before either of you start debating numbers, i chose that value to ensure the ability was not abused in RP, this coating is not a magic cape of invisibility- well kinda.
Point is, a cape wont hide your 32 degree body heat in a steel corridor that clocks in at just under 20, so the ability to shift 15 either way means you can blend with the traditional range of room temperatures.
Also i do have to agree with frost I've notice many people have different interpretations when things aren't stated explicit so, i think ill go for explicit, means if its approved like that no one can call it out for being op either.