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Outdoor use of Hardibacker?

JonE's picture

Have poured my ICF basement, and put waterproofing membrane on it, now I want to cover the exposed foam above grade with either Durock, Wonderboard or Hardibacker.  It will then be covered in stucco or cultured stone.  HD here carries all three in 1/2" thickness.  I hate working with the cement board tile backer because of the weight and mess involved, but the Hardibacker says right on it that it is for only interior use.

So what's the big deal?  Both products can be used under tile in wet locations, but only the cement board can be used outside?  Can anybody offer some insight on this one?  I'd really like to use the Hardie product, it seems so much easier to use, but I'm now worried about durability and lifespan if applied to an exterior wall (even if it's covered).  The Hardie siding products get wet - why can't this stuff get wet?




(post #96256, reply #1 of 11)

The Hardie siding products get wet - why can't this stuff get wet?

Because it's a different product, made from different raw materials, using different manufacturing priducts.

It's kinda like asking why your Toyota can't be used to transport you in water. After all, the Toyota is an automobile, and it was made for transportation purposes. Why not try to use it in amphibious applications?

Any other questions?



Unless you're the lead dog, the view just never changes.

. . . I can't live proud enough to die when I'm gone, So I guess I'll have to do it while I'm here. (Phil Ochs)

(post #96256, reply #3 of 11)

Yep.  Besides, I don't have a Toyota.

My point is, the Hardibacker gets used in the exact same applications *indoors* as the Durock.  So, if that's the case, and we know tile jobs often get wet, then why can't I use the Hardibacker outside under stucco?  Just because the label says so?  I really don't give a rat's behind about the warranty issues.  Their website says what it's made of - cement, cellulose fiber, sand and "selected additives" and says it's "moisture resistant".  Is Durock waterproof?  Will the cellulose fiber decompose?  If I get this crap wet, will I have a big spongy mess on my wall?

I guess what I need to know is, regardless of warranty or "recommended use", will this stuff fall apart within twenty years or fifty years if used under stucco or stone veneer as an exterior substrate?   Hell, I might use it anyway, and I'll report back to y'all in 2014 to let you know how it's holding up.



(post #96256, reply #4 of 11)

Hardie does make a sheet material for exterior use.

(post #96256, reply #2 of 11)

Its one thing to get it wet. Its another thing to get it wet and freeze.

(post #96256, reply #5 of 11)

You've asked a good question- I'm not looking forward to buying, carrying or cutting all that cement board either- and I can't afford to get someone to stucco the thing for me. 

Now I've got another one:  what's the best termite barrier detail to use for the transition between the top of your ICF basement/foundation wall and the sill plate?  Is a piece of galvanized flashing over the entire top of the ICF wall, bent over the cement board on the exterior, good enough?  Termites are relatively uncommon up here but not completely unknown, and the brand of ICFs I'm interested in doesn't come with borate treatment.

When I talked to the local ICF distributor, what he recommended was ripping off a 6" swath of the exterior foam layer after the concrete is cured, and filling it with fresh concrete to form a break in the foam the bugs couldn't bridge without being detected.  That idea sounded pretty dumb to me. I'm going with ICFs for thermal performance, and the last thing I want to do is bridge the exterior foam and put a big concrete fin onto the wall, exposed to the coldest part of the exterior environment (i.e. above grade)!  It'll also look crappy since local code requires that I protect the foam above grade with cement board, but that concrete strip would have to remain exposed so you can see termite shelter tubes if they should form.

(post #96256, reply #6 of 11)

You already committed to an ICF brand? How about switching to one with borates, or to a Rastra-type block? Or you could cut a swath out and fill it with the bug-resistant foam, mortared hard to the concrete so they cannot pass in the hidden places...the bugs, that is.


(post #96256, reply #7 of 11)

Nope.  Haven't committed yet, but like this one the best so far- except for the borate issue.  Again, termites aren't a big problem in my area, but they're not unknown in my city and the consequences are so dire that I can't just ignore it.   I'd hate to switch to a harder-to-use brand just for this issue, if there's an easy way to overcome it. 

The galvanized flashing wouldn't be too tough to do or cost too much, but would it work?  Easier than ripping off foam and replacing it with other foam...

(post #96256, reply #9 of 11)

If your ICF wall has monolithic concrete slab between two foam faces about 2" thick, it seems to me that you could kerf through the foam and slightly into the concrete with a skilsaw, inject the slit with good acrylic caulk, and then insert a stainless steel flashing into the kerf, being sure to push it all the way to the back. For that matter, you could run a downward sloping kerf as above, then fill it with epoxy. That might work without the metal, and the thermal bridging of either solution is inconsequential.


(post #96256, reply #10 of 11)

Sounds like a good idea- easy and effective  Will stop the little buggers dead in their tracks for sure.  Now to convince Mr. Building Department...

(post #96256, reply #8 of 11)

I doubt you need Hardi under stucco... can't they find a way to attach the stucco wire to whatever you'd attach the Hardi to? As far as cultured stone goes, are you sure that wants to hang on the wall rather than sit on a lip of the foundation?

(post #96256, reply #11 of 11)

How about expanded galvinized diamond metal lathe with a scratch coat then your stone product? Attach the metal to your ICF furring strip devices whatever they may be.