On the topic of insulation and energy usage, we have a small "utility cupboard" in our downstairs bathroom which houses our washing machine and the previous owners also had a dryer in here too. They left the big gaping hole where their dryer used to vent out through (much like what you have fit
@UberTiger) which I have just sealed over using some silicone and a plastic cover i had lying around.
On the outside of the vent on the external wall there is a flappy vent where it vented out, would it be diligent to shove some spare insulation in the hole and then seal that side up too on the outside (with gaffer tape or a similar plastic cover)? it's already an abnormally cold room compared to the rest of the house so anything I can do to aid insulation and keep it warm(er) is helpful!
We aren't likely to get a dryer in the immediate future and if we did it would be a heat pump so no need to externally vent.