a chart showing the relative warmth of different grades of Polartec fleece versus other materials used in clothing. The "CLO" value is a standard way to measure insulation value for clothing (like R value for home insulation)
The warmest grade of Polartec fleece, 300 weight, it is 11 oz/sq yd and has a CLO value of around 1. If you look at the lightest grade of Climashield APEX, it is 2.5 oz/sq yd and has a CLO value of 2.1.
So Climashield is twice as warm and 1/4 the weight - so ounce for ounce, Climashield is 8x warmer than the warmest grade of Polartec fleece.
Temperature ratings for different CLO values are more art than science - but here is one set of ratings, assuming you are in a well sealed bag or quilt. He only starts counting at a CLO of 2 for 40 degrees, so maybe a CLO of 1 is only good for around 50 degrees?
Of course, fleece feels great and drapes nicely, its just heavy and bulky if you need to carry it around for cold weather use.