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Home Energy Magazine Online November/December 1999

Unventing Attics In Cold Climates

By Joseph Lstiburek

Joseph Lstiburek is an engineer and the principle investigator for the Building Science Consortium, a partner in the Department of Energy's Building America program.

Build your cold-climate attic with no vents--the shingles may not last quite as long, but you'll get big payoffs in performance and energy savings.

Vapor, Not Vents

Although there clearly are potential benefits from attic vents in heating climates, there are also disadvantages: Vents can be prone to snow and rain entry that can wet the insulation, and cold air blowing through eave vents can degrade the thermal performance of attic insulation.... In heating climates, attic ventilation usually provides a measure of protection from excessive moisture accumulation in the roof sheathing, but if indoor humidity is high and humid indoor air leaks into the attic, the use of attic vents does not guarantee that attic moisture problems will not develop. Therefore, moisture control in attics in heating climates depends primarily on maintaining low indoor humidity levels during cold weather and on ensuring sufficient airtightness and vapor resistance (i.e. a vapor retarder) in the ceiling.

--1997 ASHRAE Handbook, Fundamentals, 23.6

As Home Energy readers know, venting attics in hot, humid climates brings a great deal of moisture into the structure (see "Conditioned Attics Save Energy in Hot Climates," HE May/June '97, p. 6). Not venting the attic avoids this problem.

What is less well understood is that venting causes many problems in cold (dry) climates, as well. For example, it allows a great deal of snow to blow in--especially the really fine snowflakes that weigh less than raindrops. Not venting also avoids this problem. Finally, as most builders know, venting roof assemblies can be extremely difficult for roof designs with complex geometries. Not venting avoids these difficulties, too.

Overcoming the Objections

I can hear the objections: What about moisture? What about sheathing temperature and shingle temperature in the summertime? What about the energy costs? What about the code?

First, take moisture: People usually vent attics in cold climates to prevent moisture accumulation in the roof sheathing and control ice dams. In cold climates, moisture in roof assemblies typically comes from inside, and the key to problems with moisture is the temperature of the roof sheathing.

Unvented attics have higher temperatures on the underside of the roof sheathing. If this area--typically the first condensing surface--is kept above the dew point temperature of the interior air-vapor mix, condensation and moisture accumulation will not occur (see Figures 1 and 2).

Ice damming can be controlled by reducing heat flow to the shingles through air sealing and insulating to more than R-40, rather than by flushing heat away from the roof shingles with venting. The net effect is the same--the roof shingles are cold--but by eliminating venting, we save a great deal of energy.

Warming Up to Unvented Roofs

The underside of the roof sheathing is where the real benefits of not venting roof assemblies are found. Our field measurements and computer modeling show that, without attic venting, the temperature of the underside of the roof sheathing increases by 10°F-20°F.

In cold climates, this is an advantage. Unventing roof assemblies in most cold climates decreases the heating load by about 10%. That answers the energy question: Unventing attics in cold climates saves energy.

What about shingle temperature? Well, the answer to that question is, Don't use asphalt shingles. They have many disadvantages anyway. They burn. They are sensitive to ultraviolet light. They can't be made to last more than 15 to 20 years--despite what the warranty says. Hail just kills them, and they off-gas horrible stuff. But they are cheap. And in cold climates, they are the roof covering of choice.

When attics with asphalt-shingled roofs are left unvented, the operating temperature of the shingles increases slightly--on the order of 2%-3% of absolute temperature. This means that a black asphalt shingle roof that is typically at 150°F will be at 153°F-155°F. That 3°F-5°F increase can be important, since it translates into an approximate 15% reduction in the useful service life of the shingle. On a 15-year shingle roof, that means you may lose 2 to 3 years in service life.

Why is there only a 3°F-5°F increase in asphalt shingle temperature? Because radiation is the dominant factor in heat transfer through roof assemblies, and venting the roof does not affect the radiation heat transfer. Also, the underside of the roof sheathing is not an efficient plywood-to-air heat exchanger, so venting is of little importance in reducing shingle or sheathing temperature.

Code Catch-Up

I have about 1,000 unvented shingled roofs under my belt. Most of them are in Canada--yeah, I know, the laws of physics are different up there--but a lot of them are in New England, Michigan, and Colorado. More than a third of them are now over ten years old, and they are doing fine.

The biggest problem with building these unvented attics has been building codes. The codes do not like unvented roof assemblies. But changes are coming. First it was the 1997 edition of ASHRAE Fundamentals--it likes unvented roof assemblies (see "Vapor, Not Vents"). Then we (the Building America guys and gals) changed the building code in Las Vegas. We have more than 300 unvented roof assemblies constructed there so far.

I predict that, in five years, the codes everywhere will have changed.

Figure 1. Potential for condensation in a roof assembly in Chicago, Illinois. The roof assembly has R-30 fiberglass batt insulation and a vented attic space. By reducing interior moisture levels, the potential condensation is reduced or eliminated.

Figure 2. Potential for condensation in a roof assembly in Chicago, Illinois. The unvented cathedral ceiling has R-12 rigid insulation above R-28 batt insulation. The R-12 insulating sheathing raises the dew point temperature at the first condensing surface so that no condensation will occur with interior conditions of 35 % relative humidity at 70°F.

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