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This article discusses How to Specify the Proper Roof Intake and Outlet Vent Area Ratios to Stop Building Heat Loss and Provide Proper Attic Venting to Avoid Condensation, Ice Dam Leaks, Mold, & Roof Structure Damage. Adding under-roof ventilation is usually a great idea, but if the relative sizes of the intake and outlet vents are not proper, the building will suffer increased heat loss and thus an unnecessarily high home heating bill.

This is a section the article series, Roof Venting: Correct Inadequate part of our discussion of 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.

These recommendations are based on roofing industry standards, roof covering manufacturer recommendations, and on review of the literature on building insulation and ventilation, as well as on 30 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. We have also measured changes in airflow, temperature, and moisture before and after installing roof venting.

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Vent Area Ratios: Attic Ventilation Soffit Intake to Ridge Outlet Air Flow or Square Inches Ratio

Continuous un-blocked soffit or eaves intake venting combined with continuous roof ridge venting (or equivalent area if the building framing does not permit a ridge vent) are needed to avoid ice dams, attic condensation, attic mold, rot, or insect damage from accumulating attic moisture.

But the ratio of intake air to outlet air is of critical importance too.

The ratio of soffit intake to roof outlet should be at least 2:1 to avoid unnecessary these heat losses from the building. A serious error is a roof outlet vent net free area that exceeds the air inlets at lower roof edges or eaves. When this occurs in a climate where building heating is needed during part of the year, warm air leaking into the attic or roof space and exiting at the ridge vent (or other vents high on the roof) creates a convection air current that draws excessive heat out of the building during the heating season, leading to unnecessarily high heating costs.

But don't "fix" a bad intake to outlet air vent space ratio by reducing the ridge vent opening. Making this mistake can result in too little air flow under the roof surface, leading to indoor condensation and mold.

Roof intake venting with no outlet vent openings won't work because there will be no air flow through the roof cavity. In a few cases, very wide, open soffit vents at building eaves seem to result in a dry attic, but the design relies on a prevailing wind pattern that sends air through the attic. Even in this case most air flow will be across the attic floor, and an inspection of the attic near the ridge may reveal evidence of unwanted condensation and moisture staining or even attic mold.

Roof outlet venting with no intake venting won't work because the absence of sufficient intake of outside air to satisfy the negative pressure from air leaving at the ridge will cause draw warm air up from the building interior, increasing heating costs and possibly mold or allergen movement through the building.

Providing more soffit or eaves intake venting than ridge outlet venting assures that the airflow required by attic air exiting at the ridge is satisfied by incoming outside air rather than by pulling air up from the building where it not only brings up building moisture, it also increases building heating or cooling costs.

Table of Types of Ridge Vents and Net Free Venting Area per Linear Foot

Comments & Opinion About Statements of Net Free Ventilation Area of Various Roof Venting Products

Besides the rated air ventilation area described by various vent product manufacturers, other roof and vent opening details can significantly affect the actual airflow and level of under roof ventilation at a building.

While roofing product companies give useful general guidance on the amount of roof ventilation are recommended as a function of the square feet of attic space, here are some factors that could significantly change the actual recommended under-roof ventilation for a specific building:

• How wide was the cut made on either side of the ridge board to permit airflow into the ridge vent?
• Is the roof over an open attic, a partial attic with knee walls, or a cathedral ceiling?
• Is the building subject to usually high indoor moisture levels for any reason?
• What intake ventilation has been provided under the roof at the building eaves?
• How long and how open is the air flow pathway from building eaves (soffit) to ridge?
• How uniformly will the roof be ventilated by a given product? Will some roof areas or rafter bays be left un-vented?

Looking at a linear foot of a typical thick mesh-type ridge vent and before considering that power-nailing compresses the mesh to further reduce airflow:

If we cut a 1.5" gap between ridge board and remaining roof deck, 12" long, on each side of the ridge board, that's
12" x 1.5" x 2 = 36 sq.in. of open vent area (before any covering with the ridge vent material).

Suppose a roof vent product company indicates that their product is giving you 17 sq.in. of roof venting in a 12" length - roughly that's a 50% airflow restriction over the free opening, before allowing for other obstructions (rafters, air flowing downhill) - by this analysis.

But another step is needed:

This is how we think about vent area with a roll-out mesh ridge vent material:

The exposed *edge* of the mesh vent is all that can possibly vent out - that's typically about 1/2" to 3/4" high between the roof surface and the underside of the cap shingles on the roofs we have walked recently.

For a linear foot, after the cap shingles are installed, and counting both sides of the ridge, that's about 12 sq. in. of available space (1/2" x 12" x 2 sides),

We then cut that area in half to factor in the 50% mesh-restricted air flow rate that we found above, so we're really seeing an effective vent outlet, in the best case, of 6 sq.in. per foot.

Which is too little compared with the intake.

The appeal of the low profile roll-out type mesh ridge vent materials that are covered with cap shingles is aesthetic - the ridge vent looks nicer from the ground, and it's convenient on the truck - doesn't get dented, rolls up and stores nicely for transport, and installs over a non-straight ridge line, something that's a problem with the old vent type. So we understand why it's a popular product. It just does not pass as much air as the older vent type. We asked one manufacturer's mesh-type roll-out ridge vent vent tech-ref-salesman about their actual airflow tests and airflow venting rates at a JLC conference in the 1980's: he was flabbergasted - replying that he had no idea about any actual tests or measured numbers. Happily most roofing product manufacturers such as the GAF are kind enough to provide their estimates of the amount of ventilation provided by each product.

A low profile mesh type and some other plastic ridge vents do not pass much air compared to an older (uglier) higher-profile rigid aluminum ridge vent. Where we are having difficulty obtaining good airflow under a roof (such as where there is limited air space between insulation and the roof deck, aggressive intake venting and properly sized outlet venting at the ridge can help assure that the limited vent space under the roof would have adequate airflow. That's why we often suggest that uglier alternative exit vent, as well as suggest making sure that the roof decking slots for outlet venting at the ridge are cut correctly on both sides of the ridge board.

In general, you want 2x as much intake venting (at the eaves) as outlet (at the ridge) but keep in mind that if you use a mesh type "ridge vent" the ridge opening is obstructed by the mesh and the air flow will may be insufficient, so you can't just measure the sq.in. of vent opening, you have to also adjust the calculation for the degree to which the vent opening is obstructed by mesh, screening, and any other airflow obstructions such as under-sized cuts into the roof deck.

On older homes where rafters are wider apart than standard modern framing specifications (16" o.c.), a baffle that extends the full width between the rafters is the best you're going to get unless the owners opt for the more labor intensive and thus more costly approach of a site-built vent path that uses furring strips alongside rafters and solid foam insulation sheets to give a deeper vent path under the roof than provided by a baffle.

You'll want to look at the baffle selected to be sure it won't be compressed when insulation is added into the remaining roof space between the rafters.

About ice dams and roof ventilation

Increased air flow under the roof will prevent, not cause, ice dams, provided that insulation is also completely installed.
Ice dams occur because lost heat at the eaves melts snow there where the snow melt runs further down the roof to the cold overhang where it freezes. If we can vent enough air under the roof surface to keep the roof uniformly cold you won't ever see ice dams.

Take a look at Comparing Two Houses where we compare two under roof venting schemes on houses that happened to be side by side. We installed continuous soffit intake and ridge vent on the house at left; the house at right had almost no soffit intake venting. See ICE DAM PREVENTION for details about this topic.

You'll want to be sure air FLOWS continuously from soffit to ridge- if the baffles compress or the air space is too little (say less than 1/2"), or if the ridge outlet is obstructed by low-flow plastic mesh, then the risk of ice dams is increased - not because of the soffit inlet but because of inadequate outlet.

Put it another way, if you had no roof venting at all, heat lost into the roof cavity will cause ice dams.

In sum the building design least likely to give ice dams includes

• good soffit intake venting
• good ridge outlet venting
• good air path soffit to ridge
• good insulation installation,no voids, extending all the way to the to plate over the walls
• proper balance between intake and outlet vent openings\
• assurance that airflow moves under all roof slopes, not just on one side of the building

Last: don't forget the importance of also avoiding excessive interior moisture levels (a key factor in attic condensation and thus mold) - the dirt crawl space needs to be addressed.

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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, Thermal Tracking
Blocked Soffit Intake Vents
CATHEDRAL CEILING INSULATION
Comparing 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

• 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).
• 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=

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

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More Information on Building Diagnostic Inspections and Repairs

More Reading about Dealing with Attic Mold, Identifying, Removing, and Preventing Mold in Attics

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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