Draw the dew point / condensation potential isotherm line with LB/HB using THERM?

therm
thermal-bridge

#1

I have a series of questions about using THERM within grasshopper.

The readTHERM component produces a gradient mesh to show the temperature through the wall assembly. This is great, but what most architects would care about most is the dew point or condensation potential location within the assembly to make sure it doesn’t infiltrate your envelope. What I would prefer to see is the isotherm line of this location. See this image from standalone THERM:

Firstly, what is the best way to calculate my dew point temperature, whether using LB or online calculator or weather data?

Secondly, does anyone have an idea for how to draw this isotherm curve through the wall assembly inside of gh? I was thinking maybe the Mesh Threshold Selector component could draw that line because it’s similar to how it would work on a daylighting analysis.

Let’s figure out a way to do this easily (if I’m not just ignorant and there’s already a way).


#3

Update:
I thought this logic made sense and I could remap temps to percentages, but I’m doubting it now. You’ll see that the dew point location wasn’t where I expected it to be. I think it’s because the percentToKeep is a percentage of the results, not a value threshold. Because the temperatures are not distributed linearly due to different materials, the percentToKeep can’t be used here.

How can I draw this line in an alternative way? I would basically just point out where 43 degrees (or whichever user specified dew point temp) occurs.


#4

@JimMarsh ,
There’s an example file for that:
http://hydrashare.github.io/hydra/viewer?owner=chriswmackey&fork=hydra_2&id=Analyze_THERM_Results_for_Condensation_Risk&slide=0&scale=1&offset=0,0

You can see that you can just use the Ladybug Dewpoint calculator and the cull mesh component to draw a condensation isotherm for any relative humidity you like! I can also say that video #10 of this series walks through that example file (though I realize the video is not free to the public yet):
https://www.ladybug.tools/video-tutorial.html#Therm


#5

Awesome! Thank you. I’ll take a look.
Edit: It’s working well. So my understanding of a paper I just read is that the heating design temperature is not actually the coldest the climate will get, it’s an average during the coldest month (in some cases 3 months)?
Do you have any go-to papers or resources for understanding condensation risk?


#6

@JimMarsh - the average of the coldest month is a typical practice for assessing condensation potential. As you are aware, the climate may certainly get colder than this average temperature. The justification for this method, versus using the minimum temperature in the climate file, is to design around the most typical cold condition and that we may “over-design” if we use the absolute minimum temperature value…over-design meaning excessive insulation. You are probably thinking, “but I know that my climate gets colder and there will be condensation if I plan for the monthly average”. The explanation I have heard is that a single temperature minimum is not going to prompt immediate condensation in location you see on a THERM model. Considering specific heat and thermal lag of your construction materials, your wall will react slowly (hopefully!) to the minimum temperature. An extension on this thought is that if condensation does occur at this minimum temperature, it will have time to dry-out considering temperatures in that month are typically greater. I admit this theory breaks down in cold-spells which is why long-term dynamic modeling such as WUFI is helpful to understand the durability of your wall assembly. We all know WUFI is expensive and offers a steep learning curve, therefore we can rely on THERM as a best-practice to avoid condensation risk - just dont forget that air barrier!


#7

@KitElsworth - Thanks for the thoughtful response! Thermal lag makes complete sense.
Speaking of air barrier, how can I model that? Is it a boundary or a polygon? If it’s a polygon does it need thickness?


#8

Hi @JimMarsh

You will not need to model the air barrier in THERM since it does not posses any thermal resistance and THERM does not model air leakage. The standard indoor air boundary layer in THERM is sufficient for your thermal transfer from indoors to the first layer of material. The placement of the air barrier is more of a design detail and construction quality concern.

Regarding the monthly mean versus minimum, I believe this is the original methodology in The Glaser method. The 2017 ASHRAE Handbook - Fundamentals include this brief statement in Chapter 27:

'Heat and moisture storage effects are not included in a dew-point analysis. Experience shows they play a significant role in heat and moisture performance of assemblies. To account for storage, it is recommended to use average values (e.g., monthly average temperature) rather than more extreme design temperatures."

Or, as the guys over at BSC explain it:

“notice that we are using “average monthly temperatures.” Why average temperatures? Ah, because they seem to work and give you the right answer. How do we know they give the right answer? Easy, the real world tells us that they do. It seems quaint considering what we do today, which is pretty much sit at the computer, but in the old days we would actually go out and look at real walls an measure them in situ. We even did laboratory experiments, and wait for it, built test huts.”
https://buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion


#9

@KitElsworth Ok, that was my first thought on air barriers: since it has basically no R-value, it’s not necessary in the model. Here’s the outcome of my first study. I think I did everything right. The coolest thing about this tool is that you can slide the humidity up and down and watch the dew point move. I think after a few of these studies I’ll have a much better intuition for how an assembly will behave before I test it.


#10

@JimMarsh ,
I see @KitElsworth has done an awesome job explaining the reasoning behind the condensation risk methodology and I just wanted to add that my one experience with controlled physical experiments gave me a lot of insight into the relevance of steady state models like THERM. Speficifically, it’s very clear from trying to approximate steady state conditions in the real world that real buildings are in a constant state of flux as outdoor temperatures go up and down and the sun rises and sets. To get conditions that approximate steady state assumptions like those of THERM, you really need a full 24-48 hours of steady conditions on the indoors and outdoors for thermal gradients to develop across construction assemblies. So, if you take a temperature like the 1% annual heating design day, you only have 3 days of the typical year when the temperature is below this value. Given that these temperatures probably don’t come in one event, you realistically don’t end up reaching the full 48 hours needed to create a thermal gradient like you THERM model. This is why, if you use a temperature like the 1% heating design condition, you aren’t likely to have condensation in a typical year for your climate. Again, all of this is much more eleganlty summarized by citing “thermal lag” as @KitElsworth does.


#11

Hello All,

I have been using the THERM dew-point method for several projects and I think for design purposes its great to plan and be conservative with. I had one project where we did a WUFI as well through Rockwool Building Science (free service) and their analysis was less conservative than the THERM. as stated previously the two methods are using different approaches and climate data. In all, I think the THERM method is a great tool while designing and documenting to make sure you will be ok.

I was wondering though if it was possible to input air pressure to the component to assist in its calculation. Am I right to assume that the THERM method is purely based of temperature difference and not potential air pressure associated with the HVAC system?

Thanks!


#12

@emmanuelgee do you mean using HVAC pressure in order to model the vapor pressure difference across the envelope? I don’t think THERM has a way to do this.

I’m a novice THERM user, so as far as I know in THERM, only radiation, convection (through coefficients), and conduction loads will participate in the construction assembly’s heat balance. As such, THERM’s condensation potential merely represents the potential of a surface temperature reaching it’s dewpoint temperature within the scope of that simplified, steady-state thermal environment (at a specified internal relative humidity and temperature difference).

The main barrier in modeling vapor drive is that THERM doesn’t take into account the vapor pressure difference, material permeability that determines vapor drive across materials.

That being said, it’s technically possible to do a steady state calculation to model vapor drive with provided material permeability, and vapour pressure difference information. And with WUFI, do it while taking into account transient (changing) heat and mass storage. I just don’t think THERM has those capabilities.

S