COLUMN BUILDING SCIENCES FIGURE 5 Insulated Dropped Ceiling. Insulate the top of the lay-in ceiling tiles with fiberglass or mineral wool batt insulation. Cladding 2 × 6 Metal Stud Continuous Exterior Insulation Fiberglass, Cellulose or Mineral Water Control Layer/Air Control Layer Wool Cavity Insulation Sheathing Gypsum Board Cellular PVC Protection Board Extruded Polystyrene Extruded Polystyrene Rigid Insulation Installed Over Drainage Mat Projects Horizontally (2 ft to 4 ft) R-10 Minimum Stone Paver Traffic Surface Capillary Break Floor Finish Podium Slab " Chairs " Plaza Deck Waterproofing Faced Fiberglass Or Mineral Wool Rigid Insulation Fiberglass or Mineral Wool Batt Insulation Lay-In Ceiling Tiles Suspension Grid FIGURE 6 Insulated Dropped Ceiling Bathtub. Think of the insulated dropped ceiling below the slab as a " bathtub " that you fill with fluffy stuff. FIGURE 7 More Bathtub. Won't the sides and bottom of the bathtub have to be airtight? The sides yes, the bottom no. Lateral airflow through the " sides " is controlled and upward airflow does not occur because there is not enough of a height distance between the " leaky " bottom and the airtight top. FIGURE 8 Convection is Height Dependent. Convection can happen in fibrous insulation in a wall but rarely in fibrous insulation on a ceiling or in a floor. Column Supports Podium Slab PHOTO 4 Lay-In Ceiling Tiles. Insulate the top of the lay-in ceiling tiles with fiberglass or mineral wool batt insulation. The ceiling tiles are great for acoustics and contribute to the R-value. closed-cell spray foam is an excellent air control layer. The closed-cell spray foam is also a vapor control layer and thermal control layer. The condensing surface now becomes the underside of the closed-cell spray foam rather than the underside of the concrete slab. Does the facing need to be vapor semi-permeable in Miami? No. It can be vapor semi-impermeable or vapor impermeable (a " vapor barrier " ). What about Figure 3 in a hot and humid climate? Yes, that works as well. In fact, Figure 3 works in all climates-as does Figure 4. The only stipulation with respect to Figure 4 is that once you get into IECC Climate Zone 4 and higher the facing on the fiberglass or mineral wool insulation layer should be vapor semi-permeable. Again, in " Joe speak " it should be on the " high side " of vapor semi-permeable (5 to 10 perms). Can we do better? Yes, particularly in cold climates. Floors directly above garages in cold climates can get pretty darn uncomfortable even with a bunch of insulation. Folks want to be comfortable-and have made this aggressively known to the folks who design and build buildings over garages. It is pretty common to heat the underside of the structural slab between the building and the parking garage. The most common way of heating the underside of a structural slab over a garage is to install an insulated dropped ceiling below the slab (Figure 5). Insulate the top of the lay-in ceiling tiles (Photo 4) with fiberglass or mineral wool batt insulation. Whoa, " Danger, Will Robinson, Danger! " What about airflow? Relax. The air pressure driving forces are trivial. Think of the insulated dropped ceiling below the slab as a " bathtub " you fill with fluffy stuff (Figure 6). FEBRUARY 2021 ashrae.org ASHRAE JOURNAL 33http://ashrae.org