Air-Cooled heatsinks use case-temperature air to cool themselves... The warmer the room temp, the warmer your case temp(how much warmer is dependant on how good yoru case cooling is). And since CPU temp is really Cpu-Temp-Rise over ambient air temp, an 18F room temp rise probably also results in an 18F or thereabouts, again dependant on case cooling, CPU temp rise.
In other words, its normal.
Mike