Thermal Transmission calculation of U values

[aease]

Hello.

I am simulating the simplest of structures, a solid (Fraunhofer) brick with interior plaster, and I am looking at the transient U values as calculated by the thermal transmission module. For 2 months out of 12, specifically July and August, the U values are shown as equal to zero - see attachment. How can this be possible? And how can it be that the corresponding R values (also attached) are not the reciprocal of the U values?

Many thanks,
Apostolos

[Thomas]

Hi aease,

these data are results of an evaluation of the transient heat flows through the component, and under these conditions the relationship between R- value and U-value is more complex than for the steady-state conditions which are usually considered.

The transient R- and U-values may even become zero or negative. The help file for the postprocessor says:

The post-processor is intended to assess heat losses. Under warm outdoor conditions, heat may flow into the building, resulting in negative R- and U-values. These negative values are set to zero, a red arrow at the side of the diagram alerts the user to this.

The red arrow in your second screenshot shows that this has happended here.

Kind regards,
Thomas

[aease]

Thanks, Thomas, your explanation makes sense, although I can't imagine why WUFI would artificially U-values to zero if negative and not keep the absolute value, which would carry physical meaning.

How can I find out more about how transient R-values are calculated, here?

Many thanks, again,
Apostolos

[Thomas]

although I can't imagine why WUFI would artificially U-values to zero if negative and not keep the absolute value, which would carry physical meaning.

Hi Apostolos,

that's because the U-values are usually used as a measure for the transmission heat loss of a building. Negative heat losses should be distinguished from positive heat losses, but they are of no real use to you because they do not compensate the losses that have already occurred. Beyond that it is somewhat arbitrary, yes.

How can I find out more about how transient R-values are calculated, here?

Please refer to Topic 3 of the "Thermal Transmission" post-processor's help file which explains the calculation in detail.

Kind regards,
Thomas

[aease]

Thanks for those pointers, Thomas.

I've read through topic 3 in the documentation for the Thermal Transmission post-processor but would still like to clarify the following:

1) Are the moisture-dependent steady-state R-values (straight lines for dry, u80 and typical built-in moisture levels) in the post-processor graph the resultants of the wall component R-values only or do they include the heat-transfer resistances of the interfaces? I'm trying to add-up all the possible components but can't seem to arrive at the R-values calculated by the post-processor.

2) The documentation seems to state that, for the transient U- and R-values, reciprocity should hold, since they are both defined in terms of the ratio (or its inverse, respectively) of the average heat flow density divided by the average temperature difference over the calculation period. However, the monthly R-values are not the reciprocal of the corresponding U-values.

Any clarifications would be very useful and appreciated.

Thanks,
Apostolos

[Thomas]

1) Are the moisture-dependent steady-state R-values (straight lines for dry, u80 and typical built-in moisture levels) in the post-processor graph the resultants of the wall component R-values only or do they include the heat-transfer resistances of the interfaces? I'm trying to add-up all the possible components but can't seem to arrive at the R-values calculated by the post-processor.

Hi Apostolos,

no heat transfer resistances are involved in calculating the R-values. For example, if your assembly only consists of a 0.3 m thick layer of Baumberger sandstone, the water content corresponding to 80 % RH is 35.6 kg/m3 (as determined by the moisture storage function), and the moisture-dependent thermal conductivity for this water content is 1.9445... W/mK (interpolating in the relevant table). The R-value of this simple assembly is therefore 0.3 / 1.9445... = 0.15427... m2K/W, the post-processor says the R-value for u80 is 0.1543 m2K/W.

Please note that due to a bug in the data exchange between WUFI and the post-processor, the post-processor may show wrong R-values if there are monitor positions within the assembly. If so, please remove all monitor positions from the assembly. See the list of known bugs, tab post-processors.

2) The documentation seems to state that, for the transient U- and R-values, reciprocity should hold, since they are both defined in terms of the ratio (or its inverse, respectively) of the average heat flow density divided by the average temperature difference over the calculation period. However, the monthly R-values are not the reciprocal of the corresponding U-values.

There is no reciprocity between the transient R- and U-values because the transient R-values use the mean temperature difference between the component surfaces, while the transient U-values use the mean temperature difference between indoor and outdoor air.

Kind regards,
Thomas