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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

Posted on Dec 6 2013 by Martin Holladay

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.

In most cases, ventilated attics require ventilation baffles near the eaves to create an air gap between the underside of the roof sheathing and the top of the insulation layer. The air gap should allow the free flow of air from the soffit vents to the attic. According to section R806.3 of the 2006 International Residential Code (IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code.), “A minimum of a 1-inch space shall be provided between the insulation and the roof sheathing and at the location of the vent.”

Many experts advise that 2-inch-deep vent cavities are better than 1-inch-deep cavities; if that's the route you want to go, size your baffles accordingly. (For more information on this topic, see Site-Built Ventilation Baffles for Roofs.)

More information on building code requirements for attic venting can be found in these two articles: Creating a Conditioned Attic and How to Build an Insulated Cathedral Ceiling.

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.”

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Image Credits:

  1. Fine Homebuilding
  2. Morrison Hershfield

Dec 6, 2013 10:30 AM ET

A vented attic is like a sloped rainscreen...
by Lucas Durand - 7A

Seems like the same rationale that applies to recommending a rain screen gap should also apply to recommending a vented attic.

Dec 6, 2013 10:49 AM ET

Response to Lucas Durand
by Martin Holladay

There are similarities between vented cathedral ceilings and walls with a rainscreen gap -- and there are differences. Attics, of course, are a whole 'nuther kettle of fish.

1. Roofs can have snow on them; walls generally don't.

2. Air leakage into roof assemblies is much more likely than air leakage into wall assemblies, due to the stack effect. That means that cathedral ceilings face a bigger moisture load due to air leaks than walls do.

The analogy is useful, but we shouldn't let the similarities between these assemblies blind us to the fact that there are important differences. We need to look at data, not analogies.

That said, examples of rotten OSB roof sheathing in unvented cathedral ceiling assemblies certainly show that a ventilated air gap can make a big difference -- in some cases, the difference between a system that works and failure.

Dec 6, 2013 11:05 AM ET

by Ron Keagle

Regarding this statement by Jeff Gordon:

“You cannot cool the upper part of a cathedral ceiling roof with venting. “

The statement is made to sound unconditional, and yet he qualifies it with the following explanation:

“If a cathedral ceiling cavity is vented top and bottom, and faces south, then air moves through the cavity driven by buoyancy. Air enters at the soffit. As it moves up the vent slot it becomes heated and exits the ridge as heated air. If a cavity has a slot [that is, an air gap between the top of the insulation and the underside of the roof sheathing] but no vents, the air in the slot will have a certain temperature. The air in the vented slot will reach that same temperature after, say, 10 feet. The remaining higher part of the slot will receive no cooling effect from the moving air.”

His explanation begins with a proper description of a vent system that includes an air passage plus inlet and outlet vents. But, then he says that if the vent system has an air passage, but no vents, it will not work to cool the roof, thus supporting his original premise that you cannot cool the upper part of a cathedral ceiling roof with venting.

It seems to me that the explanation is saying you cannot cool the roof with venting if no venting system exists. A venting system requires an air passage plus and inlet and outlet. Without the inlet and outlet, no venting system exists.

Dec 6, 2013 11:22 AM ET

Response to Ron Keagle
by Martin Holladay

Jeff Gordon may have expressed the idea inelegantly or clumsily, but he was reporting actual temperature measurements made in vented cathedral roof assemblies. The data come from a study performed by Bill Rose; here is a link to his paper: Measured Summer Values of Sheathing and Shingle Temperatures for Residential Attics and Cathedral Ceilings.

I reported on this study in the July 2002 issue of Energy Design Update:

"Rose has developed his own statistical method for comparing temperatures from roof to roof. The data were represented in a form of correlational analysis. This was done using linear regression of individual cases against the base case. The slope of the regression line was used to permit the temperature of the comparison case to be expressed as “hotter than” or “colder than” the base case as a percent. The statistical method behind these comparisons is fairly complex. The method’s most important feature is its usefulness in ranking factors in order of their effect on shingle temperature. ...

"Rose also 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,” writes Rose.

"There are many good reasons to vent a roof — to maintain a shingle warranty, to remove moisture from the roof assembly, and to comply with code requirements. However, when it comes to lowering shingle temperatures, ventilation is far less important than roofing color."

Dec 6, 2013 2:19 PM ET

Second response to Ron Keagle
by Martin Holladay

Because Jeff Gordon's explanation of the temperature gradients in cathedral ceiling ventilation gaps was confusing, I have rewritten that section of my article. Thanks for your comments.

Dec 6, 2013 4:25 PM ET

Cooling Distribution on Cathedral Ceiling Roof
by Ron Keagle


Thanks for that clarification of Jeff Gordon’s statement. I looked at the paper by Bill Rose, and his finding of the strong temperature gradient in the vent air passage of cathedral ceilings. I will study it further, but did not yet notice if the area of the air passages had been stated. But, I suspect that the strong temperature gradient is the result of the area of the air passages slowing the upward airflow due to constricting the natural convection flow.

In other words, the restriction of the passage slows down the upward airflow, so it remains in the passage for a longer time. The longer it remains in the passage, the more it is heated. The warmer the air becomes, the slower the heat transfers from the shingles to the air.

Thus, the lower air is heated rapidly due to the temperature difference, and the heat transfer slows as the rising air gets hotter. The slower the heat transfer from the shingles to the ventilating air, the hotter the shingles become from the solar gain.

If you increase the cross sectional area of the air passage, a greater quantity of cooling air will enter and move through the passage, and thus have added capacity to absorb more heat transfer from the shingles higher in the air passage; therefore, reducing the temperature gradient of the shingles between the lower and higher regions of the roof shingles.

So, while there will always be a temperature gradient between the lowest and highest part of the roof, that gradient can be minimized by reducing airflow constriction in order to maximize the airflow rate. I see no reason why the gradient cannot be made so small that considerable cooling of the upper part of the roof can be achieved.

So, I disagree with Jeff Gordon when he says, “You cannot cool the upper part of a cathedral ceiling roof with venting.”

Dec 6, 2013 5:32 PM ET

Third response to Ron Keagle
by Martin Holladay

Bill Rose explains the gap in his experimental cathedral ceilings this way: "Nominal 10 in. R-30 fiberglass batt insulation with kraft facing is fastened at the bottom of the 2×12 framing cavity. The resulting airspace between the top of the batt and the underside of the sheathing was not controlled to a uniform dimension. There were no air chutes in the cathedral ceilings."

Since 2x12s measure 11.25 inches, the ventilation gaps were roughly 1.25 inch high.

If you are hoping that ventilation channels will significantly change the temperature of your asphalt shingles, you are likely to be disappointed. All research data on this topic are consistent: cathedral ceiling vents only lower asphalt shingle temperatures very slightly. The color of the shingles matters much more than whether or not there is a ventilation gap under the sheathing. Even when shingles are installed over a vented attic -- one in which the vent channel is basically between 6 inches and 8 feet high -- venting the attic is less important than shingle temperature, geographical location, and the orientation of the roof slope.

Dec 6, 2013 7:17 PM ET

Vented Cathedral Ceiling
by Chris Barnes

(I apologize in advance if this is too far off topic)

I have been designing a home with a vented cathedral ceiling for some time now, and I am all but complete at this point (I am working on the final drawings). I recently revised the overhangs on the home so that they are separate construction that are fastened until the side of the home so that the sheathing can run continuously up the side of the exterior. Then it occurred to me that the house wrap could be installed underneath the overhang structure. Furthermore, the house wrap could then be extended over onto the roof into the ventilated cavity.

The be clear, current design has 2x4 "furring strips" installed on the flats on the first layer of roof sheathing. Then a second layer will be installed on top of the 2x4s creating a 1.5" gap.

Would it be reasonable to use house wrap underneath the 2x4 "furring strips", just above the bottom roof sheathing? The primary purpose of this would be to eliminate any wind-washing issues.



Dec 6, 2013 8:23 PM ET

Response to Martin.
by Lucas Durand - 7A

Your points about the differences between assemblies are understood...

It seems like the benefits of ventilated exterior wall cladding are well enough established that they are seldom seriously questioned...
Is it fair to say then that if the benefits of ventilated exterior cladding are (for all intents and purposes) beyond debate, that roof assemblies should also (for all intents and purposes) benefit from ventilation in the same ways?

I agree that data and research provide the best guide for decision making in this regard.
However, in terms of expressing what the data and research have to say, I think "rules of thumb" are a more effective means of communication than direct references to the actual science (ie, a litany of evidence-based reasoning is seldom very useful outside a "nerdy" niche audience if it can't be reduced for consumption by a wider "less nerdy" audience).
The trick, I think, is to develop new "rules of thumb" that accurately reflect the state of the underlying science and then work them into the existing culture over time until they are essentially matters of faith (which is all a lot of the present "rules of thumb" are).

To be clear, I'm just musing here and wouldn't want you to take my comments as being critical of the detail within your blog post.

Dec 7, 2013 7:03 AM ET

Response to Chris Barnes
by Martin Holladay

If I understand you correctly, you are building your house without roof overhangs, in order to simplify the air-sealing details at the exterior air barrier. This is a good approach.

Most builders who follow this approach use tape to create the air barrier. The easiest way to do this is to use Zip System sheathing and Zip System tape. If you follow this approach, you won't have any air leakage at the sheathing seams. Of course, you also need to seal the seam at the top of the wall, where the wall meets the roof sheathing. As long as you follow this approach, you don't need to fold your housewrap onto your roof.

For more information on this approach, see Airtight Wall and Roof Sheathing.

Dec 7, 2013 7:16 AM ET

Edited Dec 10, 2013 12:22 PM ET.

Response to Lucas Durand
by Martin Holladay

Basically, I agree with you. But there are a few caveats.

Rainscreen gaps on walls have many benefits and few downsides (other than cost and the need to develop new flashing details). If you add a ventilated rainscreen gap on the outside of your wall sheathing, the gap won't cause any problems.

The same cannot be said for a cathedral ceiling. Increasing the vent openings (soffit vents and ridge vents) that connect to a cathedral ceiling has definitely been known to tip a functioning roof assembly into failure. That's because a new ridge vent can pull more interior air through ceiling leaks, overloading the assembly with interior moisture.

All of these issues are connected. A ventilation gap in a cathedral ceiling will only work if the ceiling is airtight. Without the airtight ceiling, the ventilation makes things worse.

I'm sure that you understand these issues, but many builders don't. When we give advice to builders, we need to encourage whole-systems thinking, so that builders know that the thigh bone is connected to the knee bone.

When it comes to attic ventilation, I tend to agree with Bill Rose. My attitude is, "Include it if you want to, or omit it -- but don't sweat the details. It doesn't matter very much."

When it comes to cathedral ceilings, a ventilation channel is much more important than it is for an attic. But cathedral ceilings are extremely risky, and need to be detailed with care. If you insist on having cathedral ceilings in your house, you had better get the details right. And I think that a ventilation gap is a good idea, because it can make the difference between a roof assembly that works and one that doesn't.

None of this advice makes any sense, however, unless you have an airtight ceiling. So that's the message we need to keep repeating: Get your ceiling airtight.

Dec 7, 2013 11:19 AM ET

Response to Martin.
by Lucas Durand - 7A

Your conclusion makes sense.
Thanks for helping me find perspective...

Next time I'm at the lumber yard talking with the guys and the subject of attic ventilation comes up (as has sometimes happened), I'll know what direction to steer the conversation in ;-)

Dec 8, 2013 12:35 AM ET

by Malcolm Taylor

…as Lucas smiles to himself knowing his house has a service core on the ceiling with an airtight layer of OSB and will do just fine.

Dec 8, 2013 8:43 PM ET

Insulating a Knee Wall
by Patrick Dunnigan

I have a 50's Cape Cod here in the North East. Knee Walls upstairs with a Cathedral ceiling running about 60% the length of the house with an un-insulated attic space for the rest. I know I have batt insulation behind the cathedral ceiling, but no venting for air passage to the ridge vent. I have batt insulation behind the knee wall with batt insulation carrying down the rafters to the soffit vents, but know insulation on the floor behind the knee walls. I can see my top plate and want to blow in insulation down my walls and on the floor behind the knee walls and in the attic space above my second floor. I'm not sure how to block out the air from the outside. I know I have to block out the end of the floor joists to keep air from going under my floor and ceiling. but what am I doing where the roof meets the top plate?

Dec 9, 2013 5:35 AM ET

Edited Dec 9, 2013 5:43 AM ET.

Response to Patrick Dunnigan
by Martin Holladay

Above the top plate of your exterior walls, you need to install a ventilation baffle on the underside of your roof sheathing between each pair of rafters or trusses. You can make your own baffle out of thin plywood or rigid foam, spaced off the sheathing 1 inch or 2 inches, or you can buy a commercial product like AccuVent.

Then you need to install an insulation dam between the top plate of the wall and the underside of your ventilation baffle. The easiest material to use for an insulation dam is rigid foam. Once the insulation dam is installed, seal the perimeter of each dam with canned spray foam so that the installation is airtight.

Of course, this approach assumes that you have access to the small attic behind the kneewall, and that there is enough room back there for you to work. If the access is too tight from the interior, you may have to remove the soffits on the exterior of your house, and work from the exterior.

Here is a link to a video by Mike Guertin showing one way to do the work you are asking about: How to Ventilate Rafter Bays When Adding Insulation.

I will post some photos and drawings below to illustrate what an insulation dam looks like.


Soffit dam 1.jpg Soffit dam 2 - Energy Vanguard.jpg Soffit dam 3.jpg

Dec 9, 2013 8:12 AM ET

Response to Martin Holladay, GBA Advisor
by Patrick Dunnigan

Thanks for the response, I attached a couple of pictures of my knee wall, it is big enough to get behind. I have insulation in the knee wall and going up to the peak(with a ridge vent), but no existing rafter baffles in the cathedral ceiling part of the roof(about 60% of the roof, the other 40% I can get to the attic above). I would have to rip out all the drywall to get those in the ceiling. can I do the knee wall without having to do that? I have a 2x4 on top of my rafters behind the knee wall, somehow I would need that rafter baffle to go from between the rafters all the way to the roof sheathing? Can I fill behind the Knee wall with cellulose once the baffles are placed?

Back Crawlspace (3).JPG Back Crawlspace (11).JPG

Dec 9, 2013 8:52 AM ET

Edited Dec 9, 2013 8:54 AM ET.

Response to Patrick Dunnigan
by Martin Holladay

Your photos show a typical situation in this type of Cape home. However, just because it is typical, doesn't mean it isn't a disaster.

The photo shows that you have the worst-performing type of insulation available: fiberglass batts.

The photo shows that your kneewall lacks an attic-side air barrier (although, since the kneewall is entirely inside your home's thermal barrier -- kind of -- this doesn't really matter).

The photo shows that there is no air barrier on the underside of your roof insulation.

The photo shows that there are gaps that provide plenty of opportunities for air to flow from your soffit vents into the conditioned space of your home.

Your description tells us that you have no ventilation baffles, so there is no guarantee that there is an air gap between the top of you roof insulation and the underside of your roof sheathing.

To fix all these problems is a lot of work -- and yes, to fix them the right way will require you to take down the drywall on your sloping ceiling.

You can either do it right, or you can do it halfway and cross your fingers. Maybe you'll have a moisture problem affecting your roof sheathing, and maybe you won't. Maybe you'll have ice dams, and maybe you won't. One thing's for sure, though: if you don't address these problems, your energy bills will be higher than they should be.

More information here: Two ways to insulate attic kneewalls.

Dec 9, 2013 10:16 AM ET

A couple of observations from the DC area
by Douglas Horgan

Martin, as usual your article is thorough and comprehensive. There are certainly a number of misunderstandings “out there” and you address many of them.

I’ve had the pleasure of working on hundreds of homes in the DC area and would like to mention a few things I’ve seen over the years.

I agree that the vent area calculations don’t seem to matter much. We work in many older houses with no soffit vents and small gable vents, or houses where the gable vents are partially blocked off, or where ridge vents were installed without cutting the plywood or the felt, so they don’t actually vent at all. All of these attics perform fine, in terms of moisture management. In this climate, a very small amount of ventilation seems to be perfectly adequate. In fact many houses around here have bath vents terminated inside the attics, and I have seen a grand total of zero problems from this detail out of hundreds of fans. I know this is not the case in colder climates, but it does seem to be the case here with 4-5000 HDD.

Also, it is certainly true that effective ventilation does not solve some problems. For example, we remodeled a house once, and the insulation pieceworkers didn’t have enough vent channel with them, so they left it out of a cathedral ceiling area which consequently had little air movement. We had a cold winter a couple of years later, and the plywood started warping and expanding enough to see it through the shingles. We removed the plywood, found it soaking wet; we fixed the venting to ‘solve the problem’, and put it all back together. The next winter we found we had the similar problems (though much less severe). With the help of Alex Moore, one of the foremost weatherization pros in the US (rest in peace Alex), we found a number of bypasses and air leaks. In particular, a new humidifier we had installed on an old HVAC system with leaky ducts was pumping tons of humidity into the cavities. Effective air sealing on the ductwork and bypasses ultimately solved the problem, not correcting the ventilation. So I concur that no amount of ventilation will make up for major moisture sources, these should be prevented or fixed.

There are three points where I would put a different slant on the information you provide.
First, I know of several houses where roof longevity is very different on unvented roof areas versus vented roof areas, including the house I’m sitting in right now—my own—where the original house has gable vents but a garage addition has no ventilation at all. The shingles on the garage have failed (curled and lost their granules) while the ones on the house are fine. The house section actually gets more solar exposure and the other details of the roofs are similar (pitch, sheathing, etc.), so the ventilation is the only factor that would account for the difference. As I say, I have seen this on several other houses as well. I have no doubt that ventilation helps with shingle longevity, even if other factors also count.
Second, in our hot summers, ventilation can make a big difference in comfort on the top floor. Many clients have told me they made a big change in their houses by adding power fans or effective (baffled) ridge vents. I agree that the fans probably use as much or more energy as they save, but energy savings is not what these clients wanted, they wanted comfort. We don’t recommend power vents because we’ve seen them depressurize the whole house and cause massive infiltration, moisture, and comfort problems in houses, and because proper air sealing and insulation are about as effective (not to mention correcting lousy ductwork). But there is no denying that well-vented attics are much cooler, and that can make a big comfort difference in some houses.
Third, in my observation, all roofs leak. Period. Most types of roofing will leak a bit of water under some conditions (windy storms, ice or snow, etc.). Also, most people only repair or replace roofs after they see water inside. Therefore roof assemblies should be built with a high drying ability whenever possible, as they are much more likely to last without major problems. For us, even spray foamed roofs should be vented for this reason. We ask the insulators to put in vent channels, lapping them so any leaks run down toward the soffits. While this is not a perfect setup (as you mention, ideally there would be more space than vent channels create), in my opinion it is much less vulnerable to inevitable small or intermittent leaks, compared to an unvented assembly.

Dec 9, 2013 10:32 AM ET

Edited Dec 12, 2013 9:56 AM ET.

Response to Douglas Horgan
by Martin Holladay

All other factors being equal -- they rarely are -- a ventilated roof will be slightly cooler than an unventilated roof. Whether the difference in temperature will affect the longevity of the shingles is a matter of debate. (It is certain, however, that shingle color matters more than ventilation.) Some experts estimate that the shingles on a vented roof may last two years longer than the shingles on an unvented roof. This has not yet been proven, but if it matters to you, by all means vent your roof.

In any case, most asphalt shingles are replaced before they wear out -- often because of unsightly algae growth.

If your garage shingles are curling and have lost their granules, my first questions would be: (a) are you sure that the shingles are the same age as those on your house, and (b) are you sure that the shingles are the same brand?

If clients are made uncomfortable because of high attic temperatures -- in other words, if their ceilings are getting hot during the summer -- I can only conclude that their ceilings have lousy insulation. If a ceiling is airtight and has code-minimum levels of insulation, properly installed, then the ceiling is never going to get hot enough during the summer to make the occupants uncomfortable. If homeowners told me that their ceilings were radiating so much heat that they were uncomfortable, my first remedy would be to improve the insulation above the ceiling.

On the third point, I agree completely. Roofs leak, and it's always good to be able to inspect the underside of the roof sheathing. That's why the best arrangement is an unconditioned attic with insulation on the attic floor. Every roofer prefers to be able to climb up in the attic and inspect the roof sheathing. The best homes will have no cathedral ceilings.

Dec 9, 2013 11:43 AM ET

Edited Dec 9, 2013 12:08 PM ET.

Cold-Side Ventilation
by Ron Keagle

With a cathedral ceiling, I feel that a bit of backup redundancy in removing any vapor that happens to get into the insulation cavity is prudent. It also seems prudent to remove any small amount of heat loss through the insulation that might develop ice dams. In the future, I will use metal roofing, so I am not worried about solar heat gain diminishing shingle life. With sufficient insulation, the problem of living space summertime heat gain could be overcome, but again, a bit of backup in the form of ventilation seems prudent.

Since there is a limit to the practical effectiveness of air sealing in the walls, and because that limit can justify the backup measure of adding insulation outside of the sheathing, it seems equally prudent to vent cathedral ceilings because the same limit also applies to the effectiveness of making the ceiling airtight.

I also disagree with the generalization that "The best homes will have no cathedral ceilings."

Dec 9, 2013 12:25 PM ET

Edited Dec 9, 2013 12:58 PM ET.

Response to Ron Keagle
by Martin Holladay

I agree with you that a ventilation channel provides more safety for cathedral ceilings. That's what I wrote in Comment #11: "When it comes to cathedral ceilings, a ventilation channel is much more important than it is for an attic. ... [For a cathedral ceiling,] I think that a ventilation gap is a good idea, because it can make the difference between a roof assembly that works and one that doesn't."

It's OK if you disagree with me on the advisability of cathedral ceilings in general. There are plenty of people on your side of the fence (the pro-cathedral-ceiling side).

In general, cathedral ceilings are risky, and they make inspection of the roof sheathing impossible. That said, it isn't always possible to put an attic under every roof. If there is a situation where a cathedral ceiling is unavoidable, there are ways to detail cathedral ceilings so they work. But in my opinion, an unconditioned attic is always preferable.

Dec 9, 2013 5:42 PM ET

We agree...mostly
by Douglas Horgan

I think we're mostly on the same page, just making some limited, finer points about my climate.

"If homeowners told me that their ceilings were radiating so much heat that they were uncomfortable, my first remedy would be to improve the insulation above the ceiling."

This is absolutely what I recommend. It makes so much more sense than adding a fan.

However, in my opinion it's better for professionals to be aware that those fans do help with summertime comfort in some situations. When it's time to have that conversation with the client, you don't want to be telling them "it doesn't help anything" when it often actually does, and the clients may even have direct personal knowledge that it does.
Insulation, duct sealing, and attic air sealing help more, work all year round, and pay for themselves, which is why they make more sense. But, venting or fans do actually achieve some limited goals, at a high cost, so let's be aware of that when it's time to talk about it.

Dec 10, 2013 6:18 AM ET

Edited Dec 10, 2013 12:53 PM ET.

Whole House Fans Work great in some climates
by Kevin Dickson, MSME

Whole house fans are usually installed in the ceiling plane and blow upward into a well-ventilated attic. Their main function is to pull cool summer night air into the house through open windows. For this reason they are often wrongly called attic fans. Installed this way, however, they sure do vent the attic!

Since it's difficult to make them airtight, they have no business in a tight well-insulated home. They are also very difficult to insulate. But many thrifty folks in the Southwest love them if they don't have AC.

Another good reason not to install them in a well-insulated home is because they won't cool during the day. Therefore, you probably want air conditioning. If you install, say, a 24 SEER minisplit, then you can't save much money by running a whole house fan at night. If you can't afford a minisplit, then a cheap swamp cooler won't cost much more than a whole house fan, and at least works during the day.

But that's a whole nother topic.

Dec 10, 2013 8:17 AM ET

Response to Kevin Dickson
by Martin Holladay

My take on whole-house fans is a little different from yours. I think that whole-house fans can be an excellent way to cool a house in a dry climate -- especially if you live somewhere where temperatures usually cool off at night.

The key is to buy a whole-house fan with motorized, insulated shutters. The best whole-house fans are manufactured by Tamarack.

Whole-house fans have nothing to do with attic ventilation, as you accurately point out. Their purpose is to ventilate the house under the attic, not to ventilate the attic. I have written an article that tries to untangle the confusion some people have between whole-house fans and powered attic ventilators. Here is the link: Fans in the Attic: Do They Help or Do They Hurt?

Dec 10, 2013 1:06 PM ET

At least consider a swamp cooler
by Kevin Dickson, MSME

A 1450 CFM Tamarack fan is $429, but a 2800 CFM evaporative cooler is only $286

Dec 10, 2013 1:17 PM ET

Response to Kevin Dickson
by Martin Holladay

I agree that evaporative coolers often make sense, especially west of the Rocky Mountains.

GBA has a section on evaporative coolers in our encyclopedia: Fans and Natural Cooling.

Dec 10, 2013 3:33 PM ET

Fire prone areas
by Jane Babin

I'd like to see an article that addresses the issue of attic ventilation in California and other fire prone areas where soffit vents are prohibited in new construction due to the risk from wild fires.

Dec 10, 2013 3:51 PM ET

Response to Jane Babin
by Martin Holladay

As far as I know, soffit vents are not prohibited in California, but the building code may require soffit vents to resist the intrusion of burning embers. If you live in an area with such restrictions, you can design an unvented attic, or can specify soffit vents that meet code requirements.

Here is a link to a product that claims to meet code requirements for use in fire-prone areas:
Vulcan Vent.

Dec 10, 2013 5:51 PM ET

All About Attic Ventilation
by Reg Burns

Fantastic article!! Very well written! Thank you Martin Holladay. I'll be passing this article around to all my production and sales staff, as you nailed a lot of the biggest misconceptions about ventilation.

Reg Burns
Cherry and Clark Roofing
Mississauga, On

Dec 11, 2013 7:55 PM ET

Gable vs Ridge Venting
by Jay Walsh

An observation I made a couple winters ago when we (here in Western Mass) were having heavy snowfall was that many homes with ridge vents, which were covered with 8-12 inches of snow for several weeks all has significant ice dams and ice-cycles. While many (most) those with Gable venting did not. While internally many other issues could be occurring, I concluded that with the airflow blocked at the ridge vent by snow accumulation, this stopped the attics from venting and the leakage of heat from the homes at the exterior wall plates (where insulation is least and many times airflow blocked) was causing snow melt resulting in the ice dams and ice cycles. Whereas the houses with the gable vents, which are always clear of snow, were functioning as designed and venting the accumulation of attic heat, from the airflow though the soffit vents and out the gables. Any thoughts on this? Keep an eye out for this and see what you conclude.

Dec 11, 2013 8:20 PM ET

Snow blocking ridge vents
by Ron Keagle

I have wondered why the issue of ridge vents being blocked by snow does not seem to concern many people. Maybe it is due to the possibility that many people believe that attic venting is only for cooling during summer. That would be the naturally intuitive conclusion. The reasons for wintertime venting are not so obvious. I have worked out a design for tall ridge vents to address the snow blocking issue.

Dec 12, 2013 7:47 AM ET

Edited Dec 12, 2013 8:11 AM ET.

Response to Jay Walsh and Ron Keagle
by Martin Holladay

Jay and Ron,
There are three reasons that snow above ridge vents doesn't concern me:
(a) Snow is not an air barrier;
(b) Because of wind, thick snow doesn't stay on a ridge for long;
(c) Attic ventilation isn't very important and is often counterproductive.

Attics in adjacent houses are rarely identical.

Some attics have ductwork; others don't.

Some attics receive tremendous amounts of escaping heated air from the leaky ceiling below; others don't.

Some attics have thick layers of insulation on the floor; others don't.

If you are walking along a sidewalk looking at roofs in winter, you can sometimes deduce what is going on in the attic. But often, defects are hidden, and the causes of an ice dam are uncertain to the pedestrian.

One thing is for sure: years of experience have taught home-performance contractors that you can't solve an ice damming problem by improving attic ventilation.

Dec 12, 2013 9:09 AM ET

Amen & The resilience of wood planking!
by Greg Labbe

Mr. Holladay,

Thanks for the thorough treatment this topic deserves! I appreciate your research and summary of this hot-potato topic. It confirms everything I've experienced in the hundreds of attics I've had the pleasure to be in and snoop around. I wrote a piece for Home Energy some time ago about roof topography, which looks at roof shape and acreage as a contributing factor too:

One artifact/anecdote I've been ruminating on over the years is the benefit of solid planking for sheathing vs OSB/plywood. I've seen 150 year old wood plank look great - even when wet - seems to be more resilient than OSB/PW. I've seen OSB/PW replaced after only 3 years of service because ventilation was increased, but air leakage from living space ignored. The dimensional lumber planking with expansion gaps between planks seems more vapour open than the more modern alternatives.

I just wanted to urge anyone with solid lumber sheathing to not be bullied into having to replace it as part of an ice dam remediation!

Dec 12, 2013 4:28 PM ET

Ventilation and Ice Dams
by Ron Keagle


Regarding your statement:

“One thing is for sure: years of experience have taught home-performance contractors that you can't solve an ice damming problem by improving attic ventilation.”

At first I was going to say that I disagreed with that, but when I parse the words, I am inclined to agree. The meaning pivots around the phrase, “an ice damming problem.” One might say that any ice dam is a problem; but probably the most common way of looking at it is that no ice dam is a problem until water starts coming through the ceiling and running down the walls. For an ice dam to get to that point of being a “problem,” there must be a major amount of heat and heated air escaping into the attic. I agree that an ice dam of that magnitude is not going to be easy to solve with ventilation, especially when the ridge vents are plugged with snow.

Dec 12, 2013 9:09 PM ET

Attic Venting
by Thomas Barrett

Martin, great article. I did an attic venting research project here in California back in the 1990s and came to the same conclusions for the same reasons. Since then I've battled many "building experts" who claim attic venting is necessary to cool attic spaces but when I tell them about my study they can't get their heads around the facts. I monitored attics and interiors of homes with temperature probes and heat flow sensors and complied months of data with dataloggers and found no heat transfer with a well insulated attic deck even when attic temperatures hit 160 - 170 degrees.

We use one home to experiment on various venting strategies typically found in California and found very little cooling or air movement effect from venting.

One of the interesting things about heat buildup in attics and asphalt shingle warranties tied to the amount of attic venting needed to keep the attic cool, is that I recorded temperatures immediately under shingles between 180 - 190 degrees on south facing slopes and just above ambient temperature under wood shingles. The problem with heat is the shingles not the venting. White shingles are better, but the asphalt matrix gets hot and transfers that heat to the attic. It is a great deal for the shingle manufacturers when their shingles fail because of the heat they can attribute it to inadequate venting and void their warranty.

Now can we get the building code changed to reflect reality?

Dec 13, 2013 8:38 AM ET

Response to Thomas Barrett
by Martin Holladay

Anyone who looks closely at the way asphalt shingle manufacturers introduced attic venting requirements into their warranty rules reaches the same cynical conclusion about the manufacturers' motives.

Suffice it to say that the manufacturers' attic venting requirements are not evidence-based.

Dec 13, 2013 11:35 AM ET

Venting Affect on Shingle Warranties
by Ron Keagle

From what I understand, it is generally believed that heat does shorten shingle life, but only by a couple years or so. Therefore this type of failure would be well beyond the warranty time frame. If failures occur within the warranty period, the manufacturer would have to show that the ventilation was insufficient to meet their warranty in order to deny a claim. In such a case, if the ventilation is in fact insufficient, why would one conclude that the cause of shingle failure was not due to insufficient ventilation?

If shingles within the warranty period fail on a roof lacking adequate ventilation according to the manufactures requirements; and if the manufacturer denies the claim because they say the lack of ventilation caused excess shingle temperature; is the only alternative explanation that the shingles would not have been at a lower temperature even if ventilation was adequate according the manufacturers requirement?

Dec 13, 2013 3:00 PM ET

About those shingle warranties
by Ed Voytovich

Please raise your hand if you have ever even *heard* of a successful shingle warranty claim on a roof that did not have **serious** visible defects when the bundle was opened. Anybody? Anybody at all??
I'm not saying it couldn't happen, but P=<0.05. That old argument for attic ventilation is right up there with "Eat your broccoli or you won't grow up nice and tall."

This is an *excellent* article, and anybody who has anything to do with residential construction, renovation, or maintenance should be required to pass a test on its content. Thanks, Martin!

Dec 13, 2013 6:23 PM ET

Edited Dec 13, 2013 8:25 PM ET.

Moisture & Ventilation in Photo Image 2 of 2
by Ron Keagle

In image 2 of 2, the caption says that this is a well-ventilated, well-constructed, relatively new building. The caption also implies that this is an example of a moisture problem in a ventilation zone, thus proving that ventilation is not capable of mitigating moisture, and therefore a reason why ventilation is unnecessary.

But where is this moisture coming from?

I would submit that this is not a well-constructed building, as the caption says, in that it has serious air leakage from the conditioned space into this attic space; and that the visible moisture is from the condensing of humid air leaking in from the heated space below.

I would not expect ventilation to solve this problem. The article itself confirms that ventilation should not be expected to solve a moisture problem. Specifically, for example, Bill Rose says, “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.”

It is probably true that the darkened zone of wetting is the coldest due to the incoming ventilation outdoor air. But if that much water vapor is available through air leakage from the heated space, it will find a condensing surface in areas other than just that incoming air zone. I conclude that this mold problem would be much worse if no ventilation were provided.

Overall, I would say that image 2 of 2 is an example of why cold-side ventilation is necessary as a backup feature to offset the difficulty in achieving 100% effective air sealing.

Dec 14, 2013 10:09 AM ET

Response to Ron Keagle
by Martin Holladay

I wrote, “This photo shows sheathing mold near the ventilation baffles in the attic of a well-ventilated, well-constructed, relatively new building in the Pacific Northwest.”

You wrote, “I would submit that this is not a well-constructed building, as the caption says, in that it has serious air leakage from the conditioned space into this attic space; and that the visible moisture is from the condensing of humid air leaking in from the heated space below.”

It is possible that you are completely familiar with this building in British Columbia, and that you know more about the building than the researcher who took the photo. Possible, but not probable.

I will quote from the paper where the photo was published (“Highly Insulated, Ventilated, Wood-Framed Attics in Cool Marine Climates,” by Patrick Roppel, Neil Norris, and Mark Lawton): “The attic venting area, including both intentional and unintentional openings, is presented in Table 2 for the measured pressure differential and is compared to the applicable required venting area required by code for the construction of the test attics (1:300 per insulated ceiling area). The measured venting areas are higher than the average areas reported in the BLP (1991) and Sheltair (1997) studies and exceed the building code requirement by as much as three times.

“The measured air leakage area for the attic ceiling, calculated normalized leakage area (NLA), and observations of the smoke test are summarized in Table 3. These values were derived using equation 43, Chapter 16 Ventilation and Infiltration, of the ASHRAE Handbook—Fundamentals to convert to a 10 Pa pressure differential basis. Comparison of the NLA values in Table 3 to the reported values in past Canadian studies (BLP 1991; Sheltair 1997; NRCan 1997) suggests that the ceiling airtightness of the units in this current study can be considered to have at least average airtightness levels, by Canadian standards, and a convincing argument can be made to classify the ceiling to attic interface as airtight.”

You asked, “Where is this moisture coming from?” The researchers concluded that this is exterior moisture. The researchers observed and photographed similar mold on the underside of roof sheathing on roofs over exterior mailboxes (like the roofs you see over a picnic table at a state park).

Dec 14, 2013 11:44 AM ET

Response to Martin Holladay
by Ron Keagle


Thanks for your explanation. You are correct that I have never seen the building in the photo. I concluded that the moisture was coming through air leaks from the conditioned interior. I had not considered that the moisture could be coming from the exterior, as you say was concluded by the researchers.

I am curious about the mechanism for delivering this exterior moisture into the attic space and depositing it on the underside of the roof sheathing. Is this from the intrusion of rain or snow being drawn into the eave vent openings? Or is it from water vapor being drawn into the eave vent openings and then condensing on the underside of the sheathing?

If the latter, I don’t see how the underside of the roof sheathing would be a condensing surface for exterior vapor in either winter or summer-- because in either season, the underside of the roof deck would not be colder than the outside temperature from where the vapor originated. So why would any feed of outside vapor condense in the attic?

Did the researchers explain why wetting occurred on the underside of open roofs over mailboxes and picnic tables? I suppose there is a possibility that roofs could lose heat by radiation to deep space on cloudless nights, and thus become colder than the ambient air temperature. If that were the case, it might also explain outside vapor being drawn into eave vents and condensing on the underside of the roof deck.

What did the researchers recommend as the remedy for preventing the wetting effect on the underside of the roof deck?

Dec 14, 2013 11:58 AM ET

Response to Ron Keagle
by Martin Holladay

The outdoor air in the Pacific Northwest is very humid. When the air is humid for much of the year, exterior wood work can get moldy (unless it is exposed to regular sunlight).

The researchers concluded that, in the climate of the Pacific Northwest, moldy roof sheathing in vented attics may be unavoidable, and we may need to educate real estate agents (telling them that mold on attic sheathing is not a health risk).

There are several possible reasons that the mold shows up near the soffit vents. It's possible, for instance, that the upper parts of the attic stay dryer because those areas are warmer. It's also possible that the frequent streams of outdoor air in this part of the attic keep the sheathing damp in that area.

Dec 16, 2013 6:49 AM ET

additional bracing for walls
by Thomas Baldes

Years ago, when my father was custom building houses and foam sheeting was new to the market . we added metal strapping(Banding as is used on loads of lumber) on the diagonal to all exterior walls both interior and exterior. The banding was self tightening in that driving an 8d nail into each stud tightened and tensioned the strapping. This was done without an engineers advice but could easily be used underneath drywall without causing lumps and bumps and was very inexpensive. It seemed to add a great deal of rigidity to the assemblies. The cost was negligible when purchased on the bulk spool. Any thoughts?

Dec 16, 2013 8:53 AM ET

Response to Thomas Baldes
by Martin Holladay

I'm not sure what your question has to do with attic venting -- perhaps you accidentally posted the question on the wrong page -- but I'll be glad to respond.

Many builders use diagonal steel strapping to brace walls. Rather than using a banding machine, however, there is an engineered solution: you can use Simpson Strong-Tie’s L-profile strapping (the product designation is Simpson RCWB). For more information, see Four Options for Shear Bracing Foam-Sheathed Walls.

Dec 16, 2013 1:57 PM ET

All About Attic Venting
by Michael Koelsch

I've been building my own log home with a cathedral ceiling and so have been following this and related blogs for some time. My ceiling is currently a cold metal roof with a 3" vented air space above 1/2" plywood covered with 30# felt. The 2x12 rafters are filled with r-38 fiberglass. I was planning on covering that with t&g boards until I reviewed Martin Holladay's July 2012 article in Fine Homebuilding. I now plan on the vented roof design with the rafters being capped with the rigid foil faced foam with taped joints and the t&g fastened to strapping installed on top of the foam. The only material difference is the venting is on top of the roof sheathing instead of below. Is this a viable design or do I need to change something to improve it?


Dec 16, 2013 2:07 PM ET

Response to Michael Koelsch
by Martin Holladay

It's perfectly acceptable to put a vent channel above the roof sheathing.

It's important to pay careful attention to airtightness with this type of roof assembly, however. If you are depending on a layer of interior rigid foam to be your air barrier -- and it sounds like you are -- you need to do a meticulous job of sealing the perimeter of each sheet of foam. It's also important to seal all penetrations.

For more information, see How to Build an Insulated Cathedral Ceiling.

Dec 17, 2013 11:24 AM ET

Cathedral Ceiling Insulation
by Michael Koelsch

Thanks Martin, as I've undertaken building this house as a ten year and counting retirement project with my wife and I doing everything from digging the foundation to building cabinets from lumber on the property, I've relied heavily on experts like yourself and others to avoid catastrophic errors.

As you suggest, I will do my best at achieving a six sided air tight seal in every rafter space. I'm glad I did not rush the finishing of the ceiling until I watched it for a couple of years and did notice it lack of performance for the reasons you outline. Thanks again for the service you provide and it's obvious benefits for us and the planet.

Jan 5, 2014 2:20 PM ET

Unvented Cathedral Ceiling?
by Hari Kamboji

Thank you for your very informative posts. We live in an older home (built 1915) near Bangor, ME. The attic is very large and we are planning to fix it up. Currently we have very old fiberglass (R-11) and rockwool on the high cathedral ceiling. We previously blew insulation into the floor. The plan is to rip out all the old ceiling insulation, reinsulate, and then add sheetrock, new flooring, and a little lavatory. I believe that there is no ridge vent and that the soffits are unvented. I'm not entirely sure about this, but I reached into the soffit beyond the floor, and I could not feel any outlet to the outside, and the space is filled with the blown insulation we added to our attic floor. The contractor suggested leaving a gap, then adding R-23 Roxul batts (our rafter beams are only 2x6), and then in the summer getting someone to drill holes from the outside into the soffit (this seems a little strange to me). If it turns out that there's no ridge vent as suspected, they propose adding gable vents across the top. After reading your posts, I am now leaning towards an unvented ceiling, something like this (in this order):

-Add foil-backed polyiso (3-4 inches) right against the sheathing, foil side down.
-Caulk the polyiso well along the sides
-Add batts of Roxul up to the edge of the rafters.
-add vapor barrier
-Attach foam board under that to underside of rafters
-Add sheetrock etc.

I don't think this will get us all the way to the recommended R-49 for zone 6 but we don't plan on using the space a lot (only heating it when in use), and the floor is pretty well insulated.

Does that sound reasonable? One more thing- we really do not want to use spray foam for health reasons. Thanks again for this valuable information!

Jan 6, 2014 11:04 AM ET

Edited Jan 6, 2014 11:07 AM ET.

Response to Hari Kamboji
by Martin Holladay

The insulation method your propose is called cut-and-cobble. For more information, see Cut-and-Cobble Insulation.

I don't recommend the cut-and-cobble method for sloped roof assemblies or cathedral ceilings; read the article to learn about some recent failures using this method.

If you want to insulate a sloped roof assembly, I recommend that you install at least enough R-value to meet minimum code requirements. You really do want to install R-49 if possible. If you are worried about head room, you can install some of your insulation in the form of rigid foam above your roof sheathing. Of course, that means that you will need to install new roofing.

For more information on your options, see How to Build an Insulated Cathedral Ceiling.

Jan 6, 2014 5:27 PM ET

Re: Response to Hari Kamboji
by Hari Kamboji

Dear Martin,
I had not seen your post on "cut and cobble"- very helpful. Thanks a lot. I got this idea after reading your post on cathedral ceilings and then reading the 2009 IRC code (R806.5.3 of the 2012 code still seems to suggest it). I wondered why the code seemed to invite cut-and-cobble for conditioned attics, while this was not mentioned in the conclusion of your cathedral ceiling article. Now it is clarified (there remains risk of vapor penetration).

The idea of rigid foam above the sheathing is appealing. In that case, the Roxul can go right up against the underside of the sheathing, right? Our roofing was installed in 1997, so maybe it's getting due for replacement anyhow. But it actually still looks pretty good to my untrained eye.

Head room is not a huge concern. I've seen mention of various techniques to extend the insulation space, but I haven't seen a dedicated explanation (e.g. what's "furring"?). Something like that, in combination with foam board atop sheathing could get us to R-49 or beyond.

Thanks for your reply.

[Editor's note: To read the response to this comment, and other comments that follow, go to page 2 by clicking the number 2 below.]

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