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VENTILATION in BUILDINGS

AIR BYPASS LEAKS
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ANIMAL ODORS IN buildings
ATTIC LEAKS, CONDENSATION & MOLD

BASEMENT CEILING VAPOR BARRIER
BASEMENT HEAT LOSS
BASEMENT LEAKS, INSPECT FOR
BATHROOM VENTILATION
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BLOWER FAN OPERATION & TESTING
BRICK WALL DRAINAGE WEEP HOLES
BUCKLED FOUNDATIONS due to INSULATION?
BUILDING NOISE DIAGNOSIS & CURE

CATHEDRAL CEILING VENTILATION
CEILINGS, DROP or SUSPENDED PANEL
CHIMNEY INSPECTION DIAGNOSIS & REPAIR
COMBUSTION AIR for TIGHT buildings
COOLING LOAD REDUCTION by ROOF VENTS
CONDENSATION or SWEATING PIPES, TANKS
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CRAWL SPACES

DECK & PORCH CONSTRUCTION
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
DRYER VENTING

FIBERGLASS INSULATION
FIREPLACES & HEARTHS
FLAT ROOF MOISTURE & CONDENSATION
FORMALDEHYDE HAZARDS
FRAMING DETAILS for BETTER INSULATION
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FREEZE-PROOF A BUILDING
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HOUSEWRAP AIR & VAPOR BARRIERS
HOUSE DOCTOR, how-to be
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ICE DAM PREVENTION
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LOG HOME GUIDE

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ODORS & SMELLS DIAGNOSIS & CURE

PAINT FALURE, DIAGNOSIS, CURE, PREVENTION

RADIANT BARRIERS
RADIANT HEAT
ROOF VENTILATION SPECIFICATIONS

SHEATHING, FOIL FACED - VENTS
SOFFIT VENTILATION
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STAIN DIAGNOSIS on BUILDING EXTERIORS
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STUCCO WAll FAILURES DUE TO WEATHER
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SWEATING (CONDENSATION) on PIPES, TANKS

THERMAL MASS in buildings
THERMAL TRACKING Indicates Heat Loss

VAPOR BARRIERS & AIR SEALING at BAND JOISTS
VAPOR BARRIERS & HOUSEWRAP
VAPOR CONDENSATION & BUILDING SHEATHING
VENTILATION in BUILDINGS

WIND WASHING INSULATION At EAVES
WINDOWS & DOORS
WINTERIZE A BUILDING
WOOD, COAL STOVES & FIREPLACES
WOOD STOVE SAFETY

ZONE DAMPERS
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More Information

Ridge vent mesh type Why Both Ridge & Soffit Venting are Needed under Building Roofs
     

  • How do a soffit intake and ridge outlet vent work together on a roof and why are both needed?
    • Adding a ridge vent alone can increase building heating costs
    • Adding soffit venting alone is ineffective
    • How to Correct Inadequate Attic Venting to Stop Attic Condensation, Ice Dam Leaks, Attic Mold, & Roof Structure Damage
  • Questions & Answers about attic moisture, condensation & ventilation: why are both soffit intake and ridge outlet needed?
  • References

Click to Show or Hide Related Topics

  • ROOF VENTILATION SPECIFICATIONS - home
  • AIR & HEAT LEAKS
  • AIR CHANGE RATE ACH HEAT SAVINGS
  • AIR LEAK DETECTION TOOLS - home
  • ATTIC LEAKS, CONDENSATION & MOLD
  • ATTIC MOISTURE or MOLD
  • ATTIC VENTILATION
  • ATTIC CONDENSATION CAUSE & CURE
  • BASEMENT MOISTURE to ATTIC
  • BLOCKED SOFFIT INTAKE VENTS
  • BLOWER DOORS & AIR INFILTRATION
  • CATHEDRAL CEILING VENTILATION
  • COOLING LOAD REDUCTION by ROOF VENTS
  • FLAT ROOF MOISTURE & CONDENSATION
  • HEAT TAPES & CABLES on Roofs for Ice Dams
  • HEAT LOSS in BUILDINGS - home
  • HEAT RECOVERY VENTILATORS
  • HOT ROOF DESIGNS: Un-Vented Roof Solutions
  • HUMIDITY LEVEL TARGET
  • ICE DAM PREVENTION
  • ICE DAM CURE: Comparing Two Houses
  • INDOOR AIR QUALITY & HOUSE TIGHTNESS
  • RIDGE VENT, ATTIC INSPECTION
  • ROOF VENTILATION SPECIFICATIONS
  • Inspect Building Exterior - Roof Venting
  • ROOF VENTIILATION INTAKE-OUTLET RATIO
  • ROOF VENT LOCATIONS
  • ROOF VENT SOFFIT & RIDGE NEED
  • ROOF VENT if NO SOFFIT
  • ROOF VENT SOFFIT, CONTINUOUS
  • ROOF VENTING ENERGY SAVING DETAILS
  • ROOF VENTING NEEDED?
  • ROOF VENTING ENERGY SAVING DETAILS
  • ROOF VENTING NEEDED?
  • SKYLIGHT VENTILATION DETAILS
  • SOFFIT INTAKE BLOCKED
  • SOFFIT VENTILATION SPECS
  • UN-VENTED ROOF SOLUTIONS
  • VENTILATION in BUILDINGS - home
  • VENTILATION DESIGN PROBLEMS & SOLUTIONS
  • VENTILATION, WHOLE HOUSE STRATEGIES
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Here we explain why a functional roof venting system needs both intake venting at the eaves or soffits and outlet venting at the roof peak or ridge. This article series explains How to Correct Inadequate Attic Venting to Stop Attic Condensation, Ice Dam Leaks, Attic Mold, & Roof Structure Damage. Our photo at page top shows a modern synthetic mesh type ridge vent (with modest airflow capacity) and our photo at left shows a typical installation of continuous soffit or eaves intake venting at the lower roof edges of a building.

Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.

Both Ridge and Eaves Venting Are Necessary

Soffit vent stripBoth Ridge and Eaves or Soffit Ventilation are Needed in buildings. 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.

This article is part of the series ROOF VENTILATION SPECIFICATIONS and also ATTIC CONDENSATION CAUSE & CURE.

Beware of adding a ridge vent without soffit vents (the worst) or soffit vents without a ridge vent (bad) or only gable-end vents (usually bad). The up-draft of air from the building (convection current of rising warm air which moves up through most buildings) will be increased and will mean unnecessary heat loss if you have a ridge vent to vent air out without also providing good intake venting at the soffits or eaves.


Sketch of an effective ridge vent designInstalling only a ridge vent and no or inadequate soffit venting is likely to result in unnecessary heat loss from the building as convection currents of rising warm air in the attic, unable to easily draw in air from outside, will "suck" warm air from the building, thus increasing unnecessary heat loss and increasing the risk of ice damming. Providing a lot of easy air intake at the building eaves avoids this problem.

Installing only soffit venting and no ridge vent works better than a ridge-only vent and this design was used on many residential buildings.

But it's not nearly as effective as a ridge and soffit vent combination, first because air moves through the attic space only when wind is blowing in the right direction, towards one of the walls of the Building which has a vented roof overhang, and second, because there is no high exit point to permit warmer air to exit by natural convection.

To be scrupulously fair, on a few buildings with very large gable end vents, lots of insulation in the attic floor, and perhaps lucky house siting, I have seen attics that were perfectly dry and free of condensation, ice dams, and mold. But these have been the exception, not the rule, at least for inspections in northern climates subject to cold winters and hot humid summers.

A High-Capacity Ridge Vent Moves More Air than a Low-Profile Good-looking Vent

Ridge vent mesh type Ridge vent closeup showing mesh

The popular plastic mesh type ridge vent shown in this photograph is popular where ridge vents are installed on new homes or as retrofits. Builders and owners like the ability to nail roof shingles over the easily-stored, transported, and installed roll-out plastic mesh that is simply laid across an opening cut along the building's ridge.

But we have observed that this ridge vent design passes much less air than the older, uglier aluminum ridge vent shown in our sketch above. We prefer the older product which moves air.

Continuous High Capacity Eaves or Soffit Intake Venting Provides Adequate Intake Air Under Roofs

Soffit intake venting Soffit vent strip

As we show in this pair of photos, continuous soffit intake venting will provide optimum intake air flow between every rafter pair.

Installing Continuous High Capacity Eaves or Soffit Intake Venting Works Best to Avoid Attic Moisture, Mold, & Ice Dams

Here are examples of inadequate intake ventilation: vents at the soffits are intermittent or "spot vents" or are simply too small.

Home made soffit vent

 

Don't install intermittent or occasional or faux soffit intake venting or vents with too little opening area such as we show in the photo at left.

Not only are the openings too small to pass enough air (obstructed further by the louvers and insect screens), intermittent soffit intake vents or little round or rectangular soffit spot vents are singularly ineffective in providing good under-roof or attic ventilation.

Continuous soffit/eaves intake venting is the proper location for the intake air, in order to assure that the entire under-side of the roof sheathing is vented and kept dry. Not what is shown in our photo (left).


Poorly vented home soffit

Where we inspect attics with "spot vents" in the soffits (those little round louvered vents ranging from about 3/4" diameter (photo at left) to 2" in diameter or even 4" in diameter are completely ineffective, never moving enough air.

In these attics of buildings in climates where moisture is often a concern, we find moisture stains in the attic at the building eaves, sometimes moldy building insulation, and on occasion serious attic moisture and condensation problems.

Where we inspect attics where even larger vent openings are provided in the soffits or eaves, if the openings are intermittent, we see wet and often moldy roof sheathing on those roof sections where no venting is provided (the "in-between-vent" roof sections), even though at other roof sections where vents are present the sheathing often looks clean and dry.

This is compelling visual evidence that air is not moving up the under-side of the sections of roofing where no intake vents are present at the building eaves. Venting needs to be provided between every rafter pair at the eaves and ridge.

Just as we want continuous intake venting along the building eaves, continuous ridge venting is the optimum exit path for warm rising air in an attic, thus pulling new cooler, drier outside air into the under-roof area from between every rafter pair. (C)Daniel Friedman - copyright protection trap.

...


Frequently Asked Questions (FAQs) about the requirement for both soffit intake and ridge outlet venting in roof ventilation designs

Question: Why don't I see soffit vents in Texas?

Solid soffits, no venting (C) D FriedmanAs a newcomer to texas [san antonio] I don't see soffit vents on any houses around here why? - Robert Tait 8/15/11

Reply:

Robert for sure construction practices vary in different parts of the U.S. and Canada and also practices vary by age of the building. For example in the 1940's it was common to have almost no attic or under-roof venting, and in fact people even closed off venting to try to save heat during the heating season in cool climates.

In the 1960's and 70's it was common practice to install venting in the gable ends of the home - a practice that may cool the attic but didn't do much for roof life.

In current building practices continuous airflow intake at eaves and outlet at ridge gives a cooler attic and longer roof shingle life. An alternative "hot roof" design used in some areas omits all under-roof ventilation and instead crams the roof cavity with insulation. Which has application in some climates but fails to cool a hot roof (shorter shingle life) and fails to dry out small leaks (leading to often serious hidden leak and rot damage).

Considering how hot San Antonio is, I'd expect at least some builders to be sold on roof venting. But perhaps you are seeing entire neighborhoods built around the same time, using the same convention for their era, or using the same few builders who simply were not sold on the benefits of under-roof venting in a hot climate.

Finally, while roof venting is usually good for the structure, cooling the attic, dumping un-wanted heat and moisture, improperly-installed venting can actually increase heating or cooling costs. For example, providing exit venting at a ridge without adequate intake venting at the eaves produces a design that sucks conditioned air out of the home, increasing both heating and cooling costs. (Typically we want to see 2x soffit air intake area as ridge outlet area.)

Or there is some consideration we haven't come across (problems with prevailing wind, for example might lead to changes in roof venting strategies). Let me know what you see when driving around neighborhoods of one family gable roof homes of different ages.

Question: my roofer is providing ridge vents but not improving soffit venting above a very minimal current intake set-up. Will this be OK?

Round soffit intake vents (C) D FriedmanI'm badly in need of your advice on ridge vent. I have a 32-year old 3-story townhouse that has 9 small soffit vents on the eaves: 3 square (4" x 4") vents with screen and 6 white round 4” vents with grill that seem to have been clogged with several layers of paint.

My roofing contractor (A) offers in the contract 2 ridge vents, i.e., "Cut out the peaks and install new CertainTeed Certi-vent 11 shingle over ridge vents.”

I got worried, and asked him if the outside air intake from our tiny soffits would be sufficient for the ridge vents to work efficiently. His reply is "Your house was not designed to ventilate the attic but the existing soffit vents should be adequate.": he has not measured the size of attic nor calculated soffit vent space.

Another estimator (B) says that we cannot have ridge vents because air intake from those tiny soffit vents won't be enough.

I asked the contractor (A) to provide me with data supporting his theory by performing a feasibility test on our attic and soffit ventilation system by measuring the length and width of the attic, calculating the area of ventilation needed, and calculating soffit vent space needed.

I would be grateful if you would send me your feedback on this issue. - S.C., Arlington VA 5/10/12

Reply: balance soffit intake with ridge outlet venting as follows:

Perforated soffit vents (C) D FriedmanIt is very common for roofing contractors to include in their roof bid the installation of a ridge vent but to ignore soffit intake venting. We think that's because the roofer is working on the roof surface where adding the ridge vent is a quick and easy task during re-roofing.

In contrast, working under the soffits or eaves to fix or provide soffit intake venting is a different operation, needing different ladder or scaffolding set-up, and involves quite a bit more labor. Nevertheless, as a building owner you indeed need your under-roof venting design to work properly.

A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem with the design of both intake and outlet ventilation for a roof. That said, here are some things to consider:

A ridge vent allows air to exit from the building at the highest point - the ridge - as air is warmed and rises through the structure by natural convection. But if there is not adequate intake venting at the soffits or eaves, the exiting air at the ridge will increase building heating or cooling costs unnecessarily by exhausting air from the conditioned space below the attic floor or cathedral ceiling cavity.

Fabric type ridge vent (C) D FriedmanOn the other hand, with a ratio of about 2x soffit intake vent area for the ridge vent exit area, the roof venting design will provide sufficient outdoor air to ventilate the under-roof area year-round, reducing energy bills and avoiding both attic condensation problems and a hot attic that shortens roof life and increases building energy costs.

Precisely measuring the intake and exit vent areas can be a misleading step if your measurements do not consider the degree of blockage of the intake vents at the soffits or eaves due to the reduced area provided by perforated or slotted vent openings, insulation in the eaves or other factors.

Similarly, in our OPINION, the most popular (cheap, easy and quick to install) roll-out synthetic fiber low profile ridge vents (photo above left) pass less air in cfm than the older, uglier, aluminum ridge vents that we prefer.

Ultimately if you provide continuous, un-blocked soffit intake vents along the house eaves, using conventional perforated soffit covering panels or louvered continuous soffit vent intake strips and a conventional ridge vent of either type along all of the ridge lengths, stopping 6" or a foot before the gable ends of the home, your ridge venting system should work just fine.

...

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Technical Reviewers & References

Related Topics, found near the top of this page suggest articles closely related to this one.

  • Carson, Dunlop &
Associates Ltd., TorontoCarson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.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:
    • Commercial Building Inspection Courses - protocol ASTM Standard E 2018-08 for Property Condition Assessments
    • Home Inspection Education Courses including home study & live classes at eleven colleges & universities.
    • Home Inspection Education Home Study Courses - ASHI@Home Training 10-course program.
      Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
    • The Home Reference Book, a reference & inspection report product for building owners & inspectors.
      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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      Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter inspectaehrb in the order payment page "Promo/Redemption" space.
    • 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.
    • The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones.
  • 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 and 12/08
  • Daniel Friedman -edited and added to comments from John Annunziata, P.E. - NY Metro ASHI during informal chapter discussions about roof and attic ventilation options (1986-1996).

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

  • Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
  • Home Reference Book - Carson DunlopThe 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.

    Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.
  • "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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