This might be a crazy thought/question...

My house sits on a slope from front to back - front is ground level, and the back of the house is 5-6' off the ground, so that much foundation is exposed on the back of the house, with a deck at house level and stairs down to the lawn.

When it gets cold like it is today (anything below 32-24*), the floor of the back 1/3 of the house is really cold. IR thermo measures it at 58* and the floor towards the front of the house tend to be just under house temp, 65-67*.

Is there such a thing to "wrap" the foundation wall on the back of your house and keep it warm, like a large elec blanket? Sounds crazy but maybe it exists and i haven't google it yet

There ARE some foam boards that can take weather exposure ... some with just use an earth berm

Best add a sloped berm of earth/mulch to add to insulation value and hold the higher-R board in place
Caveat from any pest control guy is being able to monitor & prevent termite invasions from under a deck, esp. when materials above supply them a nice nest.

Hopefully your deck posts are ground treated

Mobile homes use a skirt that helps a bit - you would apply such to deck perimeter, even though it still breathes above

The rich can afford rubber & other fancy mats but we don't need to go there

thanks

deck posts are on concrete, as is bottom of staircase, both with some variation of these