(316) 927-2233Your attic space is an integral part of your total roofing system. In warm weather, the attic acts as a barrier between your living space and the tremendous heat generated from the sun hitting your roof. In cold weather, the attic keeps condensation and humidity out of the home. In both situations, air movement is critical to the proper functioning of the attic and the health of the home. Proper roof ventilation is a relatively cheap and easy fix with big impact on your energy bills, protection of the roof system, and prevention of problems such as mold and mildew.
Before we can get into the causes, symptoms, and fixes of various ventilation issues, you’ll need to know what type of ventilation style and roof system your home/building has.
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- Ventilated open attic, with insulation between the ceiling of the top floor and an open attic space above. This is the most common type of attic ventilation.
- Vented & unvented cathedral ceilings (vaulted ceilings) are both common.
- Unvented open attics, which are common over covered porches.
- Insulated roof decks, which can be both vented and un-vented.
- Uninsulated attics
- Permeable “breathing” roofs, which is used for wood shake roofs.
- Combinations of ventilation styles throughout the home.
We have a lot more detail in a dedicated article covering how to identify What Type of Roof/Attic Ventilation System Do I Have, which is both a good inspection resource and more advanced ventilation guide especially for those with a combination of different ventilation methods.
Note: both this article below and the one linked above discuss ventilation of steep-slope roofs, as found on most residential homes in the US. Flat roofs (called “low-slope” within the industry) have different insulation/building envelope standards.
Changes to Attic/Roof Ventilation Systems
Ventilation building codes have changed over time as understanding of building science has improved over the last 50 years. Insulation standards for homes have increased over that same time period, which has led to tighter air sealing standards and higher insulation requirements. Additionally, the increased prevalence of interior air conditioning and humidity controls have further highlighted the need for effective ventilation systems at the peripheral of the building envelope to control temperature variances and humidity.
The effect of all these changes is that even some recently built homes do not meet current building codes for ventilation. If any remodeling work has been done and insulation was added, that will change the performance of both roof and ventilation systems. Changes to roof type – most notably, when a natural wood roof is replaced by an asphalt shingle roof – change how air moves through an attic.
Finally, developer builders are notorious for building homes with minimal cost, meaning “invisible” features such as ventilation systems rarely get much attention from buyers and are built to the bare minimum standard legally required by whatever city/county building codes apply (if ventilation is even checked at all during final inspection).
Systemic Design: Achieving Adequate & Balanced Ventilation
We have many specific fixes to common problems detailed below, but first, it’s worth looking at the system as a whole. System-wide, ventilated sloped roofs need to have adequate and balanced ventilation. Note that there are un-vented “hot roofs” (see our Types of Attic/Roof Ventilation Systems article) – but for most buildings where there is ventilation, these standards apply.
Addressing Insufficient Ventilation
Ventilation is calculated in net free area (NFA) – the “amount” of ventilation can be described as the surface area of the vent openings. Each building has a target amount of ventilation, based on the size of the roof. A basic NFA calculation follows the 1:300 rule; for every 300 square feet of attic floor area, there should be 1 square foot of net free ventilation area (NFA), split between intake and exhaust (ideally 60% intake and 40% exhaust area, but more on that below). Steeper roofs (and therefore larger attics) tend to have more NFA relative to the 2-dimensional footprint. A roofing professional can give a more detailed analysis of a particular building, or tools such as the Air Vent Calculator by ventilation manufacturer Lomanco can give some guidance – see https://ventselector.lomanco.com/.
Power-vents are usually given an equivalent net free area, similar to passive ventilation types like ridge vents, by the manufacturer to make comparison/calculation easy.
If your home/building does not have enough ventilation area, you may need to add additional intake or exhaust vents.
Addressing Imbalanced Intake/Exhaust
Assuming you have sufficient ventilation as measured by area, the next element of the ventilation system to consider is the balance of intake and exhaust. On a straightforward gabled home with an open attic, a roof where the top has a continuous height (e.g., a roof with one ridge), and passive ventilation with no power-vents, the ratio of intake to exhaust vent area should be 60 percent intake and 40 percent exhaust. 50/50 was the industry standard for many years, and is still found/acceptable today, but 60/40 is considered a safer ratio.
In general, having “too much” intake ventilation is okay, as the extra area simply will not take in extra air, and will leave your attic space with a slight positive pressure, which can be good. However, having too much exhaust ventilation is bad, because it will “short circuit” the ventilation system, meaning that some of the intended exhaust vent opening(s) will become intake vents – usually on the upwind side of the house – drawing in outside air at the top of the roof rather than the bottom and failing to circulate air through the whole space, which is the whole point of attic ventilation. You also don’t want air getting pulled into exhaust vents because it can suck in snow or rain, unlike soffit/intake vents which are positioned/designed to avoid this. Too much exhaust ventilation leaves the attic space with a slight negative pressure, which is bad because it can draw conditioned air from the house up into the attic through lighting, electrical, or plumbing penetrations.
If your balance ratio of net free area is above 50 percent exhaust vent area, you should either add intake vents or close off some of the exhaust vent(s) until the ratio is appropriate.
If your balance ratio of net free area is above 60 percent intake vent area, you may be okay. Again, too much intake is not usually a problem. However, double check that you have enough total ventilation by doubling the exhaust NFA (i.e., pretending you have a 50/50 ratio) and check that against the 1:300 ratio to see if you have sufficient ventilation. Homes with continuous perforated soffits, for example, have lots and lots of intake ventilation (which is okay), but require using this adjusted calculation to see if the total air movement is appropriate.
How to Improve Attic Ventilation & Fix Common Ventilation Issues
If your overall system design is adequate (adequate ventilation area & correct balance, as detailed above), but not working properly, here is a detailed list of troubleshooting items to improve attic ventilation.
We have run into lots of ventilation challenges over the years and done our best to categorize them below with information on causes, symptoms, and fixes. This is roughly sorted in descending order of frequency that we run into these problems.
Systemic Design – Ventilation Volume & Balance
| Goal | Solution if Needed | |
| Adequate Ventilation Volume (Net Free Area/NFA) | General rule of 1:300 ratio of ventilation area (combined intake/exhaust) to total attic footprint | Add ventilation while keeping intake/exhaust balance |
| Intake/Exhaust Balance | 60% of total NFA as intake, 40% as exhaust. No more than 50% exhaust |
Specific Ventilation Problems and Optimization
| Problem | Identification/Inspection | Solution |
| Blocked Soffit/Intake Vents | Cannot see daylight from inside attic | Clean or replace vent covers, or add baffles/chutes to prevent insulation obstructions |
| Vent Cuts Not Matching Vent Cover Sizing | From attic: size/shape of openings does not match covers | Cut opening to appropriate size |
| Bathroom Vents Terminate in Attic | “Candy-cane” duct ending in attic, or no duct | Route 4” duct to roof or soffit termination |
| Gable Plus Ridge Vent | Both active (open) gable vents and ridge or box vents are present | Seal gable vents |
| Vaulted/Cathedral Ceilings w/ Permeable Insulation but no Chutes/Baffles | 2” air baffles against roof deck are not visible from soffit/behind knee wall | Add chutes – ceiling and/or insulation will have to come out/get reinstalled |
| Vaulted/Cathedral Ceilings with Trapped Rafter Bays | No shared upper attic at ridge nor continuous ridge venting above vaulted ceiling | Add continuous ridge venting. If hip roof, closed-cell spray foam to unvented ‘hot roof’ design. |
| Exhaust venting at different heights | Multiple ridge heights with shared attic space and exhaust (ridge or box) vents on each | Divide attic spaces into independent systems or seal lower ridge vents if appropriate |
| Rain/Snow Getting Pulled Into Ridge Vents | Snow in attic | Repair damaged vent, adjust intake/exhaust balance, or fix multiple height/type exhaust venting |
Blocked Soffit Vents/Intake Ventilation
Thankfully, the number one issue we see with existing ventilation systems is also the easiest/cheapest to fix. Intake ventilation is at the bottom of a sloped roof system – most commonly soffit vents – and those are prone to getting blocked by blown insulation, bird nests, construction debris, clogged screens, etc.
While soffit vents themselves are just holes cut into the soffit, the vent covers (metal grills you see from the outside) typically have screens to keep insects out of the attic space. While a foot of blown insulation is a common blockage we see when insulation has been added to an attic, it’s more often a thin layer of lint or blown cellulose material that clogs these screens. Most of the time (power-vents are the exception), attic ventilation moves air only from the gentle movement of warm air rising. This means that even a relatively thin layer of lint or debris can dramatically reduce the amount of air that comes in a soffit vent.
Inspecting and fixing this problem is straightforward.
From inside the attic, look for daylight coming through the soffit vents. If you have a count from the outside of how many vents there are, hopefully you can see the same number from the inside. If your eave is accessible, you can also inspect the screens from the inside. Some debris, like cottonwood fuzz, tends to stick to the outside of the screens, whereas insulation or dust will accumulate on the inside. Shining a flashlight up into the soffit vent, can you clearly see the wood of the roof decking above?
To fix clogged vents, we find that using a tool with a handle can make access a lot easier. For blown insulation, garden rakes are effective. Nylon brushes, especially with a handle like a deck scrubber, brush attachment on a car window scraper, or even a toilet brush all are effective and have a handle to help with reach into the narrow soffit.
In more severe cases, you may need to add ventilation chutes. If wood baffles were added when blown insulation was installed, there should be a gap between the roof decking and the eave. If that is a narrow space, as is often the case on a shallower roof (under ~6:12 pitch), of it more blown insulation was added later to get a higher R-value, then you may need ventilation chutes to keep the space open for airflow. These chutes are a few inches thick, relatively inexpensive, made of rigid foam or plastic, and sized to fit between 24” or 16” truss/rafter spacing against the roof deck.
Finally, if the metal vents themselves are beat up or the louvers have been flattened/painted over, the vent covers can be replaced from the outside.
Soffit Vent Cuts Not Matching Size/Shape of Vent Cover
Unfortunately, this result of time/cost saving and poor workmanship is more common than you would expect. The symptom is that, instead of a 4×16” rectangular cutout for the soffit vents, for example, the opening behind the vent cover is actually cut with a 4” round hole saw, or simply an opening smashed through the soffit material with a hammer. In either case, the actual net free area will be less than expected, resulting in insufficient air intake.
The size and quantity of intake ventilation is calculated in terms of net free area (NFA, in square inches) for the house. When the home is built, this is supposed to be a part of the final inspection. This is often done from the ground by counting the number of soffit vents. However, the actual opening behind the vent covers is not typically visible from the ground, and this cost-cutting install/code violation may not get caught.
The fix here is straightforward but will take some time and skill. Double check whether the vent design/quantity was done appropriately to start with. A basic NFA calculation follows the 1:300 rule: for every 300 square feet of attic floor area, there should be 1 square foot of net free ventilation area (NFA), split either 50/50 between intake and exhaust, or (preferably) 60 percent intake and 40 percent exhaust. If the intake ventilation is appropriate for the home size and ventilation structure, then proceed and cut the openings to match the vent covers. Note that a 4″ x 16” vent describes the screen size, as the cut will be about 1” smaller so that the screen occludes the opening. Be sure to check for wiring behind each cut, especially if there is soffit lighting.
Bathroom Vents Terminating into the Attic
While this isn’t technically a roof system ventilation issue, it is certainly a problem.
In warm climates (specifically, homes that have air conditioning but don’t require heating), bathroom vents can be terminated into the attic, and humidity is carried away with the air exchange of the attic. In cold climates, like the northern US, this is an absolute no-no, as warm, humid air from a shower will condense onto the cold roof deck of the attic, dripping down from the nails and fostering mold. Here in the lower Midwest (greater Wichita area), you see both. Occasionally, homes get away with it, such as vents from an infrequently-used bathroom, or vents on the sunny side of homes. Often, however, condensation creates mold blooms.
In Wichita (Sedgwick County), the law is unambiguous. Code requires external ventilation of all bathrooms fans. We certainly agree, and have seen the messy result of many, many failures to do so. Inexplicably, then, we still find hundreds of bathroom vents terminated into the attic each year. Whether it’s DIY renovations from people who moved here from warmer parts of the country, historic homes, or corner-cutting builders trying to save a few dollars on flex ducting and a soffit or roof termination, they’re still around. Check yours.
To fix this, 4” flex ducting needs to be run to a roof termination (preferred) or soffit termination (acceptable and better if on a sunny side of the house). With either application, care should be taken with the run of the flex ducting. When warm, humid air from a bath/shower is piped through a cold attic, water will condense onto the sides of the duct and flow downhill. For a roof termination, simply make sure that there are no local low points in the run – meaning that it only ever flows uphill all the way to the roof vent. This will allow any stray drips to come back into the bathroom, and/or simply evaporate as air moves past. For soffit-bound duct runs, make one dominant high point immediately above or near the bath fan, and then a consistent downhill slope all the way to the terminal vent in the soffit with no local low spots. This will allow condensation drips to get all the way outside. If there are low spots, water will puddle and, over time, drip. By far, the #1 “false alarm” call we get from customers thinking they have a roof leak is from a low spot in a flexible bathroom vent duct collecting water condensation and dripping through the ceiling below.
Conflicting Ventilation Systems – Gable Vents with Ridge Vents
Gable vents, high up in the attic, are intended to be exhaust vents. They are more common on historic homes. However, gable vents are also sometimes used to perform both intake and exhaust ventilation in unconditioned buildings such as barns, but this is only appropriate because the building is unconditioned (no humidity buildup risk). In the case of an unconditioned space, consistent prevailing winds can help with movement.
In the case of a home, gable vents are riskier because wind can “short circuit” a ventilation system by leaving stagnant air lower in the attic if wind causes cross-ventilation at the top (one gable vent acting as intake and the other exhaust). It can also draw snow in. Gable vents are virtually never installed on modern homes for this reason – they are simply a worse system than low intake and box or ridge exhaust vents near the top. When we encounter active gable vents, we typically recommend sealing them up and using a modern approach to exhaust ventilation.
To seal a gable vent, you can either staple synthetic roofing underlayment over the opening from the inside or cut OSB and cover the hole from the inside. The external vent cover/louver can stay, as it is usually a decorative element of the home. We call a sealed gable vent “inactive.” As with any time you are adding or sealing vents, recalculate the net free area of the total system and make sure it’s in accordance with the appropriate amount for the house as a whole (see 1:300 calculation above).
The worst offenders, as suggested in the title of this section, are when both gable vents and box vents or ridge vents are present within an attic space. In this case, the gable vents, which are slightly lower and exposed to more direct wind, become consistent intake vents, leaving no air intake from lower in the attic where it should be drawn in, such as soffit vents. Without air coming in lower in the attic space, most of the area will be stagnant, and the system fails to draw humidity up along the roof deck and out of the building.
Conflicting Ventilation Systems – Insulation at Both Attic Floor and Roof Deck
While this version of a conflicting/double system is less common, it is certainly a problem. We see this happen when DIY homeowners want to add insulation to their home and use the opposite space to the one that is already insulated – e.g., putting fiberglass batts between trusses at the roof deck when the floor of the attic is already insulated. First of all, covering the roof decking with air-permeable insulation such as fiberglass batts with no chutes for air movement traps condensation moisture against the roof deck. Additionally, blocking the thermal transfer of the sun hitting the roof in an open attic system will prevent warm air from rising along the roof deck and prevent the draw of fresh outside air in. The insulated pocket in between risks becoming stagnant, warm, and moist – the perfect conditions for rapid mold growth.
Missing Vent Chutes or Baffles in Vented Cathedral Ceilings
Cathedral Ceilings, also called vaulted ceilings, can either be vented or unvented. An unvented system is called a “hot roof” and needs to be air impermeable, such as closed-cell spray foam, to beyond the dew point – see our article What Type of Roof/Attic Ventilation System Do I Have. Any vaulted ceiling with air-permeable insulation – such as fiberglass batts, net-and-blow insulation, open-cell foam, etc. – must have ventilation chutes or baffles against the roof deck to allow for adequate air flow. These chutes are typically 2” thick, relatively inexpensive, and made of rigid foam or plastic. They simply hold the insulation away from the roof deck and create a channel for rising warm air to pass. We frequently find that these are missing. Usually by the time we’re getting the phone call, mold or rot has set in from trapped humidity.
The fix for adding ventilation chutes is unfortunately expensive, even if the chutes themselves are inexpensive when installed during the house’s construction. If the ceiling is enclosed, the ceiling will have to come down, insulation removed, chutes stapled into place, and insulation/drywall re-installed. In an attic with primary insulation at the roof deck, such as a patio home where HVAC and plumbing equipment is installed in a conditioned space above the ceiling, the roof decking will be more accessible because only the insulation has to come down to add the chutes.
Cathedral Ceilings with Trapped Rafter Bays or Non-Continuous Venting
Read the section above for an overview of how vented cathedral (also called vaulted) ceilings work.
Because each rafter bay is an independent path for air movement, each bay needs to have access to exhaust ventilation at the top. Sometimes this is done with a small attic space along the ridgeline of the roof, which reconnects the exhaust vents back into a single air cavity. If that is the case, your vaulted ceilings will usually have a flat section at the top rather than meeting in a point. Continuous ridge venting can also be used, since the venting will provide an opening between each set of rafters (see Ridge Vents vs Box Vents for more detail). Box vents, often called turtle vents, can theoretically be used, but would need to be spaced every 24” (or 16” if that is the rafter spacing) and on each side of the ridge, which would not look very good from the outside and may make for too much net free area to keep an appropriate amount/balance of venting in the attic.
A problem we see is where a ceiling has been retrofitted into vaulted finished ceilings, but the ventilation system remains unchanged. If box vents provided the exhaust, and are spaced every few rafter bays, then all the bays in between won’t have a place for air to go. These trap humidity and grow mold/rot the decking. We also see this problem occur on hip roofs, where individual rafter bays terminate into the hip and don’t have a way for air to get out. For a hip roof with vaulted/cathedral ceilings, the only viable option is usually an unventilated ‘hot roof’ design – see the Types of Roof/Attic Ventilation Systems article for details.
Exhaust Vents at Different Heights – Multiple Ridge Lines
If the attic spaces are connected, all exhaust vents need to be at the same height. If they are not, then the lower ones will act as intake vents and ‘short-circuit’ the ventilation system design by drawing in air near the top rather than the dedicated intake vents at the bottom of the roof. This leaves stagnant air lower in the attic and does not move air/humidity up as it passes along the roof deck, leading to mold risk.
We tend to see two situations that lead to exhaust vents at different heights. The first is a mixed vent system, as detailed above in the section on “gable and ridge vents.” This also happens when two different ridge lines or peaks share an attic space, as on larger homes with complicated architectural features.
We find that the best way to address multiple height ridges/peaks is to separate each attic system into independent air cavities, as long as each area has sufficient intake and exhaust ventilation (and somewhere to put those vents). Dividing attic spaces can be achieved by stapling plastic sheeting inside the attic to partition off the space.
In some cases, there is nowhere to put intake venting in one of the rooflines — no soffits for soffit vents, and/or lower slopes are all valleys, so off-eave eyebrow vents aren’t possible. In this case, combined attic areas may be the only option. Power vents can be used to add equivalent net free area of exhaust venting to the top of the upper connected roofline, and the lower section left unvented. The goal here is to draw more air through the combined space and up to the upper roofline as the only exhaust point.
Rain/Snow Getting Pulled into Ridge Vents
Precipitation should not come into your ridge vents. Barring damage to the vent itself, if precipitation is coming in, that is a sign that some of the exhaust vent area is acting as intake. This can be caused by two things: either there is too much exhaust area (imbalanced system, see above on systemic design/balanced system), or because there are mixed types/levels of exhaust ventilation. This second example is most often seen when both gable and ridge venting is present (see details in its own section above), or when multiple rooflines of different heights are connected by one open attic area (details on addressing or dividing attic spaces also above in its own section).
How Much Does Good Attic Ventilation Matter?
Roof system ventilation isn’t something homeowners think about on a daily basis. If you’re lucky, you never have to think about it at all; hopefully everything is working properly and quietly taking care of thermal and humidity exchange.
When ventilation systems go poorly, mold, rot, and leaks can have striking consequences. Health concerns, financial costs, and disruption for repairs are a steep price to pay for preventable issues. We cover a lot more details, case studies, and remediation in a separate article on Why is Proper Attic Ventilation So Important.
If you have recently purchased a home and are interested in learning how your ventilation/roof system works, or want to make sure everything is performing as intended, Rhoden Roofing offers free inspections in the greater Wichita area. If you are experiencing active problems or damage as a result of poor ventilation, we can also help with repairs.
This article is part of our ‘How To’ Series. Learn more about:
- Installation of Roof Components
- How To Fix Your Gutters: Most Common Repairs
- How Do You Properly Flash a Brick Chimney?
- What Is the Proper Installation & Flashing for a Heater Flue Pipe On a Pitched Roof?
- What Is the Best Temperature to Install Composition Shingles?
- How Do You Properly Flash a Curb Mounted Skylight on a Residential Roof?
- Building Science & Roofing Systems
- How to Improve Your Attic Ventilation
- Can You Install a Roof in the Winter in Kansas?
- What Is the Difference Between Open and Closed Valley Installation?
- Continuous Venting Ridge vs Static/Box Vents: Which is better?
- Reroof vs Recover; What Is the Difference and What Should I Do?
- Residential/ Steep Slope Materials Series
- Inspection & Hazards
