Thermal Diffusivity: The Overlooked Key to Future-Ready Passive Building Design

Passivhaus, NCC, BASIX, and global high-performance standards have radically transformed energy-efficient design. But a critical thermal property remains underused: thermal diffusivity. This guide reveals how this missing metric can bridge the gap between compliance and climate resilience — and why it’s the next evolution in advanced building physics.

Passivhaus buildings are designed to resist heat flow and minimise energy loss through U-values and R-values, but these are steady-state metrics. They don’t account for the rate at which heat enters or exits—a dynamic that becomes critical under real-world conditions like heatwaves, solar spikes, or sudden cold fronts. Thermal diffusivity introduces a time-based layer of control, acting as a “thermal shock absorber” for lightweight or solar-exposed structures.

What is Thermal Diffusivity?

Thermal diffusivity (α) measures how quickly heat moves through a material. It’s calculated as:

Where:

• k = thermal conductivity
• ρ = density
• cₚ = specific heat capacity

A low diffusivity value means heat penetrates slowly — which reduces internal heat spikes, stabilises comfort, and protects against peak thermal loads. This makes diffusivity especially important for lightweight walls, metal structures, and retrofits.

Why U-Values Alone Are Not Enough

U-values measure total heat transfer, but they assume steady-state conditions. Real buildings experience dynamic spikes: morning sun, winter cooling, heatwaves and night-time drops.

Thermal diffusivity reveals how fast heat enters or exits.

Inside and Outside: Diffusivity in Action

Externally, a low-diffusivity coating like Super Therm^® block and delay incoming solar radiation of UV, visual and infrared.

Internally, low diffusivity slows heat loss, creating thermal lag that enhances comfort during cold nights or in fluctuating climates.

Super Therm^® is effective on:

• Metal
• Concrete
• Timber systems

…delivering high thermal performance on both sides of the building envelope.

Real-World Validation: Super Therm® Case Study

Super Therm^® has been tested under ASTM E1461 (Laser Flash Method) on a metal plate — the correct substrate for real-world application — and achieved a thermal diffusivity of just 0.256 mm²/s.

For reference:

• Aluminium: ~80–100 mm²/s
• Concrete: ~1 mm²/s
• Fibreglass batts: 0.5–1.5 mm²/s

Super Therm® outperforms them all and was tested at 23, 50, 75 and 100°C, while also offering:

• Solar reflectance (ASTM C1549)
• High emissivity (ASTM C1371)
• Proven field performance (Australia, Japan, USA)

That’s one of the lowest globally for any insulation coating.

Additionally:

• Reflects and blocks 96.1% of total solar load – Emissivity, reflectance and diffusivity
• Blocks 99% of infrared radiation

Unlike competitor coatings tested on plastic, Super Therm^® provides real-world, substrate-correct data.

Passivhaus Integration

Thermal diffusivity enhances passive performance by controlling peak load dynamics, even when U-values are marginal. In warm-temperate or hot-humid climates, where Passivhaus is harder to meet using bulk insulation alone, low-diffusivity coatings help deliver performance without architectural compromise. Super Therm^® can be used in EnerPHit retrofits, especially on corrugated metal or concrete, where traditional insulation depth isn’t feasible.

Passivhaus relies on:

• Airtightness
• Superinsulation
• Thermal bridge control
• High-performance windows
• MVHR systems

Passivhaus Element Breakdown: Where Diffusivity Helps

How diffusivity enhances these:

• Slows and reduces thermal bridging across junctions
• Reduces internal temp swings
• Reduces dramatic humidity swings
• Supports EnerPHit retrofits where wall cavity depth is limited
• Stabilises solar-exposed elements both roof and walls

Passive House Planning Package (PHPP) doesn’t currently model diffusivity – but it should.

NCC and BASIX Alignment\For professionals designing under NCC, BASIX, or Passivhaus frameworks — especially those leading performance-first builds — the details matter. Thermal diffusivity offers an elegant yet overlooked upgrade to dynamic thermal management. Here’s where it fills the gaps and supports advanced project outcomes across architecture, sustainability, and building science.

NCC (National Construction Code):

• Super Therm^® enhances NCC compliance by reducing roof and wall heat loads, enabling thinner assemblies to meet performance goals.
• Supports compliance with Section J without bulky insulation
• Contributes to fire safety and energy efficiency objectives
• Thermal diffusivity provides dynamic performance that complements Section J’s goal of reducing HVAC energy loads — especially in lightweight commercial or Class 2–9 buildings.
• Fire-resistant coatings like Super Therm^® also contribute to compliance with flame-spread and fire resistance requirements.

BASIX (NSW):

• Especially valuable on metal roofs, in hot or coastal climates
• Enhances thermal comfort performance under NatHERS
• Super Therm^® can lower internal summer temperatures, especially on lightweight metal roofs, directly improving the thermal comfort and energy usage metrics in BASIX.
• When combined with solar PV and ventilation strategies, Super Therm^® supports high-performance, low-cost BASIX scores without thick wall builds.

Opportunities for Integration into Standards:

Expert-Level Considerations

Permeability & Moisture

• Prevents trapped moisture behind substrates
• Super Therm^® is water-resistant but breathable, helping prevent trapped moisture while shielding against rain and condensation.
• Water-resistant but breathable – Perm Rating (ASTM D1653-13): 250 microns/0.25mm = 8 perms
• Vapour permeability WVT (Water Vapour Transmission) ratings) (ASTM E96) ensures internal wall drying — essential for timber frames, modular systems, or retrofits in humid climates
• Critical for maintaining structural durability and indoor air quality.

Condensation & Thermal Lag

• While thermal lag reduces daytime heat gain, it must be paired with correct dew point design.
• In winter, delayed surface warming can increase condensation risk if inner surfaces stay cool.
• Best results occur when used in ventilated cavities or confirmed via WUFI modelling — especially in climates like Melbourne, Hobart, or Alpine Victoria.

Mould Resistance

• Low-diffusivity materials stabilise interior surface temperatures and reduce cold bridging, preventing condensation that leads to mould.
• Super Therm^® is non-organic and does not support fungal growth (ASTM D3273).
• Ideal for Passive-certified social housing, aged care, and education projects requiring robust IAQ strategies.

Acoustic Performance

• Reduces sound transmission by up to 50-68% – ASTM E90
• Useful in lightweight façades, metal buildings, or near high-noise areas like airports, highways, or urban infill sites.

Fire Compliance

• AS 1530.3 – non-combustible
• ASTM E84 – no flame spread/smoke
• Zero VOCs – safe for BAL zones and Class 2–3 buildings

Contains low VOCs

Suits use in:

• BAL-rated zones
• Multi-storey façades
• Class 2 and 3 buildings, especially in post-Grenfell regulatory environments like Victoria and NSW.

Embodied Carbon

• Ultra-thin system = low material volume and transport energy.
• Long recoat cycle = less maintenance, reducing operational and embodied carbon over decades.
• Supports performance credits under Green Star, NABERS, and Climate Active frameworks — aligning with Net Zero pathways.

Substrate Compatibility

• Adheres to metal, concrete, fibre cement and primed timber
• Used at eaves, junctions, parapets, and membrane transitions

Impact on U-Values and Dynamic Heat Load

U-values quantify how much heat transfers through a material, but not how fast it happens. A wall can meet a U-value target yet still allow rapid internal heat spikes if its materials have high diffusivity. By reducing the rate of heat transfer:

• Low-α materials like Super Therm® create more stable internal temperatures
• Reduce HVAC peaks
• Deliver better real-world performance, even when U-values are matched

Why Diffusivity is Ignored

• It doesn’t fit the code-based models (NCC, PHPP, ISO)
• It requires ASTM E1461 (Laser Flash Method) — few products are tested
• It exposes flaws in conventional insulation and reflectance logic
• Marketing prefers to sell R-values and SRI — easy, familiar, low friction

What the Industry is Missing

Without thermal diffusivity:

• Projects misjudge thermal stability
• Lightweight buildings underperform during heatwaves
• SRI-based coatings mislead on internal temps
• Professionals miss a simple solution to peak load risk

Diffusivity is the truth behind thermal behaviour.

Addressing Common Myths

Myth: “If the U-value is low, I’m covered.”
Truth: U-value doesn’t account for thermal response speed.

Myth: “Diffusivity isn’t in the code, so it doesn’t matter.”
Truth: Codes are lagging. Diffusivity reflects real performance under dynamic heat loads.

Myth: “All white coatings perform the same.”
Truth: High reflectance doesn’t mean low heat penetration. Diffusivity determines whether that heat spike reaches inside.

Myth: “PHPP doesn’t model diffusivity.”
Truth: PHPP is a steady-state tool. Diffusivity addresses what it misses — heat velocity and spike absorption.

Myth: “It’s not on the datasheet, so it’s not important.”
Truth: Most coatings don’t test for it. Super Therm^® does — independently and under ASTM E1461.

The Future of Passive Design

In a volatile, warming climate:

• The rate of heat transfer matters just as much as resistance
• Comfort, performance, and resilience all depend on dynamic control
• Super Therm^® provides that — today

If you’re building to perform — not just comply — thermal diffusivity is your edge. It doesn’t replace what you know. It completes it.

It’s not an alternative to insulation.
It’s the next layer of performance.
And the smartest buildings in the world will use it.

R-values tell you how much heat flows. Airtightness seals it in. But only diffusivity tells you how fast heat moves.

In a warming climate, where buildings face rapid thermal shifts and extreme solar exposure, thermal diffusivity gives you the time delay needed to maintain comfort, reduce mechanical loads, and preserve envelope performance.

Super Therm® leads this evolution with:

• Independent ASTM-tested diffusivity
• Solar reflectance and IR blocking
• Global performance data

It doesn’t just meet the demands of today — it’s designed for the climate challenges of tomorrow.