Shrinkage in MDF Crown Moulding

Ideas for installing MDF crown so as to minimize shrinkage, and its harmful effects. August 7, 2006

I install mainly 4-7" MDF crown moulding and am wondering how to preventing separation of material at scarf and inside joints. Should I be coping the inside corners? Some jobs have separated at the scarf joints after about 6-8 months. Also, are there any sample contracts out there for installing crown? How is this shrinkage addressed in the contract?

Forum Responses
(Cabinet and Millwork Installation Forum)
From contributor A:
First, let the MDF sit in the house for a few days to acclimate. Then when you are cutting make the inside corners long and snap them into place. I usually don't use a scarf joint with MDF because of shrinkage. I use a butt joint with a biscuit and cut the molding long (about 1/8") so that it snaps into place.

From contributor B:
If you aren't already doing it, glue that scarf joint its the only way to keep it from moving.

From contributor C:
I've seen inside corners come apart a lot out here. (southern California). I think that it's the way it is installed, not actual shrinkage. If the crown doesn't meet the ceiling or the wall at the corner properly, it gives it room to twist when the caulking shrinks. Making sure the crown is up against the wall on both sides of an inside corner and then putting a small shim above on both pieces close to the joint at the ceiling will push the crown down tight against each other preventing it from twisting up when the caulking shrinks. Any moisture in MDF will cause it to swell but I haven't seen it shrink back to shape when dry. Coping is always the best way but solid wood may shrink, causing a gap no matter what. In this case I would probably also make the piece a little longer and snap it in. Learn to be quick if you use Titebond glue. It tends to set quicker than one thinks.

From contributor D:
Also take into consideration the expansion / contraction of the home or dwellings shell.

Expansion (-Contraction) = Di * SFt or r * (MCair - MCwood / FSP)
where Di = the initial dimension
SFt or r = the shrinkage factor, either tangential or radial
MCair & MCwood = the moisture content of the air and wood
and FSP = the fiber saturation point (typically, 28 percent)

From contributor E:
I recently did a wide crown molding install using simple butt joints between the 8' pieces of crown. I used counter-top joint bolts let into the back of the crown pieces to make sure they would never show a crack.

From contributor B:
Wow that crown must be real thick, great idea.

From contributor E
Seal it or prime it before installing it - it will help (from a finisher).