Image Source: Natural Building Systems
Evaluating building shell options for new build Passivhaus homes requires balancing embodied carbon, operational energy demand, overheating risks, and construction costs. We analysed 8 wall systems to understand the pros and cons of the options available. Timber I-joist frame systems (PH15) minimize space heating demand (7,559.8 kWh/a), while 450mm cast hempcrete offers optimal moisture regulation and biogenic carbon storage (-12,774 kgCO2e).
Key Takeaways
- Biogenic Carbon Storage vs. Emissions: For a ~200sqm dwelling, bio-based systems like 400mm EcoCocon straw panels (-22,214 kgCO2e) and 450mm cast hempcrete (-12,774 kgCO2e) serve as active carbon sinks under EN 15978 / RICS standards, contrasting sharply with traditional concrete cavity masonry (+23,669 kgCO2e).
- PHPP Operational Heating Demand: Passivhaus Planning Package (PHPP) modeling indicates deep timber I-joist walls filled with blown wood fibre yield the lowest heating demand at 7,559.8 kWh/a, compared to 10,293.3 kWh/a for 300mm cast hempcrete.
- Decarbonisation ROI: Natural building envelope systems carry a cost premium of £2,000 to £4,000 per tonne of CO2e avoided over standard cavity blockwork—a metric aligned with official UK public-sector carbon reduction benchmarks rather than speculative offsetting.
- Procurement & Delivery Risk: Single-source prefabricated options, such as the Natural Building Systems (NBS) ADEPT panelized system (£279,833), eliminate contractor interface friction and reduce supply-chain volatility.
How do wall system constructions options compare in biogenic carbon storage under the RICS standards?
Evaluating whole-life carbon under EN 15978 and RICS Whole Life Carbon Assessment guidelines highlights a stark divergence between traditional masonry and bio-based wall systems. While standard compliance frameworks model the release of sequestered carbon after 60 years, factoring in biogenic carbon storage positions the building envelope of a 200sqm home as a net carbon sink.
- EcoCocon Straw Panel (400mm): Stores -22,214 kgCO2e net biogenic carbon, delivering the highest atmospheric carbon capture of all options analyzed.
- Cast Hempcrete (450mm): Locks away -12,774 kgCO2e net carbon through the combination of industrial hemp shiv and carbon-capturing lime binder petrification.
- PH15 / Kithurst Timber I-Joist Walls: Store -6,394 kgCO2e net carbon via FSC-certified timber frames and blown wood fibre insulation.
- Natural Building Systems (NBS) ADEPT: Achieves net-neutral embodied performance (-6 kgCO2e) through a hybrid panel combining wood fibre insulation, hemp insulation, and a cast hemp-lime core.
- Concrete Block Cavity Wall: Emits +23,669 kgCO2e upfront, offering zero carbon sequestration capacity.
Which wall system delivers the lowest annual heating demand for a 200sqm UK net zero property?
Using Passivhaus Planning Package (PHPP) calculations tailored to a 200sqm residential footprint, annual space heating demands vary by up to 2,733.5 kWh/a across structural options:
| Structural Build-Up Method | Heating Demand (kWh/a) | Estimated Build Cost (£) | Carbon Storage (kgCO2e) |
|---|---|---|---|
| PH15 Timber I-Joist Wall | 7,559.8 | £261,758 | -6,394 |
| Kithurst Timber I-Joist Wall | 7,559.8 | £304,864 | -6,394 |
| Concrete Block Cavity Wall | 8,052.6 | £245,800 | +23,669 |
| EcoCocon Straw Panel | 8,301.6 | £260,236 | -22,214 |
| Durisol / EcoBrix Woodcrete | 8,301.6 | £250,805 | +12,826 |
| Cast Hempcrete (450mm) | 8,351.5 | £298,160 | -12,774 |
| NBS ADEPT Prefab System | 9,568.8 | £279,833 | -6 |
| Cast Hempcrete (300mm) | 10,293.3 | £269,460 | -8,446 |
Although lightweight timber I-joist walls achieve the lowest space heating demand, they lack thermal mass, raising summer overheating risks unless paired with shaded glazing strategies. Monolithic hempcrete and straw-bale envelopes supply superior thermal inertia and hygric regulation, smoothing daily temperature fluctuations. Paired with a standard domestic solar PV array capable of generating 15,000 kWh/a, all options easily achieve net zero operational energy status.
What are the build costs and procurement risks when specifying hempcrete, straw, or timber I-joists?
While conventional block cavity masonry (£245,800) and EcoBrix insulated concrete forms (£250,805) show lower initial supply costs, their high embodied carbon contradicts net zero carbon objectives. When evaluating bio-based envelope options for a 200sqm project, procurement strategy dictates real-world cost outcomes:
- Cast Hempcrete (450mm – £298,160): Involves temporary shuttering, on-site wet trades, and weather-dependent curing periods, requiring specialized UK contractors.
- EcoCocon Straw Panels (£260,236): Highly competitive material cost, but manufactured in Lithuania and requiring craned installation by a limited pool of UK certified installers.
- PH15 Timber Frame (£261,758): Standardized construction techniques familiar to mainstream UK joiners, backed by supplier-provided Passivhaus engineering and thermal calculations.
- NBS ADEPT System (£279,833): Minimizes cost uncertainty by providing a single-source contract covering off-site panel manufacturing and on-site assembly.
To see how these wall build-ups and Passivhaus performance metrics are implemented in a real-world project, explore the Natural Building Studio 69 Constable Road Case Study.