Insulated Raft (Passive Slab) Calculator
Calculate materials for energy-efficient shallow foundations. Estimates XPS insulation boards, concrete volume, and underfloor heating pipes.
About
The Insulated Raft (often called a Passive Slab or USHP) is the gold standard for energy-efficient construction. It eliminates thermal bridges by wrapping the entire concrete foundation in high-density XPS foam. However, estimating the materials is complex because the system consists of distinct "L-elements" (edge profiles) and flat base sheets, along with a complex network of radiant heating pipes embedded directly in the concrete.
This tool breaks down the components. It calculates the concrete volume net of insulation (saving you from over-ordering) and provides a specific breakdown of XPS boards needed for the perimeter versus the core. It also estimates PEX tubing length for the radiant floor, accounting for feeder loops and standard spacing.
Formulas
The calculator separates the edge "L-Element" volume from the main slab area.
Heating pipe length is estimated using the formula:
Reference Data
| Material | Typical Spec | Function | Thermal Conductivity (λ) |
|---|---|---|---|
| XPS Foam (Edge) | 100mm - 300mm | Prevent perimeter heat loss | 0.035 W/mK |
| EPS Foam (Base) | 200mm - 400mm | Load bearing insulation | 0.038 W/mK |
| Concrete (Core) | 100mm - 150mm | Thermal mass & structure | 2.3 W/mK |
| PEX Pipe | 16mm / 20mm | Radiant heating | N/A |