Why Both Ridge & Soffit Venting are Needed under Building Roofs
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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
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 Both 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.
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 Installing 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.
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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.
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). |
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.
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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?
As 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?
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.
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- "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
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- "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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