Stupid question, but are they actually open?
Reason I say this. Moved into a new house that had only been refurbished a few years prior, so everything was new. Trickle vents though had been installed the "correct" way up. In other words the bit that opens (the ugly part) facing the ceiling. The problem was that half of our vents had been damaged (by fingers trying to find the latch I assume), so even though the latch was down, a good portion of the vent is open.
I hate trickle vents. I swear we've yogurt pots in the fridge that are made from more robust plastic. In the same year for the autumn/winter period I sealed up the bedroom ones too (blue and electric tape layered over the holes).
I replaced all ours (turns out several others were broken/half open too) and flipped them "upside down". You see the holes/grill but most of our windows have blinds so unless you go looking for these you never notice. The benefit is I can tell if they're open or not. This year I've not taped up the vents over winter and everything is fine. One thing to note when installing, the plastic is so cheap that if you tighten them up too much they won't close properly as they get caught on the window or a burr left by the screwhole, so again test/check this.
I did research better thermal/accustic vents but they seemed super expensive and not easy to purchase. I don't really get it.
