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Difference between adiabatic surface and high R value

asked 2021-04-06 10:36:00 -0600

sashadf1 gravatar image

updated 2021-04-14 19:58:10 -0600


I am trying to minimize conductive heat transfer through the envelope.

I can do this by setting the surface to adiabatic through the measure Set Exterior Walls and Floors to Adiabatic....

Figure 1: Adiabatic Measure with conduction still occuring from ground into building. adiabatic measure

Or, for envelope members that are not the ground (roof and walls), I can increase the R-value to a very high percentage to approximate adiabatic, using the following measure.

Figure 2: R Value Increase Measure with Large (>>100%) Increase.

r value measure r value measure 2

Making a surface adiabatic removes windows and stops ALL heat transfer through that surface.

If I still want fenestration in my walls and solar, radiative, and convective gains on my walls and roofs, can I still use the adiabatic surface measure? I am assuming no, but would like to confirm.

If I cannot set the walls and roofs to adiabatic, what is the max % increase I can input into the above measures without the simulation breaking? @David Goldwasser?

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answered 2021-04-06 14:24:48 -0600

"If I still want fenestration in my walls and solar, radiative, and convective gains on my walls and roofs". If you wanted to have windows you could build a base surface around it so that base surface is not adiabatic but others are, but what is the use case for this? If you want adiabatic walls but still want convective gains, I'm not sure if you want adiabatic walls, as there will not be an external heat gain that makes it into the zone through adiabatic surfaces.

When you increase the R value with the measures shown, it is increasing the thickness of the least conductive material in your layered construction. or for no-mass material just changing the Thermal Resistance value. If you really want to take it far I would use a construction with a no-mass material.

As I mentioned in another post there is also a newer version of Set Exterior Walls and Floors to Adiabatic measure that gives a lot more control by having bools to enable/disable the surfaces impacted by the model by cardinal direction. It also supports roofs, floors, and has an inclusion list and exclusion list option if you want to customize more for specific models and use cases.

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This is a PAT analysis through ResStock, so I don't actually build the OS models myself, just apply measures to them. So I don't write the constructions myself.

I have the newer version of the measure :)

How do I figure out what surfaces are in my model and what their boundary conditions are? --> self answer: look in the idf file.

sashadf1 gravatar image sashadf1  ( 2021-04-08 03:40:13 -0600 )edit

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Asked: 2021-04-06 10:36:00 -0600

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Last updated: Apr 12