If you are using MDF Baseboards/ casing... please read

[Mitch Hrycan]

I used 4 inch mdf baseboards which came primed on one side bought from Windsor Plywood. Having never used MDF before I just painted the side that came primed from the factory and put it up in the house. I put it up in summer. Now in winter, every inside mitred corner has open up (some as much as 1/4 inch on long runs).

In my head I'm thinking MDF is supposed to be more stable than wood (so I always heard)...so...causes...

1) I painted and primed the BB in my garage/shop and once cut brought them into the house for direct install...you MUST acclimatize it to the space. Keep it in the room to be installed for a couple of days before you cut and install it. OK, so that was my first mistake.

2) I didn't seal the cut ends of the BB after i cut them, again, I'm thinking this stuff is stable...ha! Seal all cut edges of MDF with Titebond III...or any waterproof glue. It dries very fast...I mean 30 seconds or less because it gets soaked in the the cut end. Also, glue the inside mitres and glue and pin nail outside mitres. Coping was not an option with this BB design and it wouldn't have helped me anyway.

3) I'd never heard of "back priming" and since the BB came primed on one side, didn't even think about it. Back prime/prime every edge. (Except the cut edges you seal with Titebond III) Yes, prime the side that faces the wall. Using a non water based primer on MDF is best (doesn't raise the grain) and I would think it'll be more moisture resistant in use.

I routed my own casings and made corner blocks from MDF for all the doors and windows (saved big $ and they look great!)...guess what...what started as a tight block to a casing...is now maybe 1/16 gap space there. All caused by what I'm certain is moisture affecting the MDF.

If you do what i suggest above, I hope it'll save you the troubles I now find myself in.

Fix for this??? I'm seriously looking at redoing all the long BB runs (i'll leave the closets and caulk the joints there) and popping off and backpriming, glueing cuts edges of all window and door casings. We're selling this house (already moved actually due to wife's medical condition), but I don't want to screw over the next buyer and want to do it right. Why not just caulk the space at the window casings and doors? I popped off the clorner blocks and reset them "tight" when the spaces appeared. I'm guessing in summer, if I don't control this now, the casings will absorb moisture and POP those corner blocks.

I made my own solid wood casings and corner blocks for the bathrooms to match the MDF ones in the house. The bathrooms ones I did prime all sides etc...due to it being in a "wet" area. Everything has stayed put.

One more tip for the corner blocks, Titebond's Moulding glue works great... Don't go crazy with it, but put a couple dabs behind those corner blocks and you'll never have to nail them. All the above advice applies to MDF corner blocks as well. Acclimatize, prime all sides etc...

Hope this helps someone out there.

Mitch in Saskatoon...back to the house now, cause time is money on this one, I'm so angry I could throw up. This is/was a BIG reno for me, wall taken out, Kitchen island biuld, solid wood drawer fronts, Stile and rail cabinet doors to match existing, Melamine cabinet construction, 100LBS drawer slides install, pantry drawers, install Cabinet crown moulding, move stove hood exhaust in attic, paint whole house, BB and corner blocks/casings etc..

[Erik Christensen]

good tip mitch will keep that in mind for the future. I don't mind a few extra minutes on an install if it will avoid problems down the road. Not sure how relevant it is to here in SoCal we get moisture swings but not like winter climes with lots of heating. I have MDF window casing up for at least 4 years with no problems like you are having but take those extra steps next time just to make sure.

[Mitch Hrycan]

Hi Erik,

I've also read that using MDF lite won't "pucker" around nail holes like regular mdf. Finishing those nails holes is another topic altogether in normal MDF.

Mitch in Canada

[David Helm]

MDF is a bad choice for any location that gets moisture, either through condensation or direct (window frames, base molding in baths and kitchens etc.). It absorbs moisture way faster than wood and, since it's essentially paper, it swells; never to return. Doesn't matter if regular MDF or MDF lite.

[fRED mCnEILL]

I have MDF baseboards and window molding and have never exerienced that problem. Our climate is moderate but WET (west coast). also we used Zinser lacquer based primer on the cut mdf to seal it. Worked very well.

[jim vancampen]

I use pva drywall primer rolled on all sides before install. It's cheap and works good.

[Phil Thien]

I wonder if the stuff they sell as base and trim has a higher moisture content or is somehow manufactured differently?

MDF and particle board typically require a great deal of stability otherwise products with vinyl wrap will fail.

[johnny means]

Mitch, I doubt MDF is your problem. What you describe sounds pretty normal in any house with any material. Structures expand and contract a lot more than we think they do. Caulk your gaps and be done with it. Also, all the extra work to seal and protect that MDF trim is unnecessary. If you have enough moisture to cause peoblems with that MDF, you need to be thinking about tearing it and the wall out for mold treatment.

[John TenEyck]

Priming, even painting, doesn't stop moisture gain/loss, it just slows it down. The reasons you had shrinkage problems was because it was at a higher MC than your house when you installed. As best I can determine the seasonal expansion/contraction of MDF is about 3 - 4%, which is on the low side for solid wood, and about half of most. It must have been at about it's max. when you installed it.

John

[Jeff Duncan]

I won't use mdf for trim work, there's a certain level of quality I won't go under and using mdf in that application is under it. I either get them to go with poplar, or I pass on the job and let someone else muck it up.

Now as far as joinery opening up, I'm not sure it would have prevented it in mdf, but using biscuits between joints in solid wood also helps greatly in keeping things together. Actually there's a lot of good info out there on installing trim and I highly recommend you check some out before redoing anything. Gary Katz's book has a lot of good info and may save you from redoing a third time

good luck,
JeffD

[Michael W. Clark]

Mitch,
Good info, but I tend to agree with Johnny above. Your house is going to expand/contract seasonally. The best scenario would be a material that matched this movement exactly. It sounds like your wood trim did that better than the MDF. I've seen wood trip that has opened up more than the MDF you have. I think the biggest issue with your MDF was that you stored it in one extreme and didn't allow it to acclimate. If you had stored it inside for a few days, then took it outside to cut it, I doubt you would have this issue.

I'm not a huge fan of MDF trim either, but I did use it when finishing our basement to match the rest of the house. It was the same pre-primed stuff and after seveal years no gaps. It was stored and cut in the same space, no back priming or glue. I'm a big fan of the Titebod mold and trim glue as well.

[Dave Beran]

I used of MDF trim in my home. The stuff installed in winter stays tight all year. Summer installs open in the winter. The cause in my mind is the changes in the structure itself. I life in northern MN where the summer/winter swing in humidity is huge. A little painter's caulk and paint solves the problem. I've also filled the open joints with light weight spackle. The only reason to use MDF is if the trim is to be painted so solutions abound.

[john bateman]

" every inside mitred corner has open up "
Even with real wood trim it's common practice to cope inside corners, not miter them. I think that magnified your problem greatly.

[Rich Enders]

Jeff,

Three cheers for your attitude about MDF. I also prefer solid wood instead of plywood in cabinets, and my favorite of all time is plaster instead of drywall. Getting back to the thread, I think back priming whether you use wood, or MDF will reduce the moisture exchange.

[Granger Ridout]

I usually lurk, but actually have an opinion and experience on this one, so hopefully I add something useful.

Anyway, for painted trim I prefer to use pre-primed whatever wood species it is that the big box stores carry (Lowe's mostly since they sell by the piece easier than Home Depot which is by the linear foot -- if I am cutting it to size I want to do it on site). I tried MDF in a previous house, using baseboard and crown molding, but my experience with the baseboard and other areas that can get bumped into by hard objects like furniture and children's play(!) is that MDF is just too soft, easily being dented and also absorbing water like a sponge when something gets spilled or leaks. I did not back coat the MDF, so that may help, but I also know that you can never 100% seal it, what with nail holes and scratches on the soft surface during installation. I never had an issue with the joints opening up on the MDF, but I did cope the joints, and we live in the humid South (granted with a/c and hardly ever open windows). The MDF baseboard and shoe molding was quickly dented in our house, looking like garbage in no time. And the sponge like nature of it was also readily apparent after a few spills from our little ones (because we adults never spill anything...). When using a nailer, the stuff also left behind huge dents, not matter how much I played with pressure. And wood will soak up spills, but not quite like MDF.

When picking out trim I usually run my fingernail over it to get an idea of how soft it is and both the MDF and wood stuff at the big box stores is soft, but the MDF is much softer in my opinion. I am biased, but I also like the feel of wood over MDF, finding it easier to work, especially when coping corners.

That all said, I don't mind MDF for crown, since it does not get any contact with hard objects and such, but the cost savings is not all that much compared to the aggravation and time factor, so I tend to not use MDF much in trim.

My DIY two cents worth, anyway.

Back to lurking and soaking up the knowledge!