Hi Mouse,
I try to shortly explain some of your points direct below your statements.
But on further examination the technique, while meeting its design goals, does not seem to fulfil its potential. But I may of course have misunderstood. On reflection it seemed to me:
Yes you are right, there are much more influencing factors on the infiltration (US: exfiltration – from the inside to the outside) that the current IBP Model uses, in the future there is maybe an improved model to refine the processes taking place. But, the more simply IBP infiltration model is designed to be on the safe side. The amount of moisture the model put into the construction must be dry out by diffusion. If this works the construction is robust enough to withstand the present humidity conditions and it can be assumed that all other factors play a rather subordinate role.
1. Only the moisture element of the convective flow is used in Wufi, the air and heat elements which I would think should be available from the calculations made, are not used.
Please see point 5. and e)
2. Only one direction of air flow is considered, inside to out
Yes, that’s right, because in colder climate you have nearly most of the time a thermal buoyancy inside the building, what is actually the only driving force for the IBP Model. Further you have to take this in account according to the German Standard for wood protection DIN 68800. For the infiltration from the outside there is no standard as far as I know. But we plan to also allow the air flow from the outside into the construction in future versions.
3. Only convective and fan driven transfer is considered, but climate information might allow wind-driven transfer to be considered as well? (However given the indirect and empirical way the 1/15th figure is determined possibly an average value for this is taken into account?).
Also right, the empirical determined moisture relevant share of the overall building airtightness contains also a certain share for the wind influence. In a possible future version of the model the wind influences will be considered, but this is much more complex as is seems, so at the moment this is only with the wind speed and direction data of the climate file not possible. The real incoming flow also highly depends on the building situation, which in most cases is unknown or difficult to capture.
4. Only moisture which is predicted to condense within the structure is considered, not that transmitted in vapour form
When the temperature at the place where the infiltration source is set is below the dew point, the model only put the corresponding share of condense water into this place. The model does not consider the increase in the water content due to the transported vapour and sorption processes. But the diffusion of the unvented construction is calculated, also the condensed water will move on by diffusion and liquid transport if possible.
5. The heat released by the predicted condensation is not considered by Wufi
The latent heat of the condensation process as well as the heat transported by a real airflow is so small compared to the surrounding thermal storage mass that it is rather neglectable. See also point e)
6. There are only 3 air tightness classes, though this situation is mitigated by the ability to specify an envelope air tightness figures. More classes would be useful, for example to cover older properties or greater and lesser leakiness.
That is a restriction of the trial version. The full version of WUFI has 3 pre-defined air tightness classes implemented and also the ability to set a user defined value.
If this is correct, Wufi is not receiving all the information it could use from the IBP air infiltration model. But I am I am not sure how to supply the missing flow information to Wufi.
Considering just convectively and fan driven flows for the present (and not addressing airtightness classes issue) I was wondering whether to use the IBP air infiltration formulae instead of the full model, and input the air flow data that can be calculated from these formulae in the form of air sources (which include moisture and heat) instead of as a moisture sources.
That’s for sure a way you can go. But again, the heat transported by the airflow is rather neglectable.
To do this I suppose I perhaps could:
a. Take the median envelope leakage @50Pa in m3/m2*h for the class and divide by 15 to get the component value
Yes that’s possible!
b. Correct this figure for to reflect the pressure difference generated by temperature differences, air column height and fan pressure instead of the standard 50Pa value. To do this I could perhaps calculate using the formula Wufi uses in its IBP air infiltration model. I could carry out this calculation for each month of the year using mean temperature values derived from Wufi internal/external climate data. (Or more simply I could calculate just two mean difference values, one for winter one for summer).
For the first part, yes, this is possible. The formula for this is: V’=C*ΔP^n, The IBP Model uses an n of 1. But you will need hourly values for the simulation in WUFI. A simulation with monthly data or data for half an year is certainly not purposeful because the pressure conditions change over the course of the day. E.g. higher thermal buoyancy in the nighttime, where condensation conditions are given.
c. Create 2 text files, one for the infiltration and one for the exfiltration air flows. (This could be simplified into one text file if Wufi interpreted negative values in such text files as a reversal of direction but I am not sure it does?)
No, this is not possible at the moment. WUFI Pro can´t handle these values. An air change source can only be connected with the interior or the exterior climate. But it is possible to set two air change sources one connected with interior and one with the exterior climate. But they must be placed on different locations inside the construction.
d. Correct these files to ACh using the dimensions of my construction by multiplying by Construction Area/(Air) Layer Volume (or in 1D divide by layer thickness, in m) .
For sure it is possible to convert these values in an ACH.
e. Use the files as internal and external monthly air inputs to the Wufi construction(s), possibly into air layers. (But perhaps it would be more realistic (but a lot more work) to divide the airflow up evenly between all layers?).
At first: WUFI is a tool for the hygrothermal simulations and dimensioning of building constructions. For accurate airflow simulations you will need a much more complex CFD tool, with even more unknown parameters.
Concerning your suggestion you need to distinguish two things: a) a real airflow through a construction will e.g. cool down on the way from the interior to the exterior, somewhere inside the construction the temperature falls below the dew point and condensation occur. Except sorption processes all other parts or layers are only minor influenced by the airflow. B) An air change source in WUFI brings heat and moisture from the corresponding climate (interior or exterior) directly to the place where the source is set. Depending on the source strength this can lead to deviating conditions.
In suggesting this I am aware that the mixing process described for air sources is not quite the same thing as the IBP model’s infiltration process. But perhaps that does not matter as it seems a reasonably realistic process?
Please see the point above. For sure it is possible to set the airflow as an ACH-rate but the results are not validated, so it can be more realistic but also wrong and therefore no longer on the safe side as concerning the dimensioning of the construction.
Also that it would be better to use finer grained temperature data – possible I guess but too much like hard work! (Making use of fine-grained real time temperature data is I guess an advantage of the current IBP infiltration model approach).
YES!
On reading the paper to which the help file refers I am however a little worried that the 1/15th figure might not be very robust – might there be some way to refine this?
As mentioned the IBP model is designed to be on the safe side and the input parameters are easily available. The 1/15 is based on empirical studies and up to now a more accurate ratio between the overall building airtightness and the component airtightness is pending.
Many thanks for any help you can give in anticipation. I realise I may have completely misunderstood the model– if so my apologies in advance for my errors.
Kind regards
Mouse
If you are interested in more information about the topic please see also:
Kölsch, P. Et al.: Airflow through Lightweight Wall Assemblies - Influence of Size and Location of Leakages. Thermal Performance of the Exterior Envelopes of Whole Buildings XIII International Conference. Clearwater Beach, Florida, USA. 2016.
https://www.researchgate.net/publicatio ... f_Leakages
Kölsch, P.; Künzel, H.; Zirkelbach, D.: Konvektiver Feuchteeintrag in Leichtbaukonstruktionen
Leckagearten, Tauwasserverteilung und Schlussfolgerungen für die Praxis. Bauphysik (2019) 41. Jahrgang, p. 269 – 278.
https://wufi.de/wp-content/uploads/Konv ... k-2019.pdf
Kind regards
Philipp
