EN ISO 13370:2007

SLOVENSKI STANDARD
01-junij-2008
1DGRPHãþD
SIST EN ISO 13370:1999
7RSORWQHNDUDNWHULVWLNHVWDYE3UHQRVWRSORWHVNR]L]HPOMR5DþXQVNHPHWRGH
,62
Thermal performance of buildings - Heat transfer via the ground - Calculation methods
(ISO 13370:2007
Wärmetechnisches Verhalten von Gebäuden - Wärmeübertragung über das Erdreich -
Berechnungsverfahren (ISO 13370:2007)
Performance thermique des bâtiments - Transfert de chaleur par le sol - Méthodes de
calcul (ISO 13370:2007)
Ta slovenski standard je istoveten z: EN ISO 13370:2007
ICS:
91.120.10 Toplotna izolacija stavb Thermal insulation
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN ISO 13370
NORME EUROPÉENNE
EUROPÄISCHE NORM
December 2007
ICS 91.120.10 Supersedes EN ISO 13370:1998
English Version
Thermal performance of buildings - Heat transfer via the ground
- Calculation methods (ISO 13370:2007)
Performance thermique des bâtiments - Transfert de Wärmetechnisches Verhalten von Gebäuden -
chaleur par le sol - Méthodes de calcul (ISO 13370:2007) Wärmeübertragung über das Erdreich -
Berechnungsverfahren (ISO 13370:2007)
This European Standard was approved by CEN on 7 December 2007.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the CEN Management Centre or to any CEN member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CEN member into its own language and notified to the CEN Management Centre has the same status as the
official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,
Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2007 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 13370:2007: E
worldwide for CEN national Members.

Contents Page
Foreword.3

Foreword
This document (EN ISO 13370:2007) has been prepared by Technical Committee ISO/TC 163 "Thermal
performance and energy use in the built environment" in collaboration with Technical Committee CEN/TC 89
"Thermal performance of buildings and building components", the secretariat of which is held by SIS.
This European Standard shall be given the status of a national standard, either by publication of an identical
text or by endorsement, at the latest by June 2008, and conflicting national standards shall be withdrawn at
the latest by June 2008.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 13370:1998.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Cyprus, Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain,
Sweden, Switzerland and the United Kingdom.
Endorsement notice
The text of ISO 13370:2007 has been approved by CEN as a EN ISO 13370:2007 without any modification.

INTERNATIONAL ISO
STANDARD 13370
Second edition
2007-12-15
Thermal performance of buildings — Heat
transfer via the ground — Calculation
methods
Performance thermique des bâtiments — Transfert de chaleur par le
sol — Méthodes de calcul
Reference number
ISO 13370:2007(E)
©
ISO 2007
ISO 13370:2007(E)
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ii © ISO 2007 – All rights reserved

ISO 13370:2007(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions, symbols and units . 2
3.1 Terms and definitions. 2
3.2 Symbols and units. 3
4 Methods of calculation. 3
5 Thermal properties . 4
5.1 Thermal properties of the ground. 4
5.2 Thermal properties of building materials. 5
5.3 Surface resistances. 5
6 Internal temperature and climatic data. 5
6.1 Internal temperature . 5
6.2 Climatic data. 5
7 Thermal transmittance and heat flow rate. 6
7.1 Thermal transmittance . 6
7.2 Thermal bridges at edge of floor. 6
7.3 Calculation of heat flow rate. 6
7.4 Effect of ground water. 6
7.5 Special cases . 7
8 Parameters used in the calculations . 7
8.1 Characteristic dimension of floor . 7
8.2 Equivalent thickness . 8
9 Calculation of thermal transmittances . 8
9.1 Slab-on-ground floor . 8
9.2 Suspended floor. 9
9.3 Heated basement . 12
9.4 Unheated basement. 14
9.5 Partly heated basement . 14
Annex A (normative) Calculation of ground heat flow rate . 15
Annex B (normative) Slab-on-ground with edge insulation . 20
Annex C (normative) Heat flow rates for individual rooms. 24
Annex D (normative) Application to dynamic simulation programmes . 25
Annex E (normative) Ventilation below suspended floors . 26
Annex F (informative) Periodic heat transfer coefficients . 29
Annex G (informative) Thermal properties of the ground. 33
Annex H (informative) The influence of flowing ground water. 35
Annex I (informative) Slab-on-ground floor with an embedded heating or cooling system . 37
Annex J (informative) Cold stores . 38
Annex K (informative) Worked examples. 39
Bibliography . 48

ISO 13370:2007(E)
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, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 13370 was prepared by Technical Committee ISO/TC 163, Thermal performance and energy use in the
built environment, Subcommittee SC 2, Calculation methods.
This second edition cancels and replaces the first edition (ISO 13370:1998), which has been technically
revised.
The following principal changes have been made to the first edition:
⎯ Clause 4 contains a revised text to clarify the intention of the initial part of the former Annex A; the rest of
the former Annex A is now contained in ISO 10211;
⎯ 7.2 no longer contains a table of linear thermal transmittances: it is now recognized, as with other thermal
bridging, that the wall/floor junction often needs to be calculated;
⎯ 9.1 provides an alternative formula for well-insulated floors;
⎯ 9.2 provides clarification for low-emissivity surfaces;
⎯ Annex A contains formulae for cooling applications;
⎯ Annex B has incorporated minor revisions to the text for edge-insulated floors;
⎯ Annex D has been revised;
⎯ Annex F (formerly Annex C) has been changed to informative status.
iv © ISO 2007 – All rights reserved

ISO 13370:2007(E)
Introduction
This International Standard provides the means (in part) to assess the contribution that building products and
services make to energy conservation and to the overall energy performance of buildings.
In contrast with ISO 6946, which gives the method of calculation of the thermal transmittance of building
elements in contact with the external air, this International Standard deals with elements in thermal contact
with the ground. The division between these two International Standards is at the level of the inside floor
surface for slab-on-ground floors, suspended floors and unheated basements, and at the level of the external
ground surface for heated basements. In general, a term to allow for a thermal bridge associated with the
wall/floor junction is included when assessing the total heat loss from a building using methods such as
ISO 13789.
The calculation of heat transfer through the ground can be done by numerical calculations, which also allow
analysis of thermal bridges, including wall/floor junctions, for assessment of minimum internal surface
temperatures.
In this International Standard, methods are provided which take account of the three-dimensional nature of the
heat flow in the ground below buildings.
Thermal transmittances of floors give useful comparative values of the insulation properties of different floor
constructions, and are used in building regulations in some countries for the limitation of heat losses through
floors.
Thermal transmittance, although defined for steady-state conditions, also relates average heat flow to average
temperature difference. In the case of walls and roofs exposed to the external air, there are daily periodic
variations in heat flow into and out of storage related to daily temperature variations, but this averages out,
and the daily average heat loss can be found from the thermal transmittance and daily average inside-to-
outside temperature difference. For floors and basement walls in contact with the ground, however, the large
thermal inertia of the ground results in periodic heat flows related to the annual cycle of internal and external
temperatures. The steady-state heat flow is often a good approximation to the average heat flow over the
heating season.
In addition to the steady-state part, a detailed assessment of floor losses is obtained from annual periodic heat
transfer coefficients related to the thermal capacity of the soil, as well as its thermal conductivity, together with
the amplitude of annual variations in monthly mean temperature.
Annex D provides a method for incorporating heat transfers to and from the ground into calculations
undertaken at short time steps (e.g. one hour).
Worked examples illustrating the use of the methods in this International Standard are given in Annex K.
INTERNATIONAL STANDARD ISO 13370:2007(E)

Thermal performance of buildings — Heat transfer via
the ground — Calculation methods
1 Scope
This International Standard provides methods of calculation of heat transfer coefficients and heat flow rates for
building elements in thermal contact with the ground, including slab-on-ground floors, suspended floors and
basements. It applies to building elements, or parts of them, below a horizontal plane in the bounding walls of
the building situated
⎯ for slab-on-ground floors, suspended floors and unheated basements, at the level of the inside floor
surface;
NOTE In some cases, external dimension systems define the boundary at the lower surface of the floor slab.
⎯ for heated basements, at the level of the external ground surface.
This International Standard includes calculation of the steady-state part of the heat transfer (the annual
average rate of heat flow) and the part due to annual periodic variations in temperature (the seasonal
variations of the heat flow rate about the annual average). These seasonal variations are obtained on a
monthly basis and, except for the application to dynamic simulation programmes in Annex D, this International
Standard does not apply to shorter periods of time.
2 Normative references
The following referenced documents are indispensable for the application 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 6946, Building components and building elements — Thermal resistance and thermal transmittance —
Calculation method
ISO 7345, Thermal insulation — Physical quantities and definitions
ISO 10211, Thermal bridges in building construction — Heat flows and surface temperatures — Detailed
calculations
ISO 10456, Building materials and products — Hygrothermal properties — Tabulated design values and
procedures for determining declared and design thermal values
ISO 14683, Thermal bridges in building construction — Linear thermal transmittance — Simplified methods
and default values
ISO 13370:2007(E)
3 Terms, definitions, symbols and units
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 7345 and the following apply.
3.1.1
slab on ground
floor construction directly on the ground over its whole area
3.1.2
suspended floor
floor construction in which the lowest floor is held off the ground, resulting in an air void between the floor and
the ground
NOTE This air void, also called underfloor space or crawl space, may be ventilated or unventilated, and does not
form part of the habitable space.
3.1.3
basement
usable part of a building that is situated partly or entirely below ground level
NOTE This space may be heated or unheated.
3.1.4
equivalent thickness
〈thermal resistance〉 thickness of ground (having the thermal conductivity of the actual ground) which has the
same thermal resistance as the element under consideration
3.1.5
steady-state heat transfer coefficient
steady-state heat flow divided by temperature difference between internal and external environments
3.1.6
internal periodic heat transfer coefficient
amplitude of periodic heat flow divided by amplitude of internal temperature variation over an annual cycle
3.1.7
external periodic heat transfer coefficient
amplitude of periodic heat flow divided by amplitude of external temperature over an annual cycle
3.1.8
characteristic dimension of floor
area of floor divided by half the perimeter of floor
3.1.9
phase difference
period of time between the maximum or minimum of a cyclic temperature and the consequential maximum or
minimum heat flow rate
2 © ISO 2007 – All rights reserved

ISO 13370:2007(E)
3.2 Symbols and units
The following is a list of the principal symbols used. Other symbols are defined where they are used within the
text.
Symbol Quantity Unit
A area of floor m
B' characteristic dimension of floor m
c specific heat capacity of unfrozen ground J/(kg·K)
d total equivalent thickness – ground below suspended floor m
g
d total equivalent thickness – slab-on-ground floor m
t
d total equivalent thickness – basement wall m
w
H steady-state ground heat transfer coefficient W/K
g
h height of floor surface above outside ground level m
P exposed perimeter of floor m
Q quantity of heat J
R thermal resistance m ·K/W
R thermal resistance of floor construction m ·K/W
f
R internal surface resistance m ·K/W
si
R external surface resistance m ·K/W
se
U thermal transmittance between internal and external environments W/(m ·K)
U thermal transmittance of basement floor W/(m ·K)
bf
U thermal transmittance of basement walls W/(m ·K)
bw
U' effective thermal transmittance for whole basement W/(m ·K)
w thickness of external walls m
z depth of basement floor below ground level m
Φ heat flow rate W
λ thermal conductivity of unfrozen ground W/(m·K)
ρ density of unfrozen ground kg/m
θ temperature °C
Ψ linear thermal transmittance associated with wall/floor junction W/(m·K)
g
Ψ linear thermal transmittance associated with edge insulation W/(m·K)
g,e
4 Methods of calculation
Heat transfer via the ground is characterized by:
⎯ heat flow related to the area of the floor, depending on the construction of the floor;
⎯ heat flow related to the perimeter of the floor, depending on thermal bridging at the edge of the floor;
⎯ annual periodic heat flow, also related to the perimeter of the floor, resulting from the thermal inertia of the
ground.
ISO 13370:2007(E)
The steady-state, or annual average, part of the heat transfer shall be evaluated using one of the methods
described below.
a) A full three-dimensional numerical calculation, giving the result directly for the floor concerned:
calculations shall be done in accordance with ISO 10211. The result is applicable only for the actual floor
dimensions modelled.
b) A two-dimensional numerical calculation, using a floor that is infinitely long and has a width equal to the
characteristic dimension of the floor (floor area divided by half perimeter, see 8.1): calculations shall be
done in accordance with ISO 10211. The result is applicable to floors having the characteristic dimension
that was modelled.
NOTE The largest heat flows usually occur near the edges of the floor, and in most cases only small errors result
from converting the three-dimensional problem to a two-dimensional problem in which the width of the building is taken as
the characteristic dimension of the floor.
c) The area-related heat transfer calculated by the formulae given in this International Standard (see
Clause 9), together with the edge-related heat transfer obtained from a two-dimensional numerical
calculation in accordance with ISO 10211.
d) The area-related heat transfer calculated by the formulae given in this International Standard (see
Clause 9), together with the edge-related coefficients obtained from, for example, tables prepared in
accordance with ISO 14683.
For c) and d), the steady-state part of the heat transfer is given by Equation (1):
HA=+U PΨ (1)
gg
where Ψ is obtained by numerical calculation in method c), or from a table of values in method d).
g
In both cases, the method is applicable to a floor of any size or shape. U depends on floor size, but Ψ is
g
independent of the floor dimensions. Equation (1) is modified in the case of a heated basement (see 9.3.4)
and in the case of application of Annex B (see B.1).
For annual periodic heat flow, see 7.3 and Annex A.
5 Thermal properties
5.1 Thermal properties of the ground
The thermal properties of the ground may be specified in national regulations or other documents, and such
values may be used where appropriate. In other cases, the following apply:
a) if known, use values for the actual location, averaged over a depth equal to the width of the building and
allowing for the normal moisture content;
b) if the soil type is known or specified, use the values in Table 1;
c) otherwise, use λ = 2,0 W/(m·K) and ρc=×2,0 10 J/(m ·K).
NOTE Annex G gives information about the range of values of ground properties.

4 © ISO 2007 – All rights reserved

ISO 13370:2007(E)
Table 1 — Thermal properties of the ground
Heat capacity per volume
Thermal conductivity
ρc
Category Description
λ
W/(m·K)
J/(m ·K)
1 clay or silt 1,5 3,0 x 10
2 sand or gravel 2,0 2,0 x 10
3 homogeneous rock 3,5 2,0 x 10

5.2 Thermal properties of building materials
For the thermal resistance of any building product, use the appropriate design value as defined in ISO 10456.
The thermal resistance of products used below ground level should reflect the moisture and temperature
conditions of the application.
If thermal conductivity is quoted, obtain the thermal resistance as the thickness divided by thermal conductivity.
NOTE The heat capacity of building materials used in floor constructions is small compared with that of the ground,
and is neglected.
5.3 Surface resistances
Values of surface resistance shall conform to ISO 6946.
R applies both at the top and the bottom of an underfloor space.
si
6 Internal temperature and climatic data
6.1 Internal temperature
If there are different temperatures in different rooms or spaces immediately above the floor, a spatial average
should be used. Obtain this average by weighting the temperature of each space by the area of that space in
contact with the ground.
To calculate heat flow rates, this International Standard requires:
a) annual mean internal temperature;
b) if variations in internal temperature are to be included, amplitude of variation of internal temperature from
the annual mean; this amplitude is defined as half the difference between the maximum and minimum
values of the average temperatures for each month.
6.2 Climatic data
To calculate heat flow rates, this International Standard requires:
a) annual mean external air temperature;
b) if variations in external temperature are to be included, amplitude of variation of external air temperature
from the annual mean; this amplitude is defined as half the difference between the maximum and
minimum values of the average temperatures for each month;
ISO 13370:2007(E)
c) for suspended floors that are naturally ventilated, the average wind speed measured at a height of 10 m
above external ground level.
If the ground surface temperature is known or can be estimated, this can be used in place of the external air
temperature, in order to allow for effects of snow cover, solar gain on the ground surface and/or longwave
radiation to clear skies. In such cases, R should be excluded from all formulae.

se
7 Thermal transmittance and heat flow rate
7.1 Thermal transmittance
Thermal transmittances for floors and basements are related to the steady-state component of the heat
transfer. Methods of calculation are given in Clause 9 for the various types of floor and basement. The
formulae use the characteristic dimension of the floor and the equivalent thickness of floor insulation (see
Clause 8).
If the transmission heat loss coefficient for the ground is required, take this as equal to the steady-state
ground heat transfer coefficient, H , calculated using Equation (1).
g
7.2 Thermal bridges at edge of floor
The formulae in this International Standard are based on an isolated floor considered independently of any
interaction between floor and wall. They also assume uniform thermal properties of the soil (except for effects
solely due to edge insulation).
In practice, wall/floor junctions for slab-on-ground floors do not correspond with this ideal, giving rise to
thermal bridge effects. These shall be allowed for in calculations of the total heat loss from a building, by using
a linear thermal transmittance, Ψ .
g
NOTE The linear thermal transmittance depends on the system being used for defining building dimensions (see
ISO 13789).
The total heat loss from a building is then calculated on the basis of a separating plane
⎯ at the level of the inside floor surface for slab-on-ground floors, suspended floors and unheated
basements, or
⎯ at the level of the outside ground surface for heated basements.
NOTE In some cases, external dimension systems define the boundary at the lower surface of the floor slab.
The thermal transmittance of elements above the separating plane should be assessed in accordance with
appropriate standards, such as ISO 6946.
7.3 Calculation of heat flow rate
Heat transfer via the ground can be calculated on an annual basis using only the steady-state ground heat
transfer coefficient, or on a seasonal or monthly basis using additional periodic coefficients that take account
of the thermal inertia of the ground. The relevant equations are given in Annex A.
7.4 Effect of ground water
Ground water has a negligible effect on the heat transfer, unless it is at a shallow depth and has a high flow
rate. Such conditions are rarely encountered and in most cases no allowance should be made for the effect of
ground water.
6 © ISO 2007 – All rights reserved

ISO 13370:2007(E)
When the depth of the water table below ground level and the rate of ground water flow are known, the
steady-state ground heat transfer coefficient, H , may be multiplied by a factor, G .
g w
NOTE Illustrative values of G are given in Annex H.
w
7.5 Special cases
The methods in this International Standard are also applicable to the following situations, with the
modifications described in the relevant annex:
⎯ heat flow rates for individual rooms (see Annex C);
⎯ application to dynamic simulation programmes (see Annex D).
NOTE This International Standard can also be used for slab-on-ground floors with an embedded heating system (see
Annex I) and for cold stores (see Annex J).
8 Parameters used in the calculations
8.1 Characteristic dimension of floor
To allow for the three-dimensional nature of heat flow within the ground, the formulae in this International
Standard are expressed in terms of the “characteristic dimension” of the floor, B', defined as the area of the
floor divided by half the perimeter:
A
′
B = (2)
0,5 P
NOTE For an infinitely long floor, B' is the width of the floor; for a square floor, B' is half the length of one side.
Special foundation details, e.g. edge insulation of the floor, are treated as modifying the heat flow at the
perimeter.
In the case of basements, B' is calculated from the area and perimeter of the floor of the basement, not
including the walls of the basement, and the heat flow from the basement includes an additional term related
to the perimeter and the depth of the basement floor below ground level.
In this International Standard, P is the exposed perimeter of the floor: the total length of external wall dividing
the heated building from the external environment or from an unheated space outside the insulated fabric.
Therefore,
⎯ for a complete building, P is the total perimeter of the building and A is its total ground-floor area;
⎯ to calculate the heat loss from part of a building (e.g. for each individual dwelling in a row of terraced
houses), P includes the lengths of external walls separating the heated space from the external
environment and excludes the lengths of walls separating the part under consideration from other heated
parts of the building, while A is the ground-floor area under consideration;
⎯ unheated spaces outside the insulated fabric of the building (such as porches, attached garages or
storage areas) are excluded when determining P and A (but the length of the wall between the heated
building and the unheated space is included in the perimeter; the ground heat losses are assessed as if
the unheated spaces were not present).
ISO 13370:2007(E)
8.2 Equivalent thickness
The concept of “equivalent thickness” is introduced to simplify the expression of the thermal transmittances.
A thermal resistance is represented by its equivalent thickness, which is the thickness of ground that has the
same thermal resistance. In this International Standard:
⎯ d is the equivalent thickness for floors;
t
⎯ d is the equivalent thickness for walls of basements below ground level.
w
The steady-state ground heat transfer coefficients are related to the ratio of equivalent thickness to
characteristic floor dimension, and the periodic heat transfer coefficients are related to the ratio of equivalent
thickness to periodic penetration depth.
9 Calculation of thermal transmittances
9.1 Slab-on-ground floor
Slab-on-ground floors include any floor consisting of a slab in contact with the ground over its whole area,
whether or not supported by the ground over its whole area, and situated at or near the level of the external
ground surface (see Figure 1). This floor slab may be
⎯ uninsulated, or
⎯ evenly insulated (above, below or within the slab) over its whole area.
If the floor has horizontal and/or vertical edge insulation, the thermal transmittance can be corrected using the
procedure in Annex B.
Key
1 floor slab
2 ground
w thickness of external walls
Figure 1 — Schematic diagram of slab-on-ground floor
8 © ISO 2007 – All rights reserved

ISO 13370:2007(E)
The thermal transmittance depends on the characteristic dimension of the floor, B' [see 8.1 and Equation (2)],
and the total equivalent thickness, d (see 8.2), defined by Equation (3):
t
dw=+ λ()R +R+R (3)
tsifse
where
w is the full thickness of the walls, including all layers;
R is the thermal resistance of the floor slab, including that of any all-over insulation layers above, below
f
or within the floor slab, and that of any floor covering;
and the other symbols are defined in 3.2.
The thermal resistance of dense concrete slabs and thin floor coverings may be neglected. Hardcore below
the slab is assumed to have the same thermal conductivity as the ground, and its thermal resistance should
not be included.
Calculate the thermal transmittance using either Equation (4) or (5), depending on the thermal insulation of the
floor.
′
If dB< (uninsulated and moderately insulated floors),
t
⎛⎞
2λ πB′
U=+ln 1 (4)
⎜⎟
πBd′ + d
tt⎝⎠
′
If dBW (well-insulated floors),
t
λ
U = (5)
0,457×+B′ d
t
NOTE 1 For well-insulated floors, it can be written alternatively as
U =
g
()R++RR +w λ+R
fsi se g
where R is the effective thermal resistance of the ground given by
g
′
0,457 × B
R =
g
λ
The thermal transmittance shall be rounded to two significant figures if presented as the final result.
Intermediate calculations shall be undertaken with at least three significant figures.
NOTE 2 The thermal transmittance can be small for large floors, so that more decimal places are needed.
The steady-state ground heat transfer coefficient between internal and external environments is obtained
using Equation (1).
9.2 Suspended floor
A suspended floor is any type of floor held off the ground, e.g. timber or beam-and-block (see Figure 2). This
clause deals with the conventional design of suspended floor in which the underfloor space is naturally
ventilated with external air. For mechanical ventilation of the underfloor space, or if the ventilation rate is
specified, see Annex E.
ISO 13370:2007(E)
The thermal transmittance is given by Equation (6):
11 1
=+ (6)
UU U +U
fg x
where
U is the thermal transmittance of suspended part of floor, in W/(m ·K) (between the internal
f
environment and the underfloor space);
U = is the thermal transmittance for heat flow through the ground, in W/(m ·K);
g
R
g
U is an equivalent thermal transmittance between the underfloor space and the outside accounting for
x
heat flow through the walls of the underfloor space and by ventilation of the underfloor space, in
W/(m ·K).
Key
1 floor slab
h height of floor surface above outside ground level
R thermal resistance of floor construction
f
R effective thermal resistance of ground
g
Figure 2 — Schematic diagram of suspended floor
The calculation of U shall include the effect of any thermal bridging. It may be calculated in accordance with
f
ISO 6946 or by a numerical method. In the case of a low-emissivity surface on the lower side of the floor, the
surface resistance may be modified using the procedure given in ISO 6946. Surface resistances for
downwards heat flow apply in the case of a heated building, and surface resistances for upwards heat flow
apply in the case of a cooled building.
10 © ISO 2007 – All rights reserved

ISO 13370:2007(E)
Calculate U by means of Equations (2), (7) and (8):
g
dw=+ λR +R+R (7)
()
gsifse
⎛⎞
2λ πB′
U=+ln⎜⎟1 (8)
g
⎜⎟
πBd′ + d
gg
⎝⎠
where R is the thermal resistance of any insulation on the base of the underfloor space, in m ·K/W.
g
If the underfloor space extends to an average depth of more than 0,5 m below ground level, U should be
g
calculated according to Equation (E.2).
If edge insulation is applied around the base of the underfloor space, U should be modified according to
g
Equation (B.3).
Obtain U from Equation (9):
x
hU εvf
ww
U =×2 +1450× (9)
x
B′′B
where
h is the height of the upper surface of the floor above external ground level, in m;
U is the thermal transmittance of walls of underfloor space above ground level, in W/(m ·K), calculated
w
in accordance with ISO 6946;
ε is the area of ventilation openings per perimeter length of underfloor space, in m /m;
v is the average wind speed at 10 m height, in m/s;
f is the wind shielding factor.
w
If h varies round the perimeter of the floor, its average value should be used in Equation (9).
Annex E gives equations for the calculation of the average temperature in the underfloor space.
The wind shielding factor relates the wind speed at 10 m height (assumed unobstructed) to that near ground
level, allowing for the shielding by adjacent buildings, etc. Representative values are given in Table 2.
Table 2 — Values of the wind shielding factor
Wind shielding factor
Location Example
f
w
Sheltered City centre 0,02
Average Suburban 0,05
Exposed Rural 0,10
The steady-state ground heat transfer coefficient between internal and external environments is obtained
using Equation (1).
ISO 13370:2007(E)
9.3 Heated basement
9.3.1 General
The procedures given for basements apply to buildings in which part of the habitable space is below ground
level (see Figure 3). The basis is similar to that for the slab-on-ground, but allowing for:
⎯ the depth, z, of the floor of the basement below ground level;
⎯ the possibility of different insulation levels being applied to the walls of the basement and to the floor of
the basement.
If z varies round the perimeter of the building, its mean value should be used in the calculations.
NOTE 1 If z = 0, the formulae reduce to those given in 9.1 for the slab-on-ground.
This International Standard does not directly cover the case of a building having partly a floor on the ground
and partly a basement. However, an approximation to the total heat loss via the ground from such a building
can be obtained by treating the building as if it had a basement over its whole area with depth equal to half the
actual depth of the basement part.
NOTE 2 Basements that are partly heated are treated in 9.5.
The procedures described give the total heat flow from the basement via the ground, i.e. through the floor of
the basement and through the walls of the basement below ground level.
NOTE 3 The parts of the walls above ground level can be assessed by their thermal transmittance calculated in
accordance with ISO 6946.
9.3.2 Basement floor
To determine U , calculate the characteristic dimension for the basement floor using Equation (3), and
bf
include any insulation of the basement floor in the total equivalent thickness, d , given by Equation (10):
t
dw=+ λR +R+R (10)
()
tsifse
where
w is the full thickness of the walls of the building at ground level, including all layers;
R is the thermal resistance of the floor slab, including that of any all-over insulation layers above, below
f
or within the floor slab, and that of any floor covering;
and the other symbols are defined in 3.2.
The thermal resistance of dense concrete slabs and thin floor coverings may be neglected. Hardcore below
the slab is assumed to have the same thermal conductivity as the ground and its thermal resistance should be
neglected.
Use either Equation (11) or Equation (12), depending on the thermal insulation of the basement floor.
′
If dz+<0,5 B (uninsulated and moderately insulated basement floors),
()
t
⎛⎞
′
2λ π B
U=+ln 1 (11)
⎜⎟
bf
′
πBd++ 0,5z d+ 0,5z
tt⎝⎠
′
If dz+ 0,5 WB (well-insulated basement floors),
()
t
λ
U = (12)
bf
′
0,457B++dz0,5
t
12 © ISO 2007 – All rights reserved

ISO 13370:2007(E)
Key
1 floor slab
R thermal resistance of floor construction
f
R thermal resistance of walls of the basement, including all layers
w
w thickness of external walls
z depth of basement floor below ground level
Figure 3 — Schematic diagram of building with heated basement
9.3.3 Basement walls
U depends on total equivalent thickness for the basement walls, d , given by Equation (13):
bw w
dR=+λ R+R (13)
()
wsi w se
where R is the thermal resistance of the walls of the basement, including all layers, and the other symbols
w
are defined in 3.2.
Obtain U from Equation (14):
bw
⎛⎞
0,5d ⎛⎞
2λ z
t
U =+1ln +1 (14)
⎜⎟
⎜⎟
bw
πzd +z d
tw
⎝⎠⎝⎠
The formula for U involves both d and d . It is valid for d W d , which is usually the case. If, however,
bw w t w t
d < d then d should be replaced by d in Equation (14).
w t t w
ISO 13370:2007(E)
9.3.4 Heat transfer from whole basement
...