I'll be a little more blunt then the others.
On a 38* night, you won't even know the space blanket is there. You will be the first guy the Donner party will eat.
You won't be able to maintain a uniform air gap between the space blanket and the hammock and even if you did, without insulation in that gap, the convection currents would quickly rob your heat. The slightest air movement will push the blanket against the hammock.
If it would work, we would all be saving ourselves 1.5 lbs and $100-$275.
As WV stated, use a method known to work and try it out/practice it well ahead of your trip. Failure to do so will result in some miserable all-nighters by the fire.