We improve previous calculations of the cosmic microwave background spectral distortions due to the decay of primordial magnetic fields. We focus our studies on causally generated magnetic fields at the electroweak and QCD phase transitions. We also consider the decay of helical magnetic fields. We show that the decay of nonhelical magnetic fields generated at either the electroweak or QCD scale produce μ - and y -type distortions below 10 −8 which are probably not detectable by a future PIXIE-like experiment. We show that magnetic fields generated at the electroweak scale must have a helicity fraction f ∗ >10 −4 in order to produce detectable μ -type distortions. Hence, a positive detection coming from the decay of magnetic fields would rule out nonhelical primordial magnetic fields and provide a lower bound on the magnetic helicity.