HVAC contractors or insulation contractors have their own windows on the topic, but rarely see the bigger picture. Not all comfort problems are due to the HVAC, nor are they all about insulation or window performance. They have to work as a system, and the systems have to be in balance to work well. Micro-zoning the hell out of the place can sometimes work, but more often it creates other problems and ends up costing more in the end.
Before you do anything, try to figure out WHY it's running 6F-10F colder (I'm assuming that's in winter?) than the rest of the house. Take the time and run room-by-room load calculations for this room and a few rooms adjacent to it using a freebie online Manual-J-ish type tool such as
LoadCalc or
CoolCalc (or if you want to play junior duct designer, use the load calculator on the
BetterBuiltNW tool- it's free, but requires you to create an account.) Be aggressive on air tightness & R-value assumptions- try to be accurate, but think optimistically when guesstimating (is it R19 or is it R25 under the floor? etc.) The biggest errors are created by underestimating the performance of the insulation or windows or air tightness. For this exercise assume the house and ducts are hermetically sealed, completely air tight, unless you know from experience that it feels very drafty in a particular room.
Then ake a look at both the sizes and lengths of the duct run(s) to those rooms. Are the duct sizes (in cross sectional area) roughly proportional to the calculated loads? Are the lengths from the plenum roughly the same? If the colder room is undersized or at the end of a long or twisty run it might be fixable with a duct tweak.
To the extent possible, inspect for loose/disconnected ducts or squashed flex duct. If it's mostly flex duct, is the duct stretched pretty tight, or is it all twisted & floppy? Flex that is not stretched tight can easily have twice the friction (or more) that it was designed for, yielding very low flow.
Does the room have a dedicated return duct & return grille? How big is the return relative to the supply duct? A doored off room with no dedicated return doesn't get enough flow when the door is closed and it pressurizes the room relative to the outdoors. In the (rare) case that the return duct is 3x or more bigger than the supply duct the room could be significantly
depressurized relative to the outdoors, forcing outdoor air infiltration into the room whenever the air handler is running. If the returns are inadequate they can usually be improved by variations on "jump ducts" , creating paths for the return air to allow higher overall flow. Door cuts at the threshold are never enough except in very high performance homes (not your house.)
It's also worth inspecting for gaps in the insulation and air leaks in from the outdoors into the walls, especially if there are exterior walls or parts of exterior walls that feel colder than others in winter. This is easier to do with infra-red cameras and calibrated blower doors, but a $50 pistol-grip infra-red thermometer and a large window fan can still get you there. (FLIR has
a decent $200 IR camera that uses a smart phone or tablet computer as a display, if you're the type of person who loves gadgets. It's currently on sale for $150.) While it's easier to look for hot/cold spots in the walls during the fall when the outdoor temps are 20F or more cooler than the indoors, it's still possible to find stuff during the summer. Depressurizing the room with a window fan (blowing out), will cause leaky areas to change temperature toward the outdoor air temp, showing up as hot/cold spots at the air leak points. Missing insulation shows up as large patches of dramatically different wall or ceiling area.
You may be able to know from the experience of living there roughly what the oversize factor on the furnace is. When it's ~ +5F outside (the approximate
99% outside design temp for Denver & surroundings) does the furnace run at least 40 minutes out of every hour (~1.5x oversizing factor) or does it run 8-10 minutes then stop for 15-25 minutes between cycles (2-3x oversized)? If you don't know but have last winter's fuel bills handy, you can calculate the "block load" aka "whole house heat load" by looking up heating degree-day data from a nearby weather station and running some arithmetic,
as outlined here. (The AC oversize factor can be more readily estimated by measuring the duty cycle on afternoons when it's crossing through the ~91F mark, roughtly the 1% outside design temp for Denver.)
If the furnace is 3x or more oversized
and the room in question is at the end of a longer run the equipment oversizing could be preventing the room's temp from tracking with the rest of the house due to a low-low duty cycle. Bigger is not better- ASHRAE recommends a 1.4x oversize factor for the 99% design load, which would mean when it's +5F outside the furnace would be running (1/1.4= ) 71% of the time- that 43 minutes out of every hour at that temp (when the sun is down. since solar gain can lower that duty cycle). While a furnace & AC swap for something more appropriately sized would be an expensive fix, it's sometimes a
necessary step in getting the room to room temperatures to balance.
Nate Adams is a contractor in Cleveland OH (a location with comparable design temps to Denver) who has made a business out of fixing comfort issues looking at the whole house issues, not just the HVAC, and has written a decent book on the topic. In your situation it's worth reviewing his the short videos and free downloadable chapters from the book here:
Home Comfort 101
HVAC 101
HVAC 102
After ruling out gross errors in the duct design attacking these problems it's always important to fix the building issues first, starting with air sealing. Unless the house is reasonably air tight controlling heat & moisture flows within the house. The biggest leaks in the house aren't always obvious, but unless you're going there I won't go into it in detail. Fixing the insulation is best done AFTER the air sealing, since getting to the air leaks sometimes requires removing & replacing pre-existing insulation.