General Interpretation of Results

[bek15212]

I know that in general when we look at our WUFI results with respect to a particular assembly what we hope to see is a general trend of lowering moisture contents until it reaches a state of equilibrium.

However I am also interested in determining what water contents in a given material would be destructive, i.e. may cause efflorescence or spalling in masonry, rotting in wood, etc.

Is this simply a judgment call or does one refer to the diagram of the moisture storage function for that material as explained in Hartwig Kunzel’s dissertation? Would worrisome moisture contents be associated with humidity levels above 95%, i.e. the super-hygroscopic region in a capillary active material? Am I on the right track?

[Thomas]

Dear Bek,

you are on the right track, but I'm afraid the details are more complicated than just looking at the moisture storage function.

Yes, WUFI only gives you 'raw data': the development of temperature and moisture content over time. Sometimes this is enough to reach a conclusion: if an initial high moisture content quickly falls to a harmless level, the assembly under investigation is obviously okay. If the moisture level keeps increasing, the assembly will fail sooner or later.

And you are right in pointing out that there are more specific potential damage mechanisms which may be of special interest, such as the danger of mould growth, of frost damage and so on.
WUFI itself cannot say anything about these risks. Evaluating these specific damage risks is therefore a task left to the user. This is why we emphasize that a certain background knowledge and experience are needed for using WUFI.

Obviously, for most users the evaluation of these risks, based on the WUFI results, is the main reason why they are using WUFI in the first place, so we are trying to assist them by giving some guidance. For example, the moisture content of wood or wood-based materials should not exceed 20 mass-percent over a longer period of time.
See the section "Criteria for Evaluating Hygrothermal Performance" in the "Questions and answers" topic of WUFI's online help for other hints.

Unfortunately, often the criteria for a specific damage risk are not that straightforward, and in many cases they are not even well enough known to be formulated in terms of a general rule (see, for example, the short discussion of the frost damage risk here). Different damage mechanisms may depend on the hygrothermal conditions in very different ways and must be considered individually.

This will be one of the major topics for our future research. Now that WUFI can tell us which conditions to expect in a construction, the natural question is: what does this mean for the component? The findings will be implemented in postprocessors which take WUFI's raw data and evaluate them in terms of aging, specific damage processes etc. WUFI version 5.1 now has a data interface for communicating with these postprocessors, and two of them are already available: 'ThermalTransmission' for evaluating the steady-state as well as the transient heat loss through the assembly, and 'WUFI-Bio' for assessing the mould growth risk at a specific location on or within the assembly. Further postprocessors will follow once we know enough about the various damage mechanisms to be able to model them.

Regards,
Thomas

[bek15212]

Thanks Thomas.

I think you told me what I already knew but it was good to hear it from someone who has a better understanding of building physics.

I myself have thought with regards to spalling of masonry one would have to know something of the compressive or tensile strength of that particular type of brick before one could say that the expansive forces of water freezing could overcome it.

On another sub-topic - I understand that if the temperature shown on the graph touches the dew point temperature I will most likely have condensation occurring. It seems to me that this would be an issue in the stud cavity, especially if we have fiberglass insulation, but what about when it is occurring in other materials. It appears often on or in brick, it even may appear in plywood sheathing. It seems to me that it is not as big a concern in these locations - what are your thoughts?

[Thomas]

I myself have thought with regards to spalling of masonry one would have to know something of the compressive or tensile strength of that particular type of brick before one could say that the expansive forces of water freezing could overcome it.

The tensile strength versus the pressure developed by the freezing water is part of it, but the details are more complicated. In particular, the water content and the pore size distribution enter the equation as well.

When the pores are partially filled with water, ice crystals forming in the larger pores will draw water from the smaller pores and, if enough water becomes thus available, the pressure exerted on the pore walls by those crystals will lead to mechanical damage. So the probability and the extent of frost damage will depend on the amount of water available for redistribution, the tensile strength of the walls of the large pores, the pore size distribution (if all pores have the same size, less water redistribution will occur, and many small ice crystals will grow instead of a few large ones), and a few other factors. So it will be difficult to determine the susceptibility of a material to frost damage from basic principles. Extensive experimental data will be needed which so far only exists to a very limited extent.

On another sub-topic - I understand that if the temperature shown on the graph touches the dew point temperature I will most likely have condensation occurring.

You certainly will have condensation under these conditions. Even more so if it happens inside a hygroscopic porous material, because in fine pores water starts to condense at relative humidities well below 100%.

It seems to me that this would be an issue in the stud cavity, especially if we have fiberglass insulation, but what about when it is occurring in other materials. It appears often on or in brick, it even may appear in plywood sheathing. It seems to me that it is not as big a concern in these locations - what are your thoughts?

If it does occur in those types of materials, it may be a concern, depending on the duration and on the materials' susceptibility to rot or other damage mechanisms. But it will occur relatively rarely.

When the temperature curve touches the dewpoint curve this means that the relative humidity at that location is 100%. However, if this occurs in a hygroscopic material, a relative humidity of 100% means that this material has a very high moisture content. The moisture storage function of a hygroscopic material describes the one-to-one relationship between the relative humidity of the pore air and the resulting adsorbed water content in the material. A relative humidity of 100% means that the pore spaces are filled with water - for most materials this is a significant amount of water.

Now if the hourly changing temperatures and humidities in such a material conspire to create what would nominally be condensation conditions, not much will usually happen. Relative humidity will not reach 100% instantly, because relative humidity is determined by the water content (and vice versa), and the water content is the same as before. Vapor flow will set in to transport moisture towards the condensation spot, but unless the material is very vapor-permeable, the water content will rise very slowly. Usually, condensation conditions will have ceased to exist before the slow accumulation of condensation water has reached high levels. Furthermore, capillary liquid transport (which, in contrast to vapor transport, is not driven towards a condensation spot but from wet spots towards dry spots) acts against the accumulation of condensation moisture and tries to disperse it.

The concept of comparing the temperature curve and the dewpoint curve is valid only for materials with low hygric response times and low capillary transport (e.g. air spaces or mineral wool insulation), or for situations where condensation conditions prevail for a long time (long enough for slowly accumulating moisture to reach alarming levels). In these cases, simplified steady-state calculation methods evaluating dewpoint temperatures can be adequate.

In more realistic cases, with hourly variation of the climatic conditions, significant hygric inertia of the materials and various transport mechanisms acting simultaneously, a transient simulation of the hygrothermal processes is necessary. In these cases, you will often find that short-term condensation conditions may not last long enough for much moisture to accumulate. If they do last long enough and the relative humidity approaches 100% at some spot in your construction, you probably do have a problem there.

Another remark on condensation conditions: if the temperature of a volume of air is lowered, this may lead to condensation conditions. The absolute moisture content of the air remains the same, and at lower temperatures it corresponds to a higher relative humidity. If the temperature drops low enough, the relative humidity will reach 100% and condensation will occur.
This is not true for hygroscopic porous materials whose hygric behavior is described by a moisture storage function. The moisture storage function is indpendent of temperature (in reality, there is a slight dependence, which is ignored by WUFI), so the relative humidity in a material depends on the water content only and not on the temperature. If the temperature of such a material is lowered, its water content is unaffected by the temperature change and the relative humidity therefore remains the same, too. So a sudden temperature drop by itself will not create instant condensation conditions in such a material. (It may indirectly lead to higher moisture contents by creating moisture flows or by creating condensation in neighbouring air layers which is then taken up by the hygroscopic material.)

So what you should look at in hygrothermal simulations is mainly the resulting water content, not so much the dewpoint temperature (which may, however, remain useful as an indicator of how much safety margin you have in your construction).

Regards,
Thomas