Slab

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

Slab Types

Slab-on-Grade

Concrete slab directly on the ground:

• Heat loss primarily at perimeter
• Edge insulation is critical
• Moderate interior heat transfer

Basement

Below-grade spaces with walls and floor:

• Floor heat transfer to ground
• Wall heat transfer to ground
• Ground temperature varies with depth

Crawlspace

Space between ground and floor:

• Ventilated or unventilated
• Floor is suspended above grade
• Different thermal behavior than slab

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). When you choose a Slab Type, it assigns that type — along with walls and roofs — to unassigned spaces on the lowest occupied level, across All Levels or the current level. 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.

F-Factor

The slab type’s F-factor sets the perimeter heat loss per linear foot of exposed edge. It 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.

U-Value

For floors above unconditioned spaces (rather than on grade), the slab type’s U-value is used with the temperature difference to the space below.

Perimeter Length

The length of the slab edge exposed to outdoor conditions. Calculated from space geometry.

Heat Transfer Calculation

Perimeter Method

For slab-on-grade, heat loss is calculated based on:

• Perimeter length
• F-factor (heat loss per linear foot)
• Indoor-outdoor temperature difference

The F-factor depends on:

• Edge insulation R-value
• Insulation depth
• Climate zone

Ground Temperature

Ground temperature varies with:

• Depth below surface
• Climate zone
• Season

Deep ground temperature is relatively constant year-round.

Below-Grade Walls

For basement spaces, below-grade walls:

• Use ground temperature for exterior
• Heat transfer varies with depth
• Different calculation than above-grade walls

Best Practices

1. Include perimeter insulation - Critical for heating-dominated climates
2. Verify perimeter length - Check against actual building perimeter
3. Distinguish floor types - Use appropriate type for each condition
4. Consider ground coupling - Basements may have cooling benefit in summer
5. Check crawlspace ventilation - Affects heat transfer significantly