Revit 2016 Heat Transfer Coefficient (U)

[Anonymous]

to all.

im creating a few wall types for our office standards, going into the Wall assembly i found that i was able to add in the Heat Transfer Coefficient (U), however when revit takes into account all of the materials in the wall and adds up all of the U values  it doesnt match the numbers we get when we do the math manually, is there a way for me to locate the formula that revit is using to ensure that its adding things up correctly? or am i not doing something right?

thanks for you reply

James C.

[RDAOU · Accepted solution]

Hello @Anonymous

You are most probably doing it right. This is an old observation and the discrepency in calculated values appears to be due to the way cavities are treated by Revit in the wall layers. Example: Add the default "Thermal/Air layer" with a typical thickness 40mm and material set as "Air" is calculated in Revit as having a conductivity of 0.0250 W/(m·K) ie: the air cavity appears to have the same Thermal Conductivity/Resistance as a solid insulating material of the same thickness... If you are doing your manual calculation based on BRE standards; then you will find some discrepancies. (PS: I assume that if that discrepancy applies to air cavity it most probably applies to other material)

To solve that you will need to edit the xml file and enter your own U and R values as per the standard you need to follow. THIS POST HERE discusses this in detail and it might help you solve that issue.

https://forums.autodesk.com/t5/revit-mep/custom-construction-type-r-and-u-values/m-p/3412897/highlig...

[Anonymous]

oh wow.. ok thanks for the tip ill dig my heels in and see if i can get this thing to work

regards

James C.

[RDAOU]

My pleasure; you're welcome

Sent from my iPhone

[RDAOU]

@Anonymous

Just a follow up in reply to your msg:

1. Find the file in C:\Program Files\Autodesk\Revit 2016\en-US (Example: I am using Revit 2016 English US)
2. Either using XML editor or copy the file to another location and rename it to Constructions.txt 
3. Define Construction Upper section
1. Either modify existing or add new
2. Note the unique materialIdRef
4. Scroll down for material definition section
   1. Most material exist (u can adjust the R and U value
   2. If you need to add new material ensure that its IDref is unique and use same naming format convention (replace the red below and leave black as is) same applies for construction format

 <Material id="mat-ADD UNIQUE NUMBER HERE">
    <Name>TYPE NAME HERE ex: 6 in metal studding</Name>
    <Description>ex: 6 in (150 mm) metal studding</Description>
   
    <Thickness unit="Meters">0.140</Thickness>
    <Conductivity unit="WPerMeterK">0.14</Conductivity>
    <Density unit="KgPerCubicM">650</Density>
    <SpecificHeat unit="JPerKgK">1200</SpecificHeat>
  </Material>

NOTE: Once done

1. Rename the .txt file  back to Constructions.xml
2. Close Revit if it is running
3. Rename the existing/original XML to ConstructionsOLD.xml (found under C:\Program Files\Autodesk\Revit 2016\en-US again I am using Revit 2016 English US yours can be different)
4. Copy paste the new Constructions.xml to the above directory
5. Open Revit and try

Good Luck

[Anonymous]

@RDAOU 

Hi, thanks for the explanation, but I have two doubts about the calculations:

1 - In your example, (25mm gypsum+ 100mm air + 25mm gypsum), In my opinion the calculations should be:

Wall R-value = Gypsum R-value + Air R-value + Gypsum R-value = 0.11030 + 0.19019 + 0.11030 = 0.32979

So, U-value is = 1/R-value = 3.03223, but in your example is 1.3289, ¿is that correct?

2 - ¿Which value uses Revit, the calulated value (2.43326) or the manually defined value (1.3289)?

Thanks for your help!

[omarselim]

Here’s what Revit is actually doing and why your hand calc may differ:

How Revit computes U for a wall type

• In Edit Type → Structure → Edit… → Thermal (button in the assembly dialog), Revit computes layer-by-layer thermal resistance as
  Rᵢ = thicknessᵢ / kᵢ (SI) or Rᵢ = thicknessᵢ / kᵢ in IP units (with the unit system handled by Revit).

  • kᵢ comes from the material’s Thermal Asset → Thermal Conductivity.

  • Zero-thickness “Membrane” layers contribute ~0 to R.

• It then sums R_total = Σ Rᵢ for the layers and reports U_revit = 1 / R_total.

• Important: That does NOT include interior/exterior air film resistances and does NOT model parallel heat flow (studs vs insulated cavity). It assumes each layer is homogeneous across the whole area.

Why your manual math is different

Most office hand calcs do one or both of these:

1. Add surface films (R_si and R_se).

2. Parallel-path (framing) correction (e.g., 25% studs, 75% insulated cavity; or ASHRAE/BS EN methods).
   Revit’s assembly U ignores both by default, so it will usually be lower (more conductive) than your corrected U.

What you can do 1) Verify the inputs Revit uses

• Open the wall: Type Properties → Structure → Edit → Thermal and check each layer’s k and thickness.

• In Material Browser, make sure every material in the wall has a proper Thermal Asset (conductivity units consistent with your project units).

2) Decide which “U” you want to standardize on

• Match Revit’s internal method (no films, no framing): use the value shown in the Thermal dialog / “Heat Transfer Coefficient (U)”.

• Match office/standard method (with films and framing): keep using your hand calc, and treat Revit’s U as the base, layers-only value.

3) Make Revit display the “corrected” U your office wants

Revit can’t natively add films or parallel paths to the wall type’s U, but you can show a corrected value in schedules:

• Create a Wall Schedule with:

  • the built-in Heat Transfer Coefficient (U) (call it U_Revit), and

  • a Calculated Parameter (Project parameter for Walls) for your corrected U.

For example (SI units):

• Let R_layers = 1 / U_Revit.

• If your standard films are R_si = 0.12 m²·K/W (interior) and R_se = 0.04 m²·K/W (exterior), then
  U_with_films = 1 / (R_layers + R_si + R_se).

• If you also do a framing correction with framing fraction f (e.g., 0.25), stud path U U_stud and cavity path U U_cavity, then:
  U_corrected = f·U_stud + (1−f)·U_cavity
  (You can compute U_stud and U_cavity from two parallel R sums, or keep them as parameters you enter per type.)

Tip: If you prefer IP units, typical still-air films used in practice are about R_si ≈ 0.68 h·ft²·°F/Btu, R_se ≈ 0.17 h·ft²·°F/Btu (but use the values from your code/standard).

4) Common gotchas to check

• Units mismatch (project set to IP but manual calc in SI or vice versa).

• Missing thermal assets → Revit assumes defaults or zeros, skewing U.

• Air spaces modeled as “Air” material: Revit treats it like a solid with a conductivity—not the same as radiation-dominated cavity R used in standards (low-e, emissivity, orientation).

• Membrane layers: zero thickness → no R.

• Studs/thermal bridges: Revit’s assembly U ignores them unless you explicitly model parallel paths (which is impractical for standard types).