I have heard of problems of mold inside walls have been caused by Hardiboard directly over Tyvek, that the moisture can't escape from inside the house and so mold forms inside the walls. I understand that the solution is to add furring between the Tyvek and the Hardiboard so moisture can escape.

Is this a problem with stucco? I have shear siding in place and was going to put Tyvek for the first line of a drainage plain and then the regular felt paper that the stucco contractor uses after that. Will that cause inside moisture to build up in the walls?

What do I need to do so that I don't get mold in the walls. I don't think I have room for furring because of the thickness of the stucco.

In northern climates, interior water vapor that works its way past a warm side vapor barrier will condense within a wall cavity on the inside surface of the sheathing if the dew point is reached there (usually on very cold winter nights.) The condensate, often in the form of frost, melts during the day wetting the inside surface of the sheathing (e.g., OSB). This liquid water is not a problem provided it can escape through evaporation to the outside in a few weeks time. Usually the cold inhibits the growth of mold in this period. Any impermeable material applied to the outside of the sheathing will inhibit evaporation to the point that rot and mold growth occurs. Tyvek is a highly permeable (breathable) water barrier which will not trap moisture in the sheathing. Hardiboard, when installed correctly, will also not inhibit evaporation. I've heard of people caulking between boards which is a no-no because it will prevent proper ventilation. Cement based stucco is also highly permeable and should not present a problem installed over Tyvek. In fact, stucco must be installed over a water resistive barrier to prevent water intrusion from the outside. Acrylic stucco (Dryvit) is another matter entirely. It is highly impermeable and must be installed with a drainage layer over the Tyvek. Hope this helps.

Thank you for your reply! I live in Santa Cruz, California. It gets below freezing about two nights per year. Cold winters are about 45 degrees at the low end. Summers are aroung 65 to 70 degrees. I am not doing Hardiboard, but the information I got about mold inside the framing itself with Hardiboard and Tyvek got me worried. They said there needed to be a space between them for moisture to evaporate.

I am not sure what to do. If I don't use Tyvek, will that mean I won't get moist air trapped in the framing? Should I just put an extra layer of felt as a secondary drain plane?

When I read about the correct way to do stucco, they say there must be two separately installed drain planes to insure no water reaches the house itself. When talking to local stucco contractors, no one seems to have heard of this and no one seems to do it. They use a roll that has two layers and install them at once.

I am very confused and, no matter what, I want to avoid MOLD. I want to do over kill if possible, but I'm afraid adding Tyvek might increase my risk.

Thanks again.

What exactly are the layers that the locals use?

Patricia, the Dupont website makes this statement:

"DuPont™ Tyvek® StuccoWrap® is recommended under stucco because it has been specially designed to work with both traditional stucco and synthetic stucco applications. The engineered surface with special grooves is designed to assist with drainage as part of the synthetic or EIFS systems. In traditional stucco, DuPont™ Tyvek® StuccoWrap® has been shown to aid in curing, helping reduce scratch coat cracking and promoting flexural strength for improved stucco integrity. DuPont™ Tyvek® CommercialWrap® will perform well under traditional stucco, however it does not offer the same surface feature."

According to this you can use Tyvek CommercialWrap or, better still, Tyvek StuccoWrap which incorporates a drainage layer.