Outgassing from an electromagnet core installed in an accelerator vacuum system is a problem, since it causes a poor vacuum, thus shortening the beam lifetime while also inducing electrical breakdown. Outgassing measurements by a throughput method were carried out for stacked laminations with a few types of insulating coatings. The vacuum performance does not always depend on a small conductance in a narrow gap spacing of a few micrometers, but on the materials coated for electrical insulation. A high-temperature cured inorganic and plasma-sprayed ceramic coating shows a lower outgassing rate than a quasi-inorganic one by an order or more. An outgassing reduction effect by a prebaking treatment is also examined. Furthermore, a time lag in the pumpdown characteristic of the stacked laminations is discussed. (C) 2004 American Vacuum Society.