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 Post subject: General WUFI Questions
PostPosted: Fri Sep 23, 2011 3:50 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
Since we our two WUFI courses last week (in Glasgow and Dublin), I've received some questions by email. Rather than responding directly, I'll post the responses here so hopefully others will benefit as well:

When using a Psychrometric chart, is the temperature measured in a room equivalent to wet or dry bulb temperatures or is it somewhere in between?
Thermostats and thermometers measure dry bulb temperature.

In case you’re curious, wet bulb temperatures are measured by a device called a ‘sling thermometer’, which is really just a thermometer on a stick or chain that can be swung around. The thermometer has a wet piece of fabric at the measurement bulb, from which moisture evaporates as it is swung around. The drier the air, the more moisture will evaporate; if the air is saturated, none will evaporate. The evaporation at the bulb results in evaporative cooling and a lower temperature reading on the thermometer. Therefore, the wet bulb temperature, will always be less than the dry bulb temperature, unless the air is saturated in which case they are the same.

I’m modifying the material properties for a membrane. I have an sd value of 100 m. What’s the mu value?
Strictly speaking, the mu value is the sd value divided by the thickness of the material. In WUFI, very thin materials (i.e. less than 0.001 m) can lead to convergence problems and calculation errors. For this reason, the Fraunhofer recommends a minimum thickness of 0.001 m for all layers. Therefore, even if your data sheet says a membrane is thinner, it is best to use a 1 mm thickness, and calculate your mu value based on that. For your membrane, this would mean mu = sd/thickness = 100 m/0.001m = 100,000.

How do you disable the saving of the film file results to make files sizes smaller?
In the menu at the top, click ‘Options’, then ‘Result Data’. Check or uncheck the boxes as you like. Unchecking both gives you the smallest files, but you have to re-run every time you reopen the file to see any results. Saving Courses/Profiles, means that all of your Quick graphs will be there when you re-open the file, but you’ll have to re-run in order to see the film or to do a WUFIBio assessment using the film.

When simulating a retrofit, after simulating the existing wall and exporting the profile of the final water content then manually adding the new layers to the file, how do you reimport that file to set up the starting conditions for the retrofit buildup analysis?
Once your water content profile with the manually added layers is saved as a text file (i.e. in notepad or something similar), in the left-hand from of WUFI go to ‘Component’, ‘Initial Conditions’, select ‘Read from File’ then click the ‘File’ button to go find the text file you created. Once you select the file, profile of the curve will appear which you can use as a sanity check to make sure that that you’ve entered the additional layers correctly.

In the course notes you show an example of a traditional roof build-up (Dew Point Calculations and Their Limits section of the notes). The build up shows slates on battens counter battens bitumous felt sheating insulation vapour retarder and battens then gypsum board. No Rafters are shown but the fibre glass is called up as being between the rafters.
This is given as an example of how to use (1D) wufi. Isn’t this a 2D problem?

Strictly speaking – yes, this is a 2D problem. That said, the Fraunhofer has investigated the impact of timber studs on hygrothermal performance by comparing 1D and 2D assessments and physical measurements and advise the following:

If the timber studs are cold (i.e. timber on the outside of the insulation), their impact is significant and this should be simulated in WUFI 2D.

If the timber studs are warm (i.e. timer on the inside of the insulation), their impact is very low and this can be treated as a 1D case. For timber bridging the insulation layer (i.e. insulation between the studs), they have found 1D to be a reasonably accurate representation.

How do you check the moisture content of timber within an insulation layer to make sure it is below the 20%-M threshold?

For solid sheets of wood (plywood, OSB, etc.), you can simply check the moisture content by %-M on the right-hand side of the water content quick graph for those layers.

For timber studs or rafters with insulation in between them (assuming the 1D asssement is valid – see above), the timber does not get modelled directly. Instead the insulation layer is modelled. In this case, because the timber is at approximately the same temperature as the adjacent insulation, the RH levels will be approximately the same. Therefore, you can monitor the RH level in the timber/insulation layer, then using the moisture storage function for softwood find the water content in the wood that corresponds to the RH. This moisture content is in kg of water/m3. The density of the material is given in kg of dry wood/m3. Therefore, dividing the water content by the density give kg of water/kg of dry wood, which is the %-M value.

Note: this is an approximation based on assuming equal RH so there is inherent error built into this, but it is a reasonable way of estimating. If more precision is required, WUFI 2D should be used.


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