Agree with
@3KillerBs -hard to have too much ventilation.
you need to provide them with draft-free roosting area. They are roosting below the opening, so out of the wind. Depending on position of ventilation to the prevailing winds in winter will also determine what you do.
For us, the west side of our coop is entirely blocked by the barn -most winter winds come from North or west. The two triangular openings (larger than yours as we have a 2-foot drop from one side to the other) are large! They are on the south and north sides of the coop. We use a fitted piece of plywood to completely cover the north triangle during Jan/Feb-the coldest months. We used to cover the south side, but now we don’t bc we have 6 ft of roofing over the south side, so no weather intrudes and winter winds do not come from the south. The East and west sides have open eaves, and the east side has a window that is closed in deep winter, but it is not where the roosts are, so it stays open most of the time as it will not blow on the birds. FWIW, the top roost of our ladder style roost is only about 12-14” below the south opening. When roosting, they are fully below the opening. When awake, I can see most of their bodies as they stand on the top roost and watch me tend to that side of the run, snd considering the coop is both a walk-in snd elevated, they are quite a bit higher than me when I am in the run.
heat lamp: we have one in the coop positioned over the middle of the roosts (so not over the preferred top roost). We have 5 roost bars that are ladder style. We have the heat lamp plugged into a thermostat that we can program. It happens to be one that has 2 plug-in, one for heat, one for cooling and is a unit popular with people who brew beer or grow mushrooms -so the right temps are maintained. We only use the one plug-in and set it so that the heat lamp comes on if coop temp falls below 15F. It hardly comes on, unless sustained cold weather, as the thermometer is positioned at roost level, out of the wind. I have found that when we have had -15 to -20F periods of cold, the heat lamp helps keep the coop around zero degrees. Sounds cold to us, but the chickens tolerate it well and they are not getting “warmed” up by the heat lamp. We do not see them huddling under the heat lamp either as we positioned it so that they would be less likely to jump into it, and winds won’t hit it.
good luck!