1. Thermal transmittance
Properties of constructions
Thermal transmittance

2. Thermal transmittance
basic thermal property of building construction
This simple formula can be used for constructions without thermal bridges!
[W/(m2K)]
internal surface resistance external surface resistance
thermal resistance
layer thickness
thermal conductivity
[m2K/W]

3. Thermal transmittance
Requirements of ČSN :

4. dew point temperature (in following lectures)
Thermal transmittance
Requirements of ČSN :
dependent on internal RH and on prevailing
design internal temperature:
table
from 18 to 22 C
other temperatures
up to 60 %
over 60 %
eliminition of internal surface
condensation:
dew point temperature (in following lectures)

5. Thermal transmittance
Requirements of ČSN :
dependent on internal RH and on prevailing
design internal temperature:
table
from 18 to 22 C
other temperatures
up to 60 %
over 60 %
eliminition of
condensation:
recalculation:

6. Thermal transmittance
Requirements of ČSN :
basic table values:

7. Thermal transmittance
Requirements of ČSN :
for refurbished buildings:
same values as for new buildings
exceptions:
historical buildings with status of monuments
provable technical or legislative problems
alternative for large floor constructions adjacent to ground:
transmission heat loss accord. to
EN ISO for Δθ = θim- θe
requirement according to
ČSN

8. Thermal transmittance
Evolution of requirements:
revisions

9. Thermal transmittance
Calculation (EN ISO 6946):
Rules for inclusion of layers:
thin layers can be omitted (foils, membranes)
- only internal deck in ventilated constr.
[m2K/W]
only layers under waterproofing
with exception:
inverted roofs
only layers above waterproofing
in floors (excp.: XPS)

10. Thermal transmittance
Calculation (EN ISO 6946):
[m2K/W]
Thermal conductivity [W/(m.K)]:
it characterizes the ability to conduct heat
it depends on number of air voids …
(thermal conductivity is usually lower for materials with
lower density; exception: mineral wool)
… and on ratio between heat transfer in voids and in material frame itself
for isotropic materials the same in all directions; otherwise different (timber, hollow bricks)

11. Thermal transmittance
Calculation (EN ISO 6946):
every material has specific dependence (various curves)
water (better conductor than air, λ = 0,5 až 0,7)
[m2K/W]
Thermal conductivity [W/(m.K)]:
depends on moisture (significant increase)
and temperature (moderate increase, mainly for high temperatures, not important for BP)
bult-in state (compression, moisturizing)
Producers present declared
values – cannot be used directly –
recalculation necessary!
If some data missing, increase declared
value for approx. 10 %.
for calculations:
characteristic value (internal constructions without condensation)
design value
- tables in ČSN (normal spaces)
- calculation for moist spaces (ČSN or EN 10456)

12. Thermal transmittance
Calculation (EN ISO 6946):
[m2K/W]
Typical values:

13. Thermal transmittance
Calculation (EN ISO 6946):
Air spaces and layers:
taken into account by means of equivalent thermal conductivity
[m2K/W]
Types of air layers:
unventilated (openings less than 0,05 %)
slightly ventilated (opening 0,05 to 0,15 %)
ventilated (openings more than 0,15 %)
included in calculation
not included;
only layers from interior to ventilated air space are included, ventilated air layer is modeled by means of different Rse (see following)

14. Thermal transmittance
Calculation (EN ISO 6946):
Air spaces and layers:
taken into account by means of equivalent thermal conductivity
[m2K/W]
unventilated layers
basic values from tables in EN ISO 6946
according to heat flow direction
usable for border materials with usual
emissivity (above 0,8)
- and for thicknesses of layers under 300 mm

15. Thermal transmittance
Calculation (EN ISO 6946):
Air spaces and layers:
taken into account by means of equivalent thermal conductivity
[m2K/W]
unventilated layers
more exactly:
základní hodnoty z tabulek v EN ISO 6946
podle směru toku
použitelné pro hraniční materiály s běžnou
emisivitou (nad 0,8)
- a pro tloušťky dutin do 300 mm
details in previous
lectures
convective-conductive heat transfer coefficient
radiative heat transfer coefficient

16. slightly ventilated layers
Thermal transmittance
Calculation (EN ISO 6946):
Air spaces and layers:
taken into account by means of equivalent thermal conductivity
[m2K/W]
slightly ventilated layers
traditionally:
thermal resistance of slightly ventilated layer is ½ of thermal
resistance of unventilated layer (and cannot be higher than 0,15 m2K/W)
new approach in EN ISO 6946 (2008) :
slightly ventilated layer is taken into account during calculation
of U-value (details later)

17. Thermal transmittance
Calculation (EN ISO 6946):
Non-homogeneous layers:
they can be approximately considered by
means of eqv. thermal conductivity
[m2K/W]
- more exactly according to EN ISO 6946 and EN ISO (details later) A1 A2 A3

18. Thermal transmittance
Calculation (EN ISO 6946):
[W/(m2K)]
default values from EN ISO 6946
0,10
0,17
0,00
0,13
0,04

19. Thermal transmittance
Calculation (EN ISO 6946):
Constructions with tapered layers:
Calculation possibilities:
- tapered layers are omitted (U in min. thickness)
- calculation according to EN ISO 6946 (mean U-value)

20. Thermal transmittance
Calculation (EN ISO 6946):
Constructions with tapered layers:
Calculation possibilities:
- tapered layers are omitted (U in min. thickness)
- calculation according to EN ISO 6946 (mean U-value)
4 basic shapes
Roof is separated to individual areas with tapered layers characterized as one of 4 basic shapes.
For every shape, formula for mean U-value is defined. Final U-value for whole roof is calculated as mean value over individual areas.
+ 4 formulas:

21. Thermal transmittance
Calculation (EN ISO 6946):
Constructions with slightly ventilated air layers:
calculated 2x:
- for unventilated layer
for ventilated layer
Final value:
new approach,
several problems
area of openings
total thermal
resistance from surface to surface for unventilated air layer
resistance from surface to surface for ventilated air layer

22. Thermal transmittance
Calculation (EN ISO 6946):
Constructions with systematic thermal bridges :
regularly repeating spots in construction with increased heat
flow caused by more conductive materials

23. Thermal transmittance
Calculation (EN ISO 6946):
Constructions with systematic thermal bridges:
regular thermal bridges are part of the construction
cannot be omitted!
200 mm of insulation
without bridges: U = 0,21 W/(m2K)
reality: U = 0,26 W/(m2K)
Bridges:
increase for 24%

24. Thermal transmittance
Calculation (EN ISO 6946):
Constructions with systematic thermal bridges:
regular thermal bridges are part of the construction
cannot be omitted!
more calc. methods:
orientational
approximate
exact
0,0 to 0,15 (ČSN )
or using eqv. th. conductivity
calculation
EN ISO 6946
calculation from th. field
characteristic section of the construction b
- SW for thermal field calculation
correct values of Rsi!
result: therm. coupl. coefficient L
details later

25. Thermal transmittance
Calculation (EN ISO 6946):
Constructions adjacent to ground
are evaluated:
without ground
with ground
Attention:
They can be very different,
U-value including ground
is significantly
lower!
for evaluation of requirements:
without ground
exception: alternative requirement
for energy performance
calculations:
with ground

26. Thermal transmittance
Calculation (EN ISO 6946):
Constructions adjacent to ground
are evaluated:
without ground
with ground
Thermal transmittance including ground:
EN ISO 13370
4 basic categories:
floor on ground (incl. edge insulation)
floor above crawl space
heated basement
unheated basement
pro ověření požadavku:
bez zeminy
výjimka: alternativní požadavek
pro energetické výpočty:
se zeminou
U-value includes
also constructions
under floor
and ventilation

27. special procedures for:
Thermal transmittance
Calculation (EN ISO 6946):
Constructions adjacent to ground
special procedures for:
partial basement
partially heated bas.
U-value includes
also constructions
under floor
and ventilation

28. Thermal transmittance
Calculation (EN ISO 6946):
Constructions adjacent to ground
Floor on ground:
final thermal transmittance : w
horizontal
edge insulation:
vertical

29. Thermal transmittance
Calculation (EN ISO 6946):
Constructions adjacent to unheated spaces
are evaluated:
without unheated space
with unheated space
similar as constructions
adjacent to ground
for evaluation of requirements:
without unheated space
for energy calculations:
with unheated
space

30. Thermal transmittance
Calculation (EN ISO 6946):
Constructions adjacent to unheated spaces
are evaluated:
without unheated space
with unheated space
Calculation:
simplified in EN ISO 6946
detailed in EN ISO 13789
0,06 for attics with leaky roofs
0,20 for impermeable roofs
no information: Ue = 2,0 + n=3

31. Thermal transmittance
Calculation (EN ISO 6946):
Corrections to thermal transmittance:
for mechanical fasteners:
mainly metal fasteners
it can be omitted for fully plastic fasteners
for air voids in thermal insulations:
mainly one-layered insulations with butt joints
it can be omitted for tongue-groove joints and multi-layered insulations
for precipitation in case of inverted roofs:
the worst situation for butt joints of XPS
it can be reduced using tongue-groove joints of XPS or drainage diffusion membranes or by placement of insulation also below waterproofing

32. Thermal transmittance
Calculation (EN ISO 6946):
Corrections to thermal transmittance:
for mechanical fasteners:
Influence of fasteners can be omitted for:
fasteners with conductivity under 1 W/(m.K)
fasteners in air spaces
λf … conductivity of fastener [W/(mK)],
nf … number of f. in 1 m2 [1/m2], Af … cross-section area of 1 f. [m2],
R1 … th. resistance of insulation [m2K/W], RT… total th. resistance
from surface to surface of the construction [m2K/W].
Formula cannot be used for metal fasteners connected to metal layers – for such
thermal bridges, detailed 3D calculation is necessary!

33. Thermal transmittance
Calculation (EN ISO 6946):
Corrections to thermal transmittance:
for air voids in thermal insulations:
R1 … th. resistance of insulation [m2K/W],
RT… total th. resistance from surface to surface of construction [m2K/W].
0,00 for insulation without gaps (more layers, tongue-groove
connect.)
0,01 for gaps not causing air circulation (insulation with butt
joints)
0,04 for gaps causing circulation (low-quality realisation)

34. factor for increased h.loss
Thermal transmittance
Calculation (EN ISO 6946):
Corrections to thermal transmittance:
for precipitation in case of inverted roofs:
R1 … thermal resistance of insulation above
waterproofing [m2K/W],
RT… total th. resistance from surface to surface
of construction [m2K/W].
factor for increased h.loss
0,0481 W.day/(m2.K.mm))
drainage factor
standard 83%
rate of
precipi-
tation

35. Thermal transmittance
Calculation (EN ISO 10077):
Windows and doors:
measurement
calculation EN ISO 10077:
single windows
coupled windows
double windows Ag Af lg

36. Thermal transmittance
Calculation (EN ISO and EN ISO 13947):
Windows and doors:
measurement
calculation EN ISO 10077:
single windows
coupled windows
double windows
Curtain walling:
calculation EN ISO 13947:
characteristic section
2 methods for mean Ucw
very difficult calculation