Many Passive House projects in North America spec mineral wool for the exterior continuous insulation layer. The reason is building science: passive house walls need to be air-tight, thermally continuous, vapour-permeable to the exterior, and dry from any incidental moisture — and mineral wool meets all four. This page covers why mineral wool suits passive house, which products to spec, and typical assemblies.
The four properties passive house walls require
A certified passive house wall assembly must hit specific performance targets:
- Air-tightness: the building must meet the PHI or PHIUS airtightness limit, and the seal must last.
- Thermal continuity: no thermal bridges that significantly compromise the effective R-value. Studs in a wall create thermal bridges — continuous exterior insulation breaks that bridge.
- Vapour management: moisture must be able to dry to one side. North American passive house walls almost universally dry to the exterior, which means the exterior insulation must be vapour-permeable.
- Drying potential under fault conditions: if a leak develops, the assembly should dry without trapping moisture against the sheathing. Wet OSB or plywood leads to rot.
Mineral wool hits all four. Foam boards are generally less vapour-open, so they limit drying to the exterior.
Why PowerWool RigiBoard 80 for passive house
PowerWool RigiBoard 80 is a rigid mineral wool board for exterior continuous insulation. Key specs, from the PowerWool data sheet:
| Property | Value | Why it matters for PH |
|---|---|---|
| R-value | R-4.2 per inch (ASTM C518) | Rigid mineral wool for continuous insulation |
| Density | 8 lb/ft³ | Stiff enough for furring strips + cladding without crushing |
| Compressive strength | 616 psf @ 10% (ASTM C165) | Stiff enough to hold cladding flat |
| Vapour permeance | 35.6 perm at 1.5" (ASTM E96) | Vapour-open, dries to exterior |
| ASTM E136 | Non-combustible | Fire safety at the cladding interface |
| Dimensional stability | Pass (ASTM C356) | Doesn't shrink/expand with temperature |
PowerWool RigiBoard 80 is a rigid mineral wool board suitable for passive house wall assemblies. Compare it against any other product on each manufacturer's current data sheet.
Typical passive house wall assemblies using mineral wool
Assembly 1: Standard double-stud wall (most common)
Outside ←→ Inside
1. Cladding (any type — siding, panel, board)
2. Vented rainscreen cavity (10mm+ furring strips)
3. Weather-resistive barrier (vapour-open — Solitex, Mento, or housewrap)
4. CI layer: 4–6" PowerWool RigiBoard 80, 6" in two layers (about R-17 to R-25 nominal)
5. Sheathing (½" OSB or plywood) — primary air barrier
6. SIGA Fentrim or Wigluv tape at all sheathing joints + corners
7. 2×4 outer stud cavity + R-15 Comfortbatt
8. Insulation gap or service cavity (1–2")
9. 2×4 inner stud cavity + R-15 Comfortbatt
10. Smart vapour retarder (Intello or MemBrain)
11. Service cavity / drywall
Assembly 2: Larsen truss wall (deep CI, single stud)
Where double-stud isn't an option (e.g. retrofits, design constraints):
1. Cladding
2. Rainscreen
3. WRB
4. CI: 8–10" PowerWool RigiBoard 80 in two layers (about R-34 to R-42 nominal) ← Bulk of the R lives here
5. Sheathing
6. Air sealing tape
7. 2×6 stud cavity + R-23 Comfortbatt
8. Smart vapour retarder
9. Drywall
Assembly 3: Mass timber + exterior insulation
For CLT (cross-laminated timber) or NLT (nail-laminated timber) projects:
1. Cladding
2. Rainscreen
3. WRB (vapour-open)
4. CI: 6" PowerWool RigiBoard 80 in two layers (about R-25 nominal)
5. Air sealing layer (membrane + tape)
6. CLT/NLT panel (acts as structural + sheathing)
7. Service layer (furring strips + drywall) on interior
Nominal values are thickness × R-4.2. The PowerWool data sheet notes thermal values may decrease up to 1% per inch of thickness. Your energy model sets the final wall R-value.
Available sizes: PowerWool RigiBoard 80 and RigiBoard PRO MAX
All boards are 2' × 4', sold by the bag. Both products come in these seven thicknesses online:
| Thickness | Boards per bag | Coverage per bag |
|---|---|---|
| 1" | 8 | 64 sq ft |
| 1.5" | 6 | 48 sq ft |
| 2" | 5 | 40 sq ft |
| 2.5" | 4 | 32 sq ft |
| 3" | 3 | 24 sq ft |
| 4" | 2 | 16 sq ft |
| 5" | 2 | 16 sq ft |
RigiBoard 80 is now also made in thicker boards, up to 8". For thicknesses over 5", email sales@warehoos.com or call 1-877-383-2671.
Why mineral wool over rigid foam for PH
Three reasons that come up repeatedly in PH project documentation:
- R-value stability across temperature. Polyiso (the most common rigid foam) loses part of its rated R-value below freezing — which is the temperature range passive house walls are designed for. Mineral wool's R-value rises slightly at lower temperatures.
- Vapour-open drying path. Passive house walls must dry to one side. Foam blocks vapour drive, forcing the wall into a vapour-closed assembly that's brittle to construction defects. Mineral wool lets moisture drift through and out.
- Fire performance. PH walls have continuous insulation right up against the cladding. Mineral wool is non-combustible per ASTM E136.
Sourcing mineral wool for passive house projects in North America
Warehoos stocks PowerWool RigiBoard 80; we confirm current stock and lead time when you order.
Quantity for a typical project: a 2,500 sq ft passive house with ~2,200 sq ft of above-grade wall area needs ~2,200 sq ft of CI per layer. For 6" of CI in two 3" layers (24 sq ft per bag), that's about 184 bags. For 8" in two 4" layers (16 sq ft per bag), about 275 bags. Add a waste allowance.
Cross-border shipping to USA: Warehoos ships to all 50 states, and transit time is quoted with your order. USMCA brokerage handled at order time.
Frequently asked questions
Q: Why is mineral wool the default for passive house exterior walls?
A: Four reasons: vapour-open (lets walls dry), non-combustible (fire safety at the cladding interface), dimensionally stable in cold (no R-value drop below freezing), and water-shedding (drains incidental moisture).
Q: Does PowerWool RigiBoard 80 meet PHIUS and PHI certification requirements?
A: Passive house certification is for the building. RigiBoard 80's ASTM-tested values (R-4.2 per inch, ASTM C518) go into your project's energy model.
Q: What thickness of mineral wool do I need for a passive house in climate zone 5 or 6?
A: The exact thickness comes from your energy model and PHIUS climate target, and depends on the assembly.
Q: Can I use Rockwool Comfortbatt as CI?
A: No. Comfortbatt is a cavity insulation batt. For exterior CI on passive house walls you need a rigid mineral wool board: PowerWool RigiBoard 80, Comfortboard 80, or Comfortboard 110.
Q: How does mineral wool affect overall wall U-value calculations?
A: Effective U-value of a mineral wool CI assembly accounts for fastener thermal bridges and any furring strips through the insulation. Most PHIUS energy models accept rigid mineral wool with standard thermal bridging factors — your modeler handles it.
Q: Is PowerWool RigiBoard 80 certified for any specific assemblies?
A: PowerWool RigiBoard 80 is ASTM E84 (flame spread 0, smoke developed 0) and ASTM E136 non-combustible. Its data sheet lists it as suitable for substitution in approved ASTM E119 1- and 2-hour assemblies and NFPA 285 wall assemblies. Project-specific assemblies are designed by the project's envelope engineer, not by the product manufacturer.
Q: What air-sealing tape works with mineral wool CI assemblies?
A: SIGA Fentrim 430 Grey (4", 6", 9" widths) for sheathing joints and corners. SIGA Wigluv for over-the-CI flashing and details. Both are stocked at Warehoos. Both bond to OSB, plywood, concrete, and most weather-resistive barriers.
Q: Do I need a thermal break with mineral wool CI?
A: Not at the insulation layer itself — mineral wool IS the thermal break. Z-girts or furring strips through the mineral wool create minor bridges; project engineers account for these. If using metal stud framing or steel cladding clips, additional thermal break detailing applies.
Q: How do I get PowerWool RigiBoard 80 for a passive house project?
A: Email sales@warehoos.com or call 1-877-383-2671. Provide project location, quantity, thickness, and delivery window. We reply with pricing, stock confirmation, and shipping ETA. USMCA brokerage handled for US-bound shipments.
Specifying mineral wool for a Passive House project?
- Shop PowerWool RigiBoard 80 →
- Shop Rockwool Comfortboard 80 →
- See Continuous insulation for climate zone 6 →
- Project-quote line: sales@warehoos.com or 1-877-383-2671
Disclaimer: The information on this page is taken from each manufacturer's published data and/or open source information online, and is provided for general guidance only. Specifications can change and can vary by product, thickness and market. Always confirm performance values, code compliance and suitability for your assembly against the manufacturer's current data sheet before specifying or ordering. All trademarks are the property of their respective owners.