I could get away with 1°. But it still short cycles. House is 2x6 construction and is only 17 yrs old. New windows and updated insulation. Re-did the drywall a few years ago, and put 1/2" up over existing 1/2". That includes all walls and ceilings throughout the entire home. Once heated it stays heated well.
Your statement above is sort of absurd. Your claiming your house is 'tight' because you added an extra 1/2" GWB. And updated the windows. and is only 17 years old.
Here is why that makes no real sense.
1/2" GWB only has an R Value of .45.....yes that is correct POINT 45
2x6 Wood Studs are 5.5 inches thick, but R-19 batts are placed in the cavity. You think you have R-19 insulation, but you really have R16 insulation because to achieve the full R19 the batt needs to be its full depth which is 6.5". Because it is compressed it loses some of its value.
And lastly, the 17 year old comment...R value is lost in all insulation as it ages. That is why there is a listed R value and and Aged R value associated with most insulations. Due to outgassing of the insulation it loses its ability to resist heat loss over time. Also, dirt, moisture, etc causes it to be less effective. So the very reasons you listed for your house being 'tight' are the reason why it probably isnt.
What really makes a house 'tight' is the use of things like infiltration barriers on the exterior, you know them as Tyvek. Or properly flashing around the windows. Yes new windows will help with some thermal heat transfer, but if they did no more than take the old windows out and put replacements in, with no sealing of the window opening, you didnt gain as much as you think you did. Sealing of windows uses products similar to ice and water shield for the roof. It is a sticky backed system that sticks to the substrate and the window frame and seals the opening. Even a good quality caulk, will only last 3-5 years before it needs to be replaced...and if its behind vinyl siding it will probably never get replaced.
The biggest gain you can do to your house, is to add rigid insulation to the exterior on the outside of the existing sheathing. 1" will get you a continuous R5. Now R5 doesnt seem like much, but because it is continuous and there is no thermal bridging, as there is with the cavity wall batts, it does more for your house than the cavity insulation does. Better would be to add 2" of rigid to the exterior. Not only do you get great R value, but after you build out all the window jambs you end up with really nice deep windows. Also, you will gain some additions by adding insulation to your basement. You think not, but the ground below the frost line is always 55 degrees. And if your basement isnt insulated, the concrete acts like a huge thermal sink transferring the heat in your basement to the exterior. Super insulated basements are becoming the norm in modern construction with 4 inches of rigid at the walls and up to 6-8" of rigid under the floor. But most people are not going to dig up their foundations to put in more insulation, a decent trade off is the batt blanket that can be attached to the joists and hung down the face of the wall. And another common point of heat loss in a house is at the rim board on top of the foundation. If it wasnt put down correctly with a sill sealer, you have a huge 'crack' in your house that is losing heat...and let me tell you that 17 years ago, sill sealers were just starting to hit the market....your house might or might not have one.... And just stuffing the joist spaces isnt really enough. It will slow down the heat loss but the most effective way is to spray foam the inside of the rim board from the top of the foundation to the bottom of the subfloor.
What really causes heat loss, is not heat transfer through the walls, but heat loss due to air leakage at cracks, door jambs/openings, window jambs/openings, outlets, light fixtures, etc. Even where the stud sits on the subfloor in a platform framed home can leak air. If you really want your house to be tight, you should have a 'blower door' test conducted and have the tester walk through with a smoke pen and see if you can identify areas of air leakage and seal those up. You would be amazed at what a difference that will make....simple things like new door sweeps, or plastic on the windows.