Evaluation of the risks due to humidity on the interior surf

[Antoine]

Hi,

I am assessing the risks due to humidity in a wall with a new isolation layer (interior side).
An approach is to see the evolution of the water content in the wall and in each layer. I did it.
To use the isopleth, we have to consider the interior layer (in my case cement plaster). But the conditions inside this layer are the one of my interior condition (because of the little resistance to heat and water diffusion of the cement plaster), so it doesn't seem really good.

So my question is : is it pertinent to evaluate the water content or isopleth in my interior layer while I myself put the interior limit condition and thus force it ?

Thanks in advance,
Antoine

[Thomas]

is it pertinent to evaluate the water content or isopleth in my interior layer while I myself put the interior limit condition and thus force it ?

Hi Antoine,

what is the 'it' which is forced in your question? The indoor climate (being pre-defined by the user) or the water content in the interior layer (being mainly determined by the pre-defined indoor climate)?

• The indoor climate is indeed forced by the user (i.e., no feedback action of the wall on the indoor climate is taken into account, you'll need WUFI-Plus for this). But if you specify an indoor climate which is sufficiently representative for the house/room under investigation, the existing feedback is basically already taken into account.
  
  If you investigate the effect of constructive changes made to the wall (e.g. adding some thermal insulation), then usually these changes will directly affect the conditions on the indoor surface of the wall (this is what WUFI computes) and will thus change the mold growth risk.
  
  The indirect effects due to the resulting change in the feedback on the indoor climate will usually be small enough to be negligible.
  After all, since you are not interested in the indoor climate itself, but in the mould growth risk, this would mean to evaluate the indirect change in the mold growth risk which is caused by the change in the indoor conditions which is caused by the change in the wall surface conditions which is caused by the change in the wall assembly. The effect caused by this chain of effects will presumably be smaller than the uncertainties inherent in the simulation
  
  If, nevertheless, you suspect that the changed feedback on the indoor climate might change the climate sufficiently, so that the change in its effect on the wall surface cannot be ignored, then you can do a little parametric study (by varying the indoor climate, for example) to investigate the sensitivity of the system with respect to this influence. If it turns out that it is not negligible, you will have to look for other means to quantify this effect - it is outside WUFI's scope - and then force the suitably modified indoor climate on the modified construction.
• If you want to assess the mold growth risk, you have to look at the temperature and humidity conditions at the interior surface of the wall. (You are not so much interested in the moisture conditions inside the various component layers, because mold does not grow inside the cement plaster, for example, but on its surface. Occasionally, one may have reason to look at the moisture conditions in wood materials in order to assess the risk of rotting, but that is a different question, unrelated to the risk of mold growth on wall surfaces).
  
  Yes, the conditions at the interior surface of the wall are very similar to the interior climate conditions. They are not identical, however, because the stagnant boundary air layer at the surface creates some (small) resistance for heat and vapor transport, and there is thus a small temperature and humidity differential across this boundary layer. The question now is whether the resulting temperature and humidity conditions at the surface are favorable for mold growth, and this is what WUFI computes.
  
  On average, the temperature difference across the boundary resistance is the same fraction of the total temperature difference across the component as the boundary resistance is of the total resistance of the component. The same applies to the vapor pressure differentials.
  
  deltaT_boundary / deltaT_total = R_boundary / R_total.
  
  Since you have added an insulation layer to your component, the component has a high total thermal resistance R_total, and the thermal boundary resistance R_boundary at the indoor surface is only a very small fraction of the total resistance. Consequently, the thermal differential at the indoor surface is only the same small fraction of the total temperature difference across the component (on average - the hourly WUFI calculation provides the details). The indoor surface temperature will therefore be very close to the indoor air temperature, and the risk of mould growth will be very low, unless you have unusually high indoor moisture.
  
  So in your case, the indoor surface temperatures and humidities are indeed 'forced' to be very close to the indoor air temperatures and humidities, but this is just how the physics of the situation behaves, and it is faithfully mirrored by WUFI.

Kind regards,
Thomas