How to stop hot air from engine compartment?

Steve Mcbean said
My car has no factory A/C, the heat is turned to off, and I'm pretty sure there's firewall insulation, yet it feels like lots of heat is passing thru the vents underneath. How can I keep heat from passing thru the vents? Or is it just radiant heat, not air in the vents?

If its coming out the heater vents, first thing to check is if the heater controls are shutting them off properly. They're just flaps mechanically controlled by the cables.

One here.

With the fresh air vents (doors in foot well) open, no air to speak of should be coming out the heater or defroster vents.

Yes there should be firewall insulation as well as seals for every opening.
The firewall insulation is a black hardboard with yellow fiberglass held in place by large plastic pins.
Here is a replacement set that DMT used to make.