Percival P. Cassidy wrote:
Our front door has an aluminum alloy threshold, and we have just noticed
that ice is forming on the indoor side of it; this, when it melts, will
not be good for our wooden floor.

What must be happening, I am sure, is that, because the outdoor
temperature is now well below freezing, water-vapor from the indoor air
is condensing an freezing on the surface.

Short of (a) replacing the threshold by a wooden one or (b) cutting the
aluminum one lengthwise and interposing a "thermal barrier" between the
two sections, what's a good way of solving this problem?

Perce

Is it smooth ice or frost? Frost won't damage your floor.

I grew up in a 19th Century house in VT. There were no storm doors.
When it was well below zero, door handles would sometimes turn white.
No wetness resulted.

In NC, sometimes I'll open curtains and find smooth ice on windows.
This will rot windowsills.

The difference is the dew point. White frost formed in VT because the
dew point indoors was below freezing. So the frost evaporated without
thawing. In NC, the smooth ice in windows is frozen condensation. The
water condensed above freezing and will melt when the window warms.