I've run into problems with the in-Revit thermal calculations in the way that it handles cavities. I outlined them in an earlier post: Here.
I am still having problems - and haven't been able to come up with a workable work-around.
Essentially it boils down to the fact that Revit treats a cavity as a layer of air - and calculates its thermal resistance as a product of the thickness and conductivity of the air material - this means, by default that an air gap of 50mm provides the same insulation as 50mm of insulation rather than the (in the UK at least) recommended value of R=0.18 m2K/W. This skews the results considerably.
50mm unventilated cavity : R should be 0.18 m2K/W (0.44 m2K/W for low-e foil to insulation to cavity face)
50mm Revit air material : R = approx 2.0 m2K/W
As an example - the correct U-value for a given wall construction with a 50mm non-ventilated cavity was 0.268 W/m2K, Revit calculated it as 0.1935 W/m2K - nearly 30% lower than it should be (a significant deviation).
I had tried to get round it by assigning a 'fake' insulation value to the cavity's air material to ensure that the resistance for that layer comes out at the desired figure - the trouble is - as soon as someone changes the thickness of that layer, the result is incorrect again. Also, I would rather not assign an inaccurate conductivity to a material, you never know how/if it will be mis-used further down the line.
Is it possible to have a material provide a fixed Resistance - regardless of it's thermal conductivity or thickness ? (ideally with an allowance for the emissivity rating of the external face of insulation)
Apparently Revit includes the correct internal / external face heat transfer resistance (Rsi and Rs) - but not for a cavity (Ra.)
If I could create a material that has a pre-defined Resistance - I could create the appropriate fixed R rating as needed for walls, floors etc.
At the moment - it means that the U-Values generated by Revit only work accurately if there is no cavity present.
Anyone else encounter this problem ? If so, how did you get round it ?
In case anyone else is searching, or has a similar query - Autodesk responded that they have passed the issue on to the development team, where it has been documented for possible resolution in a future version.
For the moment - I have created new materials - setup for specific cavity widths and named them as such: "Wall Cavity 50mm", " Wall Cavity 100mm" etc. and assigned the thermal conductivity as required to generate the specific R value (ie 0.18) at that specific width. It's not ideal - and is still subject to someone changing the cavity width and not changing the material - or vice-versa - but it is a work-around for the moment.
Access a broad range of knowledge to help get the most out of your products and services.
Start with some of our most frequented solutions or visit the Installation and Licensing Forum to get help installing your software.
Upgrading to a 2015 product? Make sure to check these out 1st!