In order to be eligible for the remedies of the Texas Property Code and lease agreement you should be caught up in paying your rent WHEN you give the notices. You should keep the receipts to prove you have paid the rent. Do not withhold your rent. Make sure you state that the temperature in your home is affecting your physical health and safety and the landlord's name and address, the date, your name printed, your address and then sign it. Make sure you keep a copy of the notice.
Chances are high that the landlord is violating a building code for putting citizens at risk.
Be sure to write down the name and phone number of the person that you talk to and the person that comes to visit. Also, request a copy of any report or notice of violation that the inspector may issue. If the inspector refuses to take any action of the landlord find out why. You should continue to record the temperatures even after you have delivered the First Notice to the Landlord. Wait TWO DAYS after you have given the First Notice before you deliver the Final Notice to the landlord.