Modeling Below-grade Heat Transfer Question using IES
Hi all, I have a quick question and would really appreciate some feedback/guidance.
I am currently working on a building that is entirely below ground in Saudi Arabia. We have data to suggest that the temperature will be a constant 27 degrees Celsius, once you’re about 4m deep. I am tasked with identifying if they need insulation.
What I am a bit confused about is, if I am setting the exterior wall temperature 27degrees, do I still account for the ground resistance of the ground + the walls? - doing so would obviously improve the u-values of the walls, and provide further protection from the 27 degrees, but that makes no sense to me, since the ground itself is already 27 degrees. So logically to me, the ground resistance should not be providing any benefit to the wall resistance, since it’s already 27 degrees.
Or should I just account for the ground resistance, and ignore below ground temperatures?
Obviously if I treat the walls like regular walls and not account for the ground resistance, the results show that there is a need for insulation. Where if i account for the ground resistance, obviously that u-value is significantly improved, and therefore there is no need for insulation.
I know that in the region they typically don’t insulate below ground structures, don’t really understand why.
Would love some feedback and thoughts on the matter.
Thanks in advance,
Mony
If the cooling setpoint is e.g. 24°C (constant), the ground temperature immediately next to uninsulated walls (same for uninsulated floors or ceilings?), will not be 27°C - there will be a temperature gradient across the wall construction (8in concrete?), continuing outwards across (maybe) the first meter of soil. Beyond which it stabilizes at 27°C. If one were to model an entirely underground rectangular building (constant 24°C) with a constant soil temperature of 27°C, adding a 1m soil layer as part of the construction is reasonable.
... and then adding XPS insulation between wall and disturbed soil would significantly alter the temperature gradient across this wall+(disturbed-soil) assembly. One would need more details to continue, e.g. what software are you using? soil properties? is the facility entirely underground (ceiling faces constant 27°C as well)?
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Hi Denis,
Thank you very much for taking the time to respond.
Yes, the facility is an underground train station, about 18m below ground, with a roof that is exposed to the ambient outdoor conditions. So all four walls, and the floor slabs are below ground, with the lowest floor being 18m below ground.
Due to the depth of the building, the expected thickness of the walls is to be 1.5m (60 inch) thick.
I am using IES to try and figure our if insulation is required. I am unsure of the soil type, I would assume a mixture of rock and sand.
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The current 1.5m concrete wall equates to a U-value of 1.1 W/m2.K.
IES has an option to account for the ground/soil resistance, the option is called "u-value correction layer", which is intended to take into account the depth and the resistance of the soil. Once that option is applied to the model, the walls U-values improve from 1.1 W/m2.K. to 0.1 W/m2.K. But I am struggling to understand why I would apply this correction layer, if I already know the ground temperature will be 27 degrees at that depth.
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It seems to me that I should treat the wall as it is, with a U-value of 1.1 W/m2.K, and have a constant 27 degrees on the outside of that wall, and see if insulating would have a positive effect. But I know the results will show that insulating the walls would have a positive effect on the energy, which then brings up the question of "why is no one insulating below grade walls in the region?".
I have reached out to IES for clarity and for them to share the equation, and they were unhelpful and stated "that it is up to modeler on how to account for the below grade walls".
Thanks, Mony