Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair
Ask a Question or Search InspectAPedia
InspectAPedia ® Home
VENTILATION in BUILDINGS
AIR BYPASS LEAKS
AIR LEAK DETECTION TOOLS
AIR LEAK MINIMIZATION
AIR LEAK SEALING PROCEDURE
AIR POLLUTANTS, COMMON INDOOR
AIR SEALING STRATEGIES
AIR TEST FOR MOLD: ACCURACY
AIR TEST SAMPLING CASSETTE STUDY
ANIMAL ODORS IN buildings
ATTIC LEAKS, CONDENSATION & MOLD
BASEMENT CEILING VAPOR BARRIER
BASEMENT HEAT LOSS
BASEMENT LEAKS, INSPECT FOR
BLOWER DOORS & AIR INFILTRATION
BLOWER FAN CONTINUOUS OPERATION
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
CONDENSATION on WINDOWS & SKYLIGHTS
DECK & PORCH CONSTRUCTION
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
FIREPLACES & HEARTHS
FLAT ROOF MOISTURE & CONDENSATION
FRAMING DETAILS for BETTER INSULATION
FRAMING DETAILS for DOUBLE WALL HOUSES
FREEZE-PROOF A BUILDING
FROST HEAVES, FOUNDATION, SLAB
GREEN BUILDING CONSTRUCTION CODES GUIDES
GREENHOUSE DESIGN for SOLAR HEATING
GREENHOUSE / SUNSPACE GLARE
HEAT LOSS in BUILDINGS
HEAT LOSS DETECTION TOOLS
HEAT LOSS INDICATORS
HEAT LOSS PREVENTION PRIORITIES
HEAT LOSS R U & K VALUE CALCULATION
HOT ROOF DESIGNS: Un-Vented Roof Solutions
HOUSEWRAP AIR & VAPOR BARRIERS
HOUSE DOCTOR, how-to be
HUMIDITY LEVEL TARGET
ICE DAM PREVENTION
INDOOR AIR HAZARDS TABLE
INDOOR AIR QUALITY & HOUSE TIGHTNESS
INDOOR AIR QUALITY IMPROVEMENT GUIDE
Insulation Air & Heat Leaks
INSULATION INSPECTION & IMPROVEMENT
INSULATION R-Values & Properties
LOG HOME GUIDE
MOISTURE CONTROL in BUILDINGS
MOLD: A COMPLETE GUIDE TO MOLD
NOISE / SOUND DIAGNOSIS & CURE
ODORS & SMELLS DIAGNOSIS & CURE
PAINT FALURE, DIAGNOSIS, CURE, PREVENTION
ROOF VENTILATION SPECIFICATIONS
SHEATHING, FOIL FACED - VENTS
SOUND CONTROL in buildings
STAIN DIAGNOSIS on BUILDING EXTERIORS
STAIN DIAGNOSIS on BUILDING INTERIORS
STUCCO WAll FAILURES DUE TO WEATHER
STUCCO WALL METHODS & INSTALLATION
STUCCO OVER FOAM INSULATION
STUCCO PAINT FAILURES
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
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.
A High-Capacity Ridge Vent Moves More Air than a Low-Profile Good-looking Vent
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
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.
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?
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?
I'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:
It 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.
On 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.
Questions & answers or comments about attic moisture, condensation & ventilation: why are both soffit intake and ridge outlet needed?
Ask a Question or Enter Search Terms in the InspectApedia search box just below.
Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.