In reply to BStar: I've got a pair of normal terras and they're pretty light, so check the weight of the insulated ones before presuming they are automatically heavier than thick single layer troos. One guy mentioned the Haglöf rugged trousers for example; great trousers; mine are 10 years old now and virtually undamaged despite huge amounts of use BUT they are not light and the newer models have even more features on them - hence more weight.
I don't know about the Terra thermostretch, but I'm sure they're not heavily insulated; they may be great as winter trouser. I sweat loads when I'm active even when its very cold so am aware to not wear too much on walk ins etc; but my legs are much more tolerant, so for example I was rarely too hot walking in in Scotland in Buffalo salopettes, but would always take my mountain shirt off for the walk in; so you might find lightly insulated troos are fine even when you're warm up top.