Revit Analytical Properties indicate an R value when a wall is selected?

[Anonymous]

My students are doing heat loss and gain calculations and when they select a wall "Ext. Brick on Mtl Stud" and look at the properties, it indicates that the wall has an R value of 54.0217......what is this information, its too high for the R value of a wall???? Am I missing something

[dzanta]

this is the total r value that is calculated based upon the layer material makeup of the wall.

select the wall...go to type properties...click edit under structure

you will see all the layers that make up the wall

each layer has a material assigned to it

select inside the material box and click the 3 dots box

the material dialog box will open up and highlight the material that is being used for that layer

look at the right side of the material dialog box and look for Thermal tab

the data in the thermal tab is what is used to calculate R value of the material

[Anonymous]

I'm having the same problem as the gentleman from the original post.  I did an experiment by building a wall 1" thick with nothing but batt insulation.  Revit suggests that the wall's total R-value is R 7.59.  Clearly the numbers in Revit are wrong.  You can fix the problem by adjusting each materials conductivity on the thermal tab for the desired R-value, but honestly... each material??? How can Revit be so far off.  R-Values for basic construction materials are well established... am I missing something?

[artejon]

Yes, you need to tell Revit about thermal properties of each layer separately if you want to get a valid R value.

Once you set up your material library with real life thermal properties, you are ready to get a valid R value.

Revit is not off, it makes calculations based on values for materials you put in your assembly,

'batt insulation' means just nothing for calculations if you do not assign a proper thermal asset to this material.

[MichaelPennyMP1 · Accepted solution]

I understand your pain.  I am currently creating materials with tested U-Values.

If you use the provided U-value in the Thermal Conductivity box under Thermal Properties the R-Value calculated is not correct.

I've had to derive backwards to create the proper R-value, pain in the butt but I can't find information on this anywhere on the internet.

Example:  Input Conductivity number with U-value (use proper thickness) it will calculate a R-Value.  Use that R-value and cross multiply with U-value to determine value Revit needs to output proper calculated R-Value.

Every answer I have seen mentions the thickness of the material or the other materials affecting the R-Value.  To me, this does not appear to be the case.

Surprised I haven't seen more people addressing this.

U-Value should be in ft2, but in Thermal Properties it shows btu/(hr.ft.F).  Do we need to take the square foot of the U-Value?  So confusing.

Mike

[Anonymous]

By "Thermal Conductivity" Revit is looking for a k-value.  It's stupid, but it's what Revit wants.

R (hr ft^2 degF) / BTU = Thickness ft / k BTU/(hr ft degF)

k = Thickness / R

[just.human]

I have had the same issue with wall total R-value not matching the R-value that I've calculated manually.  The solution I found was to: Go to the wall type Edit Type, select Edit box on the Structure row.  In the Edit Assembly box, click on the material properties that you want to edit, and the box with the three dots.  The Material Browser pops up.  For this example, select Mineral Wool Insulation, and then at the top right, select the Thermal tab.  The Thermal Conductivity is the U-value, which is the inverse of the R-value expressed as btu/(hr.ft.degree F).  I usually think of R-value per inch as that is what the product data sheets for materials in the US publish, but notice, this is per foot thickness.  For example, mineral wool has an R-value of 4.2 per inch thickness = R-value of 50.4 per foot.  Calculate the Thermal Conductivity as 1/50.4=0.0198 btu/(hr.ft.F).  Enter 0.0198 as the Thermal Conductivity for mineral wool.

Check the R-value for each material in your wall and roof assemblies to get the proper R-value to show up in the wall type Edit Assembly box, top left at Resistance (R).  Double check to be sure each of your assemblies are referencing the updated material properties.

Another complexity I've run into is to have too many layers in the assembly.  Either Energy Plus or Insight has a limitation of possibly 10 layers.  I was modelling double stud walls with insulation between the framed walls.  I got multiple errors based on assembly complexity.  Since then, I've reduced the model complexity by showing a 7" studs rather than two walls of 3 1/2" and by removing incidental materials.  Those complexity errors have stopped.

Please let me know if you see an errors in the above or know of a better way.

Thanks,

Richard