Summer dewpoint analysis - Modifying winter dewpoint tool for summer dewpoint

I have a grasshopper file that is expended upon the Hydra “Analyze THERM Results for Condensation Risk” example.
I have used this for a few years to successfully find the dewpoint in cold climates.
I was recently asked to find a summer dewpoint and I think it’s breaking the way I have it set up. I have the boundary temperatures set up correctly, but I think my “Indoor Humidity slider” needs to switch to an outdoor humidity slider in the case of summer/HVAC cooling.

Ideally, I would be able to set interior and exterior RH independently, but I’m not sure if THERM supports more than a single RH.

Does anyone have some insight on this?

@JimMarsh

I don’t think THERM uses RH anywhere. It’s just modeling steady-state heat conduction, so the sensible or latent energy impact of mass is assumed to be zero, and thus moisture has no impact in its calculations. You can check out a tool like WUFI if you want to model transient heat and moisture transfer over a time period.

The condensation risk calculation done with THERM is much more simple. All you are doing is checking if the temperature gradient, starting from the warm side of your assembly at an assumed RH, will hit the dewpoint temperature relative to the vapour barrier. Therefore for winter, if your warm interior has 22C and 30% RH, make sure the temperature doesn’t hit the dewpoint of 4C before it passes the vapour barrier. For summer, if your warm exterior has 30C and 60%, make sure the temperature doesn’t hit the dewpoint of 21C after it passes the vapour barrier. Note: assumes a cold climate, with vapour barrier on the inside (warm) face of the insulation.

So all you have to do to modify your script for a summer condition (assuming you’ve already set the appropriate boundary conditions) is to plug the exterior drybulb temperature into the dewpoint component[1], and set the RH to some assumption of a humid, summer day.

[1] Or just look it up on the psychrometric chart.

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