A thermal inkjet printhead with generally planar heater elements disposed in respective bubble forming chambers such that they are bonded on one side to the chamber so that the other side faces into the chamber. Each heater element receives an energizing pulse to heat ejectable liquid above its boiling point to form a gas bubble on the side facing into the chamber, whereby the gas bubble causes the ejection of a drop of the ejectable liquid from the nozzle. The chamber has a dielectric layer proximate the side of the heater element bonded to the chamber. The dielectric layer has a thermal product less than 1495 Jm−2K−1s−1/2, the thermal product being (ρCk)1/2, where ρ is the density of the layer, C is specific heat of the layer and k is thermal conductivity of the layer. The present invention reduces the drop ejection energy and the heat dissipation into the printhead IC by improving the thermal isolation between the heater and the substrate.