Many will tell you otherwise but the purpose of the air space is to allow porous insulation to dry if it gets wet. This is not considered to be a serious issue in walls. DuPont does offer a Tyvek air barrier system for roofs but the continuous drywall and lack of penetrations of most interior ceilings creates a pretty good air barrier and a vapor retarder would increase its effectiveness.

IMHO the source of the idea that air flows freely through fiberglass batt insulation is the manufacturers of other kinds of insulation especially cellulose. If you're concerned about that issue you can use spray-in dense-pack cellulose over vent baffles or the Tyvek air barrier system.

It's a geographical issue concerning venting with pourous insulations along with ridge/soffit venting. You need to research what is succesful in your area with the insulation, venting/non-venting method you choose. Improper venting in some areas will allow moisture to reach the insulation and w/o that air space, not allow for drying as Mighty stated.

Thanks.

Mightyanvil, I agree with your opinion re airflow and other insulation claims. Airflow will act like electricity and water and take the path of least resistance. It also makes sense about moisture being a bigger issue in ceilings than walls as air/moisture rises and is trapped against ceilings.

Sierraeast, my building package include plastic as a vapor barrier for walls and ceilings. My builder was adamantly against plastic on the ceilings because of the moisture issue. Or as he called it, your own interior rain forest. However, my HVAC system includes a Fantech whole house ventilation unit and should negate any interior condensation issues. This will be my first winter completely buttoned up but I'm curious to see what my interior humidity will be when we hit our normal stretch of lows at -20 to -30 F.

Your responses have assured me I am not missing anything obvious...T

the purpose of the air space is to allow porous insulation to dry if it gets wet

As well, in cold climates it is supposed to keep the roof cold and prevent ice damming; in hot climates, a vented attic reduces the heat on the roof surface and lessens the load on cooling equipment.

Fibrous insulation subject to a convective current, as in an attic, has a reduced effectiveness vis-a-vis solid insulation materials.

"Convective heat loss in insulation is
rare, but it can occur when large temperature
differences above and below insulation create
tiny air currents (called "convection loops")
within the insulation. Studies have shown that
convective heat loss can occur with lighter
density loose-fill fiberglass at the very low attic
temperatures possible in extremely cold climates.
Depending on the attic temperature, the
insulation's measured R-value could decrease
by as much as 50%" (See link below.)

I have been in vented attics in winter watching the swirling snowflakes coming in from all those mandated vents. If the whole thing were sealed in the first place, the snow wouldn't be coming in. If prices were the same, I'd go for a sealed attic next time. I don't know if I have the energy to explain to the trades how to do it. Old dogs do not like learning new tricks.

Here is a link that might be useful: Energy Guide

Venting of a roof (with insulation in the rafter bays or an attic floor) does reduce the temperature of the roof surface but not to a degree that it is important.

Carl Cash, in my opinion the best authority on asphalt roofing, conducted an experiment with vented and unvented asphalt shingled roofs in many different climates and exposures and found that venting lowered the temperature of the roof surface by .79 to 1.1 degrees F. The difference in temperature between roofs facing south vs west and those of light color vs dark was between 3 and 4 times those amounts.

The myth that venting reduces the temperature of asphalt shingles enough to affect their longevity has long been perpetuated by roofing manufacturers and is based on no testing or other evidence.

Here is a link that might be useful: Roofing Failures

The advantages of a non-vented attic vary depending on the climate, the roofing surface, and the location of HVAC equipment.

In a study done in Las Vegas by Lstiburek, Rudd and Moyer (see link) where cooling predominates and A/C equipment is located in the attic, the researchers estimated a 16% reduction in cooling energy for unvented v. vented attics.

Computer modelling also showed a peak summer-day roof temperature difference of 8ËF on black asphalt shingles between sealed and unsealed attics.

An unvented attic is also appealing because of the greater ease in adequately insulating above and around vaulted skylights and recessed ceilings, especially where the roof is low slope due to design or height limitations.

Here is a link that might be useful: Building Science Corp.: Roofing Studies

The majority of homes in the desert are by way of evaporative cooling or "swamp coolers". Venting of the attic is important with these systems as the use of up ducting allows cool air from the living space to be drawn up into the attic and circulated by way of ventilation. Cooling the attic creates a cooler envelope causing the cooling system not to have to work so hard and is more efficient.

Cooling the attic creates a cooler envelope causing the cooling system not to have to work so hard and is more efficient.

This is true when applied to a conventional attic where the thermal barrier is the ceiling.

But when the thermal and air barrier is moved to the roof, meaning a sealed unvented attic is used--the energy savings alone are indisputable in a hot climate, as the Las Vegas study above, now 12 years old, shows.

Indeed, at the minimum, hundreds of homes have been built since in Las Vegas based on these conclusions.

Here is a link that might be useful: Unvented Attics in Hot Climates

Yep and as desert cities grow and the humidity levels rise, ex.- pheonix & las vegas, evaporative cooling with vented attics become less efficient. The closed attic with a/c or heat pump units sounds like the right direction of newer homes in those areas. Thanks for the info/articles!