Expert: Bruce E. Johnson - 6/16/2011

Question
Hi Bruce,

We are remodeling a 125-year-old home from the ground up. We started by insulating with rigid Polyiso between the studs, followed by an interior layer of 7/16" OSB throughout the house on walls and ceilings for added structural strength. We then finished with traditional 1/2" drywall over the OSB. After we primered, painted, and put up crown molding on the first three rooms, everything seemed to be working fine.

Today, however, I noticed hairline cracks in the tape joints of one of the completed 16x16 ceilings. They are quite thin, but they are plainly visible in ordinary light. The ceilings of two smaller rooms are doing fine so far, as are all the walls. All of the rooms have been temperature controlled throughout the remodel. The attic over the problem ceiling is insulated between the rafters with two thick layers of rigid Polyiso (about R50), and is therefore part of the heated (and cooled) upstairs living space. The joists are spaced on 16" centers, but they are full cut 2 x 10's and hard as bone. We taped with fiberglass mesh and used standard mud for the joints.

What do you think could be causing the cracks? We have some spectacular molding (about $1500 materials) installed in this room, and I'd really hate to tear it out.

Would it be possible to rout the cracked mud out of the joints and replace it with a compound that would flex enough to prevent the hairline cracking? If so, what would you suggest?

Thanks in advance for your help.

Mike

Answer
Hi Mike, gee it could be a number of reasons why you are getting hairline cracks in your ceiling. My first guess is that the added layer of OSB to the underside of the joists is causing a sag on that 16' span of 2x10s.  Either that or the OSB itself is sagging between joists and pushing the 1/2" drywall down.  I kind of ruled out expansion based on your comment that the place has been conditioned during construction. You might hold up a long straight edge across the joists and with the joists and see if there is a joist sag (in the middle of the room and the straight edge with the joist direction) or a material sag (in the middle of the room with the straight edge crossing the joists).  If the upstairs attic space has seen some activity during construction and after the drywall ceiling finish it's possible that the bounce in the joists cracked the seam as well.  Things like this can't be helped sometime.  A good elastomeric type of paint will bridge the cracks and provide flexibility in the ceiling plane.  When dealing with stubborn ceiling cracks I try a couple of things.  First I will try rubbing a good latex painter's caulk into the crack with my finger and then wiping it with a damp sponge lightly after it dries for a few minutes.  Then after the caulk dries you may need to repaint that area.  Like I said above, a good heavy latex or elastomeric paint after the caulking trick should do the trick.  I'm sure you have pulled thick patches of paint from a heavily painted wall?  That's the same principle for ceiling paint.  A heavy coat will create it's own plane and the drywall cracking shouldn't telegraph through.  I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce johnson..bejohnsonconsulting.com