It usually starts small: a patch of haze in a corner of the glass that comes and goes with the weather. Over months it spreads, droplets appear, and eventually there is a permanent cloudy film you cannot reach, because it is not on either surface you can touch. It is between the panes.
The good news up front: this is one of the most routine window repairs there is, and it almost never requires replacing the window.
What actually failed
A modern window is not a single sheet of glass. It is a sealed unit: two (or three) panes bonded to a spacer around the edges, with the space between them sealed and filled with dry air or an insulating gas. That sealed airspace is what makes the window insulate.
The seal around the edge is the working part, and it lives a hard life. Every temperature swing makes the airspace expand and contract, flexing the seal a little each time. North Bay hands a window dozens of hard freeze-thaw cycles a year, so seals here simply wear out sooner than they do in gentler climates. When the seal finally lets go, humid air gets pumped into the airspace, and the moisture in that air condenses on the inside faces of the glass. That is your fog.
Why cleaning cannot help
The condensation, and eventually the mineral staining it leaves behind, sits on surfaces that face the sealed airspace. There is no way to reach them without taking the unit apart, and a unit taken apart cannot be resealed to factory spec in the field. This is also why the fog seems to come and go early on: it condenses when the temperature drops and evaporates when the sun hits the glass, leaving a little more residue each cycle.
It is an energy problem, not just a cosmetic one
A fogged unit has lost its insulating gas and its dry airspace, which means it has lost a good share of its insulating value. The room feels colder near the window, the furnace works a little harder, and in deep winter you may see frost forming on glass that never frosted before. Left long enough, constant interior moisture can also start on the sash and frame around the unit.
The fix: replace the sealed unit, keep the window
The repair is a thermal seal repair: the failed sealed unit is measured precisely, a new unit is fabricated to match, and it is fitted into your existing sash with fresh perimeter seals. The frame, the sash, the hardware, the trim, and the siding all stay untouched. Once the new unit is in hand, the swap typically takes about an hour per window.
Cost-wise, you are paying for a piece of glass and the labour to fit it, not for a window, an installation crew, and disturbed trim inside and out. That is why sealed unit replacement runs a fraction of the cost of window replacement, and why it is the right call whenever the frame is still sound.
One caution: if several windows in the house date from the same build, their seals are the same age. When one fogs, it is worth having the rest checked in the same visit, which is exactly what a full window service is for.
When fog means something else
Condensation on the room-side surface of the glass, the side you can wipe, is a humidity issue in the house, not a failed seal. Condensation on the outside surface on cool mornings is actually a sign of a well-insulated window doing its job. The seal question is simple: if you cannot wipe the moisture from either side, the seal has failed, and it is time to get a free assessment.