Prolonged High RH in Stucco Assembly

[Jon Barratt]

I'm analyzing a wall assembly in San Francisco, composed of:

• Exterior
  7/8" stucco
  paperback lath
  3 1/2" batt insulation
  5/8" gypsum board
  Interior

A cold year was selected for the exterior climate, and a medium moisture load sine curve was selected for the interior conditions.

When this assembly was simulated, it was found that the RH stays well above 80% at the interior side of the stucco (and therefore at the exterior edge of the insulation) for extended periods of time (approaching 5-6 months).

I tried inserting a vapor barrier at various locations, adding a layer of sheathing under the paperback lath, and adding a one-inch wide ventilated air space between the paper (or sheathing, if included) and the insulation. No configuration has been able to reduce the prolonged periods of high humidity experienced at the exterior edge of the insulation.

This type of construction is fairly common in the Bay Area, and my colleagues and I are concerned that it appears as though the software is predicting that this high RH condition will always occur in this type of construction. This has lead to some difficulty in reconciling the results of the simulation with what field experience has shown to be the case.

Has anyone else had a similar experience analyzing these types of stucco assemblies?

[Achilles Karagiozis]

What type of stucco are you using ? what is the capillary saturation moisture content of this stucco ? Is that what is used in San Francisco ?

Stucco is an absorptive cladding, as such why would it not have a high relative humidity at the interior side of the stucco ?

If you have an answer to the above two questions,it might shed light on the high values you predict.

Best Regards

ANK

[Jon Barratt]

Hi Achilles,

The stucco is "Regular Portland Stucco", chosen from the Generic North America database.

I had expected the stucco layer to experience high relative humidity, but am concerned that this condition persists for months at at time. I'm more concerned about what effect this condition will have on the Batt insulation and wood framing. Based on the results I'm getting, the RH in both the stucco and at the outer edge of the insulation stay above 80% for almost 10 consecutive months.

The results do not indicate that there is a year-to-year increase in moisture content, which is encouraging. Am I right to be concerned about the high RH in the insulation layer?