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Odd Moisture Issue Caused by the Hydro Radiant
Looking for help figuring out how to resolve a moisture issue before it comes a bigger problem. Not sure what avenues are the best to tackle the issue.
I have a 2000 sq foot ranch with 3 zone hydro radiant on the main level with 1/2 pex loops stapled within the 16" subfloor cavities with bubble barrier stapled under that. This is my second year in the house and the heat runs fine. I have noticed moisture on windows on the ranch level when it gets really cold out and it wells up considerably if its very cold and the heat runs a long time. I can manage this by wiping a few of the rooms windows every few days.
The Relative Humidity on the main floor and the basement stay very similar to the same number and are 30-40% range in the winter.
I am finishing my basement and recently installed 3.5" rockwool insulation in the joists (under the pex, under the bubble insulation.) This had an interesting effect which is the problem i'm looking for help with. I noticed that some condensation is building up on the rockwool insulation and where they meet the joists. Also, since installing, the basement temp dropped from an average of 66 degrees to 62 degrees (the open cavities were allowing the radiant to also throw some heat into the basement. We heat the upstairs to 68-70 range in the cold months. This went from a 2-4 degree temperature difference to a 6-10 degree difference.
What is puzzling me is that condensation usually forms when warm moist air hits cold dry air. Best I can tell in this situation from all my moisture readings is that warm dry air is encountering not-much-colder dry air. There's no moisture spikes... I can't figure out how this condensation is forming.
I'll note I did put a sump pit in recently with the basement Reno and ensured I sealed it tight thinking it may be releasing moisture. If I put the moisture meter on it and its same RH as the rest of the space (again 30-40 range.)
I'll also note I had closed cell spray foam installed last spring on 3 sides of the basement. The 4th side is a walkout and has a non-vapor-barrier-protected stud wall with fiberglass insulation.
I will also mention I have central AC set up and vented in the attic. I have all vents closed and sealed during the winter.
Thanks for reading!