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Severe ice dams, eaves to ground in Poughkeepsie NYRoof Intake and Outlet Vent Area Ratios for Proper Attic Venting

  • ROOF VENTIILATION INTAKE-OUTLET RATIO - CONTENTS: 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. Table of Types of Ridge Vents and Net Free Venting Area per Linear Foot. List of roof outlet ventilation products comparing equivalent ventilation area provided by each
  • POST a QUESTION or READ FAQs about attic moisture, condensation & ventilation: what is the proper ratio of soffit intake to ridge outlet venting for roofs?
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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.

Our page top photo of extreme ice formation at a roof eaves illustrates an important problem for un-vented roofs in freezing climates. Ice formation at roof eaves, exacerbated by air leaks or inadequate insulation, risks leaks into the structure, damage to building components, and in some cases is also dangerous to building occupants.



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

Turbine vent shown on rooftop (C) Daniel FriedmanAdding 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.

Our photo (left) illustrates use of a turbine vent on a house roof as a means of improving attic exhaust venting. Wind powered turbine vents are useful in special applications but in our OPINION do not provide uniform under-roof ventilation across the entire roof surface. Details comparing various methods of roof ventilation are in the table below.

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.

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.

Building Code Requirements for Roof Ventilation

As detailed in Best Practices Guide to Residential Construction, chapter on BEST ROOFING PRACTICES:

The rules of ventilation developed by researchers in the 1940s were adopted first by the Federal Housing Administration (FHA) and later by all the major residential building codes, including the 2003 IRC, with few changes. Most asphalt shingle manufacturers will void their warranties if these rules are not followed. They require:

-- Adapted with permission from Best Practices Guide to Residential Construction.

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

Roof Ridge Outlet Ventilation Product Examples

Sq.In. of net free ventilation

GAF Cobra® Ridge Runner™ exhaust vent 12.5 sq.in. per linear foot, covered by cap shingles
GAF Cobra® exhaust vent: mesh type, roof nailing-gun-nailed 14.1 sq.in. per linear foot, covered by cap shingles
GAF Cobra® exhaust vent: mesh type, hand nailed 16.9 sq.in. per linear foot, covered by cap shingles
GAF Cobra® rigid Ridge Vent-2, Ridge Vent 3, & Snow Country ridge vent products 18 sq.in. per linear foot, covered by cap shingles
Adjustable aluminum ridge vent (typical) 18 sq.in. per linear foot, covered by cap shingles
Conventional rigid aluminum ridge vent (typical) 20 sq.in. per linear foot, covered by cap shingles (est).
Roof louvers or "spot vents" (typical) 50 sq.in. per vent, does not provide uniform ventilation between all rafter bays. Vent area ranges from about 35 sq.in. to 70 sq.in. for non-powered vents.
Turbine vents (wind-powered rotary, typical 12" to 14" diameter) 120-240 sq.in. estimated equivalent vent area, does not provide uniform ventilation between all rafter bays. Varies by wind speed and turbine diameter/design. Installing without adequate air intake can result in significant building heat loss. Can be installed on sloped or flat roofs; consider for flat and low slope roofs.
Smart Vent™ by DCI for eaves with no overhang  
AccuVent™ attic ventilation roof baffle  

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:

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 ROOF ICE DAM CURE: 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 ROOF ICE DAM LEAKS 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

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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