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# Revision history [back]

Changing both the Conductivity [W/m-k] and Thickness [m] are both ways to change the R-value of the insulation material, which is calculated "by dividing the thickness of a material (in metres) by its thermal conductivity (k-value or lambda value (λ) in W/m-K)."

So the R-value for the EPS_Roof material you defined above is 0.476 m / 0.035 W/m-K = 13.6 [m^2-K/W]. These are the correct units for thickness and conductivity according to the Input-Output Reference.

"Unlike U-values, R-values do not include surface heat transfers at the boundary of the element by convection and radiation."

"U-value = 1 / (the sum of the R-values of the layers of the element + the resistance of the internal and external surfaces of the element)"

source: https://www.designingbuildings.co.uk/wiki/R-value

These internal and external surface resistances are the inverse of so-called "film coefficients." The internal and external surface resistances must have the same units as the R-value, or [m^2-K/W]. The film coefficients have the inverse units, [W/m^2-K].

https://en.wikipedia.org/wiki/Heat_transfer_coefficient

The R-value is only a measure of resistance to conductive heat transfer, and does not include film coefficients or the effects of convection and radiation at the solid-fluid interface on either side of a surface element.

Depending on the tool you are using and whether it supports Ruby scripting, you could add measures that change the R-value of certain materials directly, rather than editing Material properties by hand in the IDF. You can find such measures on the Building Component Library. https://bcl.nrel.gov/

Changing both the Conductivity [W/m-k] and Thickness [m] are both ways to change the R-value of the insulation material, which is calculated "by dividing the thickness of a material (in metres) by its thermal conductivity (k-value or lambda value (λ) in W/m-K)."

So the R-value for the EPS_Roof material you defined above is 0.476 m / 0.035 W/m-K = 13.6 [m^2-K/W]. These are the correct units for thickness and conductivity according to the Input-Output Reference.

"Unlike U-values, R-values do not include surface heat transfers at the boundary of the element by convection and radiation."

"U-value = 1 / (the sum of the R-values of the layers of the element + the resistance of the internal and external surfaces of the element)"

source: https://www.designingbuildings.co.uk/wiki/R-value

These internal and external surface resistances are the inverse of so-called "film coefficients." The internal and external surface resistances must have the same units as the R-value, or [m^2-K/W]. The film coefficients have the inverse units, [W/m^2-K].

https://en.wikipedia.org/wiki/Heat_transfer_coefficient

The R-value is only a measure of resistance to conductive heat transfer, and does not include film coefficients or the effects of convection and radiation at the solid-fluid interface on either side of a surface element.

Depending on the tool you are using and whether it supports Ruby scripting, you could add measures that change the R-value of certain materials directly, rather than editing Material properties by hand in the IDF. You can find such measures on the Building Component Library. https://bcl.nrel.gov/

https://bcl.nrel.gov/. Try searching for "R-value" or "insulation" and filter by "envelope measures."