AITA for putting a lockbox over the thermostat in my shared apartment?

I share a three-bedroom apartment with Mina and Josh. Utilities are not included in our rent, and last winter’s heating bills were high enough that we agreed to keep the thermostat at 68 during the day and 64 overnight.

Mina is always cold. She wears sweaters, keeps a blanket on the couch and has a small space heater she uses while working. Josh and I are usually comfortable.

In January, I started waking up sweating. The thermostat would be set to 74. Mina admitted changing it after we went to bed because her bedroom is at the end of the heating line and stays cooler than the living room.

We tried compromises. We raised the overnight setting to 66. Josh moved a bookshelf away from the vent in her room. I bought an inexpensive thermometer so we could compare rooms. Mina’s room remained several degrees cooler, but she continued setting the whole apartment to 74.

Our next utility bill was $96 higher than the previous month. Weather contributed, but the repeated changes clearly did not help.

I bought a clear lockbox that fits over the thermostat. It does not lock with a key; it needs a four-digit code. Josh and I chose the code and did not give it to Mina.

That was the mistake.

I thought of it as enforcing an agreement two of three roommates still supported. Mina saw two roommates creating physical control over a shared system. She removed the lockbox with a screwdriver and placed it on my bed.

The argument became much larger than temperature. Mina said majority rule does not apply when one person cannot sleep comfortably in her own room. I said one cold bedroom does not justify heating every room to 74 while the cost is split equally.

She offered to pay half of any amount above our normal bill. Josh likes that idea. I do not, because extra payment does not solve waking up overheated or the waste of heating empty rooms.

We have asked the landlord to inspect the airflow, but the visit is two weeks away. Until then, Mina is using her space heater more often, and the thermostat is uncovered at 66.

I still believe the settings needed a physical limit after repeated agreements failed. I also recognize that choosing a secret code with Josh turned a household problem into two people controlling the third person’s access.

AITA for installing the lockbox?