A method to control a heat transfer profile in a defined space, the method comprising the steps of introducing a thermally insulating packer fluid into the defined space such that the thermally insulating packer fluid forms a gelled solid and reduces a rate of heat transfer through the defined space as compared to a prior rate of heat transfer through the defined space before introducing the thermally insulating packer fluid, where the thermally insulating packer fluid comprises an acidic nanosilica dispersion and a polyamine.