To Vent or Not To Vent…
This discussion centers around buildings without a ceiling (exposed rafters) and insulation following the roof-line.
In the south attics are vented to flush heat. However, heat transfer in an attic comes mainly from radiation. Venting an attic doesn’t flush radiation effectively at all (no ceiling – insulation follows roof-line).
In a perfect world a properly built and vented attic provides a 3% to 6% air change rate. This means an effective 2 to 3 percent reduction in heat transfer to the conditioned area when compared to an un-vented attic with similar insulation and construction. Not so much.
Planning to reduce radiant heat by construction materials chosen is far more effective than venting the building. Moisture control becomes an issue in most areas. It is humid outside.
Venting crawl spaces can bring in moisture and potential mold issues. The ground surface is typically colder than the dew point temperature of the exterior air. The underside of crawlspace floor insulation is radiation coupled to the ground surface and is close to the same temperature of the ground. Moisture forms on the top surface of the poly and hangs to the bottom surface of the crawlspace floor. So vent crawlspaces when there is no insulation, no air conditioned space, and no crawlspace walls.
Unvented roof systems can be used in cold climates by insulating exterior to the roof sheathing to prevent condensation underneath the sheathing.