How does Revit calculate Wall Assembly R-Values?

[Anonymous]

I entered an Interior-5 1/2" Partition wall. The total default R-Value of the wall didn't seem to make sense, so I am trying to understand how Revit calculated R-Values. I have found at least 4 different sources that rate 5/8" of gypsum board with an R-value of at least 0.5. To test this in Revit, I reduced the metal stud layer to effectively zero and left only 3 layers of Gypsum Wall Board. According to Revit, this wall only has a resistance of 0.4160. How does 3 layers of Gypsum Wall Board have a smaller R-value than all the other sources I have found for only 1 layer? I also found another source that lists the thermal conductivity of Gypsum Wall Board at 0.17 btu/(hr·ft·°F). Revit lists this at 0.3756 btu/(hr·ft·°F). Why such a large difference? At 0.17 btu/(hr·ft·°F), the total R-Value is much closer than indicated by several other sources.

[iainsavage]

It might be to do with the air layers. Revit doesn’t automatically add the surface resistances or take account of air gaps. You need to add these yourself and you need to play about with the thermal conductivity values to get the correct resistance - Revit treats air layers just like other layers and calculates resistance from the layer thickness and thermal conductivity (rather than just a fixed value for inside resistance etc).

[iainsavage]

Be aware as well that out of the box insulation materials have NO insulation values - you need to add the thermal properties asset to the materials yourself

[HVAC-Novice]

I bet Revit doesn't do a good job with thermal bridges due to the studs. For any load or energy simulation you should use a dedicated software. This is just reason #100000 to not use Revit for that. 

[Anonymous]

I can understand that, but that doesn't explain why the default thermal conductivity given to a specific material would be so far off.

[Anonymous]

Sounds like a may have made a poor decision going with Revit for an energy simulation project, but I am stuck with it now. I'll just have to do my best to correct the thermal values where I see mistakes.

[iainsavage]

I can usually get the u-values of elements to be correct once I have (a) added thermal properties to insulation materials and (b) fiddled with the values of surface resistance and air gaps, but I have other concerns regarding the results from the analyses and if you search the forum you’ll find previous posts by @HVAC-Novice , myself and others regarding these.

[iainsavage]

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@Anonymous wrote:

I can understand that, but that doesn't explain why the default thermal conductivity given to a specific material would be so far off.

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I’m not familiar with imperial units for thermal conductivity but as far as I was aware Revit mainly uses ASHRAE data?

If you’re not happy with the default value though you can just change it, or duplicate the material and edit it to your own needs.

Re @HVAC-Novice ‘s point about thermal bridging, in my experience most programmes don’t handle this too well and generally requires some sort of fudge factor to adjust the resistance of the bridged layer.

[HVAC-Novice]

True, most commercial software ignores thermal bridging. I reviewed many designs and the designers based their load calcs on very inflated R-values. Add to that real-world construction with a lot of misc. steel added here and there to attach or brace something and you will end up with cold rooms and condensation....

What I do (in Trane Trace) I manually calculate the actual R-value by using the ASHRAE tables for studded walls, or even use a software called THERM. then I pick a wall that already has a similar U-value and copy/edit its construction to get close to my calculated value. It is a shame that professional software requires that workaround. and their default walls trick designers into thinking they already account for thermal bridges. As i said, I see many designs using the wrong values. and by wrong I mean that the actual R-value may only be a third!

Many years ago i tried a demo version of DESIGNBUILDER. that software had options to add thermal bridges. Like you could determine 12% of that insulation layer is steel or wood etc. Unfortunately that software only does energy simulation, and not loads and I ended up with a product that does both (but requires me to fake the walls and roofs to be correct)

This being wrong in Revit is just the tip of the iceberg. And kudos to OP for even noticing that it is wrong. I bet many people use Revit and take it at face value. At best you can use Revit for very schematic energy numbers. But really, just use a proper software.  If the numbers will matter later, don't use Revit. if this is just a school project, use whatever the professor tells you to. 

I could not imagine to put my stamp on a result calculated by Revit. When your client sues you because the numbers are off by a factor of 3, what exactly will be your defense? 

[iainsavage]

Re thermal bridging, in the early days of “simulation” we used MS-DOS software like Hevacomp and Cymap - they included a thermal bridging factor but like you say all it did was replace a percentage of a layer’s material with the conductivity factor for the bridging material to get an area weighted average conductivity for the layer. You could easily do this in Revit just by using a modified conductivity factor for the bridged layer, not difficult but its frustrating that you need to do this manually rather than having it built in to the Revit interface.

Re the accuracy of Revit’s calculations they should be accurate since, as you’ve mentioned elsewhere, they use the same EnergyPlus engine as other software, however I just don’t have confidence in them and find the data input and output a bit opaque in comparison to , say, IESVE so its difficult to interrogate the results (and Autodesk’s help isn’t very expansive).

I’m currently wading through the EnergyPlus help instead to try to get some understanding of how the software works.

I so far haven’t managed to identify specifically why Revit’s results might be off - maybe you have found out @HVAC-Novice but maybe that’s a topic for a different post.

The problem with using “proper software” is the additional financial burden and also the frustration of having to use different programmes rather than the one integrated design tool which Revit is claimed to be - I know that when our company invested in Revit part of the thought process was to retire IESVE but so far we have still kept using it due to lack of confidence in Revit’s energy analysis results.

Anyway, we’ve strayed away from how to calculate R-values, apologies for hijacking and no doubt we’ll explore this again in other posts.

[HVAC-Novice]

I wasn't aware Revit uses EnergyPlus, but they must have started that in 2019. 

They must have limited what features of EnergyPlus Revit uses since there is no way to actually tell Revit the details of the HVAC options. i know the heating and cooling loads are just "suggestions". but if loads are already "odd", I have no way to believe energy data (that must be based on loads) can be correct.  Revit doesn't have calendar options, ERV, AHU data, pump efficiencies,  boiler efficiencies... all the things that are important for energy consumption can be adjusted. 

I'm also not sure Revit actually uses R-values of the actual construction. in the energy settings you can set what type of building you have. Either way, buyer beware... 

I guess you can try different wall constructions and see if energy results change. 

I'm of the opinion they should focus on features usable for most people and not even have these half-baked (at best) features. 

[iainsavage]

Revit does appear to use fabric constructions in calculations but it depends on the model settings, this link might help (see part 10)

https://forums.autodesk.com/t5/revit-mep-forum/revit-mep-heating-and-cooling-load/td-p/9693670

Re EnergyPlus and systems analysis, 2020 seems to be when it changed, including adding the ability to include analytical systems and system-zones (haven't had time to explore all this yet).

https://www.energy.gov/eere/buildings/articles/autodesk-brings-detailed-energyplus-hvac-simulation-r...

https://blogs.autodesk.com/revit/2019/08/21/revit-systems-analysis/

https://knowledge.autodesk.com/support/revit-products/learn-explore/caas/CloudHelp/cloudhelp/2020/EN...

[hoda.ganji]

Would this help? "Advanced Walls in Revit 1": https://www.youtube.com/watch?v=_bjmx2JfMzQ&list=PLnQTukI1DJvzx89eu7sm7UdeTZtxfwSyJ&index=1

This video shows how we can create an exterior wall with correct thermal properties in Revit.