What relative humidity % should be used for an interior space to determine if condensation is a risk?

[Scott]

What relative humidity % should be used for an interior space to determine if condensation is a risk?  Should I is use an online dew point calculator and play around with various % at the outer edges of the comfort zone, say 35% and 65%, as worst case scenarios?  

I have modeled a roof / wall in THERM 7.8.8 and used the standard exterior boundary condition temperature of 14 deg F, and interior boundary condition temperature of 68 deg F.  

Therm model attached - any insight would be appreciated.  Thanks

Scott

[Collin Robinson]

Scott, if 14 degrees is the design temp in your area, that is an approach that could work. The lowest interior surface temp I found is 54°F, which is high enough to avoid obvious  interiorsurface condensation for 50% RH at 70°F indoor air. See attached snap. Indoor air that humid seems too high to me, unless this building will have amazing air sealing and humidification, assuming residential due to the construction detail. Even in a crowded airport, the mech system on a reject job was able to keep it under 50% RH in winter. 

Some concerns: 1. Therm 7 only looks at temperature, not air or vapor. When 70°F 50% RH air (51° dewpoint) leaks into the cold building cavity, it is more likely to cause condensation there. The backside of your sheathing gets down to 18F for example, so 51° dewpoint air would definitely condense there. Air barrier is important, usually behind interior GWB. Assuming air barrier, this condensation is not a big deal if it can dry to interior or exterior, though. Look at this with WUFI to know more about wetting and drying. 2.  It's more likely that you'll get lower temps and more-apparent condensation at window and door penetrations through this assembly. Got to look at those to know the worst-case conditions.  

I like the Condensation Index approach used in the new NRFC 500-2023, where you take simulations at standard conditions (0°F outdoor for example), find the coldest spot automatically with the CI tool, then plot it on a chart. The chart has the range of interior temps and RH, so you can see how close you are to the edge. 

[MR]

Hello,

I am early in my engineering career, so anybody correct me if I am wrong. 

In THERM for a condensation analysis we typically use 30% RH for standard interior environment. Once we completed the analysis with 50% RH, but that was a gardening school with a lot of moisture within the classes. 

If you dont have any specs and want to be conservative, I would recomend 40-50% RH to be fine. 

Best regards

Michal

Dne čtvrtek 25. září 2025 v 19:52:39 UTC+2 uživatel Scott napsal:

[Dan Bettenhausen]

The relative humidity within a building can vary significantly depending on geographic location, occupancy type and actual use.  It is a project parameter that exceeds the specifications of the software.  There are various references such as ASHRAE's HVAC applications in the US that prescribe design conditions for certain scenarios.  In a heating climate with 14 F temperature there isn't a lot of moisture in the air outdoors and subsequently interior humidity levels will only be elevated if significant humidification of the interior spaces occurs, which could come from either mechanical means or activity. For many applications a wintertime value of 30-35% humidity is likely appropriate in "severe heating climates." Values in excess of 50% would only occur for certain special applications (kitchens, hospitals, pool area ...) or in other instances where moisture is not well controlled.  

A program like THERM is useful for determining what the temperature distribution is in a given cross-section, but some care must be applied when considering the location of vapor barriers/retarders in the wall cross-section and exterior ventilation because the interior dew point only really applies inside the building and internal to the first robust line of vapor control.  Outboard of that location the vapor pressure varies in accordance with the impedance/capacitance of the wall materials and variations in weather. Subsequently, measuring temperatures relative to the interior dew point is only a good practice if you have a lot of confidence in the effectiveness of the vapor barrier in the system and just want to do a simple check to see if temperatures inboard of that location are above the dew point.  

  

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