HVAC for a Slab-on-Grade House
Keeping HVAC systems within the conditioned enclosure can be tough without a basement; one option is to use plenum trusses for the roof, which offer a space for equipment and ductwork.
I’m planning a new one-story, 1400-sq.-ft. slab-on-grade house with a vented roof and loose-fill insulation on top of the drywall ceiling. A high water table prevents building on a crawlspace or basement. The design includes a main section and a guest suite under the same roof separated by a large covered porch serving both spaces. The attic is unconditioned. I know it’s bad practice to put ductwork and air handlers in an unconditioned space, but where do you put them when you don’t have a basement or crawlspace?
— Emel; submitted via the GBA forum
Engineer and HVAC Expert Allison Bailes Replies
As you suggest, the challenge with a slab-on-grade house is keeping the equipment and ductwork out of the unconditioned attic and inside the conditioned enclosure where they will operate most efficiently. Some contractors might suggest heating and cooling exclusively with ductless minisplits, but I’d stay far away from this option.
Bedrooms and other smaller spaces built to modern standards in all but the most extreme climates need only a few hundred Btus per hour of heating or cooling. The smallest ductless units are 6000 Btu and turn down to a minimum of about 1500 Btu per hour.
If you install this type of equipment and it’s oversized, the rooms may feel uncomfortably warm or cold because of frequent on/off cycling and clammy due to insufficient dehumidification. One way to create a conditioned space for ductwork without a crawlspace is to build a framed floor above the slab, creating a con-ditioned “mini-basement” for a conventional HVAC system that runs the length of the house. This approach would make future upgrades and wiring easier than building on the slab alone, but it is an expensive option, requiring both a framed floor and a slab.
IBACOS, a Building America partner based in Pittsburgh, has done a lot of work with plenum trusses—modified trusses designed with a space to keep ductwork and mechanical systems within the building enclosure. The key to making plenum trusses work with a vented-roof assembly is a sturdy air barrier around the plenum and loose-fill insulation on the top and sides.
In climate zones 4 through 8 (the colder climate zones), ducts that reach beyond the plenum area should be insulated to R-8 and buried under 31/2 in. of loose-fill. Warm, humid climates need more duct insulation to resist condensation. Since 2018, the IRC has required that supply ducts outside the plenum in climate zones 0A through 3A (the warmer, humid zones) be insulated to R-13 and buried under 31/2 in. of loose-fill.
Unfortunately, no manufacturer currently produces R-13 duct insulation, so I’d suggest covering the ducts with 3 in. of closed-cell foam. Keeping the ducts within the conditioned space using interior walls and air-sealed chases is another option. In your case, I’d suggest a ducted minisplit installed within plenum trusses in the hallway ceiling for the bedrooms.
With this arrangement, the hallway ceiling can be the same height as the other spaces. (You could also lower the hallway ceiling, but many homeowners find it feels too low.) I’d recommend using ductless minisplits in the main living space and in the guest suite. Ductless units can be sized to match these loads and are easier and less expensive than ducted units to install.
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