Originally Posted by Boothill
i wouldn't, unless you need a vapor barrier for some reason
As Boothill said, one of the factors is breathability. If you have an UQ, it will have the ability to dissipate moisture to a certain extent. If you put down the heatsheat, you'll lock in the moisture. One of the problems that I had with a wallyworld blue mat ($10) was getting a sweaty back. With UQ's I don't have that problem, unless the heat is intense (like using a UQ with a 0* rating in 75* weather, without venting).
I got a UQProtector from 2QZQ for $35 & it added about 5* to 10* warmth, as it fits loosely around the OUTSIDE of the UQ, without compressing the UQ. It helps keep the heat in by stopping a lot of the wind from carrying away the heat of the UQ; while still allowing vapor to travel off your body.
When you get wet (even from sweat) and it's cold out, you'll often get colder.