COLUMN BUILDING SCIENCES FIGURE 1 Conditioned Unvented Attic With Vapor Diffusion Port. Conditioned unvented attics are constructed with thermal insulation at the roof deck and the attic space is thermally connected to the building. FIGURE 2 Unconditioned Unvented Attics With Vapor Diffusion Port. Unconditioned unvented attics are constructed with the thermal insulation at the ceiling line and the attic space is not thermally connected to the building. FIGURE 3 HVAC Systems in Vented Attics. Ductwork and air handlers typically leak air, which leads to significant negative pressures in buildings when they are located in vented attics, with resulting energy penalties. FIGURE 4 Conditioned Unvented Attics. Conditioned unvented attics have significant advantages over unconditioned vented attics. The building enclosure can be made significantly more airtight much easier by constructing an unvented attic, thereby making the building much more energy efficient. leading to significant negative pressures in buildings when they are located in vented attics (Figure 3). By installing insulation on the underside of the attic roof sheathing, any ductwork or mechanical systems installed in attics are now located "inside" rather than "outside," again making the building much more energy efficient (Figure 4). Unvented attics are also significantly more "fire safe" in wildfire areas and where neighboring buildings are close. Ash and embers enter typical attics through attic vents. No vents means no ash or embers entering attics, and much less risk of fire. In high wind regions-particularly in coastal areas- wind-driven rain is a problem with vented roof J U N E 2020 ashrae.org ASHRAE JOURNAL 77https://www.ashrae.org/