I am installing radiant heat in a concrete pad and using SIPs. I am planning a loft (15x30'). This is located in the Seattle area.
1/ Is it necessary to have radiant heat for the loft in the floor (i.e. between the rafters or warm board)? One thought is heat rises and the loft should be ok (for me this is an occasional hobby room of something non-sleeping).

2/ If it should be heated, the other idea is to just add a radiant wall heater (like a radiator).

Comments/ideas/experience welcome

your building will be tight but my recommendation is create a second zone for the loft. On the days when it is cold here (Seattle cold is 20 degrees for you walk in freezer regions) you will want the extra bump. I would recommend a floor panel system on top, cost effective, easy install after building is up. 15x30 could be done with 2 300 foot loops.
Dan

in SIP construction, if you are doing the typical "one layer second floor" with no joists, you will get some heat migration from the lower level. a small panel radiator would probably be all you need.

if the loft is wide open to below, you might not even need that.

if you have a joist cavity, and it's not wide open to below, then you need more.

I just finished a house with a loft in the Bothell/Mill Creek area. Part of the house is slab on grade and part has a basement. I put radiant in the loft area above the slab portion and during the last cold spell it did come on. If the outside temp is above 35 to 40 it does not seam to come on.

I would assume you would be using a condensing boiler, maybe a domestic water option as well. In order to keep your fuel efficiency at max you will want to keep delivered water temp low.
this is why a floor panel system works best. if you chose a radiator and want to keep to a one temp system you will need to oversize the radiator. normal radiator temp is 145+.
Now the primary on the boiler is set to high.
The invisible factor of in floor heat has its advantage.
That said a radiator is less cost.
Dan