Air ventilated cavities and air unventilated cavities

[Alberto Morales]

Hi,

Playing around with air cavities in WUFI, Many questions raise so I hope they make sense.


1- In WUFI Help, If I am not wrong , it is recommended to model a ventilated air cavity (e.g. 50 mm) with "New Air change sources" with a heat/moisture source along the thickness of the air cavity layer, Am I right? but

with or without additional moisture capacity?

however, the source of moisture from rain from outside and moisture from inside the room is not defined and only that of the air (in ach) that ventilates the cavity. Are any rain or/and humidity from inside neglected due to both are "removed" by the air passing through the ventilated cavity? or will it depend on the ach number entered? which is the ach value that removes any effect of rain and indoor moisture?

2. for ventilated cavities, should the rain source of moisture of 1% be added in the outermost face of the insulation (by the ventilated air cavity) or innermost face of the ventilated cavities? what about the air leakages from the interior room? does the air leakage contribute to the effect of rain moisture source in case of being defined this last one in the outermost face of the air cavity?

IMAGE 1.jpg (108.23 KiB) Viewed 4557 times

3- in which situations (for ventilated and unventilated air cavities) do we have to introduce a heat sources? What fraction of the heat source do you put according to the material in front of the air cavity? for brick, will it be no heat source?

4- would it be ok to model it as I have done since the one on the left is a moisture source from rain (1%) and the one on the right comes from the Air infiltration of the interior (where humidity can condense) (5 mm of thickness each of the moisture sources)? (for ventilated and unventilated air cavities) (see image below)

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If case I am wrong, can you explain how a ventilated and unventilated chamber would be modelled in WUFI with all its exceptions/recommendations/considerations etc etc (e.g. width of moisture sources, typical fraction of heat sources for each material in front,...)


5- for unventilated and unventilated air cavities, is it more advisable to model it following way (1 mm - 48 mm - 1mm = 50 mm) (see image below)? (where the rain source will be the left film layer of 1 mm and the infiltration air leakage the right film layer of 1 mm). I found in WUFI FORUM where the 1 mm air film layers are with additional moisture capacity (the middle one is without additional moisture capacity) (see image below)?

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6- would the properties to change of each layer of air (air layer) be only those of Basic Material Data or also those of hydrothermal functions? since in the WUFI database there are no 1mm and 48mm layers.

7- How do you calculate the thermal conductivity of a 1 mm air layer? because the interpolation is not possible since it comes out linearly from the value 0 to 1 cm.
The 48mm would be between the 50mm and 40mm air layer?

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and the last one,

8-When I simulate both moisture sources (rain and leakage infiltration in the outermost and inner most face of the unventilated air cavity) and I add a heat source along the thickness of the air cavity,I get a problem with the balances (first image below), why?

thanks in advance

[Philipp]

Hi Alberto,

I try to answer some of your questions.

1. The answer is: It depends on what do you want to do.

Where in your construction is the air layer located and do you need to evaluate the water content inside the air layer?

If you model an air layer behind e.g. an internal gypsum board, where the water content inside the air layer doesn´t matter, you can just use a “normal” air layer (e.g. Air Layer 30 mm).

When you need to evaluate the water content inside the air layer, you better use one with the name e.g. “Air Layer 30 mm without additional moisture capacity”. Sometimes this leads to numerical problems especially when dew water formation take place. And in this case it is suitable to divide these one layer into three, where the middle one is an air layer “without additional moisture capacity. Also when you like to put a portion of the driving rain into the air layer a division is appropriate.
Usually we put thin “normal” air layers (1 mm) on the boundaries and put those “without additional moisture capacity” in the middle. All three layers together should result in the desired total thickness. If you do so, you can evaluate the water content.

But please keep in mind to set all the air layers with the material name of the desired total thickness. Otherwise the effective parameters for convection and radiation are not right anymore. For example, if you like to simulate a 30 mm air gap the construction is the following:

Air Layer 30 mm 0.001 m
Air Layer 30 mm; without additional moisture capacity 0.028 m
Air Layer 30 mm 0.001 m

“however, the source of moisture from rain from outside and moisture from inside the room is not defined and only that of the air (in ach) that ventilates the cavity. Are any rain or/and humidity from inside neglected due to both are "removed" by the air passing through the ventilated cavity? or will it depend on the ach number entered? which is the ach value that removes any effect of rain and indoor moisture? “

I´m not sure if I understand your questions right. An air change source can be connected with the interior or exterior climate and put the corresponding amount of air (heat and moisture) according the ACH into the air cavity, insulation or other materials. If you like to model the moisture infiltration (or exfiltration) from the inside caused by leakages in the building envelope please use the IBP infiltration model as a moisture source. This model will put the according amount of water into the construction if the conditions at the place of the source are below dew point. If you like to consider a share of the driving rain, please set another moisture source.

There is no specific ACH value to “remove” the moisture from rain or from the interior side out of the air cavity. This depends on the specific building design and especially the design of the air inlet an outlet opening. So you need to determine the requested ACH by iteration and have to ensure this on the real building site.

2. The approach of 1% of driving rain behind the plaster comes out of the ASHRAE Standard 160. This is intended to take into account any leaking connections (e.g. windows) in an ETICS. It is therefore tested whether the construction can withstand this amount of rain, i.e. whether it can dry out.

So it depends on the construction if the driving rain source needs to be set or not and where is the right place to set it. E.g. to be on the safe side in case of ventilated wall construction with a wooden sheathing the rain moisture source can be placed in the outermost part of the material behind the air cavity. But your case seems more like a ventilated brick wall, so I guess you have already an amount of driving rain direct on the façade. In this case it is possible to set the 1% driving rain source in the 1 mm air layer behind, to represent the water leaking through the joints.

As mentioned above, for the modelling of air infiltration or exfiltration please use the IBP infiltration model. The source should be placed inside the construction (not in the air layer!) where condensation is expected.

If you place an air change source with internal climate on the outermost face of the air cavity condensation will occur there under cold climatic conditions. Otherwise ACH has nothing to do with the rain moisture source.

3. For regular constructions you don´t need any heat source. This is just for special cases e.g. wall heating.

4. Unfortunately it´s not visible in the picture what kind of sources exactly set and what kind of materials you used. Therefore it is not possible to give any statement about this construction design.

“If case I am wrong, can you explain how a ventilated and unventilated chamber would be modelled in WUFI with all its exceptions/recommendations/considerations etc etc (e.g. width of moisture sources, typical fraction of heat sources for each material in front,...)”

A detailed description would definitely exceeds our capacity. So please see my descriptions above and the following guideline for more information:

https://wufi.de/en/wp-content/uploads/s ... 202011.pdf

5. Please see 1)!
6. I guess the questions 6 and 7 are also answered with point 1) because no air layers with the thickness of 1 or 48 mm are needed.

8. Sorry, you have not posted any image. But generally there is no need for any heat source in the given wall construction. Problems with the mass balances needs a further evaluation, suitable measures are e.g. a finer simulation grid, adaption of numerical parameters and so on.

Kind regards
Philipp

[Alberto Morales]

thanks for the answer but I do not know if I understood some part of it

1- I attached an image to clarify what I need to know when I modelled an unventilated and ventilated air layer

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Are my comments right?

2- How would I model the outermost air layer if it is an unventilated cavity instead of ventilated cavity?

[Philipp]

Hi Alberto,

I´m not sure what you didn’t understand, so I try to clarify.

Starting from the exterior side of the construction, the assembly can be look like this:

- Brick wall (or other masonry) 0.1025 m
- Air Layer 50 mm 0.001 m Here you set the 1% driving rain source
- Air Layer 50 mm, without additional moisture capacity 0.048 m Here you can set your ACH for a ventilated cavity
- Air Layer 50 mm 0.001 m
- Insulation Material 0.1 m The IBP Model source can be set in the outermost 5 mm of the insulation layer
- Air Barrier / VCL 0.001 m
- Unknown material 0.06 m
- Air Layer 30 mm 0.03 m (One layer is enough! See post above)
- Gypsum board 0.0125 m
- Gypsum Board 0.0125 m

Hope this helps you!

Kind regards
Philipp

[Alberto Morales]

thanks so much, this is what I need it to know it. I appreciate it

From the guideline of typical construction that is in the website. i have see this ventilated timber frame construction

Screenshot 2022-04-26 134606.jpg (98.34 KiB) Viewed 4408 times

. it is mentioned to add a 1 % rain but it is not indicated where. where should it be placed? could you indicate in the drawing for a better comprehension please

in one of the examples from the WUFI folder corresponding to "Example USA leakage Vent"

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, why is the 1% placed in the outermost face of the OSB instead of the inner face of the brick as you indicated in your last feedback? i can understand that it is done because it is thought that the building paper can have some tears but the moisture from the rain should pass through the air cavity. can you make the difference between this type of build up and my case because both are brick + air cavity?

sorry for being so annoying i hope you understand me

[Christian Bludau]

Dear Alberto,

next time please open up a new topic if the question is not related to the header anymore.

Generally spoken a source has to be included at the position, where the temperature or moisture will appear or in this case where rain water could be run in. So just image where water, which may run in eg. at a window sill will flow.

The German Standard DIN 4108-3 Annex D writes the following:

• 1 % of the driving rain hitting the component surface should be added as moisture source in the substructure.
• It shall be applied to the outside 5 mm of the moisture-sensitive substructure (for thinner layers: entire material layer).
• If this first layer of the substructure is a foil/underlay, no moisture source is to be applied

In the case of the USA example, if you would place the source on the inner surface of the brick it is still in the external facing and will probably transported to the outside quite fast. Here it will have about no harming influence on the wooden construction. Thats the reason, why it is placed behind the building paper. This way you can check if the construction is able to handle the extra moisture load.

Best regards
Chirstian

[Alberto Morales]

Thanks for your feedback once again.

I think this last message is related to the header because I need to know the strategy where to place moisture sources and why,

Anyway, I do not know what you mean with substructure and what is the foil or underlay in the substructure of the "Ventilated timber frame contruction" so that not to include the rain water as a moisture source, Coud you explain it again please?

in my case, following your indication of "Generally spoken a source has to be included at the position, where the temperature or moisture will appear or in this case where rain water could be run in. So just image where water, which may run in eg. at a window sill will flow", why should I put the rain water source in " Air Layer 50 mm 0.001 m Here you set the 1% driving rain source" as you indicated me?

I am a little bit confused

Sorry about my ignorance,

[Christian Bludau]

Hi,

with substructure the load bearing construction is meant. So the structure below the outer finish (eg. cladding).

The rain source should be placed at the position, where intruding rain water can run in. The intrusion may happen for example at the connection of the window sill to the wall. Philipp suggested the 1% driving rain behind the first layer, and so placed it in the air gap. Instead of placing the rain source in an air gap, where intruding water could run down directly, I would place it more in the bricks itself, where the water can be distributed due to the capillarity. You also can place it in the insulation behind the air gap, if you assume, the intruding rain water may come up to there.

Christian