New stucco over foam: cementitious or synthetic color coat?
My New Mexico house is getting a whole new stucco job and the new stucco is over 1.5 inches of EPS insulation, making the wall sandwich look like this outside to in:
Cementitious stucco w/ metal lath
1.5″ unfaced EPS foam
0.25″ cementitious stucco
Latex paint (ugh)
0.75″ cementitious stucco
Single layer of tar paper
Celotex sheathing
2×4 stud wall insulated with faced mineral wool batts
Drywall
Latex paint
The cementitious base coat is on, and now I get to choose what kind of color coat to put on: cementitious or synthetic. I was all set to choose cementitious, but in the two weeks since the base coat has gone on, it’s developed a bunch of tiny cracks all over, and some slightly longer straight ones at the joints between the foam boards. None are big. The base coat was fiber-reinforced, so I suspect that some of the cracks are the result of the foam expanding and contracting, and maybe the low temperature?
So now I’m questioning the wisdom of applying a cementitious color coat if it’s just gonna immediately crack as well. My neighbor has a synthetic color coat on his house and there are zero cracks after 6 years.
Is there any reason I should be concerned about a synthetic color coat? I worry that it could trap moisture, but I live in the desert and there’s foam under it…
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Replies
Nathaniel,
Have you talked to your stucco contractor? Or are you doing the stucco work yourself?
If you have a stucco contractor, and if your stucco contractor is the one who has to handle callbacks and complaints about cracking, then your stucco contractor is the person to listen to.
Cement/concrete cracks. That is a fact. When it comes to stucco coats the strongest, longest lasting, maintenance free coating is synthetic non-cementitious coatings. That is WHY you will never see a commercial building use conventional concrete based stucco.
You pay a little more for synthetic stucco but it will save you $$ in the long run and you don't have to run around patching cracks. In addition, real cement based stucco will absorb water and will crack especially with freeze/thaw cycles. Synthetic stucco is waterproof.
Not all synthetic stucco is made with the same properties. Once a crack always a crack with regards to the base coat. Lack of expansion joints and to much water in the mix will lend itself to more cracking.
Many synthetic stuccos do not have enough elongation to flex at the cracks so you will see still the cracking. On the hand, there are also synthetic stucco finishes with enough elongation to hide the cracks. You also need to look a the perm of the finish, as it varies greatly from finish to finish.
I'm getting conflicting information. My contractor tells me it's a little bit better from the perspective of crack resistance, and it will cost me another $1600. Some people tell me it's the best thing since sliced bread. Other people tell me it won't last, and will trap water that gets behind it. I especially worry about that due to my window detailing; they are "semi-innies" with integrated sloped stucco sills. But the joint between the window and the stucco sill is a place I can see water entering and getting trapped behind impermeable synthetic stucco.