25
Journal of Sustainable Architecture and Civil Engineering 2019/2/25
Corresponding author: kehl@holzbauphysik.de
Moisture Safety of
the Construction with
VCL Using Hygrobrid
Technology Received
2019/01/09
Accepted after
revision
2019/05/02
Journal of Sustainable
Architecture and Civil Engineering
Vol. 2 / No. 25 / 2019
pp. 25-34
DOI 10.5755/j01.sace.25.2.22422
Moisture Safety of
the Construction with
VCL Using Hygrobrid
Technology
JSACE 2/25
http://dx.doi.org/10.5755/j01.sace.25.2.22422
Martin Polikarpus
Siga Cover AG, Rütmattstrasse 7, CH-6017 Ruswil, Switzerland
Daniel Kehl
Büro für Holzbau und Bauphysik, Nixenweg 14, D-04277 Leipzig, Germany
Unventilated and green flat roofs with wooden construction and insulation between the rafters are used
in new buildings. Due to the external vapour-tight construction, such roofs have no drying capacity on the
outside. The scope of this study is to assess whereas the wood degradation in the wood or OSB caused
by wood destroying fungus can take place as this can affect the structural function of the construction.
Results show, that the 3rd generation moisture variable vapour control layer, which has additional
function that the Sd-value changes according to the direction of the diffusion, has many application
areas. The behaviour of the vapour control layer has a positive effect on the moisture balance of an
external vapour-tight green roof. The moisture and direction variable performance of the 3rd generation
vapour control layer has also a positive effect on the moisture balance of an external vapour-tight
pitched roof. It is important to underline that presented results are valid for Central-European climatic
conditions.
Keywords: building physics, diffusion tight flat roof, diffusion tight pitched roof, hygrobrid, moisture
variable
Introduction
Unventilated and flat green roofs
Unventilated and green flat roofs with wooden construction and insulation between the rafters are
used in new buildings. Due to the external vapour-tight construction, such roofs have no drying
capacity on the outside. This is a typical application area for a moisture variable vapour control
layer, which has a higher diffusion resistance in winter, when the inside air is dry, and a lower dif-
fusion resistance in the summer, when the humidity is re-diffused and the room air is more humid.
Since flat roofs are a standard in new buildings, the moisture from screed and plaster should also
be considered.
Pitched roofs with vapour-tight underroof
The functioning roofs of old buildings can energetically renovated from the inside. In previous
times, such roofs were often constructed with a vapour-tight underroof (e.g. bitumen sheeting on
wooden framework). Therefore, the internal vapour control layer is of particular importance also
in this type of construction.
Journal of Sustainable Architecture and Civil Engineering 2019/2/25
26
The scope of the simulations is to assess whereas the wood degradation in the wood or OSB
caused by wood destroying fungus can take place as this can affect the structural function of the
construction.
Fig. 1
Calculated
Sd-value curves
of the 1st and 2nd
generation moisture
variable VCL-s (left).
Directional Sd-value
curves of the
3rd generation
VCL – blue curve:
room → insulation;
red curve – insulation
→ room (right)
Methods
Vapour
Control
Layer
The simplified Glaser method according to DIN 4108-3: 2014 and SIA 180: 2014 is not applicable
for external vapour-tight, green flat roofs, and vapour tight pitched roofs in wooden construction
with inward laying moisture control layer. In addition, such a simplified method cannot be used
to analyse different parameters in terms of building physics. In such cases, hygrothermal sim-
ulations could be used (EN 15026: 2007; WTA 6-2: 2014). The roof types can be examined and
analysed using the software Delphin 5.9. These analyses will demonstrate the performance of
the newly developed 3rd generation vapour control layer with Hygrobrid technology (3rd generation
MVVCL). The vapour control layer is not only moisture variable but also direction variable, and it is
compared in this examination with other moisture variable vapour control layers with small and
large spread.
Moisture variable vapour control layer
There are currently many different moisture variable vapour control layers available on the mar-
ket. The first generation of vapour control layers with moisture variable diffusion resistance has
a small spread between high and low diffusion resistance (Fig. 1). Since then, they have been
developed further, so that the 2nd generation vapour control layers have a larger spread of diffu-
sion-equivalent air layer thickness (Sd-value) (Fig. 1).
Moisture and direction variable vapour control layer
Besides the moisture variability, this patented vapour control layer has an additional function – its
material properties allow different Sd-value at the same ambient air humidity, according to the
direction of the diffusion flux. This way, the effect of the moisture variability is strengthened. More-
over, the diffusion of the water vapour in the construction is reduced due to the elevated Sd-value
(Fig. 1) at higher room air humidity (e.g. moisture from screed or plaster during construction
phase). The presented curves are based on measurements carried out by TU Dresden.
27
Journal of Sustainable Architecture and Civil Engineering 2019/2/25
Flat roof
External vapour-tight and green flat roof in wooden construction is simulated hygrothermally
(Table 1). The roof is provided with a sealing (construction site seal) above the OSB panel with ad-
ditional insulation of a further seal. As several studies have determined, construction of green roof
has an unfavourable effect on its drying capacity. The section of a flat roof is shown in Fig. 2. Layer
numbers correspond to Table 1.
Table 2
Structural building
components of the
calculated pitched
roof
Table 1
Structural building
components of the
calculated flat roof
construction
Fig. 2
Section of flat roof
Layer
number
Description Thickness [mm] Thermal conductance [W/(m·K)] Vapour resistance [-]
1 Green roof – – –
2 Roof seal 1.0 2.3 100 000
3 On-roof insulation 120.0 0.028 100
4 Bituminous sheeting
with aluminium insert
1.0 2.3 1500 000
5 OSB panel 22.0 0.13 165
6 Mineral fibre insulation
cellulose insulation
240.0
240.0
0.035
0.040
1
1.5
7 Vapour control layer 0.16 2.3 800-35000
8 Air layer (batten) 25.0 0.18 0.46
9 Gypsum plasterboard 12.5 0.2 8.3
Layer
number
Description Thickness [mm]
Thermal conductance
[W/(m·K)]
Vapour resistance [-]
1 Roof covering – red clay tiles - - -
2 Air layer (counter batten) - - -
3 Bituminous sheeting 2.0 2.3 100 000
4 Solid wooden formwork 22.0 0.09 130
5 Mineral fibre insulation
cellulose insulation
240.0
240.0
0.035
0.040
1
1.5
6 Vapour control layer 0.16 2.3 800-35000
7 Air layer (batten) 30.0 0.18 0.46
8 Gypsum plasterboard 12.5 0.2 8.3
Construction
Calculations
Pitched roof
Unventilated and externally vapour-tight pitched roof with ventilated roof covering is simulated
hygrothermally (Table 2). The roof is provided with an external vapour-tight underroof on solid
Journal of Sustainable Architecture and Civil Engineering 2019/2/25
28
Table 4
boundary conditions
for the hygrothermal
simulations of
pitched roofs
wooden formwork and represents the classic building standards of the 1960s and 1970s. The first
studies have shown (Künzel 1998) that the external and internal climate, the inclination of the
roof and the orientation are important influencing factors for the drying performance of the roof.
Convection through leaks is an additional factor to consider. The section of calculated pitched roof
is given in Fig. 3. Layer numbers correspond to Table 2.
Boundary conditions used
The results of the hygrothermal simulations are always in connection with the applied boundary
conditions (Table 3, Table 4).
Parameter Description of boundary conditions
Outdoor climate: Holzkirchen humidity reference year
Orientation: North
Indoor climate:
Internal climate according to EN ISO 13788: / WTA 6-2: 2014
20-25 °C and 35-65 % relative air humidity
Surface transfer coefficient outwards:
(convective component)
Heat transfer resistance: 0,08 m²K/W
Heat transfer coefficient: 12,5 W/m²K
Surface transfer coefficient inwards:
Heat transfer resistance: 0,125 m²K/W
Heat transfer coefficient: 8 W/m²K
Short-wave absorption coefficient a: a = 0,6 [-] (Künzel 1998)
Long-wave emissions coefficient ε: ε = 0,3 [-] (Künzel 1998)
Source of humidity
Airtightness:
Airtightness class B (q50 = 3 m³/m²h)
Height of the air column in the building: 10 m
Moisture load 250g/m2y
Start/duration of the simulation: 01.10. / 10 - 15 years to a steady state
Starting humidity:
20 °C / 80 % relative air humidity
(cellulose: 50 %)
Table 3
Boundary
conditions for the
hygrothermal
simulations for
flat roofs
Fig. 3
Section of
pitched roof
Parameter Description of boundary conditions
Outdoor climate: Holzkirchen humidity reference year
Indoor climate: Internal climate according to EN ISO 13788: / WTA 6-2:2014 - 20-25 °C
and 35-65 % relative air humidity or building moisture
Surface transfer coefficient
inwards:
Heat transfer resistance: 0,125 m²K/W; Heat transfer coefficient:
8 W/m²K
Source of humidity
Airtightness:
Airtightness class B (q50 = 3 m³/m²h)
Height of the air column in the building: 10 m
Moisture load 250g/m2y
Start/duration of the simulation: 01.10. / 10 – 15 years to a steady state
Starting humidity: 20 °C / 80% relative air humidity (cellulose: 50%)
Simulation
Boundary
Conditions
29
Journal of Sustainable Architecture and Civil Engineering 2019/2/25
Explanation of the boundary conditions
Outdoor climate: Künzel (1998) has shown that with the external vapour-tight pitched roofs, the
humidity of the outer wooden framework with the same roof inclination essentially depends on
the outside temperature. The colder it is, the bigger the damage to the construction of the build-
ing could be. Holzkirchen, together with Hof, is one of the most critical sites and covers locations
with an annual average temperature of at least 6.6 °C (in comparison Stockholm and Oslo has
an annual average temperature of 6.8 °C with annual sun radiation difference of 2-4%). Since the
climate data set in Holzkirchen (reference year for humidity) was of high quality for the simulation,
this data set was used.
Moisture load and convection: The 22 mm thick wood and 22 mm OSB is divided into two layers
in the simulation: 12 mm and 10 mm. The 10 mm layer is situated towards the inner side of the
construction and moisture load of 250g/m2y is applied to this layer. In this way leaks through
convection are taken in account.
Green roof: Green roofs have an impact on the hygrothermal performance of the flat roof. As
there is no validated material data set for the green roofs in Delphin 5.9, the climate data set was
generated from temperature and relative air humidity. In addition, the flat roof described above is
simulated using WUFI® including a 80 mm green roof (single layer substrate). The temperature
and relative air humidity above the seal has been recorded and transferred into a climate data set
for Delphin. The 80 mm layer of green roof is taken from the research project [IBP 2013] of Fraun-
hofer Institute for Building Physics and has been validated on various roofs in Europe (Leipzig,
Vienna, Holzkirchen, Milan).
Built-in moisture: As the examinations of the Fraunhofer Institute for Building Physics show
(Holm, Künzel 1999), the wet masonry walls with outside insulation dry between 2-4 years, de-
pending on the insulation material (mineral fibre and EPS). Therefore, an assumption is made that
the relative room air humidity only adapts to the normal indoor climate in the residential buildings
in the course of three years as derived in the study by Zirkelbach, Holm in 2001 using room sim-
ulations.
Orientation: According to the orientation of the roof, the solar radiation onto the pitched roof is
different - it is the highest towards the south and lowest towards the north. The pitched roof ex-
amined in this study is towards the north direction and is therefore in an unfavourable situation.
Evaluation
Criteria for
Oriented
Strand
Board
(OSB) and
Wood
Oriented strand board
In case of high material moisture, the strength parameters of OSB are reduced. This can lead to
both a reduction of the load capacity and the usability (DIN 68800-2: 2012). This is to comply with
the limit value of 18 M-%. According to the standard, it may be temporarily exceeded up to 20
M-%, if it can re-dry within 3 months.
Safety margin for OSB
The safety margin is determined by means of the limit value of 18 M-%. In this case, the smallest
margin between the OSB moisture and the limit value in the steady state is calculated. If the value
is negative, the limit is exceeded, if it is positive, then it is not exceeded. Definition of the safety
margin for OSB are shown in Fig. 5.
In addition to the safety margin of the limit curve, the amplitude of the OSB is evaluated. Damage
cases are known, where a bulge in the construction / pressing apart of the underlying walls ap-
peared due to significant changes of moisture in OSB. Therefore, it is recommended in WTA MB E
6-8: 2015, chapter “Usability”, that the humidity fluctuation between summer and winter is to be
constructively minimized (e.g. by additional external insulation).
Journal of Sustainable Architecture and Civil Engineering 2019/2/25
30
Evaluation criteria for wood
For the assessment of wood moisture content, the so-called 20 M-% wood moisture criteria is of-
ten used. As stated by Kehl (2013), such a limit value is unsuitable for the evaluation of the hygro-
thermal simulation. In such cases, a detailed demarcation is needed, which couples the height and
duration of the humidity with the temperature. For this purpose Kehl (2013) derived a limit based
on Finnish investigations (Viitanen, Ritschkoff 1991). As a result of the study, no wood degradation
took place following 12 months of storing wood samples with inoculated wood-destroying fungi
in climatic conditions that lie on the limit value line.
Fig. 4
Limit curve according
to [WTA E 6-8 2015]
for the evaluation of
the pore air humidity
and temperature
under a 10 mm thick
wood layer, which
may not be exceeded
in the daily average
Fig. 5
Example for the definition
of the safety margin
between the recorded
daily average of the
wooden composite
moisture and the limit
value in the 10th year
steady state (Left).
Example of the definition
of the safety margin
for wood between the
entered average daily
values and the limit curve
in the 5th year in steady
state (right)
Since the wood moisture content spreads due
to different absorption isotherms in the indi-
vidual programs, the wood moisture content is
not evaluated but rather the relative humidity
in the material (pore air humidity) is taken in
account. This type of evaluation has proven it-
self in numerous simulations and can be found
in the corresponding regulations (WTA MB E
6-8 2015). The temperature and relative pore
air humidity in the wood (10 mm thick lay-
er) is averaged over the day and represented
in Fig. 4.
Safety margin for Wood
The safety margin is determined in Fig. 5, which shows the border limit and an example of the
measured daily values for the 5th year. The smallest distance between the value pairs (tempera-
ture / relative pore air humidity in the material) and the limit curve in the steady state are calcu-
lated. In case of a negative value, the limit curve is exceeded, it may result in wood degradation
by wood-destroying fungi; in case of a positive value it cannot. If the safety margin is very small, it
must be observed, whether the long-term humidity level is low.
Results
Safety margin with mineral fibre insulation for flat roofs
The safety margins of the (OSB) moisture when using the different vapour control layers are
shown in Fig. 6. When using mineral fibre insulation and considering the convection (LDK B,
h = 10 m), the 3rd generation MVVCL has a safety margin up to 1.1 M-% compared to other vapour
control layers. If the safety margin is negative then it exceeds the limit value.
31
Journal of Sustainable Architecture and Civil Engineering 2019/2/25
Moreover, the humidity fluctuation of an OSB panel with 3rd generation MVVCL is lower, 0,8 M-% per
year (see Fig. 7). Due to lower amplitude, the deformations of the OSB panel are also reduced.
Safety margin of moisture building
Since flat roofs are mainly used in new buildings, the moisture behaviour within the first few years
after completion of the building is considered in Fig. 7. As already described above, the increased
moisture building is taken into account within the first 3 years. Although the moisture and di-
rection variable performance of the 3rd generation VCL slightly exceeds the limit (18.2 M-%), the
duration of the exceedance is less than 3 months, which is permissible according to DIN 68800-2.
Fig. 6
Safety margin of an
external vapour-tight flat
roof with different vapour
control layers and mineral
fibre insulation. The 3rd
generation MVVCL is the
only one with a safety
margin to the limit value.
The others exceed the
limit value of 18 M-%
Fig. 7
Humidity fluctuation of
the OSB panel between
summer and winter
(amplitude)
1,1%
-0,8% -0,2%
-1,0%
-0,5%
0,0%
0,5%
1,0%
1,5%
3. generation MVVCL 1. generation MVVCL 2. generation MVVCL
Sa
fe
ty
m
ar
gi
n
(M
-%
)
incl. convection / air humidity according to WTA 6-2: 14
Safety margin Flat roof (mineral fibre) - Holzkirchen
0,8%
1,7%
1,2%
0,6%
0,8%
1,0%
1,2%
1,4%
1,6%
1,8%
A
m
p
lit
u
d
e
O
S
B
p
a
ne
l (
M
-%
)
incl. convection / air humidity
according to WTA 6-2: 14
Amplitude OSB panel Flat roof
(mineral fibre) - Holzkirchen
18,2%
20,9% 19,5%
16,9%
18,8% 18,2%
0%
5%
10%
15%
20%
25%
3. generation
MVVCL
1. generation
MVVCL
2. generation
MVVCLM
ax
O
SB
m
oi
st
ur
e
(M
-%
)
incl. convection / increased building
moisture
Max OSB moisture (Mineral wool) -
Holzkirchen
1-3 years steady state
The lower the value, the less deformations resulting from it (left). Maximum material moisture of
the OSB panel in the first 3 years following the production (with the estimated building moisture) and
the long-term performance. With its moisture and direction variable vapour control layer, the 3rd
generation MVVCL shows a strong inclination towards building moisture (right).
Safety margin for pitched roofs
The roof is gradually tilted north, which reduces the solar short-wave radiation onto the roof and
thus the re-drying capacity of the building component. The safety margin between the 3rd gen-
eration MVVCL and the moisture variable vapour control layer of the 1st and 2nd generation in
conjunction with the roof inclination is shown in Fig. 8. If the safety margin is very small (yellow
area), the data must be analysed more precisely. The steeper the roof inclination is, the lower the
level of summer re-drying and thus the safety margin. Accordingly, the application area of the 3rd
generation MVVCL in case of a north orientation is possible up to a roof inclination of 80° and thus
also for steep mansard roofs.
Journal of Sustainable Architecture and Civil Engineering 2019/2/25
32
The moisture variable vapour control layer of
the 1st generation at 40° in a steady state has
a small safety margin of 1.2 M-% relative to
air humidity (Fig. 9). However, it exceeds the
limit curve in the first year of the simulation.
The moisture variable vapour control layer of
the 2nd generation can be constructed up to a
roof inclination of 60°. If cellulose is used in the
roof construction instead of mineral fibre, the
pitched roof with all the vapour control layers
may only be constructed up to a roof inclina-
tion of 40°. In case of a 40° and 60° inclined,
north-oriented roof, the safety margin of the
Fig. 8
Safety margin of an
external vapour-
tight pitched roof
(north orientation)
with different vapour
control layers in
function of roof
inclination (mineral
fibre insulation)
Fig. 9
Safety margin of an
external vapour-
tight pitched roof
(north orientation)
with different vapour
control layers in
function of roof
inclination
(left: 40°, right: 60°)
Fig. 10
Evaluation of the
pore air humidity/
temperature
according to
WTA-leaflet E 6-8
in 10 mm wooden
formwork for 1st, 2nd
and 10th year
3rd generation VCL will be again compared with the moisture variable vapour control layer of the
1st generation and the 2nd generation (Fig. 9).
Daily calculated pore air humidity/temperature values for 3rd generation MVVCL and the limit
line for mould growth according to WTA-leaflet E 6-8 in 10 mm wooden formwork on 1st, 2nd and
10th year are presented on Fig. 10 dealing with pitched roofs with mineral fibre insulation, North
orientation 60o pitch. Delphin calculation results for development of the pore air humidity in a 10
mm thick wooden layer and the corresponding condensing limit value according to WTA-leaflet E
6-8 are given in Fig. 11.
33
Journal of Sustainable Architecture and Civil Engineering 2019/2/25
The 3rd generation moisture variable vapour control layer, which has additional function that the
Sd-value changes according to the direction of the diffusion, has many application areas. The be-
haviour of the vapour control layer has a positive effect on the moisture balance of an external
vapour-tight green roof. Even in case of unfavourable conditions, there is still a re-drying capacity.
As compared with the moisture variable vapour control layers of the 1st and 2nd generation, it has
a high drying potential.
The moisture and direction variable performance of the 3rd generation vapour control layer has
also a positive effect on the moisture balance of an external vapour-tight pitched roof. Even in
unfavourable conditions (e.g. 80° - north – mineral fibre insulation - with convection), a re-drying
capacity still exists. As compared to the moisture variable vapour control layer of the 1st and 2nd
generation, the vapour control layer has a greater drying potential also in these constructions.
It is important to underline that presented results are valid for Central-European climatic condi-
tions. Authors of this study have conducted numerous simulations for Northern-European cli-
mate (Oslo, Tromso, Stockholm, Helsinki) which will be presented in further research.
References
Fig. 11
Development of the
pore air humidity
in a 10 mm thick
wooden layer and
the corresponding
condensing limit
value according to
WTA-leaflet E 6-8
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MARTIN POLIKARPUS
Deputy Head of Application Engineering
North Europe
Siga Cover AG, Application Engineering
North Europe
Main research area
Building physics and material behavior
Adress
Rütmattstrasse 7, CH-6017 Ruswil, Switzerland
Tel. +41 41 499 69 69
E-mail: martin.polikarpus@siga.swiss
About the
Authors
DANIEL KEHL
Consultant for building physics
Bü ro für Holz bau und Bau phy sik
Main research area
Building physics and hygrothermal behavior
Adress
Ni xen weg 14, 04277 Leip zig, Germany
Tel. +49 0341-52941138
E-mail: kehl@holzbauphysik.de