High humidity in new construction – Mitsubishi Multi System playing a role?
Hi Everyone – Longtime lurker and looking forward to being a member of GBA. Thank you in advance for advice.
Question/concern I have is high humidity in new home I’ve built for my family. Highest levels in upper level bedrooms. Project location is Western Maryland (Zone 5A I believe). Here are a few details about construction that might help:
Main Level – 1,223 sq.ft. – ( Dense packed cellulose)
Upper Level – 586 sq.ft – (Dense packed cellulose)
Basement – 1,179 sq.ft – (2 inch EPS Foam followed by 2×4 wall with unfaced fiberglass)
Attic – blown cellulose – R44
While not perfect I paid attention to proper air sealing, EPS foam in all rim joist areas with perimeter sealed with spray foam, caulked plates etc… No blower door test yet but overall I feel the house is very tight. Exterior wall profile is Cedar Lap, vertical felt strips for small air gap, tyvek, OSB, 2×6 stud wall with celllose, drywall, paint.
HVAC – Mitsubishi Multi System – H2i Outdoor Unit (MXZ-5C42NAHZ, 42,000 BTU 3.5 ton), Indoor Unit is a MVX-A36AA4 (3ton) and finally 2 Mini Splits (MSZ-FE09NA, 9,000 BTU). The Basement & Main level are served by the forced air system and the Upper Level consist of 2 small bedroom each with a mini split. Keep in mind bedrooms are only 150 sq.ft. and 200 sq.ft. with remainder of upstairs space being a small bathroom and hall/sitting nook. I do have a 6 inch fresh air duct tied into return with honeywell damper that is open about 25% and trying to consistently run upper level bath fan to help exchange air. I have a feeling this set up needs improved. ERV, HRV, more sophisticated bath fans??
My goal is to figure out if I’m simply over reacting to high humidity (65%-70%) and need to let the house go through a winter/heating cycle due to all of the moisture associated with new construction. I currently have 2 off the shelf dehumidifiers that keep things in the 50% range but not thrilled about having to run them all the time. Also thinking that the Minisplits may be oversized and simply overcooling the space vs conditioning the space, I installed access panels below each unit to check condensate drains and they seem to produce very little condensate which I thought was weird.
Anyway, I’ll stop rambling and looking forward to any suggestions…
All the best,
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