All About Attic Venting
We vent attics for four reasons, and all four goals can be better achieved by adopting measures other than attic venting
Most homeowners and builders believe that attics should be vented. If you walk down to your local lumberyard and lean on the counter, the employees and nearby customers will offer a variety of opinions about why attics need to be vented. Unfortunately, it’s highly unlikely that the statements you hear will be true.
Here are the four most common reasons people suggest to explain the practice of venting attics:
- To reduce the chance of moisture build-up in the attic or condensation on the underside of the roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. .
- To make roofing shingles last longer.
- To lower cooling bills during the summer.
- To reduce the chance of ice dams.
Although attic ventilation is sometimes able to contribute in a very small way to addressing the problems on this list, there are much better solutions to all four problems than ventilation.
What does the code require?
If you plan to install insulation on your attic floor, then most building codes require that your house be equipped with soffit vents.
The standard code formula requires 1 square foot of net free ventilation area for every 300 square feet of attic floor area, assuming that half of the ventilation openings are located in the soffit, and half along the ridge. If a roof has only soffit vents and no ridge vents, most codes require 1 square foot of net free ventilation area for every 150 square feet of attic floor area.
Manufacturers of soffit vents and ridge vents usually specify the net free vent area of their products on product packaging or in specifications available online. (Researchers have shown that the net free vent areas reported by manufacturers are exaggerated, but that is a topic for another article.)
If you plan to install insulation between your rafters, building codes require that the attic be sealed (unvented). The code allows you to install a ventilation channel between the underside of the roof sheathing and the top of the insulation installed between the rafters if you want, but this type of attic can't have any vent openings that allow outdoor air to mix with the air in the attic.
Reducing moisture buildup in the attic
William Rose is a research architect at the Building Research Council at the University of Illinois. Rose has delved more deeply into the history of attic ventilation requirements than any other building scientist or historian. According to Rose, the stated aim for the first code requirements for attic venting was to reduce moisture buildup in the attic. Unfortunately, the code requirements were not based on science or research. Rose reports, “The attic ventilation ratio ‘1/300’ is an arbitrary number selected by the writers of FHA (1942) with no citations or references.”
High attic humidity usually shows up as dampness or frost on the underside of the roof sheathing. Another sign is mold (usually on the underside of the sheathing or the sides of the rafters). In almost all cases, these symptoms are due to two construction defects: a ceiling with air leaks, and a damp basement or crawl space. The way to solve this problem is to seal the air leaks and correct the moisture problems in the basement.
Rose advises, “Don’t rely on ventilation alone to take care of moisture in the attic. The best protection against condensation and mildew in the attic is a dry basement or crawlspace. Also important is an airtight ceiling.”
One of Rose’s colleagues at the Building Research Council is Jeff Gordon, who gave a presentation on attic ventilation at the 2011 Affordable Comfort conference. According to Gordon, “The three parameters for attic condensation in cold climates [are] interior house humidity, ceiling airtightness and pressures, [and] attic ventilation. Attic ventilation will have a slight positive influence, but it is third in the list.”
Extending shingle life
I installed a lot of asphalt shingles when I worked as a roofer in the 1970s. In those days, asphalt shingle manufacturers did not require attics or cathedral ceilings to be vented. “The earliest dates for shingle warranties being linked to attic ventilation requirements could not be determined…,” Rose reports. “However, archival material at NRCA [National Roofing Contractors Association] indicates that the links may have first begun to appear in the late 1980s and early 1990s.”
Rose co-authored an article (with Anton TenWolde, a former research physicist at the Forest Products Laboratory in Madison, Wisconsin) titled “Venting of Attics and Cathedral Ceilings”; the article appeared in the October 2002 issue of the ASHRAE Journal. Rose and Tenwolde wrote, “One published rationale [for venting requirements established by asphalt shingle manufacturers] holds that venting cools shingles, and thereby affects the rate of embrittlement by reducing the rates of oxidation and volatilization of asphalt hydrocarbons. However, ventilation is a minor factor in the determination of shingle temperature. … Venting cools shingles, but the cooling effect is not strong.”
In an article titled “Roof Ventilation Update,” Rose wrote, “Many factors influence the temperature on the roof. A prioritized list might include hour of day, outdoor air temperature, cloud cover, color of the roof, roof orientation, where the measurement is taken (sheathing or shingles, top or bottom), latitude, wind speed, rain or snow on the roof, heat conductionMovement of heat through a material as kinetic energy is transferred from molecule to molecule; the handle of an iron skillet on the stove gets hot due to heat conduction. R-value is a measure of resistance to conductive heat flow. across attic insulation, roof framing type (truss or cathedral), and attic ventilation to the outdoors. As you can see, ventilation falls pretty far down the list.”
The bottom line: if you care about your asphalt shingle warranty, you may need to follow the shingle manufacturer's venting requirements. But if you care about the temperature of your shingles, the most important step you can take is to choose white shingles.
Lowering cooling bills
If a house has insulation on the attic floor, there isn’t any evidence to support the idea that attic ventilation will reduce your air conditioning bills.
Jeff Gordon, in his presentation on attic ventilation, wrote, “Cooling season energy savings? Well, we tried to measure energy use, but this did not work very well. … Basically, [any savings are] lost in the noise.” Gordon reported that research has shown that “Attic ventilation is not an effective energy conservation procedure for houses with more than 6.5 inches of attic insulation.”
Summing up, Gordon reported, “Venting will reduce the temperature in an open attic. The difference in attic temperature between a vented an unvented attic, with R-30 at the ceiling, translates into minuscule [cooling energy] savings. No savings have ever been measured. … In mixed climates, savings from delta T must be balanced by losses from delta T in the winter. In cold climates, this is clearly a net loser.”
In some homes, the HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. equipment or ductwork is located in a vented attic. This is a terrible practice, of course. Ventilating this type of attic doesn’t solve the problems associated with locating equipment and ductwork outside of the home’s thermal envelope. As building scientist Joseph Lstiburek explains, “In a situation where mechanical systems or ductwork has to be in the attic space or when there are lots of penetrations in the ceiling below the attic, it’s best to bring the entire attic area inside the thermal envelope. This way, it’s not as big a deal if the ceiling leaks air or if the ducts are leaky and uninsulated.”
Reducing the chance of ice dams
It’s very difficult to solve most ice dam problems by increasing attic ventilation. In my 2010 article on ice dams, I wrote, “By recommending ventilation, a builder is saying, ‘I wasn’t able to include enough insulation to prevent the roof sheathing from being warmed by escaping building heat. So I guess I’ll use another method to cool the roof — I’ll ventilate the underside of the roof with exterior air.’ ”
Jeff Gordon has prepared a prioritized list of the causal factors for ice dams. “The principal cause of melting is heat from mechanical equipment or ductwork in the attic. The second cause is leaky ductwork. The third cause (or first if [the attic has] no ductwork) is air leakage through openings in the ceiling. The fourth cause is inadequate insulation. The fifth cause is sun heating exposed roof. Venting cannot dilute this level of excess heat.”
Once again, we see that an attempt to increase attic ventilation sidesteps the basic causes of the problem at hand and is unlikely to solve it. That said, including ventilation under your roof sheathing probably makes sense for homes in snowy climates.
What about cathedral ceilings?
When guidelines for attic ventilation were first proposed in 1942, no one anticipated that these guidelines would eventually be applied to cathedral ceilings. Rose and TenWolde note that vent channels above cathedral ceiling insulation aren’t very effective. The authors wrote, “Venting rules for attics have been extended to apply to cathedral ceilings, but few studies have been made to confirm the validity of that extension.”
While cathedral ceiling venting can (to a limited extent) lower the humidity level of roof sheathing, it can’t really help cool roof shingles. According to Jeff Gordon, “You cannot cool the upper part of a cathedral ceiling roof with venting.” Bill Rose has collected data proving this point; his findings were reported in a 2001 paper, Measured Summer Values of Sheathing and Shingle Temperatures for Residential Attics and Cathedral Ceilings. Rose found that shingles above a vented cathedral ceiling are cooler at the eaves and hotter at the ridge than shingles above a vented attic. This is due to the strong temperature gradient, especially on the south side of the roof, which exists in the ventilation channel above a cathedral ceiling. “It becomes apparent that venting can cool the lower section of a vented cathedral ceiling quite effectively, but the cooling effect is greatly reduced for the upper part of the cavity,” Rose reported.
The main problem with venting a cathedral ceiling has to do with roof geometry. If the plane of the roof is interrupted by hips, valleys, chimneys, dormers, or skylights, as most roofs are, effective ventilation is impossible.
Confused thinking that needs to be debunked
Although I have listed the four most common explanations for attic ventilation requirements, it’s important to mention a fifth explanation — one that is particularly muddled and confused. My nickname for this explanation is, “Your ceiling is a safety valve.”
This explanation is entirely divorced from any understanding of building science. Here’s how an old-time New England builder might explain the theory: “You can’t put a poly vapor barrier in your ceiling because your ceiling has to breathe. If you put polyethylene up there, the moisture won’t have anywhere to go. It will be trapped. You want the moisture to be able to get out.”
The ceiling-is-a-safety-valve theory encompasses several misconceptions. Here are two of them:
- The purpose of attic vents is to help lower indoor humidity levels. If you encourage moisture to flow through your ceiling assembly, you will improve conditions inside your house.
- Because your attic is vented, you need to feed a continual stream of moisture towards the attic vents so that the vents have something to do.
Of course, these ideas are misguided. Ideally, your ceiling should include a thermal barrier that separates the warm, humid, interior air from the cold, dry, attic air. You don’t want to encourage any moisture flow through that assembly — whether by air leakage or by diffusion.
Sometimes, attic venting can cause problems
I’ve shown that attic ventilation isn’t very effective at solving the problems that it is supposed to address. There’s more to the story, however: attic ventilation sometimes causes problems.
One obvious problem is called “wind washing”: this refers to the degradation in the performance of fibrous insulation (especially fiberglass batts) due to the flow of exterior air through the insulation. This problem is especially acute in the areas of a vented attic that are nearest to the soffit vents. The problem can be mitigated by switching to a denser insulation and by installing insulation dams above the top plateIn wood-frame construction, the framing member that forms the top of a wall. In advanced framing, a single top plate is often used in place of the more typical double top plate. of the perimeter wall.
In cold climates, attic ventilation can also increase energy bills. For example, imagine a house without a ridge vent that is getting new roofing. Trying to improve the home, the roofer cuts back the sheathing and installs a new ridge vent. What happens next?
The (relatively warm) attic air escaping through the new ridge vent depressurizes the air near the attic floor. Since most homes are leaky, the effect is to pull more warm, conditioned air through ceiling air leaks. The net result: energy bills go up.
When attic vents are installed on a house with an attic that was previously unvented, the attic can develop new moisture problems. In an article for Home Energy magazine, Tony Woods explained, “Ventilating a previously unventilated attic has the effect of making the attic colder. If nothing is done to stop warm, moist air from entering the attic space from the living space, condensation on the now-cooler surfaces is a certainty. Mold, mildew, and eventually leakage into the living space will probably follow.”
One way to describe these problems is to note that air doesn’t always follow the “smart arrows” you see in the diagrams created by soffit vent manufacturers. According to Rose, “Many attic assemblies are built with vents to the outdoors on the presumption that outdoor air will enter the attic and dilute moisture coming from indoors or from the foundation. The further presumption is that indoor air is wet and outdoor air is dry. Both of these assumptions are often false. If there are openings in the ceiling, then air movement in the attic can induce airflow from below, or dilute air from below, or do nothing, in ways that are just plain unpredictable no matter how much research is done. Attic air movement can also induce flow into the living space below, which is a nasty problem when the air conditioning is running.”
Hot, humid climates
Another category of problems caused by attic venting occurs in hot, humid climates. In their ASHRAE Journal article, Rose and TenWolde wrote, “No scientific claims have ever been made that attic ventilation is needed for moisture control in hot, humid climates. In these climates, the outside air tends to be much more humid than the inside air. … In such climates, attic venting tends to increase rather than reduce moisture levels in the attic.”
In a paper titled “Vented and Sealed Attics In Hot Climates,” Armin Rudd and Joseph Lstiburek explained the problem in more detail. They wrote, “Ventilation can be one of the major causes of humidity problems in southern humid climates. The problem of condensation in attics in hot-humid climates is caused by humid outdoor air coming in contact with cold surfaces in the attic. Although worse in coastal areas, this problem is not confined to them. The most offending cold surfaces are usually supply ducts, but they can be ceiling drywall and metallic penetrations through the ceiling if low interior setpoints are maintained.”
It’s safe to say that ventilating attics in a hot, humid climate is just plain stupid. Danny Parker, a researcher at the Florida Solar Energy Center, is the author of “Literature Review of the Impact and Need for Attic Ventilation in Florida Homes.” Parker wrote, “Although the rationale for attic ventilation is for moisture control, this was historically based on needs in cold climates and to prevent ice dams. The justification for attic ventilation for moisture control in hot humid climates is not scientifically defensible.”
The conclusions of Rudd and Lstiburek echo those of Parker. Rudd and Lstiburek wrote, “In the hot humid climate, the best solution to eliminate the potential for moisture condensation in attics may be to keep the moisture out of the attic altogether by sealing the attic to the outdoors.”
Powered attic ventilators
Some proponents of attic ventilation assume that if a little natural ventilation is a good thing, then powered ventilation using one or more fans has to be even better. These people are wrong; I explained why in a 2012 article, Fans in the Attic: Do They Help or Do They Hurt?
Danny Parker lists three research studies that demonstrate the folly of powered attic ventilation.
Parker wrote, “Increasing attic ventilation rates in existing residential buildings is often accomplished by adding forced ventilation using attic temperature activated attic fans. However, even those who are in favor of increased attic ventilation have often warned that the energy consumption associated with the attic fan motor is likely greater than any realized energy savings from its use (Wolfert and Hinrichs, 1974). Also, an early detailed study showed that while forced attic ventilation did reduce cooling energy use, the reduction was quite small and outweighed by the energy consumption of the fan itself (Dutt and Harrje, 1979). Another study in two instrumented side-by-side homes in Texas came to similar conclusions (Burch and Treado, 1979). … Thus, the powered ventilation does not typically result in a net energy savings unless the attic is uninsulated.”
If you want to vent your attic, do it right
Some building scientists, notably Joseph Lstiburek, defend attic ventilation. If you are building a vented attic, you may want to follow Lstiburek’s guidelines, which are laid out in a Fine Homebuilding article, “A Crash Course in Roof Venting.”
Lstiburek’s guidelines differ in several respects from code requirements. “If you choose to vent the roof deck, then be serious about it and really vent it,” Lstiburek recommends. “The code calls for a minimum of 1 in. of airspace between the top of the insulation and the back of the roof sheathing. That’s not enough. For best performance, the airspace in the vent chute should be a minimum of 2 in. deep.”
Lstiburek advises builders to include more soffit ventilation than ridge ventilation. “Building codes suggest balancing the intake and exhaust ventilation,” Lstiburek wrote. “The code, however, is wrong, and I’m working hard to get it changed. More ventilation at the eaves than at the ridge will slightly pressurize the attic. A depressurized attic can suck conditioned air out of the living space, and losing that conditioned air wastes money. For best results, provide between 50% and 75% of the ventilation space at the eaves; a 60/40 split is a good sweet spot.”
William Rose is much more skeptical of the value of attic ventilation than Lstiburek. While Lstiburek implies that small changes in attic ventilation details are quite important, Rose thinks that these details hardly matter. Rose advises, “Once you’ve sealed all of the openings that lead from below into the attic, corrected the ductwork, and installed a nice thick blanket of insulation in the attic, then one venting strategy is about as good as any other. Gable venting and ridge venting are both fine. Soffit venting with baffles is fine. Combinations are fine. If parts of the roof have a lot of venting and other parts have little or none, most would agree that that’s fine too. Power venting, however, is noisy and expensive.”
Jeff Gordon leans more to the Rose than the Lstiburek side of this debate. According to Gordon, on a windy day, your attic will be ventilated; when the wind isn’t blowing, don’t expect much ventilation. Gordon wrote, “What pressures drive attic ventilation? Wind – and that is about it. [There is] little stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season. in the attic – they aren’t very tall. [There] shouldn’t be any induced pressures from mechanical systems (we don’t want duct leakage in the attic). Attic ventilation provides air flow only to the extent that the wind blows.”
As often happens, the code gets it backwards
For years, building codes have required cold-climate builders to include interior vapor barriers, while almost totally ignoring air leakage. Yet vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. causes very few problems, while air leakage is a huge problem. For all these years, the building code was focusing on the wrong issue.
The code’s obsession with attic venting represents a similar error. While enforcing requirements for soffit vents and ridge vents, most codes have turned a blind eye to ceiling leaks. The code requirements are backwards.
My favorite quote from William Rose on the topic of attic ventilation appeared in the
August 1997 issue of Energy Design Update. Rose noted, “Ventilation is like a little boy who goes around the house looking for a job. He can do some things well, but can’t do anything really well.”
In one of his papers, Rose laid bare the unscientific nature of code requirements for attic ventilation, concluding, “Professionals in the building industry — design, codes and construction — may view the support for the current regulations, described in this paper, as being strong or weak. In the opinion of the author the support is weak, and a strict interpretation of 1/300 compliance is not appropriate.”
Elsewhere, Rose and TenWolde recommended, “The focus of regulation should be shifted away from attic ventilation. The performance consequences of other design and construction decisions should be given increased consideration.”
Here’s the most important detail to remember if you want your attic to perform well: build an airtight ceiling. As Rose summed up in one of his many articles on the topic (“Roof Ventilation Update”), “So you should vent where venting is appropriate and not vent where it is not appropriate. As it turns out, the worst-performing, most mold-ridden attics I have seen were vented — with a flooded crawlspace and a direct path for air movement from the crawlspace to the attic. … The father of a colleague of mine says that when the word ‘ventilation’ comes out, people stop using their heads. Vented assemblies often perform well, but not always. Sometimes roofs appear to be vented but actually aren’t.”
Rose advised, “You can mess up a vented attic … You can mess up an unvented attic as well, usually by not providing vapor protection appropriate to the climate and indoor moisture levels. Tight ceilings would be a great first step toward moisture control, summer and winter.”
So when should attics be vented?
Attic ventilation is incapable of performing all of the magic tricks that some people assume it can perform. Nevertheless, it often makes sense to include ventilation channels under your roof sheathing:
- It's always easier to comply with the building code than it is to argue with your local inspector.
- Generous ventilation channels can reduce the risk of ice damming if you live in a location with deep winter snows.
- If you have cathedral ceilings, OSB roof sheathing is at more risk of rot when the roof assembly is unvented than when it is vented — especially if there is no rigid foam above the OSB to keep it warm.
Martin Holladay’s previous blog: “The Klingenberg Wall.”
- Fine Homebuilding
- Morrison Hershfield
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