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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
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.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
How to Stop Roof Ice Dam Leakage & How to Prevent Ice Dam Formation at the Roof Edges & Gutters
How to STOP ongoing roof ice dam leaks
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.
[Click to enlarge any image]
These are the useful approaches to curing existing roof ice dam leaks into a building attic, walls or other areas:
Use heat tapes:
during winter & freezing conditions when weather precludes actual building modifications or repairs to add ventilation or ice and water shield, you can use
Heat tapes at the roof eaves are not a great permanent fix but it's a useful stop-gap measure if you're having ongoing ice dam leaks into the building.
Fix air & heat leaks:
winter is a fine time to take a look at the attic for missing insulation, gaps in insulation, or for dark spots on insulation that mark air bypass leaks. Fixing insulation gaps and air leaks drops the attic temperature and thus cuts ice dam formation.
Turn down the heat:
A neighbor told us that as her dad's currently empty house in New York had a long history of ice dam trouble she was keeping the heat up to hope she would melt ice off of the roof!
Really?No, she was taking the wrong approach. Making the house warmer means sending more heat into the attic or roof space where the under-roof heat melts snow on the upper roof sections.
That melting snow water runs beneath the snow cover on the roof just fine, right down the warm roof deck until it hits the cold roof overhang or lower edges. There it freezes solid. The more water we send down the roof to the cold roof edge the bigger the ice dam. Don't do that.
The neighbor then asked: "OK so should I turn the heat down?" Geez! not knowing the house I was worried that if I told her to do that the pipes might freeze - we'd go from one problem to another.
lots of people risk their lives or the lives of others having somebody shovel or rake snow off of the roof when ice dams seem to be a problem.
Really? Snow raking might be appropriate where unusual conditions of deep snow threaten structural collapse of a building, but that's not a great fix for ice dams unless all of the snow is removed from the roof.
Otherwise, snow high on the roof still melts (in that hot attic), running down to the eaves where it freezes to form ice dams.
OK maybe smaller ice dams as we have less melting snow and a somewhat cooler attic due to increased heat loss through the non-snow-coverd area, but we'll continue to see ice dams nevertheless.
Emergency steps in stopping roof ice dam leakage that included removing snow from the roof are illustrated
ok so we've seen all sorts of crazy schemes: socks filled with ice-melt crystals or salt (which is hell on the aluminum gutters), chopping ice (which wrecks the roof shingles) at the roof eaves, and my favorite - squirting a jet of hot water to melt drainage channels in the ice dams to allow backed-up water off of the roof.
I confess, I've done this. It works, for a few hours. Maybe that's a way to make room for heat tapes.
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.
Do we really want to be raking a high roof like the one shown at left every winter?
Doesn't raking snow risk roof damage or a shorter roof life? Are there falling and electrocution hazards? You bet. Here are some other approaches to avoiding ice dam problems on roofs:
Ventilate the roof properly at soffits (eaves) and ridge.
We'll say more about this below. If you can't ventilate the roof you'll have to go to a hot roof design and hope that nothing ever punctures the roof covering.
Over at ROOF ICE DAM CURE: Comparing Two Houses we show the best long term way to avoid ice dam formation and roof ice dam leaks: good ventilation and good insulation and no air leaks into the roof cavity.
Inspect & fix insulation & air bypass leaks.
Details about air bypass leaks themselves can be read
even in really careful construction, challenging weather conditions or unusual building conditions (someone left the door to the attic open) can cause so much ice formation on a roof that ice dam leaks occur where they never have before.
As extra insurance against roof ice dam leaks when re-roofing, use ice and water shield at the building roof eaves, running this impermeable membrane (it even seals around nails) up the roof at least three feet, or in some climates or on problem roofs, up six feet or more.
Watch out: ice formation in gutters at the roof edges can push water back up underneath ice and water shield if it did not bond perfectly to the roof sheathing, or it may push water up underneath the roof deck itself.
Extra detailing at the roof edges, including wrapping the ice and water shield down over the fascia may be needed
Install & Use Heat Tapes at Roof Eaves & In Problem Spots:
Sometimes the most expedient, least costly, and fastest way to stop ice dam leaks is to remove accumulated snow and ice (as much as possible and safely) and then install heat tapes in those areas.
Particularly for un-vented roofs or roof spots at which adding ventilation and insulation to stop ice dam formation is very costly or difficult, the appeal of ice melting heat tapes becomes overwhelming.
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
So how much roof ventilation do we need to prevent roof ice dams, leaks, and attic moisture, condensation & mold problems?
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.
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.
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.
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.
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.
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 - firstname.lastname@example.org. 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=
Building Research Council, BRC, nee Small Homes Council, SHC, School of Architecture, University of Illinois at Urbana-Champaign, brc.arch.uiuc.edu. "The Small Homes Council (our original name) was organized in 1944 during the war at the request of the President of the University of Illinois to consider the role of the university in meeting the demand for housing in the United States. Soldiers would be coming home after the war and would be needing good low-cost housing. ... In 1993, the Council became part of the School of Architecture, and since then has been known as the School of Architecture-Building Research Council. ... The Council's researchers answered many critical questions that would affect the quality of the nation's housing stock.
How could homes be designed and built more efficiently?
What kinds of construction and production techniques worked well and which did not?
How did people use different kinds of spaces in their homes?
What roles did community planning, zoning, and interior design play in how neighborhoods worked
"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?
"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
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: email@example.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, 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. Field inspection worksheets are included at the back of the volume.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
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