Whole life carbon module B and replacement cycles

Module B is the use stage of a whole life carbon assessment, and on a long study period it often rivals upfront embodied carbon in scale. The result is driven almost entirely by two sets of assumptions: how long each component is expected to last, and how long the building is assumed to be assessed over. Neither is a measurement. Both are choices, and in most whole life carbon assessments neither is justified.

If a reviewer can change your module B total by 40 per cent by swapping one service life figure, the assessment has not established anything.

What module B covers across the building's life

The module structure follows BS EN 15978 and is applied in the United Kingdom through the RICS whole life carbon assessment methodology, second edition. Module B splits into seven sub-modules:

• B1, use, covering emissions from the building fabric and systems in place, such as refrigerant leakage and the carbonation of concrete
• B2, maintenance, covering cleaning, servicing and redecoration
• B3, repair, covering the correction of failures short of replacement
• B4, replacement, covering the periodic replacement of components that do not last the study period
• B5, refurbishment, covering planned major works such as a fit-out cycle
• B6, operational energy use
• B7, operational water use

B4 usually dominates the non-operational part, with B2 and B5 adding a smaller amount and B1 material only where refrigerants or large concrete volumes are involved. B6 and B7 are reported separately and usually kept out of headline embodied carbon figures, which is a reporting convention rather than a judgement about importance.

Service life assumptions and their sources

Every B4 figure comes from an expected service life. Get that from a source you can name.

The usual hierarchy is manufacturer-declared service life for the specified product, then published component life data such as the BCIS life expectancy data or the HAPM component life manual, then the service life planning approach in the BS ISO 15686 series, then a documented project assumption agreed with the design team. What will not survive review is an unattributed number in a spreadsheet column.

Two habits cause trouble. The first is applying a single service life to a whole build-up, when the sealant, the fixings, the insulation and the outer leaf have different lives and different replacement implications. The second is using an optimistic manufacturer figure for a product in an exposed or coastal location without an adjustment, or any note that one is needed.

State the service life and the source for each element in a table. It takes a page and it removes the argument.

Replacement cycles for facade, roof and services

The number of replacements within the study period follows from the service life. The convention is to divide the reference study period by the expected service life and subtract one, rounded down, so a component with a 30-year life in a 60-year study period is replaced once rather than twice.

Illustrative example only, because these figures vary by product, exposure and specification:

Notice where the carbon actually sits. Services replaced three times over the period can outweigh a cladding system replaced once, even though the cladding has the larger one-off carbon figure. That is a design argument as much as an accounting one. A plant strategy with fewer, longer-lived, accessible units reduces module B in a way no amount of material substitution in the facade will match.

The rounding convention rewards durability at the margins. A component pushed from 28 years to 31 years of expected life halves its replacement count. Specifying for that, and documenting it, is legitimate and is one of the few places where the methodology and good design pull the same way.

Maintenance, repair and refurbishment carbon

B2, B3 and B5 are usually estimated rather than calculated, and the honest approach is to say which.

For B2 and B3, a proportional approach based on the maintenance regime is normally accepted: redecoration cycles, cleaning of the facade, planned servicing of equipment. Where a component has a declared maintenance requirement, use it. Where it does not, state the assumption.

B5 is where speculation creeps in. A commercial building assessed over 60 years will see several fit-out cycles, and the carbon of those cycles is real and large. Some assessments exclude B5 on the basis that future fit-out is unknowable. That is defensible if declared, and indefensible if the exclusion is buried. Where a scheme claims a lower whole life figure than a comparator, check whether the comparator included B5.

Consider designing for the fit-out cycle. Raised floors, accessible services and a structural grid that tolerates change reduce future B5, and that reasoning belongs in the design and access narrative rather than only in the carbon report.

Operational energy and water within module B

B6 covers regulated and unregulated operational energy, and it is reported separately from the embodied modules.

The figure depends on a grid carbon factor trajectory rather than on today's factor, because electricity decarbonises across the study period. Use the trajectory the methodology or the local authority specifies, state it, and show the years. An assessment using a flat present-day grid factor overstates B6 heavily and will be corrected by any competent reviewer.

Keep the operational modelling consistent with the Energy Statement. Two documents in the same application showing different regulated energy demands is the single most common internal contradiction in a planning submission, and it undermines both. Where the authority requires a be lean, be clean, be green, be seen sequence, the carbon assessment should sit on the same model outputs. The relationship between these documents is explained further in the comparison of a sustainability statement and an energy statement.

B7 is small and quick to do properly. Base it on the specified fittings and the water consumption target, not on a default.

Sensitivity to the assumed study period

The reference study period is the biggest single lever in the whole assessment. Sixty years is the normal default for a new building under the RICS methodology, and a longer period increases module B roughly in proportion for short-life components while leaving upfront carbon untouched.

That creates a perverse comparison risk. A refurbishment assessed over 30 years and a new build assessed over 60 years are not comparable documents, and the same problem appears in the baseline question for refurbishment assessments. State the period prominently, use the same period for every option tested, and run at least one sensitivity case.

A sensitivity table with three rows does more for credibility than ten pages of method description. Vary the study period, vary the two or three service lives that dominate the result, and show the spread.

Reporting the assumptions transparently

The test of a module B section is whether a reviewer can reproduce it.

Give the reference study period and its source. Give a table of elements with quantity, expected service life, source of that life, number of replacements and the resulting carbon. Give the grid trajectory for B6. State every exclusion, including B5 if you have excluded it, and say why. Add the sensitivity cases.

Councils usually expect a suitably qualified author for a Whole Life Carbon Assessment, and professional review is strongly advised before it supports a planning condition. Where a condition requires a post-construction assessment, the assumptions table is what makes the later comparison possible, and discharging a zero carbon condition is far easier when the original assumptions were written down.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.