There is a room in a lot of south valley homes that never matches the rest of the house. It is the one over the garage, or the one under a sloped ceiling at the end of the upstairs hall. It bakes in August and is the last to warm up in January, and the age of the house has almost nothing to do with it. In these subdivisions the framing is sound and the details around bonus rooms and vaulted ceilings are what create the problem. Call 801-644-9000.
A room built over a garage sits on top of an unheated space, and often on a floor that extends past the wall below it. Insulation in that floor is only as good as its contact with the subfloor, and batts that sag or fall out of the joist bays leave an air gap that undoes most of what they were doing. The band at the perimeter, where the floor framing meets the exterior wall, is a common gap. So is the wall between the finished room and the attic space on either side of it.
Those side spaces matter more than people expect. Behind the short knee walls of a bonus room there is usually an open attic sitting at outdoor temperature. If the knee wall has insulation but nothing holding it against the framing and nothing sealing the back of it, air moves straight through. The same goes for the little access door into that space, which is very often a piece of plywood with no weatherstripping. Sealing and blocking those areas is frequently the single most useful thing anyone does in a bonus room.
A vaulted or cathedral ceiling has no attic above it. The insulation sits in the rafter cavity, and in most assemblies that cavity has to hold both the insulation and a ventilation channel running from the eave up to the ridge. There is not much depth available and no room to simply add more. If that channel was blocked when the insulation went in, or the material was pushed tight against the underside of the sheathing, the ceiling can run warm in summer and collect moisture in winter where you would rather it did not.
Improving one of these is a different job from topping off an attic. The options generally involve working from inside, which means the ceiling finish comes into it, or from above during a roof replacement, which is why the honest advice is sometimes to wait until the roof is due anyway. Small openings can be made to inspect what is actually in the cavity before anyone commits to a plan. Be skeptical of any quote for a vaulted ceiling that does not include a step where somebody looks inside it first.
The upstairs room that never cools is very often a distribution problem. It may sit at the end of the longest duct run in the house, fed by one small register, with no return of its own. Close the door and the room has no path back to the system at all. Warm air collects up there while the thermostat, which is downstairs in the hall, has no idea anything is wrong. Adding insulation to a room in that situation can help at the margins and still leave you with exactly the same complaint.
This is where we tell people to stop before buying anything. Check whether the room has a return, or at least an undercut door. Ask whether the system was ever balanced, whether the dampers in the trunk lines were set, and whether the register in that room delivers anything close to what the others do. In a newer subdivision home the insulation is often adequate and the air handling is the weak link. Fix the wrong one and you have spent money to be exactly as uncomfortable as you were.
Sometimes, and often only in part. Check the floor over the garage, the knee walls on either side, and the small access hatch into the side attic, since those are the usual gaps. Then check whether the room has enough supply air and any return path at all. If it sits at the end of a long duct run with one register and a door that seals against the carpet, more insulation alone will not resolve it.
It means it went in under more recent practice, which helps, but installation quality varies and the tricky spots are the same ones that are tricky in any house: knee walls, cantilevered floors, attic hatches, and the transitions around vaulted ceilings. Newer homes also tend to have complicated rooflines, which produce more of those transitions. An inspection is still worth doing before you assume anything either way.
The options are limited. The cavity is shallow and usually has to preserve a ventilation path, so there is rarely room to add much without working from one side or the other. Some contractors will make small inspection openings to see what is in there, which is a reasonable first step and costs little. If a roof replacement is coming in the next few years, that is often the sensible time to address the assembly.
Cold rooms, drafts, ice at the eaves, or an attic you have never been in. Any of it is a place to start.