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Passive House Exterior Wall Mineral Wool

Passive House Exterior Wall Mineral Wool — Why It's the Default for High-Performance Envelopes

Passive House projects in North America consistently spec mineral wool for the exterior continuous insulation layer. The reason is building science: passive house walls need to be air-tight, thermally continuous, vapor-permeable to the exterior, and dry from any incidental moisture — and mineral wool is the only major exterior insulation product that satisfies all four constraints without compromise. This page covers why mineral wool wins for passive house, which products to spec, and the typical assemblies in use across PHIUS and PHI-certified projects in 2026.

The four properties passive house walls require

A certified passive house wall assembly must hit specific performance targets:

  1. Air-tightness: ≤0.6 ACH50 for PHI, ≤0.05 CFM50/ft² for PHIUS. The wall must be sealed and the seal must last 60+ years.
  2. Thermal continuity: No thermal bridges that significantly compromise the effective R-value. Studs in a wall create 15-25% thermal bridging penalty — continuous exterior insulation breaks that bridge.
  3. Vapor 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 vapor-permeable.
  4. Drying potential under fault conditions: If a leak develops, the assembly should dry without trapping moisture against the sheathing. Soggy OSB or plywood = 5-10 year rot trajectory.

Mineral wool hits all four. Polyiso fails on vapor management (vapor barrier on both sides). XPS fails on drying potential (drains poorly, holds water against sheathing). EPS partially passes but degrades in R-value and is more flammable.

Why PowerWool RigiBoard for passive house

PowerWool RigiBoard is the Warehoos-exclusive rigid mineral wool board. Key specs:

Property Value Why it matters for PH
R-value R-4.2 per inch Comparable to Rockwool, packs serious R into typical 2"–4" CI thicknesses
Density ~8 lb/ft³ Stiff enough for furring strips + cladding without crushing
Compressive strength ~290 psf @ 10% 40% more rigid than Comfortboard 80 — better cladding flatness
Vapor permeance 30+ perms (10cm) Vapor-open, dries to exterior
ASTM E136 Non-combustible Passes Class A fire, no thermal barrier required
Water absorption (ASTM C1763) <1% by volume Drains, doesn't hold water
Dimensional stability <1% in any direction Doesn't shrink/expand with temp
Recyclability 100% recyclable Aligns with PH sustainability ethos
Lead time In stock at Warehoos Critical — many PH projects delayed by Rockwool 8–12 week lead times

PowerWool RigiBoard is functionally equivalent to Rockwool Comfortboard 80 for passive house performance, with a slight edge in rigidity, predictable availability, and typically lower cost per square foot.

Typical passive house wall assemblies using mineral wool

Assembly 1: Standard double-stud wall (most common)

The "PHIUS Standard" double-stud wall as used in dozens of certified projects:

Outside ←→ Inside

1. Cladding (any type — siding, panel, board)
2. Vented rainscreen cavity (10mm+ — Keene Easy-Fur or Cor-A-Vent)
3. Weather-resistive barrier (vapor-open — Solitex, Mento, or housewrap)
4. CI layer: 4–6" rigid mineral wool (PowerWool RigiBoard or Comfortboard)
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 vapor retarder (Intello or MemBrain)
11. Service cavity / drywall

Total wall R-value: R-45 to R-55 (typical)
Effective whole-wall R: ~R-40 to R-50 (after thermal bridge penalty)

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 (R-32 to R-42)  ← Bulk of the R lives here
5. Sheathing
6. Air sealing tape
7. 2×6 stud cavity + R-23 Comfortbatt
8. Smart vapor retarder
9. Drywall

Total wall R-value: R-55 to R-65

Assembly 3: Mass timber + exterior insulation

For PHIUS CLT (cross-laminated timber) or NLT (nail-laminated timber) projects:

1. Cladding
2. Rainscreen
3. WRB (vapor-open)
4. CI: 6" PowerWool RigiBoard (R-25)
5. Air sealing layer (membrane + tape)
6. CLT/NLT panel (acts as structural + sheathing)
7. Service layer (firring strips + dry-wall) on interior

Total wall R-value: R-25 from CI + minor R from CLT thermal mass

Why mineral wool over rigid foam for PH

Three reasons that come up repeatedly in PH project documentation:

  1. R-value stability across temperature. Polyiso (the most common rigid foam) loses 25-30% 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.

  2. Vapor-open drying path. Passive house walls must dry to one side. Foam blocks vapor drive, forcing the wall into a vapor-closed assembly that's brittle to construction defects. Mineral wool lets moisture drift through and out.

  3. Fire performance without compromise. PH walls have continuous insulation right up against the cladding. Foam in this position is a fire propagation risk (PCM events: Grenfell Tower, others). Mineral wool is non-combustible per ASTM E136 and changes the fire dynamics entirely.

Sourcing mineral wool for passive house projects in North America

Two practical realities of PH-spec mineral wool sourcing:

Availability is the #1 supply-chain issue. Rockwool Comfortboard has had recurring lead-time problems (8–12 weeks not unusual). Project delays cost money. Warehoos stocks PowerWool RigiBoard with consistent in-stock availability — designed to be the reliable alternative when Rockwool isn't available.

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 a 6" CI assembly using PowerWool (16 sq ft per bag), that's ~140 bags. For a 2-layer 8" assembly, ~280 bags.

Cross-border shipping to USA: Warehoos ships to all 50 states from branches across USA and Canada in 2–3 business days. USMCA brokerage handled at order time.

Frequently asked questions

Q: Why is mineral wool the default for passive house exterior walls?

A: Four reasons: vapor-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). No other major exterior CI product satisfies all four.

Q: Does PowerWool RigiBoard meet PHIUS and PHI certification requirements?

A: Yes. PowerWool RigiBoard's R-4.2/inch and ASTM-verified properties meet both PHIUS and PHI material requirements. The project's overall wall R-value (cavity + CI + assembly effects) is what gets certified, not the individual product.

Q: What thickness of mineral wool do I need for a passive house in climate zone 5 or 6?

A: Most PHIUS projects in zone 5/6 use R-30 to R-45 total wall, with R-15 to R-30 of that as exterior CI. That's 4–7 inches of mineral wool depending on assembly and PHIUS climate target. The exact number comes from your energy model.

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 rigid mineral wool board: PowerWool RigiBoard, 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 the fastener thermal bridges (~3-5% R-value penalty) 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 certified for any specific assemblies?

A: PowerWool RigiBoard is ASTM E84 (FSI 0, SDI 0), ASTM E136 non-combustible, ASTM C1763 water absorption verified. Project-specific assemblies are designed by the project's envelope engineer, not by the product manufacturer.

Q: How does PowerWool compare to Rockwool Comfortboard for passive house?

A: Functionally equivalent for PH performance. PowerWool's edges: 40% more rigid (better cladding flatness), R-4.2 vs R-4.0 per inch, consistently in-stock vs Rockwool's recurring lead times, and typically lower per-board-foot price. Full comparison: PowerWool RigiBoard vs Rockwool Comfortboard →.

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 ship from Warehoos with no lead time. 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 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 respond within 1 business day with pricing, stock confirmation, and shipping ETA. USMCA brokerage handled for US-bound shipments.

Specifying mineral wool for a Passive House project?