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 Post subject: Non Insulating Air Layer
PostPosted: Tue Oct 18, 2011 5:00 am -1100 
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WUFI User
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Joined: Tue Sep 27, 2011 3:54 am -1100
Posts: 16
Hi I am wondering how best to model a non insulating air layer, that is to say a layer that is open to the atmosphere i.e. behind a rain screen cladding.

While I know I could simply ignore the rainscreen and the air layer and set the next outersmost material's rain adhering fraction to zero, I just wanted to see how I could set up the construction correctly in Wufi.

Should I add an air source with 100 air changes per hour.. or is there another way?


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 Post subject:
PostPosted: Tue Oct 18, 2011 5:49 am -1100 
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WUFI International Support Team
WUFI International Support Team

Joined: Thu Mar 10, 2005 2:17 am -1100
Posts: 326
Location: Northbrook, IL; USA
Hello David,

nice to hear from you. What is the thickness of your air layer?

_________________
Manfred
Wiss, Janney, Elstner Associates, Inc.
Official WUFI® Collaboration Partner for USA/Canada
Enjoy WUFI® :) .... It is easy and complex.


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 Post subject:
PostPosted: Tue Oct 18, 2011 9:14 am -1100 
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WUFI International Support Team
WUFI International Support Team

Joined: Wed Dec 15, 2010 6:27 am -1100
Posts: 12
Hi David,

Your approach in either case is correct. If you're assuming a very highly ventilated cavity, you can just ignore the layers outside the cavity as you said, just make sure you also turn off solar radiation in addition to the parameters you mentioned.

If you want to simulate the cavity and outer layers (as you would need to if you were concerned about the outer leaf), then you're correct, you model an airspace with an air change source and then whatever layers exist beyond it.

The trick with the second method is knowing what to use for an air change rate in the cavity. This will depend on a number of factors. There is an interesting paper out of Oak Ridge National Labs in America that discusses cavity ventilation and references ventilation rates from 0.3 to 50 ach depending on the conditions. You can find the paper at:

http://www.ornl.gov/sci/roofs+walls/sta ... rs/210.pdf

It also include references to a number of other sources that may be useful.

Hope that helps!

Calina


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 Post subject: Non Insulating air layer
PostPosted: Tue Oct 18, 2011 10:24 pm -1100 
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WUFI User
WUFI User

Joined: Tue Sep 27, 2011 3:54 am -1100
Posts: 16
Thanks Manfred and Calina,

I am actually interested in modelling 30 mm thick stone when used as a rainscreen as part of a 'ventilated' rainscreen.

I want to see if it reaches saturation and if so for how long.

Another way I thought of modeling it is to assign Irish climate data to both sides but to turn of radiation and driving rain to the 'inside' however I want to see if the adjoining construction which will provide some radiation will affect the result so that is why I am thinking of modelling the entire buildup.

Like you said Manfred easy and complex


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 Post subject:
PostPosted: Wed Oct 19, 2011 3:10 am -1100 
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WUFI International Support Team
WUFI International Support Team

Joined: Wed Dec 15, 2010 6:27 am -1100
Posts: 12
Hi David,

Setting an 'indoor' climate which matches the outdoor temperature and humidity would be another way to do it if you're only interested in the outer leaf. Obviously this would inherently assume very high ventilation rates in order to have the cavity match the exterior conditions.

In response to you comment about radiation, WUFI only calculates radiation to and from the external surface (i.e. solar/short wave radiation absorbed, and nighttime cooling/long wave radiation released). It doesn't calculate radiation heat transfer with the interior space, nor does it account for radiative effects between layers.

I'm not sure of the details of your wall, but if it is internally insulated, then the inner masonry leaf will be fairly cool and there would be very little radiation heat transfer across the cavity anyway. In a highly ventilated cavity, the heat transfer will be almost entirely heat loss to the air by conduction/convection.


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