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Here we explain how Proper location of roof intake and outlet venting prevents Attic Condensation, Ice Dam Leaks, Attic Mold, & Roof Structure Damage. This is a section of ROOF VENTILATION SPECIFICATIONS and also ATTIC CONDENSATION CAUSE & CURE. This article describes 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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Proper Roof Vent Location for Air Intake and Outlet

Intake venting needs to be at the eaves or lower roof edges. Otherwise these roof areas will not be dry and cool, and we'll find water damage, condensation damage, ice dams, and often mold in these areas.

Some people install the ridge vent (the easy part) and leave older gable end vents as "intake." This does not work very well. Simply pulling in the needed makeup air from typical gable end wall vents will not move air under the lower and center areas of roof sheathing, thus inadvertently encouraging moisture and mold to form in these areas.

In buildings where this vent design has been used we see pretty clean roof sheathing near the ridge and near the building gable walls, but at the lower roof center and looking down to the eaves, the sheathing will often be wet, moldy, damaged.

The sketch at page top is not quite right. It shows the soffit opening along the house wall where wind-blown rain may enter the soffit and wall. A better location for this opening would be at the outer edge of the soffit, just behind the fascia!

Outlet Venting needs to be along the length of the ridge. On some complex roof designs and on pyramid roofs there is no ridge.

The sketch at left is an example of the insulation placement and air flow pathway for homes that combine an attic knee wall space with a partial cathedral ceiling space. We recommend closing the gable-end vent on older homes that have one, installing a continuous ridge vent to assure air flow between every rafter pair.

On some hip roof designs the length of ridge is very short compared with the total roof length. In these cases there may be no option but to add multiple individual exit vent openings across the roof field. In this case they should be placed near the ridge so that the upper attic will be vented.

Putting them at mid roof or lower is ineffective. Usually such vents are placed on just one slope of the roof so that they are not visible from the front of the building. This is ok, provided the vents are near the ridge. Otherwise the un-vented roof slope simply won't be treated.

On lower single-plane shed roofs that abut an upper story building wall, venting can be provided by installing a half-ridge at the upper roof slope. Spot vents and gable end vents are a distant second choice for these designs for the reasons I outlined above.

For cathedral ceilings that have minimal vent space see our suggestions at CATHEDRAL CEILING INSULATION

For roofs that have no soffit or eaves overhang to provide an intake opening, see Roof Venting: Eaves Intake if no Overhang.

For roofs that cannot be vented, roofs whose structure, shape, or choice of insulation (such as sprayed foam insulation under the roof deck, or the "hot roof" design, see Roof Venting: Un-Vented Roof Solutions.

For irregular or hard-to-vent roof shapes, condensation (winter) or summer heat build-up can be relieved by a thermostatically operated attic fan - with a cutout switch to turn off the fan in case of a building fire. (Otherwise the fan spreads and speeds the fire.) But such fans do not run in cold winter weather, so you're only solving the heat problem not the winter condensation and ice dam problem.

Optimum roof ventilation design: provides continuous ridge venting, continuous soffit venting, and then, close off those old gable-end vents to force the intake air to come where you want it to flow. Otherwise the exiting air at the ridge will usually pull its makeup air from the closer gable end vents and not from the soffit or eaves vents.

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• Daniel Friedman - principal author/editor of the InspectAPedia^® Website
• Critique, contributions wanted: Contact Us to suggest corrections or additions to articles at this website, and if you wish, to receive online listing and credit as a contributor. Particular thanks are due to the many experts and also consumers who read and critique technical articles at InspectAPedia.com.
• 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).
• Technical reviewers are invited to comment or ask questions - contact us

Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

INTERIORS of BUILDINGS
AIR BYPASS LEAKS
AIR LEAK DETECTION TOOLS
AIR LEAK MINIMIZATION
AIR SEALING STRATEGIES
BATHROOM VENTILATION
FREEZE-PROOF A BUILDING
ATTIC CONDENSATION CAUSE & CURE
Air Bypass Leaks,Blocked Soffit Intake Vents
CATHEDRAL CEILING INSULATION
Comparing Two Houses
omparing Two Houses
Heat Tapes: Use on Roofs for Ice
HEAT LOSS: How to Calculate Heat Loss in a Building
HOUSEWRAP AIR & VAPOR BARRIERS
HUMIDITY LEVEL TARGET
ICE DAM PREVENTION
INSULATION INSPECTION & IMPROVEMENT
Inspect Attics for Moisture or Mold
Inspect Basements for Moisture or Mold
Inspect Building Exterior
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 Roof Solutions
ROOF VENTING ENERGY SAVING DETAILS
HEAT LOSS: How to Calculate Heat Loss in a Building
HOUSEWRAP AIR & VAPOR BARRIERS
HUMIDITY LEVEL TARGET
ICE DAM PREVENTION
INSULATION INSPECTION & IMPROVEMENT
THERMAL TRACKING & HEAT LOSS

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Be sure to review HOW TO FIND MOLD: How to Inspect Homes and Other Buildings for Mold - the Basics of How to Find Problem Mold Indoors in our Mold Action Guide. Here are other articles that will be helpful in evaluating attic mold presence, causes, and cures:

• 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?
• 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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The Mold Information Center: What to Do About Mold in Buildings, When and How to Inspect for Mold, Clean Up Mold, or Avoid Mold Problems

Use this simple, economical mold test kit by following our instructions on how to collect and mail mold samples to our lab

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