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VENTILATION in BUILDINGS
AIR BYPASS LEAKS
AIR LEAK DETECTION TOOLS
AIR LEAK SEALING PROCEDURE
AIR POLLUTANTS, COMMON INDOOR
AIR SEALING STRATEGIES
BASEMENT CEILING VAPOR BARRIER
BASEMENT HEAT LOSS
BASEMENT LEAKS, INSPECT FOR
BLOWER DOORS & AIR INFILTRATION
BRICK WALL DRAINAGE WEEP HOLES
CATHEDRAL CEILING VENTILATION
CEILINGS, DROP or SUSPENDED PANEL
COMBUSTION AIR for TIGHT BUILDINGS
COOLING LOAD REDUCTION by ROOF VENTS
CONDENSATION on WINDOWS & SKYLIGHTS
DEW POINT CALCULATION for WALLS
FIREPLACES & HEARTHS
FLAT ROOF MOISTURE & CONDENSATION
GREEN BUILDING CONSTRUCTION
HEAT LOSS in BUILDINGS
HEAT LOSS DETECTION TOOLS
HEAT RECOVERY VENTILATORS
HOT ROOF DESIGNS: UN-VENTED ROOF SOLUTIONS
HOUSEWRAP AIR & VAPOR BARRIERS
HOUSE DOCTOR, how-to be
HUMIDITY LEVEL TARGET
ICE DAM LEAKS
INDOOR AIR HAZARDS TABLE
INDOOR AIR QUALITY & HOUSE TIGHTNESS
INDOOR AIR QUALITY IMPROVEMENT GUIDE
INSULATION AIR & HEAT LEAKS
INSULATION INSPECTION & IMPROVEMENT
INSULATION R-VALUES & PROPERTIES
LOG HOME GUIDE
MOISTURE CONTROL in BUILDINGS
ODORS GASES SMELLS, DIAGNOSIS & CURE
ROOF VENTILATION SPECIFICATIONS
ROOF ICE DAM LEAKS
SHEATHING, FOIL FACED - VENTS
STAIN DIAGNOSIS on BUILDING INTERIORS
STUCCO WALL METHODS & INSTALLATION
SWEATING (CONDENSATION) on PIPES, TANKS
THERMAL MASS in buildings
THERMAL TRACKING Indicates Heat Loss
VAPOR BARRIERS & AIR SEALING at BAND JOISTS
VAPOR BARRIERS & HOUSEWRAP
VAPOR CONDENSATION & BUILDING SHEATHING
VENTILATION in BUILDINGS
WIND WASHING INSULATION At EAVES
WINDOWS & DOORS
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.
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Let's start with emergency measures for buildings already in trouble with heavy roof ice dams and leaks into the building attic, walls, or other areas.
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These are the useful approaches to curing existing roof ice dam leaks into a building attic, walls or other areas:
How to Prevent Future Roof Ice Dam Leaks
OK so we've taken emergency measures to stop ice dam leaks. Now how do we stop ice dam leaks in the future, and how do we fix an existing building or build a new one that won't have ice dam leak troubles.
Why Roof Iced Dams Form & How Water Backs Up & Leaks at Ice Dams
Cold Roofs Prevent Ice Dams
So how much roof ventilation do we need to prevent roof ice dams, leaks, and attic moisture, condensation & mold problems?
But just adding outlet vents at a ridge (or worse, at gable ends of a building) or just adding soffit vents alone does not work very well to ventilate attics or under-roof spaces. We need both air inlet at the building eaves or lower roof edges, and air outlet at the ridge in order to move air up beneath the entire roof surface, keeping the roof deck and attic cool and dry.
See ROOF VENTIILATION INTAKE-OUTLET RATOS for details about the ratio of soffit intake vent area to ridge outlet vent area.
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.
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,
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.
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
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.
Continue reading at HEAT TAPES & CABLES for ROOF ICE DAMS or select a topic from the More Reading links shown below.
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Frequently Asked Questions (FAQs)
Question: reader claims drip edge prevents ice dam formation
(Aug 14, 2014) TrueNorth said:
IMHO, this article is academic hog wash, irrelevant and points the homeowner in the wrong direction because it fails to explain the real reason for many homeownes. The article neglects to mention a cause of ice dams is faulty roof installation, where drip edge flashing was not installed.
Really? Thank you for your opinion True, but with respect, you are quite mistaken - let's not confuse your opinion with the combination of fact, research, and expertise given by generations of roofers and by folks who have inspected many ice dam problems in a variety of environments. The data in this article series is based on expert sources not theoretical speculation.
Ice dams form on the cold lower edge of a roof as water from melting snow or ice higher on the roof runs down and hits that cold edge. Ice dams can become large enough to cause water to back up under shingles even several feet back up the roof slope as well as causing water to find its way into the building by way of soffits or at other points.
A fundamental understanding of why ice forms on lower roof edges may be helpful. It is heat loss that melts snow on upper roof areas, followed by that meltwater running down to the lower and colder overhanging roof edge where it freezes to ice that causes ice dams. Faulty drip edge flashing or perfect drip edge flashing has nothing to do with ice dam formation except in the uncommon case of the combination drip-edge and Hicks Starter vent that provides intake venting at some roof edges.
For buildings where ice dams are a chronic problem or are unavoidable because of roof, insulation, and ventilation design limitations, the use of an impermeable barrier such as ice and water shield extending well up the roof from its lower edge may permit living with ice dams without leaks into the roof or wall cavity.
But no ordinary metal drip edge flashing can prevent ice dams, nor prevent ice dam leaks.
Take a look at the other ice dam articles in this series for more details. Also see the REFERENCES in each of those articles.
Questions & answers or comments about how to diagnose and fix attic moisture, condensation, and mold sources
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
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