A double is only four bytes and your CPU should be able to access the whole thing in a single go; single byte access is likely to be slower than accessing the whole lot.
Have you run some profiling and really found this to be the bottleneck, or are you just assuming (probably wrongly) that it will be? Why exactly do you think a double comparison will be slow?
One way you could solve it, although it won't increase performance, is to use a structure containing a char (i.e. single byte) flag and the double. Use the char as an indicator variable that determines whether the double has been initialised or not, then it's only a single byte comparison.