Slab

The slab refers to the floor construction for ground-level spaces. Heat transfer through slabs affects heating loads and ground-coupled cooling.

HVAKR does not have separate slab-on-grade, basement, or crawlspace slab types. Every slab uses the same Slab Type record; which formula HVAKR applies depends on whether that slab type has an External Temperature Override set — see Slab Properties.

Slab Assignment

You work with slabs on the Slabs tab in Building Model → Envelope.

Auto Create Envelope

The fastest way to add slabs is Auto Create Envelope (the ✨ button in the Envelope toolbar). It assigns the Slab Type you choose to unassigned spaces on the building’s lowest occupied level only — wall types apply to every exterior edge in the chosen scope instead, and roof types go to the highest occupied level. If you scope the run to a level that isn’t the lowest occupied level, it assigns no slabs. It never overrides existing assignments.

Assign slabs manually

1. On the Slabs tab, click Assign Slabs
2. Choose a Slab Type
3. Click spaces on the canvas to toggle the slab assignment
4. Click Done when finished

Edit a slab

Select a slab from the Slabs list, or click it on the canvas, to open the Slab Data panel, where you can change its Slab Type.

Slab Properties

The slab type — defined in Basis of Design → Building Data — supplies the values below. Perimeter length comes from the space geometry. The Summer and Winter External Temperature Override (advanced fields, hidden by default) switch methods independently per season: the Summer override affects only the cooling calculation, and the Winter override affects only heating. Set one, both, or neither.

F-Factor (default per season, when that season has no override)

For a season without its own override, HVAKR uses the perimeter method for slab-on-grade floors: the slab type’s F-Factor sets the perimeter heat loss (or gain) per linear foot of exposed edge, multiplied by the perimeter length and the difference between the space setpoint and the outdoor design (heating) or hourly outdoor (cooling) temperature. F-factor captures the combined effect of the slab construction and any edge or under-slab insulation — choose an F-factor that reflects the insulation detail in your design. Better-insulated edges mean a lower F-factor and less perimeter heat loss.

Overall U-Value + External Temperature Override

For a floor over an unconditioned space, such as a vented crawlspace or unconditioned basement below, set the slab type’s Summer External Temperature Override, Winter External Temperature Override, or both to that space’s assumed temperature. For each season with an override set, HVAKR uses the slab type’s Overall U-Value with the space area and the difference between the space setpoint and the override temperature, instead of the perimeter method — the other season, if it has no override, still uses F-Factor. In the Loads calculator, an override without an Overall U-Value on the same slab type produces an error; the Energy report instead falls back to F-Factor for that season (or contributes no slab load, if F-Factor is also unset), without an error.

Perimeter Length

The length of the slab edge exposed to outdoor conditions. Calculated from space geometry — not an editable field.

Best Practices

1. Include perimeter insulation in the F-Factor - Critical for heating-dominated climates
2. Verify perimeter length - Check against actual building perimeter
3. Use the External Temperature Override for floors over unconditioned space - set the Summer override, Winter override, or both, along with the Overall U-Value
4. Confirm which method applies per season - the Summer override affects only cooling and the Winter override affects only heating; a season without its own override keeps using F-Factor