I would look into the Big Agnes sleeping bag/pad system. The concept is that you only insulate the top half of the bag as the underside is compressed anyway. The bottom of the bags have a sleeve for the sleeping pad to hold it all in place.
This is not an endorsement as I have never tried them out, but if I was buying a bag I would start looking up reviews. I actually think I first heard about them on AO.
http://www.mec.ca/en/product/5052-50...RoCYCIQAvD_BwE