In ultra-low energy and Passive House steel frame construction, the ground interface represents one of the most critical thermal and structural boundaries. In uninsulated buildings, 15% to 20% of envelope thermal losses occur through foundation perimeters. SerVilla light gauge steel structures interface seamlessly with frost-protected shallow foundations (FPSF / Schwedenplatte] to eliminate all ground thermal bridging.
1. Thermal Bridge-Free Ground Slabs (Schwedenplatte]
The insulated raft foundation encapsulates the structural concrete slab within a continuous tub of load-bearing extruded polystyrene (XPS].
- Continuous Thermal Envelope: The concrete slab never contacts subsoil, achieving a floor U-value of U ≤ 0.12 W/m²K.
- Linear Thermal Bridging: Perimeter foundation-to-wall junction values maintain Ψ ≤ 0.010 W/mK, precluding internal corner condensation and cold floor perimeters.
- Activated Thermal Mass: Low-temperature underfloor radiant PEX loops are cast directly into the structural slab, turning 150–200 mm of concrete into a thermal flywheel.
2. Load-Bearing Under-Slab XPS & Compressive Resistance
Insulation installed beneath structural slabs must withstand permanent dead loads and dynamic seismic combinations without creeping.
- Compressive Strength: Specified XPS grades range from 300 kPa to 500 kPa at 10% nominal strain (CS(10/Y]300-500].
- Long-Term Water Absorption: Immersion and diffusion absorption limits remain <0.7\% by volume, ensuring thermal conductivity (λ = 0.034 W/mK] remains constant across 50+ freeze-thaw cycles.
3. CFS Frame-to-Foundation Interface & Anchor Engineering
Light gauge steel bottom tracks transfer building wind shear and overturning uplift forces into the slab.
- Heavy-Duty Anchors: Sized strictly per Eurocode 8 seismic parameters using ETA-approved M12 to M16 grade 8.8 galvanized drop-in or vinyl-ester chemical anchors.
- EPDM Capillary & Acoustic Isolator: A continuous closed-cell EPDM gasket is sandwiched beneath the steel bottom runner, isolating moisture and providing an acoustic decoupling barrier.
- Non-Shrink Structural Grout: Minor slab deviations are packed with high-early-strength non-shrink cementitious grout, establishing 100% bearing contact.
4. Ground Moisture, Capillary Breaks & Radon Mitigation
- Radon Gas Barrier: A monolithic 1.2 mm polyolefin radon-tight membrane with hot-air welded laps is installed beneath the slab, eliminating subsoil toxic gas infiltration.
- Capillary Drainage: A 200 mm layer of washed crushed stone (16/32 mm] forms an uncompacted capillary break below perimeter foundation drainage lines.
5. Dimensional Tolerances & Quality Handover Checklist
- Elevation Tolerance: Finished slab levelness must not deviate by more than ± 5 mm across any 10-meter span (DIN 18202 Class 3].
- Squareness / Diagonals: Overall building corner diagonals must align within ± 10 mm.
- Concrete Cure Maturity: Steel panel anchorage proceeds only after concrete reaches minimum 14-day design compressive strength (min C25/30].
