replicating a field test

[Matthew Smith]

Hello,

In order to use WUFI for an investigation into the effects of latitude, air temperature and WDR zone on summer condensation I have gathered a weather file for the time and place (global and diffuse radiation from within 100km) where the BRE conducted its original investigation. I had intended to use the results of the calculation as a benchmark for further calculations in different areas of the U.k. All my inputs were therefore intended to provoke condensation (masonry/mineral wool/vapour barrier construction oriented SW), to approximate the results from one of the wall constructions in the original experiment. The calculation only delivered two days when condensation conditions were present on the VB.

What can I do to provoke a higher interstitial RH? I have put the wall in Lund (a similar latitude) and the insulation gets soaked through, so it seems that my external boundary conditions are letting me down. I am confused about whether reflected short wave radiation is included, if not could I multiply my radiation values by a factor to compensate? Dis/enabling the explicit radiation balance doesn't make much difference; I only have cloud index, no data for long wave up and down.

Is there any way to increase my wall's response? It is mentioned in the Q&As that calculations for steep temperature gradients may be skewed by using the default moisture storage function for mineral wool. Should I play with the table to see if that changes anything?

Yours,
Matt

[Thomas]

... use WUFI for an investigation into the effects of latitude, air temperature and WDR zone on summer condensation

Hi Matthew,

you will probably find that the amount of summer condensation depends mainly on the exterior air temperature, to a lesser degree on the amount of solar radiation and almost not on the amount of driving rain.

The reason is quite straightforward to understand: in order to get summer condensation (i.e., vapour transport from the masonry across the mineral wool and onto the vapour barrier), you need high water vapour pressure on the interior side of the masonry.
The vapour pressure is the product of the relative humidity and the saturation vapour pressure. Now, in your case the RH on the interior side of the masonry will almost always be quite high and will only show small variations, even if the water content of the masonry varies wildly in dependence of the current rain load (assuming the masonry is not hydrophobic). This is due to the fact that, whatever your masonry material is, the rightmost part of its moisture storage curve will usually be quite steep so that the majority of all possible moisture contents (and in particular almost all of the moisture contents encountered in a wall exposed to rain) will correspond to high RHs (say, between 80% and 95%).

So the vapor transport across the mineral wool depends on the temperature (via the saturation vapor pressure) and the RH at the interior side of the masonry, with the RH generally being high and basically constant.
The temperature depends on the exterior air temperature and the solar irradiation. Solar irradiation may heat the exterior side of the wall to considerable temperatures, but has a much smaller effect on the interior side (where it matters for our purpose); it also acts only during a few hours per day. The exterior air temperature causes smaller temperature variations, but it acts 24 hours per day.

These are the results of our investigations on summer condensation in a double-leaf wall (see Fig. 5 in the publication, which shows the surface humidity of the inner leaf in dependence of driving rain (top diagram), global radiation, exterior relative humidity and exterior air temperature (bottom diagram)). The same basic principles should apply in your case.

So in order to provoke some summer condensation you must first make sure that the masonry is moist enough to have high RH on its interior side, but further increasing the moisture content of the masonry by somehow forcing more driving rain onto it will have little effect, because the RH can not rise much higher any more.

It will be more effective to raise the temperature level of the masonry. In order to experimentally change the exterior air temperature, enter the desired temperature shift in the climate options dialog which you can access via the "Details..." button on WUFI's climate dialog.

Another, probably less efficient way is to increase the absorbed amount of solar radiation by increasing the short-wave radiation absorptivity of the exterior surface (i.e. by making the surface darker).

I am confused about whether reflected short wave radiation is included, if not could I multiply my radiation values by a factor to compensate? Dis/enabling the explicit radiation balance doesn't make much difference; I only have cloud index, no data for long wave up and down.

In WUFI's default radiation mode, short wave radiation reflected from the ground is not taken into account; if "Explicit Radiation Balance" is enabled, it is (and you can enter the desired value in the "Ground Short-Wave Reflectivity" box).

I think it should be possible to multiply the radiation data by an appropriate factor to "simulate" the contribution of the reflected radiation, but this factor would depend on the current position of the sun in the sky and would thus have to be different for each hour.
Of course, you can always multiply the radiation values by some factor in order to check the response of your assembly to variations in the radiation load.

If you have no long-wave radiation data but cloud index data, include these in the *.WAC file. WUFI will then estimate the long-wave atmospheric counterradiation from the air temperature, the humidity and the cloud index. Similarly, WUFI will estimate the terrestrial long-wave radiation by assuming that the radiating ground has air temperature and the emissivity specified by the user in the "Explicit Radiation Balance" dialog. Details of what WUFI uses when which data are given in the WAC file are explained in the on-line help topic "The *.WAC Format for Climate Data". And don't forget to enable the explicit radiation balance.

Kind regards,
Thomas

[Matthew Smith]

Dear Thomas,

Thankyou for your reply, it was very helpful, as always. I had found since my last post an appreciable underestimation of direct radiation in the values for global radiation when those values are above 600 w/m2 which must result from different timings of measurements (they are measured in watthours/m2). I generated my own direct radiation column by adding the average discrepancy between the measured direct and that estimated by global-diffuse from the only two stations to measure direct radiation, onto the global-diffuse for each of my climate files. This, combined with setting the short-wave absorptivity a little higher to reflect the damper bricks (the file had a very damp summer) gave me results more in line with what I was expecting at that location.

My one surprise, as you predicted, has been the unimportance of the brick's moisture content. However, I am finding that radiation has as important a role as air temperature in influencing condensation conditions. So far, even without a doctored direct radiation column, my results have shown that the short term effects of radiation on the wall are crucial in raising the temperature of the internal surface of the masonry above that of the temperature of the vapour barrier (I have tried to input as realistic an internal temperature profile as possible based on the average summer temperatures at each location). I think this difference in impact on the sol-air temperature must be because of our milder summers.

The link to your publication was not active; is the paper in the IBP's conference papers section? In which case, what is it called?

This forum is a fantastic resource. My student license for WUFI runs out soon but I very much hope to reactivate it in a professional capacity as soon as possible so that I can continue bothering you.

Yours,
Matt

[Thomas]

The link to your publication was not active

Hi Matthew,

it seems we had some name server trouble. The link to the publication should work now. It's in German, I just wanted to point out the diagrams...

Kind regards,
Thomas