The silicon avalanche photo-diodes (APDs) in the CMS barrel electromagnetic calorimeter will be exposed to an integrated neutron fluence of about 2 x 10(13) n/cm(2) over 10 years of operation. High neutron fluences change the electrical properties of silicon detectors. The changes are proportional to the non-ionising energy loss in the APDs. Using the Geant4 toolkit, we have calculated the non-ionising energy loss as well as the rate of generation of primary defects in the APDs, for the expected neutron fluence. (C) 2011 Elsevier B.V. All rights reserved.