Roof Soffit Intake Vent Specifications
How to Install Soffit Intake Vents to Stop Attic Condensation, Ice Dam Leaks, Attic Mold, & Roof Structure Damage

Roof Soffit or Eave Ventilation Specifications:

Continuous intake venting is desirable in vented roof designs.

This article describes how to properly inspect, place, and size ventilation air intakes at the lower edges of a building roof - its soffit, or eaves.

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

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 40 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.

[Click to enlarge any image]

We have also measured changes in airflow, temperature, and moisture before and after installing roof venting.

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

On 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.

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.

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.

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

Venting needs to be provided between every rafter pair at the eaves and ridge. You won't achieve this if venting is intermittent along the soffits or eaves of a home.

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.

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, even though at other roof sections where vents are present the sheathing often looks clean and dry.

This is very strong evidence that air is not moving up the under-side of the sections of roofing where no vents are present.

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 violation trap.

But also remember the danger 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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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2015-09-25 - by (mod) -

Sure J,

See ROOF VENT if NO SOFFIT at https://inspectapedia.com/ventilation/Roof_Vent_Eaves_Intake.php

On 2015-09-25 by J

I have an old house that does not have soffits. The wall goes all the way up to the deck of the roof with rafters coming through the wall. This means the attic starts where the wall meets the deck also. Is there a good diagram for how to install soffits on a house like mine?

Question: ok to strip off old plywood soffit covering

could you take the plywood off and jus run the soffit? - Paul Gilbert 8/13/11

Reply:

Certainly, Paul, as long as there is adequate support for the vinyl or aluminum perforated soffit covering the plywood soffit that was in place can usually be omitted.

But do not just leave the soffits entirely open with no covering whatsoever - doing so invites an invasion by birds, squirrels, raccoons, etc.

Question:

I have a bit of a problem with your last statement; too great and intake area is going to be a problem as well, IMHO. I am curious about the engineering with regard to airflow in the case of continuous soffit venting.

Unless you have the same area (or greater) for the air to exit, does the air flow not just go slack (no proper draft) and possibly create the problem you are trying to prevent?

I have continuous soffit venting and ridge venting (which frankly is a horrible idea without the right amount of pitch and again IMHO a bad idea in general...automated extraction is a MUCH better answer!) and the air flow is pathetic - it has no direction and don't even bring in the range of factors created by a breeze, more engineering!

A continuous soffit vent of let's say 3" around the perimeter of a home is going to have a much larger opening area then the ridge vent or gable vent not creating a proper draft and allowing the air to go slack. (with exception of automated extraction fans) This issue has been over-simplified, it is much more complex than what I have read here. - Robert 9/30/2012

Reply:

Really? Not quite, Robert. We need about 2x as much intake venting at the house eaves as exit venting at the ridge in order to be sure that we don't start drawing conditioned air out of the building itself.

Decades of field experience as well as numerous building studies and, among building professionals, hundreds of thousands of onsite inspections of buildings in various configurations of venting and venting devices confirm the data.

I agree that some modern ridge vents may not be passing as much air OUT as we'd like, but having extra intake venting is not going to hurt - since the attic or roof cavity is not a flexible balloon, more air won't enter the attic than can exit at the ridge.

But Robert, your idea, of automated "extraction" of roof moisture is unfortunately either a fantasy ( the fans that "extract" attic air don't draw in cool outside air and instead extract air from the building, increasing heating and cooling costs), or works only at significant net cost (for example the cost of adding humidity controlled equipment), and in all cases, risk uneven drying of the under-roof surfaces unless continuous intake and outlet venting are there to direct the air flow where it's needed.

Some of the other problems we see with "extracting" vent systems that rely on an attic or roof vent fan include these:

• The attic or roof or gable end fan does not ventilate evenly under the roof, leaving wet areas
• The attic or roof or gable end fan operates by temperature, venting only in summer, not in winter when there are condensation worries
• The attic or roof or gable end fan creates a serious fire-increase hazard should it operate during a house fire, or reqires extra costly controls

It's often risky to be over-confident in our own opinions, no matter how carefully reasoned, as too often our own experience, study, and knowledge are just much less than that of the larger body of experts interested in the question. Take a look at the ASHRAE, DOE and other expert citations at the end of this article and you can figure that we're not just making this stuff up.

InspectAPedia is an independent publisher of building, environmental, and forensic inspection, diagnosis, and repair information provided free to the public - we have no business nor financial connection with any manufacturer or service provider discussed at our website.

We are dedicated to making our information as accurate, complete, useful, and unbiased as possible: we very much welcome critique, questions, or content suggestions for our web articles. Working together and exchanging information makes us better informed than any individual can be working alone.

Question: how can I install a vinyl soffit covering if there is dentil trim at the wall top?

I am considering replacing the soffit on the front upper overhang of my house with vinyl invisivent soffit, but have an existing dentil moulding. Is there a way to install the new vinyl soffit without covering the dentil moulding so that I can preserve this distinctive architectural feature? - C.M. U.S.A.

Reply:

http://www.certainteed.com/resources/CTS150.pdf from Certainteed describes the soffit covering product you mention.

Normally a vinyl soffit covering is supported on the outer or fascia edge by L-channel and against the building wall by L or F channel strips.

If you have space to nail the supporting F or L channel to the house wall above the dentil trim you describe, you're in business. If there is no space even for the supporting channel then you don't have room to install the soffit covering, as its thickness is not much less than that taken up by the supporting vinyl channels. You'd have to move the dentil trim down.

You need about 3/4" of space to support the edges of the vinyl soffit covering.

But all of this is speculating and arm waving - as I haven't seen the details of your existing soffit structure and house trim details. If you'd like to send us some photos I can comment further.

Question:

(Feb 6, 2014) gene said:

hi we have moisture coming out of our soffit and cant figure out why . we have gable end vents and no ridge venting, with small 3 inch holes in the soffit. its only doing it on the wall that the shower is on its about a ten foot section we tried cutting the 3 inch holes bigger but it didn't help it just made more ice cycles , so that tells me we have heat loss now?

Reply:

Gene, it would be no surprise that moisture may accumulate in a soffit, particularly if the vent openings are too small (which IMHO is the case with 3-inch openings unless you've cut a 3-inch strip for the full soffit length).

Moisture accumulates in cold weather (warm moist air entering soffit, hits cool conditions, moisture condenses out) and can form ice

I agree that your description may mean that there is more building warm air movement into the attic and out through soffits (probably through other vents & openings) than you'd want - but to make sense of the heat loss picture one would want to take a look in the attic - for the amount and care with which insulation was installed - look for openings, missing insulation &c. And in the finished area look for leaks especially around wall & ceiling penetrations.

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Continue reading at ROOF VENTILATION SPECIFICATIONS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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