December, 2017
The Word
Inspection News and Views from the American Society of Home Inspectors

The Word: Attic Ventilation


Once again, The Word invites you to travel into the dark realm of subjects that are sometimes misunderstood by home inspectors. The Word hopes you will find this trip informative and maybe a little entertaining.

Our subject this month is attic ventilation. The Word finds this subject interesting because there is a lot of misinformation out there, some of which comes from building codes.

Why Ventilate Attics?
The reasons for attic ventilation depend on where the house is located and on the season. In cold climates during the winter, attic ventilation helps keep the roof sheathing near the outside air temperature to reduce ice dams. 

Ventilation also helps remove water vapor that enters the attic to reduce condensation.

In warm climates during the summer, attic ventilation helps remove heat from the attic to reduce the cooling load, although there is some controversy about how much the attic temperature is reduced. Some claim that heat removal helps reduce premature failure of asphalt-based roof coverings; however, there is some controversy about this, too. Of course, heat removal occurs in cold climates during the summer, and temperature equalization and water vapor removal occurs in warm climates in the winter. It’s just that the need for these is less.

Where do heat and water vapor come from in the winter? Some enters the attic from below. Heat enters by conduction and, to a lesser extent, by radiation, especially where insulation is poor. Heat also enters by convection. Water vapor enters mostly by convection, traveling with air leaking through openings in the ceiling and through openings in the framing. Water vapor can enter directly through vapor-permeable membranes such as drywall, but this method is usually less important, especially when compared with convective transport.

Heat and water vapor also enters the attic when HVAC appliances or ducts are located in the attic and when exhaust ducts, such as bathroom exhaust ducts, deposit air in the attic. These can be a significant source of heat and water vapor, which is why it is much wiser not to locate these components in ventilated attics.

During the summer, heat enters the attic by conduction and radiation through the roof. Water vapor enters the attic by convection through the ventilation openings, which can create an interesting paradox when HVAC appliances or ducts are located in the attic. The benefit of heat removal may be offset by the addition of water vapor. The water vapor can condense on the appliance and ducts, and on the condensate disposal pipe. The resulting liquid water can damage components in the attic, including insulation.

Attic Ventilation a la Code
The International Residential Code (IRC) default minimum attic ventilation is 1 square foot of net free ventilation opening area per 150 feet of attic floor area. Ventilation opening location is not specified in this default provision, so code-compliant ventilation openings could be all at the eaves, all at the ridge or some combination of these. Net free opening area is the total opening area less space occupied by screens, louvers or other means used to keep critters out. Note that no authoritative source recommends locating all ventilation openings in the eaves or at the ridge, but the IRC allows it.

The IRC ventilation exception allows a ratio of 1 square foot of net free ventilation opening area per 300 square feet of attic floor area if all of the following factors are present:

  • Between 40 and 50 percent of the opening area is not more than 3 feet below the ridge.
  • The rest of the ventilation opening area is at the eaves.
  • A Class I vapor retarder (for example, polyethylene) or a Class II vapor retarder (for example, Kraft paper) is installed on the conditioned space side of the ceiling in climate zones 6, 7 and 8.

Given the code requirements and the stated reasons for attic ventilation, two interesting questions arise. Where did the code ratios come from? Do the code ratios work? The answer to the first question seems to be lost to history. The answer to the second question is that it depends on who you ask, where the house is located and several other factors. What is certain is that it is difficult to apply one prescriptive rule that applies to all houses.

One Size Does Not Fit All
So, here’s the problem with a one-size-fits-all approach: It is best applied to the type of house that was commonly built when the original code requirements were adopted. This house is a simple box with a flat ceiling, relatively few openings in the ceiling, a simple roof and no HVAC equipment or ducts in the attic. The IRC ventilation exception has worked in these houses for many years when properly implemented. One problem is that this description does not apply to all houses, especially newer houses. Another problem is that ventilation often is not properly implemented. Let’s look at a few examples where the one-size-fits-all approach may not work.

Let’s change the flat ceiling to a vaulted ceiling with the finish (drywall) applied to the rafters. To ventilate this system, the ridge and eave ventilation must be continuous across each rafter bay, and the rafter bay must be uninterrupted from eave to ridge. There must be at least 1 inch (2 inches is better) of space between the insulation and the roof sheathing for air to flow.

It can be difficult to effectively insulate rafters and maintain adequate air space, but that’s an issue for another day.

Let’s add complexity with a skylight. Now one or two rafter bays have an interruption between the eaves and ridge that stops ventilation. This is a really tough nut to crack.

Let’s add even more complexity by adding recessed lights. Unless the can is air-sealed and the opening around the can is also air sealed, we have added a hole for heat and moisture to enter the attic. Compound the felony by locating the recessed light in a rafter bay that is interrupted by a skylight. With any hole in the ceiling, you have the potential air leaks that can result in condensation, water damage and mold.

Recessed lights are not the only holes between the attic and space below. There is often an opening around every pipe, vent, chimney and electrical cable that penetrates the ceiling. While, in theory, these openings should have been sealed for fire-blocking reasons, enforcement was sporadic in many areas and still is in some. These openings can add up to several square feet in total area, especially if there is a big opening around a masonry chimney, which there often is. These openings can be a big problem in crawlspace foundations because they can provide a direct path between the warm and moist crawlspace and the cooler and dryer attic. The stack effect will move this air.

By the way, there are lots of openings between the crawlspace or basement and the house. Think about that 1 square foot or larger hole that the plumber hacks to set every bathtub and the slightly smaller hole for every shower.

Let’s change the simple roof to one with lots of intersecting hips and gables. Ventilating these more complex structures usually can be done; it just takes more care. Care is often in short supply on residential building sites.

Let’s put HVAC equipment or ducts in the attic. Actually, let’s not, but because the attic is far less expensive square footage than conditioned space, it’s done anyway. There’s no practical way to completely air seal the air handler and ducts, and to keep them sealed over time. Leaky HVAC components cause several problems. The obvious problem is putting warm, moist air into the attic during the winter. Hello, ice dams and condensation. A less obvious problem is pressure imbalance. If the leak is on the return side, the house can be at positive pressure with respect to the attic, potentially pulling (negative attic pressure) and pushing (positive house pressure) heated, moist air from the house through the holes into the attic. Again, hello, ice dams and condensation, not to mention the energy penalty.

Speaking of pressure imbalances, let’s put a power attic ventilator in the attic. They were once the rage, but they were found to be both inefficient and problematic. They often depressurize the attic, more so than all but the leakiest of return ducts. They can disrupt attic ventilation, especially if ridge and gable ventilation openings are present. The fan may draw air mostly from the high openings, leaving dead air low in the attic. They only run when the attic is hot, which makes them useless much of the year. They use energy, sometimes more energy than they save. Finally, they don’t last long. If one gets seven to 10 years from a power attic ventilator, this is good. Bottom line: Attic ventilation that relies on a power ventilator may not work well.

Geographic location and house orientation matter. Wind-driven rain and snow can enter ventilation openings. Few gable ventilation openings are without evidence of rain or snow infiltration. In addition to stains and sometimes damage, compressed insulation is often found near ventilation openings where rain or snow has entered. Compressed insulation has a much lower R-value. Ventilation openings that face west can be, in many areas, more prone to rain or snow infiltration. Ventilation openings increase the risk that a house will catch fire in areas that are prone to wildfires.

Inspecting Ventilated Attics
We are required to inspect attic ventilation. Our inspection is visual and is not technically exhaustive; therefore, we are not required to measure attic ventilation openings. We are not required to determine the adequacy or effectiveness of the attic ventilation. We are required to report if there is no attic ventilation, if ventilation is obviously inadequate or if ventilation openings are damaged, not functioning (turbine ventilator is stuck) or blocked by paint or other material.

We are required to report visual evidence that could indicate an attic ventilation problem. Examples could include evidence of condensation (such as widespread staining of framing members and rusted nails) and evidence of water intrusion such as localized staining, water damage and insulation that appears compressed by water.

Solutions for Attic Ventilation Problems
Home inspectors should not recommend how to fix problems. Assuming the liability for recommending repairs is far beyond what we should do. That said, we are often asked for recommendations. Here are some options that may be considered.

The current recommended solution is the conditioned attic, also called the unventilated attic, the closed attic and the sealed attic. In this solution, the insulation is installed at the roof sheathing instead of on the attic floor. The only openings into the attic should be an HVAC supply and return to allow removal of moisture that inevitably migrates into the attic. The insulation may be applied under the roof sheathing, above the roof sheathing or a combination of both. Design and installation of a conditioned attic is a complex undertaking. Insulation type and location, vapor retarder type and location, climate zone and roof covering type are all factors that need to be considered. The presence of combustion appliances in the attic is another complication. A highly qualified and experienced insulation contractor is required and a permit is required in some jurisdictions. This is an expensive solution.

Another solution is air sealing the attic floor. In many cases, air sealing can be more effective at both improving energy efficiency and reducing attic moisture problems than just adding insulation. Ideally, all of the old insulation should be removed; it’s usually pretty worthless in older houses anyway after initial poor installation and years of abuse. Foam insulation can be sprayed on the attic floor to provide air sealing and some insulation. The remainder of the insulation can be blown-in insulation to the recommended R-value. A qualified and experienced insulation contractor is required for this, too, especially if combustion appliances, combustion vents, chimneys or recessed lights are involved. Although it is less expensive than the conditioned attic, this is not an inexpensive solution.

Other, less expensive solutions involve air sealing large visible openings, replacing old recessed lights with new airtight IC-rated lights, repairing and improving installation of existing insulation and adding insulation after air sealing. These solutions may have the best return on investment.

The Bottom Line
An alert reader may have noticed that The Word has not mentioned improving attic ventilation as a solution for attic ventilation problems. Improving attic ventilation may well be part of the solution, but often it should not be the only part. The attic is a system and ventilation is only one component of the system. Air leaks and heat transfer are other important components. All components should be evaluated when designing solutions to attic ventilation problems.

Memo to Hestia (goddess of the home and hearth): The Word does not reside on Mt. Olympus (just at its base) and welcomes other viewpoints. Send your lightning bolts or emails to The thoughts contained herein are those of The Word; they are not ASHI’s standards or policies.

Bruce Barker, owner of Dream Home Consultants, in Cary, NC, is the author or editor of several books, including Everybody’s Building Code, Deck Codes and Standards, The NHIE Home Inspection Manual, Codes for Homeowners, The Complete Guide to Wiring and The Complete Guide to Plumbing. Bruce is an ASHI Certified Inspector who currently serves on the ASHI Board of Directors. Bruce has been building and inspecting homes for over 29 years. He is an ICC-certified Residential Combination Inspector, a licensed contractor and a licensed home inspector in multiple states.