What is the build date?
If you have standard batt insulation between studs, fully sealed poly vapour barrier and no drafts from windows and penetrations then more insulation isn’t going to be the first solution. Surely it wouldn’t be a bad idea but the current insulation done properly should be sufficient to hit a minimum threshold. Proper heat supply and cold air draw, pipe insulation on cold air intakes, etc are where I would go first. Most people close off their basement heat registers to force more air upstairs but what they end up doing is worsening their comfort. Hot air rises. If you send it down first, it brings up the basement temperature, warming the main floor structure so underfoot is more comfortable. When your feet are warm you feel warmer. Warmer air in the basement recirculates through the return putting less strain on the furnace. If it’s a well built home, dricore subfloor with good underlay under carpet makes a difference.
* There are plenty of variable factors. Homes higher out of the ground have less thermal mass around their foundation. Homes with lots of basement windows have more heat loss at lower elevations. Walk-outs. Homes with depressed side entrances. Poorly insulated joist rims. Main floor cantilevers and associated thermal bridging. Uninsulated bath fan lines. Poorly positioned or limited heat registers. Under sized furnaces. Upper level factors contributing to colder internal temperatures that sink to the lowest elevation.
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