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Modeling Below-grade Heat Transfer Question using IES

asked 2022-07-10 12:27:52 -0500

Gtfxxx's avatar

updated 2022-07-13 08:45:58 -0500

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

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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.

Denis Bourgeois's avatar Denis Bourgeois  ( 2022-07-12 05:06:53 -0500 )edit

... 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)?

Denis Bourgeois's avatar Denis Bourgeois  ( 2022-07-12 05:15:19 -0500 )edit

1/3

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.

Gtfxxx's avatar Gtfxxx  ( 2022-07-12 06:40:58 -0500 )edit

2/3

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.

Gtfxxx's avatar Gtfxxx  ( 2022-07-12 06:42:39 -0500 )edit

3/3

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

Gtfxxx's avatar Gtfxxx  ( 2022-07-12 06:43:18 -0500 )edit

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answered 2022-08-12 07:33:06 -0500

Gtfxxx's avatar

Hi all, just to follow up with an answer, in case anyone faces the same questions in the future, hopefully it can help someone out. I will post the approach I decided to take in answering the initial question.

The question was: when simulating a below-grade building, do I need to account for the heat transfer through the ground, as well the below ground temperature. Or just one of the items.

As suggested by Denis on the forum, I also believe that the simulation should either account for the below ground temperature data, or the ground resistance adjustment - but not both methods. Contacting IES for clarity they stated that both should be used, however I believe that is incorrect.

I decided for my analysis to NOT account for the ground resistance, as I already had the ground temperature data. So I treated the below-grade walls as regular walls and calculated the U-values, then ran the simulation testing out different U-values and their impact on the overall energy.

One thing to note, several codes (such as IeCC) state that in hot climates, such as climate zones 1 and 2, do not need insulation for below-grade wall structures, and simulation results agree with that statement. The results showed than insulating below grade walls in my case, when the ground/soil temperature is 26degrees, the energy impact will be quite small. When running a test to see the effects when the ground temperature is significantly colder, the energy impacts were drastic, which obviously makes a lot of sense, as the delta T between indoor and outdoor temperatures is much greater. Which again is in line with code requirements, of not needing to insulate in hotter climate zones.

Now, when the other approach was followed, and both the ground resistance and the ground temperature data were accounted for in the simulation, the cold soil temperatures had minimal effects on the overall energy, due to the ground resistance improving the U-values.

So from the results, building codes, and the reasons mentioned, I believe you only account for one of the methods, not both. Hope I was able to explain that clearly.

Please feel free to contact me if any further clarity is needed on the analysis.

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Asked: 2022-07-10 04:03:31 -0500

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Last updated: Aug 12 '22