Never had a problem being cold with a 1" Thermarest even on ski tours in the Sierra or 1/2" Insolite on climbing trips.

The bag and your metabolic rate make the difference. I've been cold when either of those were insufficient, but never from a pad.

One trick is carry about a 16"-18" square of Insolite to put under your feet if you are using a 3/4 length pad.

It also doubles as a dry, warm place to sit or a pad under the stove to keep it from melting out a hole, if you can't find a suitable rock to put it on.