External water repellent gypsum board

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johannbjorn
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External water repellent gypsum board

Post by johannbjorn » Tue Mar 19, 2019 4:59 am -1100

Dear forum,

I am interested in simulating a roof with gypsum boards as weather boards.

The type of board I am looking at is this GU-X 9 MM gypsum board from Norgips. It is a regular gypsum board with a water reppellent surface treatment (wax). (https://norgips.no/produkter/spesialpla ... endig-gu-x)

I unfortunately can't find the specific product in the WUFI material database. I only found one exterior gypsum board from LTH Lund database.

I have the following material properties of the GU-X 9mm:
Bulk Density [kg/m3] = 790
Water Vapor Diffusion Resistance Factor [ µ ] = 10
Sd – value [m] = 0.078
Thermal Conductivity [W/mK] = 0.21
Thermal resistance [(m2K)/W] = 0.04
Watertightness (EN 12467:2012): Water tight for 24hours @20mm water

Any help would be deeply appreciated so I can simulate the roof with these boards. Is there a similar gypsum board available (.xml) ? Should I model the waxing as a surface coating with a Sd-value 0.078m?

Warm regards,
Jóhann

Christian Bludau
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Re: External water repellent gypsum board

Post by Christian Bludau » Tue Mar 19, 2019 9:35 pm -1100

Hi Jóhann,

the panels are used below some tiles, right?
In this case the water absorption of the panels is not that important. If the producer guarantees, that it is not absorbing water, you can set it as an sd-value in the boundary conditions. Do not forget to switch off the rain water absorption, as a sd-value in the boundary only effects the vapor transport.

I´m wondering a little bit about the sd-value they give. The sd-value is the same what the usual gypsum boards for interior shows. I would have thought a treatment with wax would increase the sd value.

Christian

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Re: External water repellent gypsum board

Post by johannbjorn » Wed Mar 20, 2019 12:48 am -1100

Hi Christian,

The panels are used under a roofing felt. See attached image.

I agree that it is strange that the Sd-value is the same of a regular gypsum plate whereas the GU-X is wax coated. I will ask the producer regarding the value.

In the image below you can see the section of the roof.

As there is roofing felt on top of the gypsum plates, how is it possible to edit it's boundary condition (Sd-value)?

The method I choose:
1. I will assume there is large enough air exchange under the roof metal cover, so I will not model the roof facade and air gap.
2. I will therefore consider the surface of the roofing felt as the exterior surface. I will choose the material for the roofing felt as a diffusion open felt (weather resistant, Sd-value probably around 0,5m. Any ideas on what kind of material this should be? and how I should model this material, thickness etc.)
3. I set the rain to zero (rain absorption factor of exterior surface to zero).
4. If I skip the metal cover the next step is to adjust the thermal surface transfer coefficients of the roofing felt in WUFI. This is because we have to take into account radiation from the sun. Wufi guideline (red roof red tiles?): effective short-wave absorptivity as=0.6 and an effective heat transfer coefficient of a=19 W/m²K. Is it appropriate to use these values?
5. The vapor barrier: divide the vapor diffusion thickness (sd=100000?) of the vapor barrier by 1 mm, use the result as the diffusion resistance factor (Sd/0.01m=10e6=µ) and insert the membrane (1mm) in the component assembly.

Warm regards, Jóhann


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Christian Bludau
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Re: External water repellent gypsum board

Post by Christian Bludau » Wed Mar 20, 2019 2:24 am -1100

Hi Johann,

i think your way is good. Second possibility would be to build the roof with the metal cover. For that you can change the existing exterior gypsum board to fit with your values and build it into your construction followed by the roofing felt, an air layer, which you could ventilate using a ventilation source if needed and then the metal deck.

To your questions:
(2) For the gypsum board you should use a similar material and change it to your properties. You need to see, if that accumulates moisture. So start over with the sd-value of your roofing felt in the boundary conditions, the first layer then should be the gypsum board.
(4) What kind of metal is that? As absorptivity/emissivity maybe "Aluminium" from the drop down menu might fit? Or the surfaces from Rheinzink? Tiles red might be to dark? The heat transfer for roof with 19 W/m²K is the right one.
(5) I would build the barrier in with 1mm. You can transfer it the way you write, but you also could just use 1500m as thats what the German Standard defines as vapor barrier. And then I would build in the two gypsum boards. But you also can skip the barrier and the layer in the construction and just set an sd-value in the interior boundary condition.

Best regards,
Christian

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Re: External water repellent gypsum board

Post by johannbjorn » Wed Mar 20, 2019 6:17 am -1100

Hi Christian,

Thank you for your answers.

Just to clearify:

I. The second possibility:
a. "For that you can change the existing exterior gypsum board to fit with your values" ,
I am not fully sure what is meant by this, could you explain this?
b. If I use a ventilated gap element, I simply add an air layer and assign a air exhange source rectangle over the whole air element, if I am correct?
c. If we e.g. assume a small portion of the rain infiltrates the aluminum metal cover into the air layer, is it possible to model this?
d. If this second possibility is executed, it's not possible to add a boundary condition Sd-value to the roofing felt as it would only be assigned to the exterior boundary of the aluminum cover, if I am correct?

2. "For the gypsum board you should use a similar material and change it to your properties. You need to see, if that accumulates moisture. So start over with the sd-value of your roofing felt in the boundary conditions"
I should then skip the 1mm vapour open roofing felt and assign the Sd-value of the roofing felt to the exterior boundary of the gypsum. Simply increasing the Sd-value of the gypsum from 0.078m to the Sd-value of the roofing felt, is that a correct understanding?

4. The metal is painted aluminum, correct.

5. Understood.

Warm regards, Jóhann

Christian Bludau
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Re: External water repellent gypsum board

Post by Christian Bludau » Thu Mar 21, 2019 1:33 am -1100

Hi Johann,

1a) in WUFI you can create own userdefined materials by editing/changing an existing material, which is close to that you want. In the WUFI DB click with right mouse button on a material (e.g. external gypsum board) and select "+ New material". This way you can enter the gypsum board into your database.
b) correct, you have to assume some air exchange rate.
c) mhhh, this drops probably should run down. You can try to express that by using the driving rain source, but you again have to assume a source strength.
d) thats correct.

2) yes, thats right

Best regards,
Christian

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