Crawlspace humidity question — is there any interpretation of these numbers that ISN'T a problem?
Looking for honest technical opinions, including counterarguments if you have them.
Setup: older house in Portland OR. Crawlspace under one bedroom, approximately 165 sq ft. Single ~3"x6" passive vent in one corner. Plastic sheeting on the ground but not sealed to the foundation walls in any way.
I've logged temperature and RH continuously in the crawlspace for 13 days using a data logger (30-min intervals, 180+ readings), paired with an outdoor sensor for comparison. Here's what I found:
- Crawlspace averaged 75% RH, never dropped below 65% RH across the entire logging period
- Outside air averaged 57% RH over the same period
- I calculated absolute humidity (g/m³) for every paired reading — the crawlspace contained more absolute moisture than outside air in 100% of readings, averaging about 18% more water per cubic meter
- This held through both hot sunny days and rainy days — not a weather-dependent spike, a sustained baseline condition
So I'm concluding that the high RH isn't simply warm outside air cooling down in the crawlspace — the absolute humidity data seems to show the crawlspace is generating its own moisture load, which I attribute to ground vapor.
A home inspector looked at it recently and described the humidity as "slightly elevated but within normal range, especially after a recent rain." Our data was collected across varied weather including hot sunny days well before and after any rain, so we're not sure how to reconcile that report with what we logged.
Specific questions:
- Is there any legitimate building science interpretation where sustained 75% average / 65% minimum RH in a crawlspace is NOT a problem?
- Does the absolute humidity data (crawlspace consistently wetter than outside air) change how you'd evaluate this?
- What would you recommend as a fix — noting that a simple exhaust fan seems unlikely to help in our Portland, OR climate where outside air in summer is already near saturation when cooled to crawlspace temps?
Context: we're renters raising this as a habitability concern (we've been having symptoms in the room above this crawlspace), so we're trying to understand whether the data we have is as significant as we think it is, or whether we're missing something. Genuinely open to being told we're wrong.
Thanks so much!