adding radiant heat?

I have a family room addition over a slab that is currently heated by a boiler unit. The register is perhaps 4-5 feet in length. Normally, its 6-10 degrees cooler in that room than the other rooms.
Here are a couple of variables to throw in.
I am adding a bedroom to the west of this room, thus this room will no longer be exterior on three sides, just two. A large and very leaky picture window unit is being replaced with a wall and a door leading to the new, well insulated bedroom. Four small (but older) window units will remain in the family room.
Will the fact that this room will be much better insulated from the west due to the new addition warm it up? My HVAC guy says its a colder room b/c the boiler register sits on an interior wall now.
I could get my electrician to wire an additional heater in this room for probably 300 bucks if he is here anyway running wire in the new addition.
On this cool morning (30F outside) it is 72 in the kitchen, and 66 in the family room. Maybe this is not a huge difference but it feels cool to me.
Should I have him throw another unit up to warm up the room or will it warm up becoming an interior room?
Thanks
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Do a whole house heat loss. See what the requirements are for that room before and after the changes. Check the other rooms as well. Particularly the room the t-stat that controls that zone is in. If that has too much baseboard the t-stat will shut down before other rooms have received enough heat.
If the heat loss indicates there was enough baseboard there may be something mechanically wrong with the system, bad zone valve, circ on its way out, excessive loop length, excessive heat emitters on the loop.

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