We have 2-over-2 paneled interior doors in our 125 year old Italianate
house. We have only lived in the house for 2 years, but now for the
first time I am noticing that many of the panels are getting severely
I am surprised because now the house is at its maximum dryness and I
would have expected more cupping during the summer as the panels
expand against the door frame.
Is it natural to have such cupping and if so what would be causing it?
I assume that since the house is so old and since I didn't notice the
cupping in the summer months that this is just cyclical and will
resolve when the humidity returns, right?
That's likely the case.
Painters _rarely_ hit the top and bottom of a door. No one sees the
top and bottom of a door anyway, right?
Doors should be sealed on all six faces to minimize the effects of
changes in humidity. Always. If the door binds and someone trims it
with a plane, it has to be resealed.
If the door is properly built -- I'm sure it was -- there is adequate
space for the panels to expand. Sometimes people naively glue panels
into place, and this can cause big problems.
IIUC painting and varnishing of panel door should be done lightly at
the places where the panels meet the doors. So one won't see a ridge
when the panel shrinks.
As to sealing the door, shouldn't that be done when the humidity is
intermediate for the location of the door? Does that make a
If the joinery is 125yrs old it is built well enough.
Never heard of either of those suggestions. Whatever you use to 'seal' a
piece of wood it will never prevent gradual changes in moisture content.
I don't really understand how a panel can be 'cupping', but the best thing
to do is just leave it be, it's an old door in an old house. I don't know
what the climate is like where you are but I have seen a lot of old joinery
which doesn't really fit well and isn't too straight but it is important to
leave and preserve these things and not do unecessary renovation. In my part
of the world a lot of old doors have not fitted too well since the owners
installed heating and damp proofed the walls of buildings which had been
generally cold damp and draughty for hundreds of years.
Cupping in the sense of grain on panels is vertical and the panel is
(cupped or bowed) similar to the way a 1x10 piece of pine would be
said to be cupped.
I'm sure it's nothing since the house is so old, the doors are
varnished (not painted) and the last coat of varnish was at least
10-20 years ago.
The house was of course built before electricy and central heat but
again nothing has changed in the interior environment (as far as the
doors are concerned) in a long time so I am assuming this is just part
of the natural cycle.
My only question remains is what would cause cupping during the "dry"
months (here in New England it is still winter and with forced hot
air, the humidity is very low in the house maybe 20-30%) vs. the
summer when it gets quite humid since we don't (yet) have central air.
As best I understand it myself, and simplified, there's two ways to
get a cupped board:
The first is from release of stresses in the wood when it's cut.
Think of the tree like a bunch of concentric stretched rubber bands.
Then someone comes along and cuts the rubber bands. They want to
straighten out. If you don't give it long enough before milling to
final thickness you can end up with more cupping in the final board.
Because you say they weren't cupped in the summer this is not likely
to be the issue.
The other way is if one side of the board has more moisture content
than the other. As one side of the board gains moisture it expands,
but only noticeably across the grain. Now one side is wider than the
other, so the board has to bend. The same thing happens when one side
of the board loses moisture, it just cups in the opposite direction.
If both sides of the board are in the same environment, and both sides
are finished the same then it should not cup because both sides are
gaining and losing moisture at the same rate.
There's some confusion as to how you can have a cupped panel in a
door. The panel is trapped in the frame, which should prevent it from
being able to cup, or atleast only cup to the extent that there is
slop between the panel and it's groove. That's sort of the point of
frame and panel construction.
There is a third way. On a flat-sawn board, the portion on the outside
of the tree will move more than the inside of the tree in response to
moisture changes. So in dry conditions (drier than the conditions in
which the board is flat), it will want to be concave on the side of
the panel that is the outside of the tree. If the panel is glued up
with alternating ring orientations, this will result in a wavy panel.
If all boards are oriented with the rings going the same way, it will
cup (or maybe split if restrained)
Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently.
This sounds like my situation. Since each panel is made of a single,
large (24"x12" approx) but THIN board (< 1/4"), it is I guess not
surprising that slight differences in stresses could lead to
Of course, I can't wait until summer to see whether it does resolve or
whether it has been truly cupped all along and I just didn't notice it
I agree. But again I would have thought such "cupping" would occur
more during the wet months when the panel would be expanding against a
But perhaps what is really happening is that the panels are quite thin
(I would say 1/4" or less) with dimensions of maybe 36" x 12" from a
single piece of wood (no laminations or joints) so that
perhaps the natural stresses in the thin wood panels lead to such
cupping during the dry months. In fact, several of the panels have
cracks in them which presumably have occured over the past 125 years
or so do to stresses, seasonal changes, abuse???? -- I guess this is
understandable in large thin panels like our doors.
Not unless you're air conditioned to dead dry and the outside's in the rain.
Right now my relative humidity is 84% outside (snow, too), and 28% inside.
Don't think it'll ever get that bad with the opposite sides of the panel in
Got too dry on one side. Weathering? Lots of humidity cycles shouldn't
make cracks, only extremes.
Hmm, if it was just a change in humidity, the panel would expand
equally on both sides, no cupping.
Perhaps you have a difference in the finish on the back side vs. the
front, so the panel moves more on one side than the other. How are
they finished, front and back?
Both sides are varnished and I have notices this now on most but not
all of my doors (and again these are all interior doors with not
appreciable difference in atmospher on the two sides). The degree of
cupping varies by panel -- some are quite severe, others are less so.
Again the only thing I keep coming back to is that the panel is a
single large but thin sheet of wood so that it may not take much
"differential" stress to lead to such cupping.
You know, your first statement about these panels implied the cupping
was recent; but if it is just that you are now noticing the cupping,
and it was there all along, then perhaps what you have are panels that
were formed long ago from wood that was not dry enough, or wood that
was reactive enough, to warp after the panels were formed.
Otherwise, it seems odd to have panels decades old all start warping
at the same time.
Regarding your other comment, that these are interior doors and the
panels are all indoors, so why is one side different than the other
... I may be belaboring the point, but ... say you removed all the
finish from one side, and left all the finish on the other, then you
have a panel that will change humidity quickly on one side, and slowly
on the other, and the shrinkage/expansion on one side could be fast
with little change on the other, and you get a warp when the humidity
changes, and then the warp flattens out as both sides equalize
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