Poor air flow from floor vents or when multiple vents choosen

Defrost has been stronger on all types. Just the way plenums are made. Clear that wind shield first. As long as you have plenty of heat to keep the cabin area warm to hot, it's OK. I wouldn't woory about it.

What year is your Envoy ? Probably high speed relay .

sounds like vaccuum line problem or vaccuum assist

check vent flaps inside the blower unit....

Mode Door 1.
a.This door regulates air flow to either the dash vents or defrost/floor. If you have air flowing only through the vents with no defrost or floor, this door is failing. From our experience, this door is usually the first domino to fail. Not sure why, just is.
<!--[if !supportLists]-->4.Mode Door 2.
a.When mode door 1 is set to divert air away from the vents and into the second stage of the system, door #2 chooses whether air goes to the defrost vents to the windshield or out through the floor vents in the passenger and driver's side footwells. Failure is indicated by an inability to choose between defrost and floor.
b.This door turns out to be the most difficult to replace. In order to replace the door, the dash panel has to be removed. It's still better than the dealer fix because you don't have to evacuate the AC system, drain the radiator, and remove the plenum box. But it is more work than the other fixes. When this door breaks, it falls in the down position, blocking the floor vents and diverting all air through the defrost vents. From a safety standpoint, this is good since you'd most likely rather have a clear windshield than warm feet. We suspect that a large percentage of customers will just leave this door unrepaired. You will have heat through the dash vents and to the windshield, just nothing to the floor. It's inconvenient, but you'll have to make the determination if warm feet are worth the extra work to fix the door. An extra pair of socks is easier.

Got kids? Years ago, mine loaded the defroster ducts with toys, candy, etc.
Otherwise, please create another post and identify the year, make and model vehicle you're talking about before posting your question.

If you have warm air coming through the defroster vents then your core is not gone. there is something blocking or bent on the heater box mechanism which directs the air flow up or down, or on the climate control knob which controls it, or in the cable which connect the two.

common problem, check under dash by the gas pedal if control arms are muving went selecting from ac to heat cuz this it is the control for the vents to go from ac to heat and to defrost.

Could be a vaccum line leaking not changing the vent flap to defrost

The climate control was poorly designed on these cars.

There is only heat on the floor and defrost settings, due to the layout of the ductwork.