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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 the attic be vented. (For example, see Section R806.1 of the 2015 International Residential Code.)

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 lower half of the attic (generally at the soffit) and half near or at 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

Jan 6, 2014 4:39 PM ET

Response to Hari Kamboji
by Martin Holladay

"Furring" is a synonym for "strapping." It refers to pieces of lumber -- usually 1x3s, 1x4s, or 2x4s -- that are attached perpendicular to the framing (studs, joists, or rafters) or on one side of rigid foam (in which case the furring is usually parallel to, and directly above, the framing on the other side of the rigid foam).

Jan 9, 2014 8:54 AM ET

Edited Jan 9, 2014 8:59 AM ET.

vapor barriers inside and out?
by Hari Kamboji

Because Roxul insulation is unfaced, I read that one needs to install a vapor barrier (or is it air barrier?) on top of it (facing the living space). On the other hand, if one applies foam board on top of the roof sheathing, that also acts as a vapor barrier. Is it bad practice to have vapor barriers on both sides of the roof? One website I found recommended using a permeable insulation on top of the sheathing (like heavy duty mineral wool batts) if a vapor barrier has been installed inside (I realize such an assembly would violate existing code).

A somewhat unrelated question: It may be a year or two before I can redo the roof to add the foam board on top. Rather than cut-n-cobble, I'm planning to go ahead now and install the air-permeable Roxul insulation between the rafters up against the underside of the sheathing (an unvented setup), then vapor membrane, then polyiso boards across underside of rafters, then sheet rock. To mitigate condensation, until I redo the roof, I will just keep the attic unheated (there's insulation in the floor). Our house uses forced hot air heating, so the interior humidity is very low in winter. Comments are welcome if anyone thinks this is a terrible plan. The house has already had fiberglass and rock wool between the rafters for 50+ years without any moisture problems arising, so this would seem a safe thing to do in the short term. Thanks again!

Jan 9, 2014 9:15 AM ET

Response to Hari Kamboji
by Martin Holladay

You are correct that you don't want to enclose your Roxul with a vapor barrier on both sides. If the Roxul is part of a vented roof assembly, you usually want to install a vapor-permeable air barrier above the Roxul; this is your ventilation baffle. Thin plywood or stiff cardboard will work; so will thin EPS, which is fairly permeable to water vapor.

Underneath the Roxul, facing the living space, you also need an air barrier. In most cases, the ceiling drywall is an air barrier.

I don't recommend installing Roxul in an unvented roof assembly unless you are also installing rigid foam above your roof sheathing. If you proceed with your plan, your roof sheathing could accumulate moisture and develop rot or mold.

Jan 9, 2014 12:05 PM ET

Many thanks for the quick
by Hari Kamboji

Many thanks for the quick reply. Just to clarify, in a vented assembly, a vapor barrier should be applied on the underside (living space side) of Roxul, but in an unvented assembly which includes the rigid foam on top of the roof sheathing, one should not apply a vapor barrier anywhere on the interior side?

Jan 9, 2014 12:21 PM ET

Edited Jan 9, 2014 12:23 PM ET.

Response to Hari Kamboji
by Martin Holladay

You don't want an interior vapor barrier. In some climate zones, building codes require a vapor retarder -- which is a less stringent layer than a vapor barrier.

In general, there aren't usually any reasons to worry about vapor retarders in ceilings or walls (with a few exceptions). In most cases, the only reason to install a vapor retarder is to satisfy your local building inspector. If the inspector wants one, I suggest that you use vapor-retarder paint or MemBrain (a smart vapor retarder).

While vapor retarders are usually unnecessary, and vapor barriers often cause problems, an interior air barrier is always a good idea.

For more information, see:

Vapor Retarders and Vapor Barriers

Forget Vapor Diffusion — Stop the Air Leaks!

Do I Need a Vapor Retarder?

Jan 9, 2014 2:14 PM ET

polyiso under rafters as vapor barrier
by Hari Kamboji

Thanks- as always these links are extremely informative, and I'm sorry for not stumbling upon them before asking. In the comments of the first link above you mention that foil-backed polyiso acts as a vapor barrier. Does this mean I should not be attaching it below the rafters for extra R-value?

Jan 9, 2014 2:43 PM ET

Response to Hari Kamboji
by Martin Holladay

Interior polyethylene can sometimes cause problems in the summertime, because it reduces inward drying and (if the house is air conditioned) can get cold enough to be a condensing surface for any moisture in the framing bays.

Polyiso won't ever get cold enough to be a condensing surface, even if the house is air conditioned, because it has R-value. It still reduces inward drying, however, so walls or ceilings with interior polyiso should (ideally) be able to dry to the exterior.

Nov 12, 2014 10:24 PM ET

attic ventilation
by Kevin Hogan

As an energy consultant working in a cold climate (7500 HDD), I have had the opportunity to investigate many home performance problems to include ice dams and condensation issues inside attics and have quite often found and photographed soffit vent screens that were plugged or at least partially plugged with airborne "debris" that rendered the vents to be less than effective, or worse. As an instructor of building science basics, I've asked hundreds of participants (mostly remodelers) how often they make the effort to clean soffit vents. Rarely does anyone even acknowledge the need to do so. In fact, when I mention the notion of doing so, they tend to look at each other as if to convey an attitude (collectively) that I must be from another world. In addition, I've seen and have photographed roof vents semi-plugged with insect nests. I feel confident in assuming the same reduction of airflow happens with ridge vents over time. My point is passive ventilation can deteriorate over time and cannot be depended upon to a permanent working system. Even passive ventilation systems need maintenance. Perhaps a better solution is designing and constructing unvented roofing assemblies.

Nov 13, 2014 7:52 AM ET

Response to Kevin Hogan
by Martin Holladay

You raise an interesting point. You're right, of course, that soffit and ridge vents (just like the air intake vents for HRVs) can become clogged by spider webs, dried leaves, hay, and insect nests. If anyone is depending on the free flow of air through their soffit vents to keep their attic functioning well, I guess they need to get a vacuum cleaner that they can use while standing on a ladder.

Here's the main point of my article, however: if you are depending on attic ventilation through soffit vents to keep your attic dry, you've done a bad job of air-sealing your ceiling. If you build your house correctly, and pay attention to airtightness, you don't really need to depend on attic ventilation to keep your attic dry.

When it comes to preventing ice dams, the two most important factors are (a) an airtight ceiling, and (b) a thick layer of insulation. Venting matters much less than these two factors. For more information on ice dams, see Prevent Ice Dams With Air Sealing and Insulation.

Jan 12, 2015 3:36 PM ET

not enough venting
by steven pereira

I have an attic converted to living space where the roof is also the cathedral ceiling. Most bays go from soffit to ridge and are insulated with fiberglass and a foil barrier with a Duravent installed so that there is always an airspace from soffit to ridge. Those bays are mostly fine. The areas I'm having a ton of ice and moisture on the sheathing is where the bays are interrupted and there is only a ridge vent. Should I fill the bays solid with spray foam? Or should I at least foam up the ridge vent from the inside on those bays? If I do a perfect job with the foil barrier will that stop the moisture from getting into the bays? I have a damp basement and as much as I want to seal it up it is no way in my price range.

Oct 12, 2015 3:22 PM ET

Edited Oct 13, 2015 7:37 AM ET.

typical 50's ranch - awful venting, moisture, poor insulation
by Mark D'Amico

First, thank you for the informative article. Second, sorry to comment on such an old publication, but I feel compelled to do so and ask a few questions.

The majority (read: all but the last statement) discuss the disadvantages, or perhaps the lack of advantages, to a vented attic at length. Despite that, the last statement is recommending a vented attic done properly. I now have a better understanding of the attic venting and so many of the misconceptions or inaccurate claims associated.

So why not just seal it right up start to finish?

I have a small ranch in New England which was built in the 50's and has, as anyone would expect of a house built in this era sub-par insulation. When it was built, the house had two small gable vents, which it still has. About 20 years ago, the roof was replaced and a ridge vent was added at that time. There are currently no soffit vents.

The attic floor was given an R-19 treatment between the ceiling joists. This was a half-hearted attempt by a previous owner, as it's incomplete. Perhaps it's still better than nothing.

I had the roof replaced a few months back and again the roofer recommended a ridge vent "because it was there with the old roof, and any ventilation is better than none".

So, this brings me to the question again of what are the benefit and disadvantages of using a closed-cell spray foam directly on the underside of the roof? [Of course the first step to this would be to close or remove the gable vents and seal off the ridge vent.] The way I see it, it'll increase the conditioned space by including the attic cubic footage, but it's a much better insulation than the R19, which once the air escapes that, it's gone. So perhaps the added R value and better sealed 'envelope' created by the spray foam, despite the added volume of 'conditioned space', nets out to a benefit over the R19 on the attic floor? I don't know.

Then of course that brings to light the argument that the reason the underside of roofs are not painted, nor should they be insulated, is to allow the wood to 'breath' and not trap moisture in the wood. Is this as much of a concern as some claim? Is it flawed thinking that whatever moisture is in the wood is really negligible and not a concern of allowing it to 'breath'? To further that, my thinking (and very likely flawed) is that the spray foam benefits (aside from the insulation factor) would be: Once applied, whatever excess moisture is in the wood will be extracted by the foam during the curing process (as the foam needs moisture/humidity to cure) and it'll seal the wood surfaces from direct exposure to moisture and significantly reduce the amount of exposed organic surface material (i.e. wood). Is this all bunk, or is there some validity to this argument?

The last byproduct of an insulated roofline rather than heavily batted attic floor is the new found availability of semi-conditioned storage space.

So after all that, given there's already R19 in there, do I just go buy a bunch more of the r19 and lay it over the existing stuff and call it a day?

Please help shed some educated light on this mind-bending question of what's best.

Oct 13, 2015 3:38 AM ET

Response to Mark D'Amico
by Martin Holladay

It sounds like you have mold on the underside of your roof sheathing. The cause of this problem is almost always a leaky ceiling. These air leaks are allowing warm, humid interior air to enter your attic, and the moisture in the air is condensing on the cold roof sheathing.

If you fix this problem by sealing the air leaks, there will be two benefits: (1) Your energy bills will be reduced, and (2) The mold will be eliminated.

Here is a link to an article which explains what you need to do: Air Sealing an Attic.

If you won't want to do this work, you can follow the route you suggested: "using a closed-cell spray foam directly on the underside of the roof." Done properly, that approach will also reduce your home's air leakage problem and protect your roof sheathing. However, that approach is significantly more expensive than sealing the air leaks at your ceiling plane. For more information on the spray foam option, see Creating a Conditioned Attic.

Oct 13, 2015 7:59 AM ET

"...significantly more expensive..." indeed!
by D Dorsett

A square foot of R20 closed cell foam costs about $3-3.50, and even in zone 5 (southern New England) you need at least R20 for dew point control with an R49 code-min stackup.

A square of R20 cellulose costs about $0.60, and the attic floor has fewer square feet than the roof deck. To hit code min R49 you'd have to add ~R35 ( at about a buck-a-foot) over the low density R19s, which will get compressed a bit by a 10" overburden of cellulose, Target total depth is 15-16" to allow for some amount of settling over time down to 14".

If you have enough space to increase the total insulation depth to 15", after air sealing at the ceiling plane blow cellulose over the existing R19s and you'd hit current code-min. But even if you can only bring it up to 10" that's still a huge improvement. Make a bunch of paper/cardboard depth gauges to keep track of it, making it smooth & even depth everywhere. You'll still have to keep the insulation an inch away from the roof deck at the eaves- use the purpose-mad de asphalted cardboard chutes for that, not the cheap thin foamies. Open blown cellulose is a fairly easy DIY with a box-store rental blower (usually free rental with a minimum purchase of cellulose.)

An overlayer of R19s is a much worse performing solution, due to the very high air permeance of the low density batts. R19s are more of an air-filter than an air-retarder, and take a serious performance hit unless there are air barriers on BOTH sides. Cellulose is about an order of magnitude more air retardent than low density fiberglass, and performs well even without a topside air barrier. As little as 3" of cellulose overtopping will "restore" the performance to the R19s, but it's worth installing more than that in any New England location.

Mar 28, 2016 8:01 PM ET

Edited Mar 28, 2016 8:03 PM ET.

Venting challenges in Colorado
by Pete Holzmann

Hi! This has been an extremelhy helpful article. I have never had trouble with attic venting until we purchased a brand new home at 7000' in Colorado, with a vented attic. Many may not be aware that even "in town" here, a storm with 40-50mph wind and 60-70+mph gusts is not that unusual. Gusts in the 100+ and even 200mph range have been measured. Builders must be careful to finish sheathing homes quickly after they begin! :)

After a Nov '15 blizzard, we experienced several of the issues you describe (including snow/ice injected directly into the middle of the insulation due to improper perimeter barriers.) I've retrofitted with proper barriers/baffles (Accuvent worked nicely).
The good news: the new barriers work. The insulation behind the barriers stays in place and is dry.

The bad news: in last week's blizzard, while no snow/ice was injected into the middle of the insulation, the nice big ventilation channels allowed significant amounts of snow/ice to still enter the attic at the leading edge of the storm, and create snow piles on top of the insulation... enough that I couldn't remove it in time, we had ceiling leaks in our new home, and a full 4x8 sheet of ceiling is in precarious condition after getting wet.

I have some ideas and questions, and am wondering what you think.

a) Even though the storm came from the west and north, we ONLY had snow enter in the NW corner. b) This corner sticks out about 8' from the home next door; we're on the side but at the downwind end of a culdesac, with no homes (just a gentle hill) downwind of us for several hundred feet.

Idea: I'm thinking that
a) the street and especially home only 10' away essentially "focused" the wind.
b) The air had to go somewhere, and due to Bernoulli effect, it likely hits the exposed wall of our home at higher than normal pressure (Florida Building Dept work on Wind-Driven Rain research estimates pressures easily reaching 2x or 3x normal
c) Thus, that corner of the home may be exposed to far higher wind speed than anticipated, causing snow-laden wind to easily enter the attic.

Question: Does this make sense or am I just blowing smoke?

My thought: now that I've properly blocked the edges, use waterproof spray-in insulation foam to seal the baffles in that corner, so that any snow or rain trying to enter will melt and fall back out into the soffit area.

Question: does that make sense?

(I do not have the equipment to reach the soffit in that corner externally. It is 15'+ above a steep riprap slope.) My home IS still under construction warranty... so far the builder has mostly said "hey, it passed code inspection so what do you want?" ... although they HAVE agreed to blow in replacement insulation after I'm done properly repairing the blocks/baffles. TBH, I don't trust the contractors to fix it right after I've seen other work they've done... I am MUCH more aware now than a few months ago, thanks to you!

Final question: do you know of any research or rules of thumb for preventing wind-driven snow/rain issues in a properly blocked/baffled attic?

We are quite concerned. So far, we have been here when the blizzards have arrived. What happens if we are traveling?!

Thanks MUCH. I am happy to invest more of my time helping all of us learn more. I have photos illustrating my own situation... even some accidentally taken during a framing walk-through. If I had known then, I would have pointed out the "blocks" made of loosely placed insulation batts. Completely inadequate of course.

The interesting thing: in the November blizzard, EVERY home builder here got slammed. They are modifying their home designs and practices to help... yet still all have vented attics with baffles providing air flow... so I am not sure the problem described in this message is really being addressed.

Mar 29, 2016 4:39 AM ET

Edited Mar 29, 2016 4:40 AM ET.

Response to Pete Holzmann
by Martin Holladay

As my article explains, the fetish for venting attics is mostly a historical accident, rather than a necessity supported by building science. Builders and homeowners need to use common sense. If your vent openings are allowing snow to enter your attic, the obvious solution is that the problematic vent openings need to be sealed.

If you ceiling is relatively airtight -- and I hope that it is -- no don't need to ventilate your attics. The only person who cares, really, is your local building inspector. If the inspector was satisfied, you can do whatever you want now that it's your home. (When it comes time to sell your home, however, you may need to discuss the status of your soffit vents and ridge vents again... but I suggest that you cross that bridge when you come to it.)

In a very windy area with lots of cold, fine, powdery snow, roof vents can allow snow to enter the attic. I've seen it in Vermont many times -- but usually the amount of snow is smaller than the amount you describe.

Do you know if your ceiling is airtight? Did your home ever have a blower door test?

Assuming that your ceiling is relatively airtight, I suggest that you seal all of the problematic vents. Good luck in your negotiations with your builder.

Oct 18, 2017 11:46 AM ET

Just got some continuous
by Steve Lenertz

Just got some continuous soffit venting from the lumber yard. Any reason the vent is only 94 1/2" long?

Oct 18, 2017 2:11 PM ET

Reply to Steve Lenertz
by Martin Holladay

It's probably the same reason that a 1-pound can of coffee now contains only 13.5 ounces.

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