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Severe ice dams, eaves to ground in Poughkeepsie NYRoof Ventilation Design & Specifications
How to Avoid Inadequate Attic Venting, Attic Condensation, Ice Dam Leaks, Attic Mold, & Roof Structure Damage

  • ROOF VENTILATION SPECIFICATIONS - home CONTENTS:, How to Correct Inadequate Roof Ventilation or Attic Venting to Stop Attic Condensation, Ice Dam Leaks, Attic Mold, & Roof Structure Damage
  • POST a QUESTION or READ FAQs about ways to correct or improve under-roof ventilation including at special problem cases where there is no roof overhang or where eaves-trough gutters are installed
  • REFERENCES
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Attic ventilation fixes: here we explain how to correct improper or inadequate attic or under-roof ventilation in buildings.

This article series 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.



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Roof Ventilation Choices, Design & Specifications

Turbine vent shown on rooftop (C) Daniel FriedmanThese attic or roof ventilation recommendations are based on a survey of building science literature combined with 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 any image for a detailed, enlarged version.]

Roof system ventilation is required by the model building codes such as International Residential Code (IRC) which includes IRC-R806 “Roof Ventilation” that requires ventilation for attics and cathedral ceilings and ICC R905.1 “Requirements for Roof Coverings,” that requires builders to follow the manufacturer’s installation instruction. The latter implies a requirement for ventilation as that is specified for many roof coverings such as asphalt shingles.

Best Practices author Steve Bliss points out that most asphalt shingle manufacturers will void their warranties if roof venitlation rules are not followed. They require:

Article Series Contents

We have measured very large changes in airflow, temperature, and moisture before and after installing roof venting. We include references to building and industry experts as well.

Our photo (left) of heating tapes along a roof eave are a reliable clue that the building owners have been concerned with ice dam formation and leaks at the roof edges - a problem found in poorly vented or un-vented roofs or poorly-insulated buildings in cold climates. In such buildings we may find costly leak damage, mold contamination in the attic, walls, and ceilings, and loss of the R-value of wet building insulation. As explained in Best Practices Guide to Residential Construction, chapter on BEST ROOFING PRACTICES:

All residential building codes require some form of roof ventilation. These rules were first developed in the 1940s, when attic spaces first started to develop problems with mold and mildew due to excess moisture. (See Inspect Attics for Moisture or Mold for details.)

With the growing use of plywood, asphalt shingles, insulation, and better doors and windows, houses were being built tighter. The tighter spaces retained more of the normal household moisture generated by cooking, bathing, household plants, crawlspaces, and exposed basement slabs. As the stack effect drove this moisture up into attic spaces, problems ensued.

Building Code Requirements for Roof Ventilation

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.

How to Correct Improper or Inadequate Attic or Under-Roof Ventilation in buildings

this simple sketch shows the optimium attic vent design with inflow at eaves and outflow at ridge

Home inspectors and building owners should be especially concerned about insulation placed directly under the roof sheathing such as between the rafters. This can trap heat and moisture and damage sub-roofing as well as roofing shingles.

Also, cathedral ceilings without vents in the soffit and up at the ridge or without adequate air path between the roof insulation and the underside of the roof sheathing can lead to major condensation problems, rot, insect damage, and severe structural damage in just a few years.

Do look for those dark rafter lines and don't underestimate the damage that can exist.

See Inspect the Soffit Vent System from the Attic for details.

The best "fix" for cathedral ceilings and un vented roof cavities is to assure that there is an air path into the attic up the under side of the roof along the building eaves.

If insulation blocks the soffit intake venting, it is corrected simply by installing soffit vent baffles at the house eaves between every rafter pair (cardboard or Styrofoam pieces made for that purpose and sold at most lumber yards) and adding vents if not in place.

SeeSOFFIT INTAKE VENT BLOCKED for details.

Watch out: when correcting insulation-blocked soffit intake venting, don't just pull back insulation into the attic, or you risk creating a cold spot over the interior ceilings near building exterior walls, increasing heat loss, and making ice dam problems worse.

Also see INSULATION AIR & HEAT LEAKS.

Un-Vented "Hot Roof" Designs Risk Costly Hidden Damage

Two approaches for insulating cathedral ceilings and flat roofs (C) Carson Dunlop Illustrated HomeAn alternative un-vented "hot roof" design (the right-hand cathedral ceiling structure shown at the right in Carson Dunlop's sketch) is touted by some building experts who argue that under-roof venting is not needed whatsoever.

Roof surface temperature affects the life of the roof covering. Studies in Florida confirmed that asphalt roof shingle surface temperature varies more as a function of shingle color than as a function of whether or not the under-roof cavity is ventilated.

Mark Cramer informs us that an insulated roof is only 7 deg. F. hotter than an un-insulated roof. By this view roof life may not be reduced significantly by an un-vented design. But here are some concerns that the "hot roof" design leaves incompletely addressed:

Details about hot roof designs are found at HOT ROOF DESIGNS: UN-VENTED ROOF SOLUTIONS

Leaks and moisture problems are common on buildings in most climates and these conditions are likely to occur over the life of a building. Good building design resists water damage.

Use of Gable End Vents with Soffit and Ridge Venting?

Question: should I add gable end vents to my attic space?

I will be siding my house in the near future. I currently have soffit vents along with a ridge vent.

Will adding gable vents to the two sides of the house add or hinder the air flow? - Mike Martino

Reply:

We recommend against adding gable end vents where a continuous ridge and soffit vent system is installed. Adding gable end vents will prevent proper roof ventilation so long as you have un-blocked soffit intakes and ridge outlet openings. When warm air in the attic moves upwards by natural convection and exits at the ridge, cooler drier air is drawn into the attic or under-roof space at the building eaves or soffits.

We want that incoming air to treat the entire under-roof surface, from the lower edges or eaves up to the ridge, by flowing along the underside of the roof deck and out at the ridge.

If you were to add gable end vents, some of the draft created by air exiting at the ridge will be satisfied by incoming air at the gable ends, as the gable vents are naturally closer to the ridge at either end of the building than are the soffit intake vents.

As a result, less air flow will be provided for the lower center portions of the roof, and the benefit of cooling and drying the entire roof underside surface will be impeded.

This attic air movement theory can often be confirmed in practice by careful observation. Home inspectors sometimes observe the combination of soffit vents, ridge vent, and gable end vents on a home where they also find that the more serious condensation and least effective under-roof ventilation is apparent at the lower center portions of the roof slopes, while on a similar home with similar interior moisture conditions, materials, age, construction, but using only soffit and ridge venting, having omitted or blocked off the gable end vents does not suffer the same problem.

In fact during an under-roof ventilation retrofit in which continuous soffit and ridge vents are being added to a building, if there are existing gable-end vents installed we recommend that after the new vents are provided the old gable-end vents should be closed off.

Details are at Roof Venting: Proper Locations.

Attic Roof Vent Configuration Designs, Alternatives & Advice

Turbine vent shown on rooftop (C) Daniel FriedmanNo intake nor exhaust venting: best advice is to add continuous soffit intake and ridge line outlet venting, with an air path up the under-side of the roof decking in an attic or cathedral ceiling

Soffit vents only: limited effectiveness as a roof venting, varying with wind direction. Best to add continuous ridge venting.

Ridge vents only: a bad idea, this condition will increase heat loss from the building and increase building heating costs. Add continuous soffit vents at the eaves.

Also see INSULATION AIR & HEAT LEAKS.

Gable vents only: traditional design, better attic venting is achieved with continuous soffit intake and ridge outlet venting; after these are installed, close off the gable end vents for best venting performance as discussed above. Just stapling plastic over gable end vents from inside the attic should be sufficient.

Details are atROOF VENT LOCATIONS

Roof Turbine vents or Spot vents: shown in our photo at left, turbine vents or spot vents on roofs give a more complex venting pattern and are not problem free. Often builders install soffit and ridge vents but leave the turbine vents in place.

Leaving a turbine vent or spot vent in place when a ridge vent is added may not be a problem and in areas of modest snow depth that covers ridge exit vents this approach may actually help vent a roof in some conditions.

Turbine vents are also used as "spot vents" to try to ventilate difficult roof structures.

Watch out: but beware: if you do not provide a source of outdoor air to satisfy the exhaust effects of the turbine vent(s), building heating costs may be increased in some cases, just as occurs if we add a ridge vent or gable vent with no eaves intake vents.

Attic Ventilation Details - Best Practices

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

Ridge & Soffit Vents - Importance of Balanced Roof Ventilation

Figure 2-55: Balanced Roof Ventilation (C) J Wiley, S Bliss

For both attics and cathedral ceilings, roof ventilation works best when it is balanced between high and low to take advantage of natural convection (Figure 2-55 at left).

This configuration also tends to evenly wash the entire underside of the roof surface with ventilation air. That's why we insist that soffit intake venting and ridge outlet venting on buildings should be continuous rather than intermittent or "spot" vents or intermittent louvered soffit inlet vents.

Ridge vents should either have external or internal baffles to minimize infiltration of windblown rain and snow.

Watch out: Do not use ridge vents or other rooftop vents without low vents to provide makeup air. The suction created could help pull moist household air into the attic.

And do not install ridge vents nor any other roof venting outlet without providing adequate outside inlets to the roof cavity at soffits or eaves. Making that mistake will draw conditioned (heated or cooled) air out of the building and thus will increase building heating or cooling costs as well.

Ridge Vent Alternatives

Where ridge vents are not an option, combine any type of upper vent such as gable-end vents, roof vents, or turbines, with soffit vents. Where soffit vents are not possible, use gable-end vents on both ends of the roof, which will ventilate adequately under wind pressure. Avoid high ridge vents alone: you'll increase building heating and cooling costs as we explained above. .

Ratio of Soffit Intake Vent to Ridge Outlet Vent Area

Figure 2-55: Soffit ventilation details (C) J Wiley, S Bliss

The soffit-vent intake area should be larger than the ridge-vent area. As we explain
at Roof Venting: Intake - Outlet Area Ratios, many builders use a 2:1 inlet to outlet ratio.

Having a bit more soffit inlet than ridge outlet vent area assures that the building never satisfies the outlet convection air current draw by pulling air out of the conditioned spaces of the home - drawing such air from outdoors and moving it up under the roof surface is what we want.

Use insulation baffles or modified framing to make sure that the ceiling insulation does not block airflow at the eaves (Figure 2-56 shown at above left.) The sketch at left is provided courtesy Steven Bliss & J Wiley & Sons [4]

Details of soffit intake to ridge vent outlet ratios are
at
 Roof Venting: Intake - Outlet Area Ratios.

Cathedral Ceiling Ventilation Details

Figure 2-57: Cathedral ceiling venting details (C) J Wiley, S Bliss

Cathedral ceilings require the same continuous air barriers, and balanced soffit and ridge vents, as attics.

Both air sealing and ventilation are more critical, however, since any trapped moisture in the roof cavity will remain longer and potentially cause greater damage than in an open attic.

Also, since there is little or no communication from bay to bay, an effective ventilation system must reach every bay (Figure 2-57 at left).

Details about cathedral ceiling ventilation are
at CATHEDRAL CEILING VENTILATION.

Also see CATHEDRAL CEILING INSULATION.

Hips and Valleys: Roof Ventilation Details

Figure 2-57: Roof hip and valley venting details (C) J Wiley, S Bliss

Ventilating hips and valleys can be challenging with a cathedral ceiling.

One approach is to use a double or triple hip or valley rafter one size smaller than the common or jack rafters.

This will create a vent space along the top of the hip or valley rafter that can be used to supply ventilation air to the jack rafters (Figure 2-58 at left).

Roof Ventilation Details at Skylights to Provide an Air Path - Eaves to Ridge

Figure 2-57: Skylight roof venting details (C) J Wiley, S Bliss

Localized hot spots such as skylights can also lead to ice dams below, due to blocked ventilation as well as melt water from skylight heat loss.

Notching the rafters on either side of the skylight will help maintain airflow above the skylight (Figure 2-59 at left).

If icing on your skylight window interior surface in winter is still a problem, add an interior storm window to reduce heat loss through the glass in cold weather.

Figure 2-5: Flashing at Skylights, details (C) J Wiley, S Bliss

As a backup to prevent leaks at skylights, during skylight installation and even though modern skylights are usually provided with a factory-built flashing and counterflashing, it is always a good idea to seal the skylight curb and surrounding roof area with a bituminous membrane (see Figure 2-5 at left).

Also see SKYLIGHT LEAK DIAGNOSIS & REPAIR
and
see Skylight Glass Breakage.

The sketch at left is provided courtesy Steven Bliss & J Wiley & Sons [4]

Also see Ice Dam Protection for Skylights for cases where under-roof venting is not provided around a skylight.

-- Portions of the above were adapted with permission from Best Practices Guide to Residential Construction.

Attic Ventilation for Deep Snow Areas

Reader Question & Discussion: how to handle roof venting when snow covers the ridge vent

I desperately need to know once in for all, if ridge venting is feasible in a climate with large amounts of snow fall. I have read many online discussions on this topic but can't get a definite answer. I live in Saskatoon Saskatchewan and this past winter lasted 5 months with above average snow fall.

This discussion has been moved to a separate article found
at ROOF VENTING for DEEP SNOW -

Also see ROOF ICE DAM LEAKS

Steep Slope Roof Open Soffit Venting Options

Question: what type of intake venting will work best at the eaves or soffits of a steep pitched roof with no horizontal soffit covering?

I am currently constructing the roof on a small timber frame cabin. The roof has a 1:1 pitch and has a cathedral type ceiling. I plan to have a continuous ridge vent installed at the top. I will also install continuous soffit vents on the bottom sides of both eaves, running the length of the building.

The eaves are a boxed cornice type with the soffits attached to the bottom of the extended rafters. Therefore the soffits are sloped up at a 45 degree angle. I asked a prominent manufacturer of continuous soffit vents if having the soffit vents angled up at a 45 degree angle made any difference. They said they would not recommend that because they thought it might compromise the flow of air to the ridge vent.

I am wondering how that can be. Maybe louvers in the vent will be angled in the wrong direction and thus impede air from flowing in. Most continuous soffit vents have directional louvers in them. Does this mean that one should only construct a soffit which is horizontal so that the louvers in the vent will be angled the right way? Certainly there must be continuous soffit vents which are functional when placed in sloped soffits? Can you suggest any such product to me?

Reply: continuous perforated vent strips are ok, complete perforated soffit covering is more effective; be sure there's a ridge vent; close off any gable end vents; special problems occur if there are eaves troughs or no roof overhang.

Soffit vents (C) D Friedman

Tom I'd sure like to hear some specific reasoning from the manufacturer you spoke with. Either I don't understand the question and mfg's comment, or else what you report sounds like arm-waving and speculation.

Look at the vent strips in this horizontal soffit (photo at left). Can anyone really suggest the vent openings change if the vent openings are placed on an angle rather than horizontal as shown? Do the little holes know which way they are facing?

A.V.'s response about "short circuiting" the roof venting seems stuck on a misunderstanding that you were adding venting at the gable ends, not the eaves of the home.

Maybe because you talked about a sloped surface?

Angled roof overhang intake vent (C) D Friedman

Your angled roof overhang at the house eaves is a very common design (sketch at left).

As long as the air path up under the roof is not blocked by insulation, perforated soffit panels on the under-roof side of that overhang will work just fine.

In more than 30 years of inspecting the exterior and interior of buildings for under-roof ventilation and related problems I have seen (nor read about) no evidence whatsoever that there is a measured difference in the effectiveness of roof ventilation between a horizontal soffit intake and an angled soffit air intake except as I note next.

Incidentally in both designs, when wind is blowing towards the walls of the house where there is roof overhang, airflow increases inwards at that soffit. I speculate that an angled soffit of the same square-feet in area will catch and move more air up under the roof space than a horizontal soffit of the same square-foot area, for the same wind conditions.

Just be sure you provide adequate air flow space below the entire roof, from soffit to ridge, or the venting will be ineffective.

Finally, I have not found any soffit intake vent product that differentiates between horizontal soffit intakes and angled soffit intakes. I prefer to cover the entire soffit with perforated panels to maximize the air inflow, though vent strips can be adequate in many cases.

Follow-Up comments from Roof Vent Manufacturers

This is the response I got back from Air Vent Inc.

"We have no testing on installing soffit vents up the rake, but it has been our long standing opinion that this would or could short circuit the ridge vent or whatever form of exhaust at the ridge. You may want to consider an alternate form of intake, like the Edge vent at the bottom of your roof. There is no difference performance wise from the plastic or metal soffit vent." [1]

Because they stated, "up the rake", I thought they might have misunderstood that I meant to put the vents along the eave, just with the eave sloped up. But when I questioned them again, they reconfirmed their initial response. Their response caused me to make multiple inquiries.

This is what the National Roofing Contractors Association had to say:

"Soffit vents are designed with louvers that should have the fins facing out away from the structure when installed. If installed on an angle they will not trap the air flow as well as on the horizontal, but they will function. Louvered vent are the most efficient, but there are many non louvered open hole soffit panels that are part of a metal panel soffit system. Louvered or open hole continuous panels should function in an angled installation." [2]

Here is what another manufacturer had to say:

"As long as the intake air flow is entering the soffit/attic space and directed towards the ridge/peak of the roof, with no other “escape routes” it should serve as effective intake ventilation and it should balance out with the exhaust ventilation of the ridge vent."

My sense is that installing louvered metal continuous eave vents will be functional and will work. It just won't have the louvers angled in the optimum direction. Since I have multiple layers of 6 mil plastic vapor barrier over the ceiling boards and will have 2 inches of free airspace between the roof sheathing and cross members (2x4s) between the 2x6 rafters (2 1/2 inches between sheathing and insulation), I think the angled vents will be adequate.

Definitions: what's the difference between gable end, rake, eaves and soffit?

Rake end of a roof (C) D Friedman

Now that we're getting clear on distinguishing between "rake" or gable-end venting and soffit or eaves venting (photo at left), I completely agree with Air Vent on this.

By "rake" they are referring to the edges of the roof that run parallel to the roof slope and seen at the building (usually triangular) gable ends of the building, not the lower roof edges or eaves that run parallel to the lower roof edges. [For a key to a sketch of various house parts including soffit, ridge, rake, gable end, see Home Inspection Definitions & Terms].

I agree that we do NOT want intake venting under the rake nor at gable end walls once there are a soffit and ridge vent system installed. And I agree that leaving gable end venting in place would short circuit airflow just as Air Vent described.

Using the same reasoning I want to close off gable-end vents on older homes (the traditional "roof venting" method) when we install soffit intake and ridge outlet venting. Otherwise I've seen, tested, and visually demonstrated (using smoke) that the air exiting at the ridge vent pulls incoming air at the nearby gable end vents (or rake venting) and stops pulling as much air in from the lower roof edges, eaves, or soffits.

Soffit intake venting blocked (C) D Friedman

What we agree we want is continuous incoming airflow along the lower roof edge, between EVERY rafter pair, and continuing up to an exit at the ridge - that's how we avoid ice dams and how we dry the entire roof underside.

Our photo (left) of a horizontal soffit covering on an older home shows ice dam leak stains - why? Because this is what we call "faux venting" - the builder installed perforated vent panels but over solid plywood or wood soffit enclosing material - there was actually no air flow under this roof.

Edge Vent™ and products like the Hicks starter vent™ and Smart Vent™[3] are products that can give an alternative air intake path at the lower roof edges by replacing the lower few inches of roof decking with a vented strip for air intake under the shingles.

If yours is a conventional (though steep) gable with modern roof framing and rafter design that provides air space clearance between the underside of the roof deck and the top of the wall top plate, you don't need those products.

But for buildings whose soffits or eaves are "blocked" by an eaves trough roof design - a starter vent may not fit nor work in that situation both because the eaves trough is in the way and because in my experience on pre-1900 homes there is often a solid beam sitting atop the house walls with rafters butting into the beam on the attic side and lookout rafters tacked on on the outer side of the house wall to support the roof extension and eaves trough. Details are
at ROOF VENTILATION INTAKE if NO OVERHANG

And finally, I agree that if you are using louvered rather than simple flat perforated vent openings at a soffit, the louvers will pick up more air if their openings face outwards, away from the house walls - presuming there is a breeze blowing in that direction. No breeze, no difference.

...


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