Question? Just ask us!
Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair
InspectAPedia ® Home
VENTILATION in BUILDINGS
AIR BYPASS LEAKS
AIR LEAK DETECTION TOOLS
AIR LEAK SEALING PROCEDURE
AIR POLLUTANTS, COMMON INDOOR
AIR SEALING STRATEGIES
ATTIC LEAKS, CONDENSATION & MOLD
BASEMENT CEILING VAPOR BARRIER
BASEMENT HEAT LOSS
BASEMENT LEAKS, INSPECT FOR
BLOWER DOORS & AIR INFILTRATION
BRICK WALL DRAINAGE WEEP HOLES
CATHEDRAL CEILING VENTILATION
CEILINGS, DROP or SUSPENDED PANEL
COMBUSTION AIR for TIGHT buildings
COOLING LOAD REDUCTION by ROOF VENTS
CONDENSATION on WINDOWS & SKYLIGHTS
DEW POINT CALCULATION for WALLS
FIREPLACES & HEARTHS
FLAT ROOF MOISTURE & CONDENSATION
GREEN BUILDING CONSTRUCTION
HEAT LOSS in BUILDINGS
HEAT LOSS DETECTION TOOLS
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
ODORS & SMELLS DIAGNOSIS & CURE
ROOF VENTILATION SPECIFICATIONS
SHEATHING, FOIL FACED - VENTS
STAIN DIAGNOSIS on BUILDING INTERIORS
STUCCO WALL METHODS & INSTALLATION
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
Attic ventilation fixes: here we explain how to Correct Improper or Inadequate Attic or Under-Roof Ventilation in buildings, part of our series of articles about ATTIC CONDENSATION CAUSE & CURE. 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.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
These 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.]
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:
Building Code Requirements for Roof Ventilation
-- Adapted with permission from Best Practices Guide to Residential Construction.
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. See Inspect Attics for Blocked Soffit Intake Vent 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.
An 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:
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.
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
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.
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 at Roof Venting: Proper 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.
Ridge & Soffit Vents - Importance of Balanced Roof Ventilation
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
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 
Details of soffit intake to ridge vent outlet ratios are at Roof Venting: Intake - Outlet Area Ratios.
Cathedral Ceiling Ventilation Details
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).
Illustration adapted from
Hips and Valleys: Roof Ventilation Details
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).
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.
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).
The sketch at left is provided courtesy Steven Bliss & J Wiley & Sons 
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.
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 ICE DAM PREVENTION
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.
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.
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?
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.
Follow-Up comments from Roof Vent Manufacturers
This is the response I got back from Air Vent Inc.
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.
Here is what another manufacturer had to say:
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?
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.
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.
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 Venting: Eaves Intake if no Overhang
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs)
No FAQs have been posted for this page. Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
Use the search box below to ask a question or to search the InspectApedia.com website.
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.