What is fNRB?

If you are developing a cookstove carbon project, fNRB is the number that determines how many credits your project will generate. Get it right and your project is credible. Get it wrong - or use an inflated value because your methodology allows it - and you risk over-crediting, buyer scrutiny, and potentially having credits rejected or heavily discounted.

Despite its importance, fNRB is poorly understood outside specialist circles, and the industry has a troubled history with it. This guide explains what it means, how it has changed, and what values are defensible in 2026.

What does fNRB stand for?

When a household burns firewood or charcoal, the carbon released was recently absorbed from the atmosphere by trees. If those trees are replaced - if the forest regenerates at the same rate it is being harvested - then burning the wood is approximately carbon-neutral: the carbon released is reabsorbed within a few years. No credit.

If the trees are not being replaced - if the harvest rate exceeds the regrowth rate, leading to net deforestation or forest degradation - then the carbon released is truly additional atmospheric carbon, not part of a natural cycle. This is the non-renewable portion, and it is what cookstove projects are credited for reducing.

The fNRB is therefore the multiplier applied to fuel savings to determine actual emission reductions. A project that saves 1,000 kg of firewood in a region with 60% fNRB generates credit for 600 kg of non-renewable carbon released - not the full 1,000 kg.

Why has fNRB caused so many problems?

For most of the VCM's history, project developers used fNRB values derived from the CDM's TOOL30, a modelling approach that produced national default values of 70–90% for most sub-Saharan African countries. A value of 85% means 85% of all wood harvested in that country is being harvested unsustainably - an implausibly high figure for many regions.

The research is clear. Multiple independent analyses, including the widely cited Berkeley study published in Nature Sustainability, found that the most common cookstove methodologies over-credit by factors of 5–9×, with inflated fNRB values being a primary driver. Average fNRB values reported by projects have been 86%, while more conservative evidence-based estimates range from 20–40% for most regions.

The MoFuSS tool and 2026 defaults

MoFuSS (Modelling Fuelwood Sustainability Scenarios) is a dynamic landscape model commissioned by the UNFCCC and developed by the Stockholm Environmental Institute and Mexico's National Autonomous University. It estimates fNRB at sub-national resolution by modelling actual biomass growth and harvest rates using satellite and ground-truth data.

MoFuSS produces values that are substantially lower than the old CDM defaults for most countries - typically in the 30–55% range for sub-Saharan Africa. The key methodologies and their 2026 fNRB requirements:

The CDM Tool 33 default of 30%

CDM Tool 33 established a global conservative default fNRB of 30%. This is now considered the minimum acceptable value by ICVCM for CCP-approved projects - meaning any project using an fNRB above 30% that cannot justify it with MoFuSS data faces credibility risk. For new projects registering in 2026, using MoFuSS country/regional values is best practice; where MoFuSS data is not available, 30% is the safe default.

fNRB by region: what values are defensible?

These are indicative ranges. MoFuSS provides values at sub-national administrative unit resolution - using the specific value for your project's districts is preferable to national averages and demonstrates more rigorous methodology to validators and buyers.

The marginal vs gross fNRB debate

One active area of methodological debate concerns whether fNRB should be calculated on a marginal or gross basis.

The gross definition (used by MoFuSS) asks: what fraction of all wood harvested in this region is non-renewable? If 40% of all wood comes from unsustainable sources, fNRB = 40%.

The marginal definition (used in the original CDM approach) asks: when a household burns one less tonne of wood because of a better cookstove, what fraction of that saving comes from the non-renewable portion of the harvest? The argument is that households reduce their most accessible fuel first - and non-renewable biomass tends to be more accessible than renewable sources, so the marginal fNRB could legitimately be higher than the gross figure.

What this means for your project design

If you are designing a new cookstove project for registration in 2026, the practical recommendations are:

• Use a CCP-approved methodology - GS TPDDTEC, VM0050, or GS Metered & Measured. These are the only methodologies that major buyers will accept for new procurement, and all are compatible with MoFuSS fNRB values.
• Use MoFuSS values, not TOOL30 or national defaults - obtain the sub-national MoFuSS values for your specific project districts and document how they were derived. This is best practice and will be expected by validators.
• If MoFuSS data isn't available, use 30% - the CDM Tool 33 conservative default. Using anything higher without robust justification creates over-crediting risk.
• Account for the Hawthorne effect in KPTs - Kitchen Performance Tests (KPTs) used to measure fuel consumption are subject to the Hawthorne effect: households cook differently when being observed. A 35% discount on KPT-measured fuel savings is recommended by Berkeley researchers to correct for this bias.
• Disclose your fNRB value prominently - sophisticated buyers now check fNRB values in monitoring reports as a first-pass quality filter. A transparent, conservative value is a competitive advantage, not a weakness.

How fNRB flows into your credit calculation

The cookstove calculator uses fNRB as part of the full emission reduction calculation. Try adjusting it to see the direct effect on your credit yield:

fNRB and project viability

One consequence of moving to conservative fNRB values is that some projects that appeared viable under old defaults are no longer viable under credible assumptions. A project that generated 10,000 tCO₂e/yr at 85% fNRB may only generate 4,000–5,000 tCO₂e/yr at 40% fNRB. At Gold Standard prices, this could push a project below the minimum viable threshold for certification costs.

This is not a reason to use inflated fNRB values - it is a reason to use the Feasibility Modeller with conservative inputs before investing in project development:

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