Understand the meeting rail
Two sliding sashes introduce a meeting rail and operating seals. Ask to see the exact model’s documented air-leakage data and try both locks on a sample unit.
Use ventilation intentionally
Opening the top and bottom sashes offers flexibility, but outdoor temperature and humidity still matter. Do not treat an operable window as a substitute for a ventilation plan.
Plan for your property
A warmer room is not always a window problem. Look at drafts, shade, orientation, and installation details together. Compare documented product ratings instead of relying on a promised energy saving.