Attic Condensation & Ice Dam Leaks in buildings
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How to stop ice dams and roof leaks on buildings
How to detect & correct roof venting deficiencies, attic insulation defects, and attic moisture or condensation problems
How much attic ventilation is needed?
How to design effective attic or under-roof ventilation
Questions & answers about how to diagnose and fix attic moisture, condensation, and mold sources

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.

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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.

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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.

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ATTIC LEAKS, CONDENSATION & MOLD - see detailed links at page left

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=

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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.

"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

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.

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

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

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

Building Exteriors Leaks and Mold vs Attic Ventilation & Moisture Troubles

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

Cold Roofs Prevent Ice Dams

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair