Limestone material properties from ORNL

[Peter Vander Heide]

Hi WUFI world,

I've been looking at an existing building in NYC because I've proposed services for a fit-out within it. I didn't have any real properties for the exterior cladding, so I assumed a high strength concrete for the precast panels, and ran a preliminary analysis. The results were terrible. But then I learned that there may be some limestone cast into the exterior surface, so I ran several cases again with the limestone properties from the Generic North American Database. All of a sudden everything is fine.

I'm puzzled. 8" of high strength concrete is no good, but substitute 2" of limestone on the outside and everything is just fine. It doesn't make sense to me.

So I tried some other stones from the Fraunhofer database as the exterior layer. Hardly anything works. How different could they be? And why? So I started to take a closer look at all the hygrothermal properties of the materials, I found what I think is an anomaly in the limestone properties. The permeability of limestone is listed as 0.158, but the moisture dependent permeability was listed as 0 until 90% RH, then went up to 125 at 100% RH. I thought maybe I corrupted my database with values like this. So I downloaded the 5.0 ORNL database. I'm getting the same numbers plus entropy with 5, so I'm thinking that this is not my doing.

Also looking around for reference material I find NRCC-50287 which says that tydal limestone has a thermal conductivity of 1.04 to 1.05 (SI) while the ORNL database says .723 (SI) for the limestone I'm looking at, Also the tyndal limestone exhibits a gentle curve in permeability at various relative humidities. Now I'm questioning the validity of the ORNL limestone values. Of course there can be large variations in natural stones, but to me this doesn't make sense.

So my question is: is there some wonderful "Limestone" on the market that can be obtained for a project with no permeability up to 90% RH, or is there something wrong with the published material properties of Limestone in the Generic North American Database? Or, of course, the ever popular I just don't understand WUFI material properties yet?

[Peter Vander Heide]

Hello again,

I'm sorry to admit that I didn't see Ned Lyon's earlier discussion about the problems with the Limestone data when I first posted this topic.

So, a couple of days more digging reveals that there is a product out there that was the basis of this data. It is Georgian Bay Limestone. The people who did the ASHRAE 1018 RP thought that it was a common North American building material, and it was the only natural stone that they included in their study. Me, I thought Indiana Limestone was a common building material. They did the Empire State Building out of it, and that's all the way out on the east coast.

But, common is local and the scientist was working in Canada. At least there is a scientific basis for the values, and the WUFI team has already addressed some inconsistencies in the data.

Given that most of the other natural stones that have been tested have far greater porosity's and that there are sink holes and great caves in limestone areas in the part of the US that I've seen, I think its unfortunate that this particular material got tabbed by WUFI as as just Limestone, instead of Limestone, Georgian Bay, so that a thoughtful user might at least inquire whether Georgian Bay Limestone was being used in a project.

I suggest that at the very minimum the name be changed in the Generic North American database.

[Thomas]

Hi Peter,

thank you for your comments. Following your suggestion, we will add "Georgian Bay Limestone" to the material name.

Also, while we do not feel entitled to change or to "correct" the data, we will add cautionary remarks to the material info, pointing out that the data may not be unconditionally suitable for hygrothermal simulations.

And finally, I noticed that the moisture storage function still has the mistyped values which had been discussed earlier in this forum and which have somehow evaded correction since. These will be corrected in the next bugfix release.

Kind regards,
Thomas