There is no single “best” Passive House insulation. The right material is the one that lets the complete wall or roof assembly meet its Passive House Institute targets for U-value, thermal bridges, airtightness, moisture safety, fire performance and buildability. In many projects, mineral wool is the safest starting point when non-combustible Class A protection is required; phenolic or PIR/PUR boards are useful where space is limited; and cellulose, wood fibre or cork can suit carefully modelled bio-based assemblies.
Use the product’s rated installed conductivity—not an optimistic catalogue value—then verify the whole junction and envelope in the project’s energy and hygrothermal models. The guide below shows how to make that choice for walls and roofs.
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What “best” means in a Passive House assembly
Passive House performance is a property of the assembly, not of an insulation board, batt or loose-fill product in isolation. The International Passive House Association explains that heat loss through an external wall, roof, floor or window is defined by the component’s U-value. Conductivity (λ) describes a material; U-value describes the finished component, including framing, membranes, fixings, air spaces, finishes and thermal bridges.
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#1 Best Overall
- 6lb/ft^3 Rockwool ProRox SL960 Mineral Wool - 3 sheets of strong, rigid stone wool insulation
- Heavy insulation is excellent at absorbing sound, is fire resistant, and insulates well. Can be used in sound recording studios as it is has excellent sound dampening qualities.
- Cuts easily with a knife. Great for sound deadening enclosures or room insulation, building fire rated light boxes, or any project that needs heavy fire rated insulation.
- Size: 3 sheets of 12" x 12" x 2"
- Similar to Owens Corning Thermafiber. Made in North America by Rockwool, a global leader in premium mineral wool.
Passive House Institute benchmarks that shape the choice
For the cited construction-system criterion, Passive House Institute certification requires:
- f·Uopaque ≤ 0.15 W/(m²K) for the opaque construction system.
- Ψexternal ≤ 0.01 W/(mK) for regular junctions.
- Internal surface temperatures above 17°C under the stated test condition, helping prevent cold-surface risks.
- Documented airtightness for the completed building.
The certification rules also require the insulation’s rated value for thermal conductivity in the installed situation and at the thicknesses used in the Passive House. The nominal λ printed on a product label is not automatically the value accepted for modelling.
Compare insulation materials by conductivity and behaviour
The Passivhaus Trust guide gives these typical conductivity ranges. They are comparison ranges, not substitutes for the declared, approved value of the specific product and installation.
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| Material | Typical λ range (W/mK) | Where it helps | Checks before specifying |
|---|---|---|---|
| Phenolic board | 0.018–0.023 | Very low conductivity where wall or roof depth is restricted | Core and facer fire classification, joint detailing, fixings, continuity of air and weather control layers |
| PIR/PUR board | 0.021–0.028 | Thin, rigid insulation for constrained cavities and roof build-ups | Combustible facers or core, thermal bridges through fasteners, joints and edge gaps, weather protection |
| Graphite EPS | 0.030–0.032 | Cost-effective rigid external insulation with lower λ than white EPS | Fire classification, UV and solvent exposure, impact protection, openings and moisture detailing |
| XPS | 0.033–0.036 | Rigid board where ground contact or high moisture exposure governs | Vapour profile, environmental requirements, compatibility and fire classification |
| White EPS | 0.036–0.038 | Widely available rigid insulation for walls and some roof systems | Fire and impact protection, UV exposure, solvent compatibility and careful perimeter detailing |
| Mineral or glass wool | 0.032–0.045 | Batts, rolls, blown fill and rigid boards; strong fire performance | Product density, wind-washing protection, compression, water exposure and installation quality |
| Cellulose | 0.038–0.040 | Loose-fill or wet-spray cavity filling with recycled-content potential | Specified density, settling control, moisture protection and fire treatment |
Wood fibre and cork are also used in Passive House work. Their λ, density, fire rating and moisture limits vary substantially by product, so use the manufacturer’s declared value and approval rather than a generic figure. Cork is generally a lower-conductivity natural option than cellulose in the cited Passive House Institute table, but it is often thicker and more regionally constrained.
Mineral and glass wool: the fire-led default
Mineral wool is usually the leading candidate when a wall or roof must achieve Construction Materials Class A fire protection. The Passive House Institute’s educational guidance specifically identifies it for that situation. It is non-combustible and vapour-open, which can simplify some wall and roof strategies, but those attributes do not remove the need for detailed design.
Rank #2
- Designed for Sound Absorption Echo Control: the dense, porous stone wool structure helps absorb sound energy and reduce unwanted reflections, echo, and reverberation; Ideal for improving room acoustics and creating a cleaner, more controlled listening and recording environment; Sound absorption performance varies by thickness, installation method, and frequency
- for DIY Acoustic Panels: build customized acoustic treatment for your space; Simply cut the stone wool board as needed, place it inside a wood or suitable panel frame, wrap it with acoustically transparent fabric, and install it on the wall or ceiling—an excellent core material for DIY broadband acoustic panels and room treatment projects
- HIGH-DENSITY 80kg/m³ STONE WOOL CORE – Made with dense 80kg/m³ (5 lb/ft³) stone wool, these rigid insulation boards provide a solid, shape-retaining core ideal for building DIY acoustic panels. The high-density fiber structure makes the boards easy to fit into frames while providing a professional foundation for acoustic treatment projects
- Ideal for Studios, Home Theaters Podcast Rooms: a versatile acoustic treatment material for recording studios, vocal booths, podcast rooms, home theaters, music rooms, rehearsal spaces, offices, gaming rooms, and more; Use it to create wall panels, ceiling clouds, and other DIY acoustic treatments where better sound control is needed
- More Than Acoustic Performance: stone wool provides a combination of sound absorption, thermal insulation, heat resistance, and dimensional stability, making these boards suitable for both acoustic and insulation projects; Easy to cut and customize for different applications; Always wear appropriate protective equipment when cutting or handling stone wool
When it is a strong fit
- Buildings where non-combustible insulation is a code, insurer or client requirement.
- Framed walls and roofs that can accept batts, rolls, blown fill or rigid mineral-wool boards.
- Assemblies needing a vapour-open layer, provided the climate-specific moisture analysis supports that arrangement.
What can go wrong
Loose or poorly cut batts create air channels. Wind washing can reduce effective performance where an exposed fibre layer is not protected. Compression lowers the installed thickness, while persistent wetting can damage performance and adjacent materials. Specify the density and product form, provide the required wind and weather control layer, and inspect every bay before closing the assembly.
Phenolic and PIR/PUR boards: performance where depth is limited
Phenolic and PIR/PUR have the lowest conductivity ranges in the comparison table. They can deliver a target U-value with less thickness than many fibrous products, which is valuable at window reveals, parapets, narrow roof build-ups or retrofit sites where every millimetre matters.
Design requirements
- Confirm the complete board system’s fire classification, including facers, joints and any cavity barriers. A low λ does not imply non-combustibility.
- Stagger and seal board joints according to the system design. Small gaps can become repeating thermal and air-leakage paths.
- Model fasteners, rails and other penetrations. Metal fixings can create significant point or linear bridges through an otherwise continuous layer.
- Provide a continuous, durable air-control layer and a weather-control layer. Rigid boards are not automatically the project’s airtightness membrane.
These products are most effective when installed as a genuinely continuous layer outside structural framing or over a fully filled cavity. A fragmented board layout around openings can erase the space-saving benefit.
EPS, graphite EPS and XPS: rigid boards with different jobs
Graphite and white EPS
Graphite EPS has the lower cited conductivity range, 0.030–0.032 W/mK, compared with 0.036–0.038 W/mK for white EPS. Both can be economical choices for external wall insulation when the approved system provides the required fire, impact, weather and vapour performance.
Protect EPS from prolonged ultraviolet exposure, solvents and site damage. Detail window and door openings, corners, base zones and penetrations so the insulation remains continuous and protected. Fire classification belongs to the tested wall system, not just to the foam board viewed by itself.
Rank #3
- 6lb/ft^3 Mineral Wool - 3 sheets of strong, rigid stone wool insulation
- Heavy insulation is excellent at absorbing sound, is fire resistant, and insulates well. Can be used in sound recording studios as it is has excellent sound dampening qualities.
- Cuts easily with a knife. Great for sound deadening enclosures or room insulation, building fire rated light boxes, or any project that needs heavy fire rated insulation.
- Size: 5 sheets of 24" x 24" x 2"
- Similar to Owens Corning Thermafiber and RockWool. Imported.
XPS
XPS is a rigid, moisture-resistant option often selected at foundations, plinths and other areas with ground contact or high moisture exposure. Its vapour resistance can be useful in one location and problematic in another. Confirm how it changes drying direction, where the vapour-control layer sits, and whether its environmental and fire requirements fit the project.
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Cellulose, wood fibre and cork: fibre-based and bio-based strategies
Cellulose
Cellulose can be blown into enclosed cavities or installed as wet spray. It fills irregular framing zones more completely than repeatedly cut batts when the installer controls density and coverage. Specify the installed density, settling allowance, moisture limits and fire treatment; the generic 0.038–0.040 W/mK range is not valid for every installation method.
Wood fibre
Wood-fibre boards and flexible or loose products support vapour-open, bio-based assemblies. Product differences are substantial: verify conductivity, density, rain exposure, fire classification and the structural backing required for the chosen board. A wood-fibre sarking board exposed beneath a temporary roof covering has different requirements from an internal flexible batt.
Cork
Cork can work where local supply, natural-material goals and moisture and fire requirements align. It generally needs more thickness than phenolic or PIR/PUR for the same thermal resistance and may be less available. Compare the actual product’s λ, water behaviour, fire rating, fixing method and total wall depth before choosing it.
How climate changes vapour-control placement
Vapour control is not a universal “inside” or “outside” rule. In cold climates, guidance from the Insulation Institute generally places the vapour retarder toward the interior, warm side of the insulation. In warm, humid climates, a very low-permeance interior layer can trap moisture; the assembly may need a different control strategy and a defined drying path.
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- High-density Mineral Wool Core: designed with a dense 80kg/M³ (5.0lb/ft³) structure from natural stone-based mineral fibers, these mineral wool panels resist sagging and deformation while maintaining their original shape; Suitable for walls, ceilings, studios, garages, and DIY insulation applications
- Effective Soundproofing Acoustic Control: the dense fiber structure helps absorb sound waves and reduce echoes, making these acoustic insulation panels ideal for recording studios, home theaters, music rooms, offices, and other spaces needing better sound management
- HIGH-DENSITY 80kg/m³ STONE WOOL CORE – Made with dense 80kg/m³ (5 lb/ft³) stone wool, these rigid insulation boards provide a solid, shape-retaining core ideal for building DIY acoustic panels. The high-density fiber structure makes the boards easy to fit into frames while providing a professional foundation for acoustic treatment projects
- Fire Resistant Stone Wool Material: constructed from stone wool insulation Board, this fire resistant insulation board is designed for heat-related applications; Suitable for fireplaces, furnaces, equipment areas, and other projects where thermal protection is important
- Multiple Applications: these mineral wool insulation boards are suitable for larger coverage and flexible use; Designed for garages, workshops, studios, theaters, renovation projects, and residential insulation upgrades
Analyse the wall or roof for the project’s climate, orientation, interior humidity, rain exposure and seasonal drying. Do not combine a vapour-tight board, membrane and finish simply because each product is individually high-performing. The stack-up must allow moisture to leave safely in the directions the model and construction details expect.
Wall assemblies: what to resolve before selecting a product
Framing and cavity continuity
Coordinate insulation with timber or steel studs, sheathing, cladding rails and service cavities. A fully filled cavity can still underperform if studs, shelf angles or rails bypass it. Continuous insulation outside the structure usually reduces repeating bridges, but the thickness, fixing pattern and cladding support must be modelled together.
Windows, doors and junctions
Set the insulation line before detailing openings. Carry it into the reveal, sill, head and jamb, then connect it to the frame’s thermal break and the wall’s airtightness layer. Check balcony slabs, floor edges, corners, parapets and wall-to-roof junctions against the Ψ external target of 0.01 W/(mK) for regular junctions.
Rainscreen and weather exposure
Where a ventilated cladding cavity is used, provide the specified wind-washing protection and maintain drainage and ventilation paths. Insulation that is exposed to driving rain or moving air cannot be assumed to perform at its laboratory value.
Roof assemblies: insulation is only one part of the risk picture
Warm pitched roofs
Keep the insulation layer continuous over rafters where possible, or combine a fully filled rafter zone with a continuous over-rafter layer. Resolve eaves, verges, ridges, skylights and service penetrations before installation. These locations often determine the thermal bridge and airtightness result more than the field area.
Best Value
- 6lb/ft^3 Rockwool ProRox SL960 Mineral Wool - 10 sheets of strong, rigid stone wool insulation
- Heavy insulation is excellent at absorbing sound, is fire resistant, and insulates well. Can be used in sound recording studios as it is has excellent sound dampening qualities.
- Cuts easily with a knife
- Size: 10 sheets of 24" x 24" x 1"
- Similar to Owens Corning Thermafiber
Flat roofs
Choose the insulation and membrane arrangement as one tested system. Check compressive strength, tapered falls, fastener effects, wind uplift and the durability of the outer water-control layer. A roof that meets a U-value on paper can still fail through ponding, membrane damage or unsealed penetrations.
Roof moisture and drying
Model condensation risk and drying direction for the actual roof build-up. Confirm how the outer layer behaves during construction, whether temporary wetting can dry, and whether a low-permeance board or membrane blocks the intended path. Roof insulation must also tolerate the project’s wind and handling conditions without gaps or crushing.
How thick should Passive House insulation be?
Passive House Institute educational guidance reports that exterior wall insulation is often 20–30 cm with λ approximately 0.035 W/(mK). This is experience-based guidance, not a universal prescription. The final thickness depends on climate, wall construction, window area, geometry, thermal bridges, heating strategy, land and reveal constraints, and the optimisation performed in the project model.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsA lower λ can reduce thickness, while a higher λ usually requires more depth for the same target. Do not convert the table’s conductivity ranges directly into a guaranteed thickness: the accepted rated installed value, framing fraction, fasteners, air layers and surface resistances all affect U-value. The energy model should compare the extra insulation cost and space against bridge reduction, simpler detailing and envelope cost.
A practical selection workflow
- Set the project criteria. Record the climate, Passive House certification route, fire classification, moisture exposure, structural system, available depth and local approval requirements.
- Define the control layers. Draw the thermal, air, vapour and water-control layers continuously around walls, roofs, openings and junctions.
- Shortlist material families. Use mineral wool first where Class A non-combustibility governs; consider phenolic or PIR/PUR for constrained depth; evaluate EPS, XPS, cellulose, wood fibre or cork where their specific system approvals fit.
- Use rated installed λ. Enter the approved value for the thickness and installation condition into the energy model.
- Model junctions. Calculate Ψ-values for openings, corners, floors, roofs, balconies, parapets and penetrations. Revise details that miss the regular-junction target.
- Run moisture and fire checks. Verify vapour control, drying, rain exposure, reaction-to-fire classification and any required cavity barriers.
- Coordinate construction. Confirm board dimensions, batt density, blown-fill installation, fasteners, tapes, membranes, sequencing and inspection access before ordering.
- Test the finished envelope. Document airtightness and investigate leaks at membrane transitions, service penetrations, window interfaces and roof-wall connections.
Common failure modes and their fixes
| Observed problem | Typical cause | Corrective action |
|---|---|---|
| Calculated U-value is good, but the junction fails | Framing, fixings or slab edges bypass the insulation | Redesign for continuous insulation; calculate the Ψ-value rather than relying on the centre-of-panel result |
| Cold interior corners or mould risk | Missing corner insulation, air leakage or a cold bridge | Improve continuity, seal the air layer and verify internal surface temperature above 17°C under the required test condition |
| Insulation is thinner after installation | Compressed batts, insufficient blown-fill density or settlement | Specify installation density and tolerances; inspect before closing cavities |
| Unexpected roof condensation | Vapour-control layer is on the wrong side or drying is blocked | Reassess the climate-specific control strategy and hygrothermal model |
| Air test fails | Unsealed membrane transitions, service penetrations or window interfaces | Trace leaks with a pressure test, repair the continuous air layer and retest |
| Fire strategy is not accepted | Insulation was assessed alone instead of as the tested wall or roof system | Use the approved assembly, including facers, fixings, cavity barriers and finishes |
| Wet or wind-washed insulation | Incomplete weather or wind protection during or after construction | Protect the layer, maintain drainage and replace material that no longer meets its installed specification |
Which material should you choose?
- Choose mineral or glass wool when Class A non-combustibility, broad product availability and vapour openness are the primary constraints.
- Choose phenolic or PIR/PUR when the assembly depth is tightly limited and the complete system’s fire, joint and fixing details are acceptable.
- Choose graphite EPS or white EPS when an economical external rigid-board system is available locally and its fire, impact, UV and moisture details are approved.
- Choose XPS for specifically moisture-exposed or ground-contact zones after checking vapour behaviour and environmental requirements.
- Choose cellulose when a properly installed, density-controlled cavity fill suits the structure and moisture design.
- Choose wood fibre or cork when a bio-based, vapour-open strategy is desired and product-specific fire, rain, structural and availability requirements can be met.
Before purchase, verify regional code approvals, dimensions, lead times, fire ratings, compatible membranes and the manufacturer’s declared installed conductivity. The best Passive House insulation is the product that remains continuous, dry, airtight and fire-safe in the real wall or roof—and can be installed and inspected to that standard.
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