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
Section titled “Slab Types”Slab-on-Grade
Section titled “Slab-on-Grade”Concrete slab directly on the ground:
- Heat loss primarily at perimeter
- Edge insulation is critical
- Moderate interior heat transfer
Basement
Section titled “Basement”Below-grade spaces with walls and floor:
- Floor heat transfer to ground
- Wall heat transfer to ground
- Ground temperature varies with depth
Crawlspace
Section titled “Crawlspace”Space between ground and floor:
- Ventilated or unventilated
- Floor is suspended above grade
- Different thermal behavior than slab
Slab Assignment
Section titled “Slab Assignment”Automatic Detection
Section titled “Automatic Detection”HVAKR assigns slab to ground-floor spaces. Verify for:
- Buildings with multiple ground levels
- Stepped construction
- Elevated sections
Manual Assignment
Section titled “Manual Assignment”To change slab assignments:
- Select a space
- Go to the Envelope panel
- Toggle slab on/off
- Select the slab type
Slab Properties
Section titled “Slab Properties”F-Factor
Section titled “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
Section titled “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
Section titled “Perimeter Length”The length of the slab edge exposed to outdoor conditions. Calculated from space geometry.
Heat Transfer Calculation
Section titled “Heat Transfer Calculation”Perimeter Method
Section titled “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
Section titled “Ground Temperature”Ground temperature varies with:
- Depth below surface
- Climate zone
- Season
Deep ground temperature is relatively constant year-round.
Below-Grade Walls
Section titled “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
Section titled “Best Practices”- Include perimeter insulation - Critical for heating-dominated climates
- Verify perimeter length - Check against actual building perimeter
- Distinguish floor types - Use appropriate type for each condition
- Consider ground coupling - Basements may have cooling benefit in summer
- Check crawlspace ventilation - Affects heat transfer significantly