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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.
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.
Rake snow off of the roof: 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.
Get Rid of the Ice: 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.
OK so either by choosing from the steps listed above in this article, or by other desperate measures shown
at ICE DAM LEAK EMERGENCY REPAIRS 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. 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.
Use Ice & Water Shield: 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.
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 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.
Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
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.
Question: Ice dam leaks on new home
(Feb 27, 2015) chris said:
I recently moved into a 30 by 60 modular home 1500 square feet! the house smell musty like mold but I just thought it was because the house was just remodeled and the air was on 50 deg! after moving in I went up into the attic and the north side roof and gable end was soaked with wetness and had signs of mold! the eastside was dry! I called two different contractor and both said the ridge vent was bad so paid the money and had it replaced! after a week I went back up and still soaked and now it had frost on the osb! the house has full eve vents with baffles that are not blocked and a fully full ridge vent! I cant for the life of me figure it out. I think its the crawl space the is a sump and the grounds not covered with plastic sheeting there is no hvac in the attic its in the crawl space! I leaning towards the crawlspace being the cause! what does you all think! please help before my house is destroyed! I have pics if it helps I can text them if u would like to see!
I forgot to say that the house gable end on the northside was running with a tea color down the siding! the house is a ranch style and was manufactured in 2002!
Our email is at the CONTACT link at page bottom. Seeing some photos may let me comment more usefully. Imagery that a wet crawl area can send water up through a house, especially if the water or moisture is being picked up by the HVAC system.
Question: how to ventilate an attic to stop condensation & mold
(Mar 23, 2015) Anonymous said:
We have an attic that is molding up due to humidity.We thought the roof leaks so we replaced the roofing material.After doing that we put up some insulation in the beams.
After just a while we realized that the insulation is falling off on it's own due to excessive humidity and the boards are getting completely black.Now there is a very thin insulation between the ceiling and attic floor(the heat access the attic anyway right now) but we were also planning to utilize the attic as a living space so it would be heated.We were told we need solar power gable fan to properly vent the roof.I am not sure of the insulation and actual heating in the attic,so it doesn't condensate and create more damage.Also the north side of the roof is excessively wet(I assume condensating,not leaking because the nails have droplets of water on them and then they freeze).So my question is how to properly ventilate and insulate heated attic so we can get rid off the mold and not create more damage?Any advice would be appreciated.Thank you!
Just above you'll see a series of articles that offer detailed suggestions, beginning at
ROOF VENTILATION SPECIFICATIONS - home
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Questions & answers or comments about how to diagnose and fix attic moisture, condensation, and mold sources
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.
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 - email@example.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=
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
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