My house will have 3 zones, each about 488 ft2 (basement and 2 floors). VERY low heat load (all ICF walls). Some are suggesting 7/8" pex, others say 1/2". Is 7/8" cheaper in the long run (no pun intended) because you can run less pex per ft2? Would I need to make a round trip in each joist bay if I use 1/2"? I plan on using thin metal fins, by the way. Does 1400' of 7/8" pex seem like a lot for this project? Thank-you! -m

there is no good reason to use huge pipe in joist bays: no good reason at all. claims about increased heat output are marginally true but not practically true, it's a bear to work with, and in the end you get a system that is significantly less effective than two 1/2" runs per joist bay and much more "stripey".

Rob is right.
!/2 inch pex will make the install time less, minimal amount of cussing. Staple up is a tedious approach at best, up and down the ladder all day.
You will want a pneumatic stapler possibly. Heat transfer plates are a plus but may not be necessary in a tight ICF type house.
Plan on up and back in each bay, a nice 8 inch pattern. We have some clients that like 3/8 pipe, this works well also and is easier to work with but you are limited to a 200 foot run. !/2 you can go 300-300 feet with out problem, less returns to the manifold. Locate your manifold central if possible, reaching out with the pipe like spider legs.
Avoid 5/8. 3/4, and 7/8 pipe . It is to stiff, and you are weaving the entire role. a total nightmare. You in the end will save nothing.
Drill 1 3/8 holes,Place insulators in the holes when done to prevent chafe on the pipe. Enjoy your work as best you can.
Dan