ISO/TR 25080:2025

Technical
Report
ISO/TR 25080
First edition
Wood and wood-based products —
2025-05
Background and examples of
calculating contributions to
carbon stored in harvested wood
products (HWP)
Bois et produits à base de bois — Contexte et exemples de calcul
des contributions au carbone stocké dans les produits ligneux
récoltés (PLR)
Reference number
© ISO 2025
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 The harvested wood product coefficient (HWP coefficient) concept . 1
5 Background and options provided by IPCC Guidelines and their applicability for
reporting at an organizational level . 2
5.1 General .2
5.2 HWP approaches to estimate greenhouse gas dynamics .2
5.2.1 Approaches .2
5.2.2 Estimating greenhouse gas dynamics based on carbon stock changes .3
5.2.3 Estimating greenhouse gas dynamics based on greenhouse gas fluxes to the
atmosphere .4
5.3 HWP methods to estimate greenhouse gas dynamics .5
5.4 IPCC tiers 2 and 3 calculations .7
6 Tier 1 calculations . 7
6.1 General .7
6.2 First order decay in IPCC tier 1 .7
6.3 Data requirements to calculate HWP coefficients in ISO 13391-1 .8
6.4 Using HWP coefficients in the first order decay model in ISO 13391-1 .9
6.5 Tier 1 HWP coefficients .11
7 Calculation of HWP contribution under tiers 2 and 3 .12
7.1 General . 12
7.2 Recycling rates and market growth . 12
7.3 HWP coefficient for roundwood .14
7.4 Product residence time . 15
7.4.1 IPCC country-specific half-lives . 15
7.4.2 Refining the IPCC country-specific half-life at an organizational level . 15
7.4.3 Altering the function used to model residence time .16
7.4.4 Tier 3 calculation of HWP contribution .16
7.4.5 Considerations based on IPCC guidelines .16
8 Data availability/Literature review . 17
9 Examples of methods for calculating HWP coefficients .18
9.1 Assumptions .18
9.2 Example 1: Using national inventory reports and country-level statistics on wood-
based products .19
9.3 Example 2: Using market development data .21
9.4 Using organization-specific data . 23
9.5 Note on sensitivity related to assumptions and limitations in the examples . 23
10 HWPs in landfill and other methods of woody carbon storage .24
10.1 General .24
10.2 Tier 1 approach for landfill carbon storage .24
10.3 Considering non-standard landfills . 25
10.4 Tier 2 methods for material entering landfill using half-life . 25
10.4.1 General . 25
10.4.2 Recycling . 26
10.4.3 Burning for energy . 26
10.4.4 Landfilling .27
Bibliography .30

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
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with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 287, Sustainable processes for wood and wood-
based products.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
ISO 13391-1 defines a framework for calculating greenhouse gas dynamics of wood and wood-based
products. The framework identifies a component for wood-based carbon (i.e. biogenic carbon stored in
wood-based products), representing the contributions to the harvested wood products (HWP) pool and
wood-based carbon storage in landfills or through biogenic carbon capture and storage (bio-CCS), see
Figure 1. ISO 13391-1 further elaborates on the calculation of these contributions based on the delivery of a
set of wood and wood-based products in a specified time period at an organizational or aggregate level. This
document provides additional background and examples to users of ISO 13391-1.
ISO 13391-1 introduces the concept of a HWP coefficient to estimate the long-term contribution of a set of
wood and wood-based products to the HWP pool. It is defined as a factor for calculating the net contribution
to the HWP pool per delivered volume of a wood-based product. Subclause 5.4 of that document elaborates
on the calculation of HWP coefficients.
Figure 1 — Illustration of the components of the greenhouse gas dynamics of wood and wood-based
products
This document provides background and examples. Clause 4 introduces the concept of an HWP coefficient,
as used in ISO 13391-1. Clause 5 considers the background to quantification of HWP storage, with particular
relevance to the IPCC methodologies used for national reporting.
Clause 6 considers the data requirements for calculating HWP coefficients and provides examples of HWP
coefficients, according to the tier 1 methodology of ISO 13391-1. These include factors for recycling.
This is followed by Clause 7, in which the details of calculating HWP coefficients are considered, when
working from market data and models. The concept of handling recycling within HWP coefficient
calculations is introduced. It also considers the other methodologies for HWP calculations, as discussed in
the IPCC guidelines, often termed tier 2 and tier 3 methods, and their counterparts within ISO 13391-1. This
provides context for ongoing research activity and thought leadership in the field, which is evolving.
Clause 8 provides a literature review showing how research has progressed on this topic.
Clause 9 gives examples of methods for calculating an HWP coefficient using national inventory reports,
market development data or organization-specific data. It also details some sensitivities related to these
examples.
v
Clause 10 discusses the long-term storage of carbon in wood and wood-based products which are disposed
into landfill, or into other long term storage options including bio-CCS, biochar etc.
NOTE The methods described in this report are largely based on IPCC guidelines; however, approaches for
organizational or national reporting can vary depending on local conditions or legislations.

vi
Technical Report ISO/TR 25080:2025(en)
Wood and wood-based products — Background and examples
of calculating contributions to carbon stored in harvested
wood products (HWP)
1 Scope
This document provides background information, methods and examples of calculating contributions to
carbon stored in wood-based products (harvested wood products, HWP), including storage resulting from
HWPs in landfill and bio-CCS, as defined in ISO 13391-1:2025. It includes background to the tier 1 HWP
coefficients for various wood-based product categories defined in ISO 13391-1:2025.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 13391-1:2025, Wood and wood-based products — Greenhouse gas dynamics — Part 1: Framework for value
chain calculations
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 13391-1 apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 The harvested wood product coefficient (HWP coefficient) concept
Wood-based products in use, including the use of recycled wood-based material, extend the time of
biogenic carbon storage until the material is disposed of, after which the wood-based carbon is released to
the atmosphere, enters landfills, or meets a different fate. The carbon storage in wood-based products is
therefore considered as a carbon pool by IPCC, as described in Clause 5.
The pool of carbon in wood-based products (or harvested wood products - HWP) has an inflow of new
woody material, and an outflow of disposed woody material. The difference between the inflow and outflow
in a given time period represents the net change in the HWP pool.
The HWP coefficient has been defined in ISO 13391-1 as the proportion of the inflow that represents a net
change in the HWP pool. This builds on the principle that it is the net change of the HWP pool that is relevant
for the greenhouse gas dynamics, just as the net change of forest carbon storage is relevant.
While the inflow of new material is straightforward to calculate based on the quantities of wood-
based products put on the market by an organization, the outflow depends on the quantities and fates of
corresponding products put on the market historically. Determining the outflow from the pool related to the
organization’s production is therefore a critical methodological aspect. As the actual outflow is difficult to
measure, this can be done through modelling.

Two main parameters for determining the outflow through modelling are:
a) The rate of decay of woody material in the HWP pool. This is usually determined by assuming an
estimated life span for each product category, combined with assumptions on proportions of recycling.
b) The historical growth or decline of market quantities for each product category.
Over the long term, an increasing market quantity will increase the HWP pool and thereby result in a
positive HWP coefficient, while a decreasing market will lead to a decrease of the HWP pool and a negative
HWP coefficient. However, as it is not meaningful to assign a negative storage effect for an organization
that delivers products, which are physically storing carbon, to the market, ISO 13391-1 states that the HWP
coefficient can be assumed to be zero in this case.
One limitation of this approach is that the calculation of HWP coefficients to be applied by an organization
will depend on products delivered in the past, whose fate the organization cannot influence.
Another limitation is that historical market developments can vary between regions, which can lead to
different HWP coefficients for similar products.
The HWP coefficient is used to estimate the present net gain of carbon in the HWP pool, but it does not
indicate a permanent net gain.
The following clauses elaborate on the use of HWP coefficients when implementing ISO 13391-1.
5 Background and options provided by IPCC Guidelines and their applicability for
reporting at an organizational level
5.1 General
The methodology to estimate the carbon storage associated with a HWP carbon pool in ISO 13391-1 is based
on the 2019 Refinement to the 2006 Intergovernmental Panel on Climate Change (IPCC) Guidelines for
[6]
National Greenhouse Gas Inventories, adjusted for use at an organizational or aggregate level.
[6]
5.2 explains the guidance provided by IPCC, as background for calculating the contribution to the HWP
pool according to ISO 13391-1. In order to calculate the carbon storage in the HWP pool under the IPCC
Guidelines, both an approach and a method need to be defined. The approaches and methods outlined in the
IPCC Guidelines are described in the following clauses.
5.2 HWP approaches to estimate greenhouse gas dynamics
5.2.1 Approaches
The IPCC Guidelines define different ‘approaches’ that can be taken to estimate greenhouse gas dynamics
of a HWP pool. The approach defines the system boundary, which indicates what will be estimated and
reported when calculating the greenhouse gas emissions and removals of an HWP. The approach is defined
to ensure that all emissions and removals are accounted for and double-counting does not occur, by being
transparent, complete, and consistent. When selecting the approach, it is important to consider the specific
question being addressed or the type of estimate that is required.
[8]
The 2006 IPCC Guidelines consider four approaches for calculating the greenhouse gas emissions and
removals of an HWP:
— ‘stock-change’ approach which estimates changes in carbon stocks in the HWP pool within the national
boundaries;
— ‘production’ approach which estimates changes in carbon stocks in the HWP pool consisting of products
made from wood harvested in a country;
— ‘atmospheric-flow’ approach which estimates fluxes of greenhouse gases from and to the atmosphere
from HWP, taking place within national boundaries; and

— ‘simple-decay’ approach which estimates fluxes of greenhouse gases from and to the atmosphere from
HWP, associated with woody biomass harvested from the forests and other wood-producing lands
within a country.
The four IPCC approaches have similarities and differences based on what is being estimated and where the
HWP is being consumed and used. As per the guidelines:
— The ‘stock-change’ and ‘production’ approaches work with carbon stock changes in HWP pools, whereas
the ‘atmospheric-flow’ and ‘simple-decay’ approaches work with greenhouse gas fluxes;
— The ‘stock-change’ and ‘atmospheric-flow’ approaches cover stock changes or greenhouse gas fluxes
associated within a consuming country, whereas the ‘production’ and ‘simple-decay’ approaches cover
those associated with a producing country.
In the context of organizational greenhouse gas dynamics (considered in ISO 13391-1) the production
approach is of greatest relevance. Organizations might find the principles of other approaches useful in
other contexts, depending on their location within the supply chain and other factors.
The following subclauses describe in further detail the differences in the system boundaries of the various
IPCC approaches in order to estimate the greenhouse gas dynamics.
5.2.2 Estimating greenhouse gas dynamics based on carbon stock changes
The two IPCC approaches to estimate the greenhouse gas emissions and removals associated with a HWP
based on the carbon stock changes in the biomass pools are the ‘stock-change’ approach and ‘production’
approach.
The two pool-based approaches contain conceptual differences which impact the carbon inflow to the
HWP pool. For instance, the annual carbon inflow to the HWP pool based on the ‘stock change approach’ is
calculated based on the domestic consumption, while the ‘production’ approach is calculated based on the
domestic production. Since the ‘stock change’ approach estimates carbon stock changes of a HWP in use
within national boundaries, the calculated domestic consumption accounts for domestic production, plus
imports and minus exports of HWP in use that are consumed domestically. Domestically produced HWP
that are exported and in use in other countries are outside of the system boundary. Therefore, the HWP pool
system boundary for the ‘stock change’ approach is within the national boundary, as shown in Figure 2.
NOTE AFOLU stands for Agriculture, Forestry and Other Land Use.
[6]
Figure 2 — System boundary of the ‘stock change’ approach
On the other hand, since the ‘production’ approach estimates carbon stock changes of ‘products in use’,
the annual carbon inflow to the HWP pool accounts for the domestic production of wood commodities

manufactured from domestic harvest. Therefore, domestically produced HWPs that are exported and in
use in other countries are within the system boundary. As a result, the HWP pool system boundary for the
‘production’ approach does not align with the national boundary, as shown in Figure 3.
[6]
Figure 3 — System boundary of the ‘production’ approach
5.2.3 Estimating greenhouse gas dynamics based on greenhouse gas fluxes to the atmosphere
The two approaches to estimate the greenhouse gas emissions and removals associated with a HWP based on
greenhouse gas fluxes to the atmosphere are the ‘atmospheric-flow’ approach and ‘simple-decay’ approach.
The two greenhouse gas flux-based approaches consider both carbon stock changes within the HWP pool
in use, as well as all cross-border greenhouse gas fluxes in wood-based products used for energy. Since the
‘atmospheric-flow’ approach considers greenhouse gas fluxes occurring within national boundaries, the
emissions from HWP and wood-based products used for energy are reported by a consuming country. In
this approach, the HWP pools are the same as the ‘stock-change’ approach. The HWP pool system boundary
for the ‘atmospheric-flow’ approach is shown in Figure 4.

Key
1 Conceptually, the atmospheric-flow approach is based on tracking all CO fluxes across the HWP system boundary.
Hence, fluxes associated with wood feedstocks directly used for energy purposes are included.
[6]
Figure 4 — System boundary of the ‘atmospheric-flow’ approach
On the other hand, the ‘simple-decay’ approach considers greenhouse gas fluxes arising from wood harvested
by a producing country. As such, the HWP pool system boundary includes domestically produced HWPs and
wood-based products used for energy that are in use and domestically consumed, as well as exported and in
use in other countries. The HWP pool system boundary for the ‘simple-decay’ approach is shown in Figure 5.
Key
1 Conceptually, the simple-decay approach is based on tracking all CO fluxes across the HWP system boundary. Hence,
fluxes associated with wood feedstocks directly used for energy purposes are included.
[6]
Figure 5 — System boundary of the ‘simple-decay’ approach
5.3 HWP methods to estimate greenhouse gas dynamics
In addition to defining the approach, the method for estimating greenhouse gas emissions and removals
associated with the HWP is also selected to determine how the inventory is going to be calculated. Methods
are divided into three tiers, with tier 1 providing a basic method and tier 3 being the most demanding in
terms of complexity and data requirements. The choice of the method depends on the availability of activity
data on HWPs (wood and wood-based products production, imports, and exports), availability of country-
[6]
specific data, and the availability of country-specific methods. The IPCC Guidelines suggest that it is

good practice to follow the decision tree in Figure 6 for selecting the relevant tier method for estimating
greenhouse gas emissions and removals arising from HWPs.
[6]
The IPCC 2019 refinement to the 2006 Guidelines refers to several different methods within the different
tiers. For instance, if no activity data is available, the ‘steady state HWP pool’ assumption can be applied.
However, the most commonly used method is the first order decay method, which is the foundation of the
tier 1 methodology. First order decay is also permitted within tier 2 and 3. Lastly, flux data methods and
stock inventory data methods are also available to be used within tier 3. These methods will be further
described in later clauses.
Figure 6 — Decision tree for choosing the relevant tier method for estimating greenhouse gas
[6]
emissions arising from HWP
Two main commodity class types are referenced in the IPCC Guidelines: semi-finished products and finished
products. Semi-finished products, sometimes referred to as primary products, are materials used to make
finished products. The default semi-finished commodity classes in the IPCC Guidelines and ISO 13391-1
are sawn wood, wood-based panels, and paper and paperboard. Finished products, sometimes referred to
as secondary products, are product end uses, such as buildings, furniture, or books. At minimum, activity
data for the three default semi-finished HWP commodity classes of sawn wood, wood-based panels, and

paper and paperboard, is required for tier 1 as per the IPCC Guidelines where the first-order decay method
is applied (further described in Clause 6). However, ISO 13391-1 also covers wood-based products used
for energy and roundwood in the tier 1 method. If relevant activity data is available in public databases
of international organizations, such as FAOSTAT (the statistical databases of the Food and Agriculture
Organization of the United Nations), then the data is suitable for tier 1 and tier 2 methods. Activity data for
the HWP is typically reported in cubic metres solid volume or metric tonnes, which can be converted into
units of biogenic carbon.
[6]
The tier 2 method, as per IPCC guidelines, can be applied when country-specific activity data or emission
factors (e.g. based on ISO 15686-1 categories of product and service life) is available for the three default
HWP commodity classes. Additionally, the tier 3 method can be applied when both country-specific activity
data and methods are available to calculate the HWP pool. Tier 3 allows finished products to be handled, or
to use flux data methods to estimate or model HWP pool changes, or to use direct inventory for some aspects
(e.g. HWPs in housing stock or structures). Tier 2 and tier 3 methods are further described in Clause 7.
If there is no information on the utilization of the HWP commodity classes, then countries using the IPCC
[6] [6]
Guidelines can consider making an assumption of a ‘steady-state HWP pool’. As per IPCC Guidelines,
this assumes that “annual inflows into HWP in use are exactly the same in magnitude as the annual losses of
carbon from HWP in use”. Therefore, although the carbon stock of the HWP is not zero, it is not changing over
time. At the organizational level, the same considerations apply, but if the absence of information applies to
all the commodity classes handled by the organization, then it would be more realistic to report this fact
rather than to assume and report a steady-state result.
5.4 IPCC tiers 2 and 3 calculations
[6]
The IPCC guidelines allow a tier 2 or a tier 3 method to be used where suitable additional data is available,
as shown in Figure 6. These tiers broadly reflect the approach taken in ISO 13391-1:2025, but with some
variations which are described in below.
[6]
The IPCC tier 2 method is country-specific and an option within it considers product categories defined
in ISO 15686-1, where the service lives of the product categories are used to calculate the half-life. These
categories allow for a more differentiated set of products in the assessment but exclude finished products
(which are addressed in IPCC tier 3). A similar approach is assumed at an organizational level in tier 2 of
ISO 13391-1, which is considered in greater detail in 7.4.1 and 7.4.2.
6 Tier 1 calculations
6.1 General
[6]
ISO 13391-1 and the IPCC guidelines share common features with regards to tier 1 processes, including
product categories and half-lives considered.
In 6.2 we consider the fundamentals of IPCC tier 1. 6.3 and 6.4 bridge into the use of HWP coefficients to
consider tier 1 situations. 6.5 provides examples of tier 1 HWP coefficients.
6.2 First order decay in IPCC tier 1
The IPCC tier 1 method is applied when activity data is available for the broad product category HWP
[6]
commodity classes (sawn wood, wood-based panels, paper and paperboard). The IPCC Guidelines provide
tier 1 default methods relevant for implementing the four HWP approaches defined in 5.2, where the first-
order decay (FOD) function is used to estimate the carbon stock at the beginning of a year, as well as the
annual carbon stock changes in the HWP.

First-order decay function is given by Formula (1):
−k
 
1−e
()
−k
 
Ci+1 =×e Ci + ×m
() ()
ll CCl,,()i in
 
k
 
 
(1)
ΔCi()=+Ci()1 −Ci()
ll l
where
i
is the year;
Ci
() is the the carbon stock in the particular HWP commodity class l at the beginning of the year
l
i (Mt C);
-1
k
is the decay constant of FOD for each HWP commodity class l given in units y (=ln(2)/HL,
where HL is the half-life of the particular HWP commodity in the HWP in years);
-1
m
is the the carbon inflow to the particular HWP commodity class l during the year i (Mt C y );
Cl,,()i in
ΔCi()
is the carbon stock change of the HWP commodity class l during the year i
l
-1
(Mt C y ).
[8]
The decay constant (k) is expressed as half-life in years, which is defined in the 2006 IPCC Guidelines
as “the number of years it takes to lose one half of the material currently in the pool”. When applying the
[6]
tier 1 method, the IPCC Guidelines provide default half-lives for the three default HWP commodity classes:
35 years for sawn wood, 25 years for wood-based panels, and 2 years for paper and paperboard.
The default IPCC half-lives for tier 1 relate to the whole range of products within that category, which works
well for national level reporting. However, difficulties can arise when working at an organizational level.
The default IPCC half-life values can lead to distorted results if applied to very narrow product groups,
for example if an organization has a very limited product range, with a lifespan that is either much lower
or much higher than the population average for the broader category. For example, if a sawmill produces
only pallets, (i.e. a subcategory of solid wood), the half-life of 35 years can considerably overestimate the
HWP effect of their product range. However, as the aim of the methodology in ISO 13391-1 is to provide an
achievable tier 1 methodology, these biases cannot be fully mitigated.
The tier 1 default method using the ‘production approach’ was used to develop the HWP coefficients
introduced and discussed below.
6.3 Data requirements to calculate HWP coefficients in ISO 13391-1
ISO 13391-1 uses HWP coefficients to express the proportion of carbon contained in wood-based product(s),
delivered to the market in a specified time period, that represent a net increase of the HWP carbon pool in
society.
Determining the HWP coefficient for a product category requires data on
a) inflow: the quantity of wood-based carbon of the product category entering the HWP pool in the
specified time period, and
b) outflow: the quantity of wood-based carbon related to the product category exiting the HWP pool.
NOTE This applies also to tier 2 and 3. Formula 2 has been used to generate the tier 1 HWP coefficients.

Based on ISO 13391-1, the HWP coefficient is calculated as given by Formula (2):
ww−
CH,,WP in CH,,WP out
k = (2)
HWP
w
CH,,WP in
where
k is the HWP coefficient, expressed as the proportion of net contribution to the HWP carbon
HWP
pool in the specified time period;
w is the inflow of biogenic carbon from the volumes of wood and wood-based products placed
C,HWP,in
on the market in the specified time period, expressed in t CO e;
w is the outflow of biogenic carbon from the HWP carbon pool from corresponding wood and
C,HWP,out
wood-based products reaching end of life, after the last recycling round, in the specified time
period, expressed in t CO e.
[6]
This follows the “production approach” in the IPCC Guidelines, which is used to report at the national level.
The IPCC method takes the perspective that the producing country accounts for the increase or decrease of
the downstream HWP pool, as described in Clause 5. Similarly, ISO 13391-1 takes the perspective that an
organization producing and putting wood-based products on the market accounts for downstream changes
in the HWP pool related to its delivered volumes.
Based on ISO 13391-1, the HWP pool contribution by an organization in a specified time period is then
calculated as given by Formula (3):
i
Δww=×()k (3)
CH,,WP BHiiWP,
∑
where
Δw
is the HWP carbon pool change resulting from an organization placing wood and wood-based
CH, WP
products on the market in the specified time period, expressed in t CO e;
1…i are the product categories included;
w
is the quantity of biogenic carbon in the woody material of product i placed on the market in
B,i
the specified time period , expressed in t CO e;
k
is the HWP coefficient for product category i.
HWP,i
The data requirement on wood-based carbon entering the HWP pool (inflow) for a product category is often
straightforwardly met. It is calculated based on the quantity of wood-based carbon in delivered products
during the specified time period, considering the market growth in that time and outflows from previous
time periods. It is also possible to include the proportion of outflow from the previous time period that is
recycled, as discussed in Clause 7.
Determining the quantity of corresponding wood-based carbon exiting the HWP pool (outflow) in the
specified time period is a bigger challenge for the organization, requiring more data, see discussion of Tier 2
and Tier 3 methods in Clause 7. Within Tier 1, the default IPCC half-lives for sawn wood, wood-based panels
and paperboard are used. Clause 10 provides examples that can be applied.
6.4 Using HWP coefficients in the first order decay model in ISO 13391-1
Using the half-lives to plot a residence time, it is possible to model HWP retention. From this a HWP coefficient
can be calculated based on HWP inflow and HWP outflow. The graph below was generated using a 1 %
market growth factor year on year (which was selected to be representative for the wood-based product
sector as a whole, based on FAO annual statistics). The HWP coefficient on the y-axis is determined using the
net quantity entering the pool (carbon stock change of the HWP pool during a specific year) divided by the
total inflow, thus in early years the value is high, but as the pool size grows, and the quantity exiting the pool

from previous years increases, it follows an exponential trend, tending towards a horizontal state at long
durations, see Figure 7.
Key
X number of years
Y HWP coefficient
1 HWP coefficient as modelled over time for sawn wood (half-life 35 years)
2 HWP coefficient as modelled over time for wood-based panels (half-life 25 years)
3 HWP coefficient as modelled over time for paper and paperboard (half-life 2 years)
Figure 7 — Simulation results for HWP coefficients with initial pool size zero
The products with a short half-life (line 3) tend towards a horizontal line earlier than the products with
higher half-lives.
From this graph it is possible to determine a coefficient at the suitable time point where the line is considered
effectively horizontal. This portion of the curve reflects the flux of product into and out of an established
market, such as sawn wood.
The coefficient therefore reflects the long-term net addition to that pool as a function of market growth. As
market growth projections from further back in time are included in the calculation, the coefficient reaches
an asymptote. The asymptote occurs when the amount of carbon leaving the HWP pool each year from the
products of interest is proportional to the carbon leaving the HWP pool from the historical products. The
coefficient calculated at the asymptote is therefore the most representative of the long-term net addition of
HWPs to the pool. In the case of product categories with long half-lives or high recycling rates, it is necessary
to include past market growth over a long time period to reach an asymptote. It is therefore recommended
that the modelling expands back as long as necessary (e.g. 200 years) for the HWP coefficient to reach an
asymptote. Assuming a 1 % growth rate for the past 200 years, the coefficients are calculated as follows:
0,33 for sawn wood with a half-life of 35 years, 0,26 for wood-based panels with a half-life of 25 years and
0,02 for paper and paperboard with a half-life of 2 years assuming no recycling.
When combined with quantities of the organization’s material placed on the market in a given time period
the coefficient can give a reasonable estimate of the HWP carbon which will remain in storage in the long
term. Note that in later clauses of this document 200 years will be used to represent the point at which time
tends towards infinity and the HWP coefficient value stabilises.

In Clause 7 several effects which have an influence on the HWP storage are discussed further. These include
the potential to select different half-lives (which is a tier 2 method), or to include recycling within the
model. Recycling is well established in many regions, and recycling rates are often available for wood-based
products. Therefore ISO 13391-1 has developed a novel way of accounting for recycled wood or fibrous
material that remains within the product category. This is discussed in Clause 7.
Other half-lives that are region-specific or product-specific could be considered where more data is
available at the relevant level. This makes it possible to apply either a tier 2 or tier 3 method to calculate an
organizational half-life, as discussed in 5.4 and 7.4.
In addition, the market growth will influence the HWP coefficient, and this is considered in a sensitivity
analysis in 9.5. Example 2 in 9.3 discusses whether a linear or a compound growth rate is appropriate for
typical HWP markets.
6.5 Tier 1 HWP coefficients
Based on Clauses 6 and 9, the examples of HWP coefficient at tier 1 level can be considered as per Table 1.
The inflow to the HWP pool is the product quantity delivered in the specified time period. The modelled
value for roundwood in Table 1 is explained in 7.3. In Table 1 the value for roundwood relates to 30 % solid
wood, and 30 % recycling of paper and paperboard. Other options are considered in Table 4.
The modelled values in Table 1 for sawn wood, wood-based panels and roundwood appear to be supported
by the Swedish national inventory reports (Figure 10, Example 1B). The reported values for Swedish paper
and paperboard however appear to be considerably higher than the modelled value.
In Table 1 a 30 % recycling rate has been assumed within the modelling of HWP value for paper and
paperboard. The approach for modelling recycling, and the use of different recycling rates are explained and
considered in 6.2. Bioenergy products are considered to not generate any HWP pool as
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