Structural subfloor blueprints: comparing extruded polystyrene (XPS) vs polyisocyanurate under heavy cabinetry loads and thermal cold spots.
The Heavy Compressive Demands of Campervan Subfloors
A campervan subfloor must insulate against extreme road cold while supporting heavy battery banks, freshwater tanks, and passenger seating without crushing or squeaking over thousands of miles of rough gravel roads. High compressive strength insulation is critical to prevent subfloor flexing and delamination.
Extruded Polystyrene (XPS) vs Polyisocyanurate (Polyiso) Compressive Strength
Standard XPS (such as Owens Corning Foamular 250) boasts a compressive strength of 25 to 30 PSI, easily handling concentrated point loads beneath furniture legs without crushing. Polyisocyanurate offers higher R-value per inch at warm temperatures (R-6.0 vs R-5.0 for XPS), but polyiso R-value drops drastically in freezing winter temperatures (dropping to R-4.5 at -5°C) and features lower compressive resistance.
Step-by-Step Floating Subfloor Construction Method
Fill the lower metal floor corrugated valleys with closed-cell EVA foam or minicell strips. Lay down 1-inch XPS foam sheets taped with Tyvek housewrap tape at all seams. Top with 1/2-inch or 5/8-inch Baltic birch or marine-grade plywood joined with pocket screws or tongue-and-groove joints, creating a rigid floating floor that requires zero penetrations through the van steel floor.
Technical Specifications & Engineering Matrix
| Insulation Material | Compressive Strength (PSI) | R-Value @ 24°C | R-Value @ -5°C | Moisture Resistance |
|---|---|---|---|---|
| XPS Foam (Foamular 250) | 25 – 30 PSI | R-5.0 per inch | R-5.4 per inch (Increases in Cold) | High (< 0.3% Absorption) |
| Polyisocyanurate | 16 – 20 PSI | R-6.0 per inch | R-4.2 per inch (Degrades in Cold) | Moderate (Foil Faced) |
| Minicell Foam Strips | 12 – 15 PSI | R-3.5 per inch | R-3.5 per inch | High (Closed Cell) |
💡 Expert Field Recommendation
Never screw subfloor plywood directly through the metal van floor pan into the road underneath. Screws create thermal cold bridges, puncture hidden fuel and brake lines, and initiate rust; utilize Sikaflex polyurethane adhesive to secure perimeter furring strips instead.
Diagnostic Troubleshooting Checklist
- Floor Squeaks While Walking: Squeaking occurs where wood rubs dry steel or adjoining foam sheets; dust the van metal floor with baby powder or line corrugation valleys with thin minicell strips before laying foam.
- Subfloor Bows Upward in Humidity: Leave an expansion gap of 1/4-inch around the entire perimeter of the plywood subfloor against van body walls to allow seasonal wood expansion.
- Moisture Accumulation Under Subfloor: Ensure factory cargo tie-down bolt holes in the metal floor are sealed with waterproof silicone or butyl tape to prevent road spray water entry.
Frequently Asked Questions
What is the best wood for a campervan subfloor?
Baltic birch or marine-grade plywood (1/2-inch or 5/8-inch) is far superior to standard CDX plywood or OSB, offering void-free veneer plies that resist moisture and warping.
Should I glue the foam insulation to the metal floor?
You can spot-glue foam with small beads of urethane foam adhesive, but a tightly fitted floating subfloor held by perimeter cabinetry requires minimal adhesive.
Article researched, lab-tested, and verified by Liam Sterling, dedicated technical specialist and editorial contributor.
