A wider, slightly warmer band for a Jack Russell
A Jack Russell Terrier kept indoors is comfortable across roughly 65 to 75 degrees Fahrenheit. This small terrier carries far less body mass than a retriever, and the smooth or broken coat offers less insulation than a dense northern double coat, so he feels a chilly room sooner. At the same time a Jack Russell generates plenty of heat in bursts, racing through games and then crashing for a nap. That swing means the room itself should stay moderate while his bedding does the fine adjustment. A Jack Russell who can burrow into a blanket in a 68-degree room will regulate himself happily, whereas the same dog in a bare plastic crate on a cold floor will not, even at the same thermostat setting.
Burrowing is this terrier's thermostat
Burrowing is a Jack Russell Terrier's own thermostat, and it changes how you should read his signals. Tunneling under blankets, wedging behind cushions, and curling into the smallest available hollow are normal Jack Russell settling behaviours, inherited from a dog bred to go to ground. Do not treat every burrow as proof the house is cold. The clearer warmth problem looks different: a Jack Russell who pants at rest, keeps abandoning his bed for the kitchen tiles, drinks frequently, and cannot settle after play is telling you the room is too warm for him. Check also where his crate sits, because a small dog crated beside a heating vent experiences a much hotter microclimate than the hallway thermostat reports.
Cold floors, drafts and a small body
Cold floors affect a Jack Russell Terrier more than they affect a large dog because so much of his resting body sits close to the ground. In winter, a bed placed on tile or near a drafty door can chill him even when the room air measures comfortably. Lifting the bed slightly, adding a washable blanket he can rearrange, and moving his sleeping spot away from exterior doors usually solves the problem without turning the house up for everyone. In summer the reverse applies: a Jack Russell will choose the cool floor on purpose, and that choice is healthy as long as fresh water is nearby and he can move back to bedding whenever he wants. Freedom to move between surfaces is the real comfort feature for this breed.
Holding one range across a busy household
Holding one range is harder in a busy household because a Jack Russell Terrier often has several people exercising, feeding and settling him across the day. Write the agreed band and the bedding setup down where everyone can find it, including the note that burrowing is normal and not a cue to raise the heat. A sitter or family member who knows he crashes hard after play will also know to offer water and a cool resting spot instead of covering him with extra blankets. That kind of shared, written routine is what KinsLe Family Sharing is built for, since his temperature range, bed setup and post-play cooling notes stay on one profile the whole household can check.