11/06 & 12/08
• John Annunziata, P.E. - NY Metro ASHI during informal chapter discussions about roof and attic ventilation options (1986-1996).
• Roger Hankey is principal of Hankey and Brown home inspectors, Eden Prairie, MN. Mr. Hankey is a past chairman of the ASHI Standards Committee. Mr. Hankey has served in other ASHI professional and leadership roles. Contact Roger Hankey at: 952 829-0044 - rhankey@hankeyandbrown.com. Mr. Hankey is a frequent contributor to InspectAPedia.com.
• The Smart Vent™ by DCI roof intake venting provides an intake at the lower edge of roof decking for difficult cases. See www.dciproducts.com/html/smartvent.htm
• The AccuVent™ attic ventilation roof baffle produced by Berger permits insulation to extend over the top plate as far forward as possible. See www.bergerbuildingproducts.com/pdfs/AccuVentAtticVent.pdf
• GAF Cobra® and other GAF roof ventilation products: see www.gaf.com/Content/GAF/RES1/ROOF/RS_whyuse_ventchart.asp?viewer=&module=

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 Elimination of Unsafe Guardrails, a Progress Report," Elliott O. Stephenson, Building Standards, March-April 1993
• "Are Functional Handrails Within Our Grasp" Jake Pauls, Building Standards, January-February 1991
• Access Ramp building codes:
  • UBC 1003.3.4.3
  • BOCA 1016.3
  • ADA 4.8.2
  • IBC 1010.2
• Access Ramp Standards:
  • ADA (Americans with Disabilities Act), Public Law 101-336. 7/26/90 is very often cited by other sources for good design of stairs and ramps etc. even where disabled individuals are not the design target.
  • ANSI A117.4 Accessible and Usable buildings and Facilities (earlier version was incorporated into the ADA)
  • ASTM F 1637, Standard Practice for Safe Walking Surfaces, (Similar to the above standards)
• "Energy Savers: Whole-House Supply Ventilation Systems [copy on file as /interiors/Energy_Savers_Whole-House_Supply_Vent.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11880?print
• "Energy Savers: Whole-House Exhaust Ventilation Systems [copy on file as /interiors/Energy_Savers_Whole-House_Exhaust.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11870
• "Energy Savers: Ventilation [copy on file as /interiors/Energy_Savers_Ventilation.pdf ] - ", U.S. Department of Energy
• "Energy Savers: Natural Ventilation [copy on file as /interiors/Energy_Savers_Natural_Ventilation.pdf ] - ", U.S. Department of Energy
• "Energy Savers: Energy Recovery Ventilation Systems [copy on file as /interiors/Energy_Savers_Energy_Recovery_Venting.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11900
• "Energy Savers: Detecting Air Leaks [copy on file as /interiors/Energy_Savers_Detect_Air_Leaks.pdf ] - ", U.S. Department of Energy
• "Energy Savers: Air Sealing [copy on file as /interiors/Energy_Savers_Air_Sealing_1.pdf ] - ", U.S. Department of Energy
• Falls and Related Injuries: Slips, Trips, Missteps, and Their Consequences, Lawyers & Judges Publishing, (June 2002), ISBN-10: 0913875430 ISBN-13: 978-0913875438
  "Falls in the home and public places are the second leading cause of unintentional injury deaths in the United States, but are overlooked in most literature. This book is unique in that it is entirely devoted to falls. Of use to primary care physicians, nurses, insurance adjusters, architects, writers of building codes, attorneys, or anyone who cares for the elderly, this book will tell you how, why, and when people will likely fall, what most likely will be injured, and how such injuries come about. "
• Humidity: What indoor humidity should we maintain in order to avoid a mold problem?
• Slips, Trips, Missteps and Their Consequences, Second Edition, Gary M. Bakken, H. Harvey Cohen,A. S. Hyde, Jon R. Abele, ISBN-13: 978-1-933264-01-1 or ISBN 10: 1-933264-01-2, available from the publisher, Lawyers ^ Judges Publishing Company,Inc., www.lawyersandjudges.com sales@lawyersandjudges.com and also from the InspectAPedia Bookstore (Amazon.com)
• The Stairway Manufacturers' Association, (877) 500-5759, provides a pictorial guide to the stair and railing portion of the International Residential Code. [copy on file as http://www.stairways.org/pdf/2006%20Stair%20IRC%20SCREEN.pdf ] -
• What Mold and Allergens Look Like: mold identification photos to help identify mold - choosing what to sample in buildings
• 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.
• Mold spores in the Home - a Photo ID Library for detection and identification of mold allergens.
• How to Find and Test For Mold in buildings A "how to" photo and text primer on finding and choosing the right spots to test for mold in buildings
• 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.
• Simple Adhesive Tape Sampling of Moldy Surfaces - how to send a mold sample to our lab
• Mold Sampling Methods in the Indoor Environment - In-depth article: detailed critique of popular mold testing methods - Is your mold test kit worth the bother?
• 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.
• Slips, Trips, Missteps and Their Consequences, Gary M. Bakken, H. Harvey Cohen, Jon R. Abele, Alvin S. Hyde, Cindy A. LaRue, Lawyers and Judges Publishing; 2 edition (April 2006), ISBN-10: 1933264012 ISBN-13: 978-1933264011
• Steps and Stairways, Cleo Baldon & Ib Melchior, Rizzoli, 1989.
• The Staircase, Ann Rinaldi
  
• Common Sense Stairbuilding and Handrailing, Fred T. Hodgson
• The Art of Staircases, Pilar Chueca
• Building Stairs, by pros for pros, Andy Engel
• A Simplified Guide to Custom Stairbuilding, George R. Christina
• Basic Stairbuilding, Scott Schuttner
  
• The Staircase (two volumes), John Templar, Cambridge: the MIT Press, 1992
• The Staircase: History and Theories, John Templar, MIT Press 1995
• Steps and Stairways, Cleo Baldon & Ib Melchior, Rizzoli, 1989.
• "The Dimensions of Stairs", J. M. Fitch et al., Scientific American, October 1974.
• "Weather-Resistive Barriers [copy on file as /interiors/Weather_Resistant_Barriers_DOE.pdf ] - ", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
• Best Practices Guide to Residential Construction, by Steven Bliss. John Wiley & Sons, 2006. ISBN-10: 0471648361, ISBN-13: 978-0471648369, Hardcover: 320 pages, available from Amazon.com and also Wiley.com. See our book review of this publication.
• Decks and Porches, the JLC Guide to, Best Practices for Outdoor Spaces, Steve Bliss (Editor), The Journal of Light Construction, Williston VT, 2010 ISBN 10: 1-928580-42-4, ISBN 13: 978-1-928580-42-3, available from Amazon.com
• ...