Do Low-E windows need special cleaning?

NGA's rule for coated glass is written as a procedure: specific glass cleaning procedures must be followed on coated surfaces, and the glass manufacturer's guidelines are to be consulted for them. Its DO NOT list puts the first step plainly: do not begin cleaning glass without knowing whether a coated surface is exposed.

Looking at the pane may not settle that. NGA describes Low-E coatings as typically neutral in color and very difficult to see, while reflective coatings are normally obvious, and it states that glass can carry pyrolytic or sputtered Low-E or reflective coatings, some of which may be on the exposed surface of the glass. The window's own documentation, or the maker of its glass, is where a particular pane's answer is found.

An insulating glass unit, in Cardinal's definition, is two or more panes separated by a spacer and sealed around the edges to create an insulated airspace. Guardian Glass states the rule for numbering its surfaces: count from the exterior, surface 1, to the interior, so the interior-facing side of the innermost pane of a triple-glazed unit is surface 6.

On a dual-pane unit that puts surface 1 on the outdoor face, surfaces 2 and 3 facing each other inside the sealed cavity, and surface 4 on the indoor face, as Guardian's double-glazed diagram labels them. The counting rule is Guardian Glass's statement; Cardinal uses the numbers without stating the rule, and the next section reads Cardinal's numbers against it.

Cardinal writes that its soft coat, or sputtered, Low-E works inside a sealed insulating glass unit, and that its regular products are applied to the number 2 or 3 surface of a dual pane. By Guardian's rule both of those surfaces face the sealed cavity, where an exterior cleaner does not touch the coating. Cardinal's indium-based i89 and Di89 coatings are a different placement: capable of being applied to surface 4 of a dual pane, designed to be exposed to the room side, and described by Cardinal as smooth, durable and easy to clean.

Cardinal's hard coat, or pyrolytic, coatings are integrated into the glass surface during float glass manufacture and permanently positioned on the number 1, 4 or 6 surfaces, with scratch resistance Cardinal states is equivalent to standard glass. By Guardian's counting rule surface 1 is the outdoor face, so some pyrolytic coatings can sit where an outside cleaner works, which matches NGA's statement that some coatings may be on the exposed surface. Cardinal's named example of a hard-coated product is HydroClear, which the same site lists as anti-corrosive glass, so Cardinal's page does not say which surface a particular home's Low-E is on.

For an exposed reflective coating, NGA says special care must be taken, because damage to the reflective surface may result in coating removal and a visible change in light transmittance. NGA's pencil test for an exposed reflective coating is scoped to reflective coatings; the same paper calls Low-E coatings very difficult to see.

On any coated surface, NGA says razor blades should never be used, and it says so twice, in the body of the paper and again in its DO NOT list. NGA's two published methods end differently, a window cleaning squeegee removing all of the solution in the routine method and a pure water rinse generally left to evaporate in the other, and for coated glass the paper adds that specific procedures must be followed and the glass manufacturer's guidelines consulted. Its direct-sunlight caution is scoped to tinted and reflective glass.

Tinted glass and exposed reflective coatings and whether a razor blade belongs on window glass each have their own answer, and how impact, Low-E, tinted and tempered glass differ sets Low-E beside the other three.