Considerations for WUFI calculations from Manfred Kehrer

Everything concerning the adequate application of WUFI in New Zealand and Australia depending on local conditions e.g. architecture, building codes, standardization, laws et cetera.
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Thomas van Raamsdonk
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Joined: Mon Jul 30, 2012 8:01 am -1100
Location: Wellington, New Zealand

Considerations for WUFI calculations from Manfred Kehrer

Post by Thomas van Raamsdonk » Mon Jul 30, 2012 1:37 pm -1100

Hi everyone,

Please find below Manfred's summary on the recent WUFI workshops as emailed to all attendees:

Considerations for WUFI calculations in New Zealand
AMENDMENTS to the calculations in the recent WUFI
workshop in Wellington and Christchurch.


Calculation of the sd value [m]
sd value [m] = diffusion-resistant factor [-] x thickness of the building component in m

The minimum thickness for material specification in WUFI is 1 mm.
For material thicknesses of less than 1 mm, these have to be converted accordingly.

Conversion of sd [m] into MVTR [MNs/g]:
1 sd [m] = 0.19 MVTR [MNs/g]
1 MVTR [MNs/g] = 5.1 sd [m]

Component ->Assembly: quality and choice of materials

A large number of material data is available. Please check the data critically. Generic materials are sometimes too general and may require the use of the material-specific data, and should not be selected based on their name only.

Fibre Glass
In the American Generic Database "Fibre Glass" has a specific weight of 30 kg / m³ and "Low Density Fiber Glass Batt Insulation" has a specific weight of 88 kg / m³.

Wall wraps
After close examination the "60 minute building paper", used by me for calculations, is not suitable as a wall wrap membrane for normal construction simulation in New Zealand. The material is not weather-stable (rather serves as an intermediate layer) and has neither sufficient strength nor adequate waterproofing. In addition the diffusion resistance is too low at high humidity levels. Therefore it should not be used as wall wrap for calculations in New Zealand. Wall wrap currently used in New Zealand are not included in WUFI databases.

Proceed with calculations as follows:
The only difference between the wall wraps are the diffusion resistance. Other data i.e. moisture storage function, liquid transport coefficients, thermal conductivities und enthalpy are zero.
In the data base "Fraunhofer IBP Holzkirchen, Germany" you will find membranes with different diffusion resistances: weather resistive barriers with an sd value of 0.5 m (approx. 2.5 MNs/g) or vapour barriers with an sd value greater than 1 m (approx. 5 MNs/g).

-> For wall wraps up to an sd value of 0.50 m (approx. 2.5 MNs/g) use the data for a weather resistive barrier.
-> For wall wraps with an sd value great than 1 m (approx. 5 MNs/g) use the data of a vapor retarder.

Select the diffusion resistance from technical data sheets or inquire about the diffusion resistance from the manufacturer. In general it is sufficient to use the nearest value in the data base. Just round the value of the diffusion resistance to the nearest in the database.
Here are some examples of diffusion resistances (sd values) from the manufactures offering wall wraps in New Zealand:

pro clima SOLITEX EXTASANA: sd = 0.10 m
Thermacraft Cover-up: sd = 0.50 m
Marshall Waterproofing TEKTON: sd = 0.25 m

Please correct the calculations that we did together in the workshop with the appropriate membranes - with and without moisture management (i.e. INTELLO). This will result in different outcomes, especially with wall wraps with a higher diffusion resistance.
The generation of new materials for your own data base is possible. Just save the new product in your custom catalog. However often the moisture data storage functions, liquid transport coefficients, thermal conductivities and enthalpy are needed and usually not known. Thus, one cannot generate useful materials. It would be a risk of false results. BRANZ is working on measuring more typical building materials in New Zealand and will put them into a BRANZ database for WUFI. The New Zealand data base will be automatically included in one of the free WUFI up-dates.


Always simulate worst case scenarios:
-> First calculation with orientation to the south (diffusion).
-> Second calculation with orientation towards the highest driving rain.

Our calculations have shown that diffusion is the more decisive factor.
Surface Transfer Coefficient Wind: Please tick for wind-exposed parts "wind - dependent".

Weather protective coatings: You can configure weather protective coatings on the exterior surface with an sd-value. According to Stephen
McNeil BRANZ have measured values of sd 1 to 7 m. I would recommend based on the values increased an average value of 5 m, unless the vapour resistance is known.

Short-wave radiation absorptivity: Not to the material description but the colour is decisive.

Adhering Fraction of Rain: If you have a weather-proof colour on the outside, then you can set the value to 0, preferably under "User Defined".
PLEASE NOTE: After you have closed one case and create a new case, it may be that the value for Adhering Fraction of Rain jumps back to 0.7. Please check before each calculation that the value is set back to 0. When choosing rough wood or plaster, you should select the proposed value of 0.7, as long as there is no measured data available.

Under "Active Time Step Control" please select the button "Enable" to reduce the number of convergence failures.


Outdoor air:
Beside the already uploaded BRANZ files weather data with an certain meteorological standard can be chosen, but would need to be converge to a WUFI format. For example Meteonorm weather data is required in Australia where there are files uploaded. How this works, I'll show you in the next seminar.

Indoor climate:
Please select "User Defined Sine Curve Parameters". You have to adjust the height of indoor temperature accordingly to the hemisphere.

Temperature: Select a temperature range of 21 + / - 1 ° C. Please note that the peak in the southern hemisphere is in February.

Relative Humidity: As Relative Humidity BRANZ recommends a Mean Value of 60% with a 10% Amplitude.
You also need to adjust the day of maximum. I recommend having the Relative Humidity maximum delayed by 1 months compared with the Indoor Temperature.

Sources Sinks for rear vents: "New Air Change Source" and "New Moisture Source"
A leak in the wall can be simulated by using the "New Moisture Source" and then with "Fraction of Driving Rain" function. The leak may exceed 1% of the driving rain. Usually the leak is in the ventilation layer, i.e. onto the wall wrap. Under "Change Air New Source" you can simulate ventilation of the cavity. Assign he source to the ventilated cavity.
For both options please select under "Spread Area" the "whole layer" option.
"New Moisture Source" and "New Air Change Source" cannot be used at the same time. They have to be placed in separate layers, "New Air Change Source" in the ventilation cavity, "New Moisture Source" can be
placed in the next layer inwards. In this case do not use a “Air Layer without additional Moisture Capacity” for in combination with “New Moisture Source”.
BRANZ recommends ventilation with an air exchange rate ("constant") of 80. This value depends very much on the ventilation openings and the structure of the facade. If a drainage cavity is used, the rate of ventilation is determined only by the wall material. Depending on how wind-proof the material is. The less air enters through cracks into ventilation cavity, the lower the number of air changes.

If in doubt, you should carry out both calculations:

- With rain leakage and ventilation of the facade and
- No rain leak in the wall and no ventilation and assume the worst case scenario.

It should be noted that around windows and doors there are still areas which, because of construction details, have less ventilation, and should therefore be calculated without ventilation.

Sources Sinks for air leaks: "New Moisture Source"
The consequences of leakage in the air tightness layer, in well insulated buildings, is becoming more and more important (moisture leakage). With this tool, "New Moisture Source" and then with Air Infiltration IBP model, the effects can be simulated.
Please ensure that you have selected under "spread area" the position where condensation due to air flow from indoor is expected.
For a well-designed and executed airtight construction (confirmation through a Blower Door test), you can assume the value being 3 (Air Tightness Class B). Passive Houses require a value of 0.6 (choose a value of 1, Air Tightness Class A) for all other buildings, I would recommend a value of 5 (Air Tightness Class C).

The parameters of the air leakage classes should be included in any calculation.

Please check the "No Failure of Convergence". This should be as small as possible, preferably at 0. "Balance1" and "balance2" should differ as little or identical.

Monitor positions
Place monitor positions at critical sections of the construction (i.e. outer layer of the insulation). Here you can see the relative humidity and temperature, as well as the Isoplets and use WUFI Bio.
Ideal mould breading conditions are at high moisture and temperatures and therefore more critical than at in colder climates. This is especially true during the typical back-diffusion in New Zealand in summer.

Result Data
You can reduce file size by only selecting Courses/Profiles and do not save the Film data.

If you have any feedback on your calcualtions or suggestions/questions please post everything here.

All the best,


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