We demonstrate a silicon photonic crystal (PhC) optical switch with a 20-mus response time controlled by a thermo-optic microheater. The switch consists of a dispersion engineered PhC directional coupler that is only 4.9 mum long. Optical and electrical isolation is provided by backfilling the holes and embedding the PhC in a silica cladding to produce a vertically symmetric structure that is more robust than a membrane geometry. No increase in optical loss is observed due to the silica cladding, despite operating above the lightline; the insertion loss for airbridge and silica embedded structures are comparable at 1-2 dB.