Exterior-Rated MDF - Any Good?

[Andrew Herrick]

So, Menards - sort of a Midwestern version of Home Depot - sells this stuff:

https://www.menards.com/main/building-m ... 731430.htm

It's an exterior-rated MDF. It's not cheap - almost as much as industrial-grade marine plywood - but it promises exceptional dimensional stability, moisture resistance, rot resistance, insect resistance and finishing capabilities. It's available down to 1/2" thickness. Anyone ever use the stuff? Thought it might work for painted cabinetry or bendable roof sheathing, if one could find it in 1/4" thickness.

[noseoil]

Too heavy & over-kill.

[KCStudly]

If it sounds too good to be true... "Must be primed and painted before being exposed to the outdoors", doesn't sound anymore waterproof to me.

As far as marketing claims go, here's an example; Advantech is supposed to be the latest and greatest exterior grade/waterproof chip board intended as a decking/sheathing material. I'm pretty sure that what they mean is if it gets wet once or twice during a home construction project it isn't the end of the world, and will dry out and and be okay once the house is done being built and weathered in. Where I work they used this as a flooring underlay in a mixer room/wash down environment, then capped it with a (supposedly) water proof industrial plastic snap lock tile finished floor, then did a pretty sketchy job sealing around the toe kicks. Within 2 years the floor became spongy and black with rot. It had to be ripped up and replaced at no small cost. (They used the Advantech again, but this time they went with a "welded" plastic sheet good and rolled it up, sealing it to the wall to form a tub like baseboard.)

So there is no free lunch. If you let moisture creep in and fester it will swell and rot just like any other wood.

I would (in fact I did) go with the marine ply in 5mm (3/16 inch); some people go as thin as 3mm (1/8 inch) for ease of bending and lighter weight. When backed up by foam board 5mm is more than adequate.

As always, it is as much about designing for and sealing the moisture out in every last detail, as much as it is the materials selected.

[aggie79]

Too heavy & over-kill.

And it won't bend.

[Andrew Herrick]

If it sounds too good to be true... "Must be primed and painted before being exposed to the outdoors", doesn't sound anymore waterproof to me.

So there is no free lunch. If you let moisture creep in and fester it will swell and rot just like any other wood.

I would (in fact I did) go with the marine ply in 5mm (3/16 inch); some people go as thin as 3mm (1/8 inch) for ease of bending and lighter weight. When backed up by foam board 5mm is more than adequate.

As always, it is as much about designing for and sealing the moisture out in every last detail, as much as it is the materials selected.

True that. I know we all love wood, but it is a finicky material, isn't it?

Thanks for the input, everybody! I'll go back to more tried-and-true products. Probably less VOC's, too!

[dmdc411]

It's too heavy, brittle, heavy, etc. My book, MDF is good for one thing, underlayment. Use a decent plywood. If it's going to be covered on both sides, BC, or AC is fine. I used 3/8" AC exterior glued, and 5mm luan for the interior, cabinets, and exterior roof. Cover with aluminum. It's very sturdy, solid! Also 1x 2 or 3" popular framing in the walls.