Vented or Unvented Residential Attics?

[rowe]

I recently read an on-line report (research from HUD) that current research indicates that residential structures may perform better when the attic spaces are UNvented in climates where ambient humidity is high (say somewhere similar to Charlotte, NC or Atlanta, GA)

The unvented attic space requires that no moisture barrier be installed in the ceiling joists. The reasoning is that, by preventing the outside humid air from entering the attic, moisture problems are prevented. And, for the air in the attic, when the moisture content is higher than the interior of the residence, the moisture can migrate thru the ceiling and is handled by the hvac system.

Because of the absence of ventilation, the temperature in the attic is reported to be 10 to 15 degrees higher than vented attics. There report stated that there is no appreciable degradation of asphalt shingles, and that if the ceiling is adequately insulated (w/o moisture barrier) then heating/cooling costs differences are not significant.

I have inspected many attic spaces with inadequate ventilation which is different from an UNvented attic. Has anyone reviewed similar research or does anyone have experience with unvented attics in this type of climate (where provisions are made for transfer of moisture thru the ceiling)?

 

[JAE]

I investigated a series of attic spaces in a nursing home in Kansas a few years ago - the attic was unvented, insulation was in between the top chords of the roof trusses.

With cooler temperatures, there was a large accumulation of moisture between the roof sheathing and the insulation - thus causing long term degradation of the sheathing - from the outside you could see lots of warping and sagging.