Stucco, Horsehair, cellulose
My house is a 100-year-old stucco-sided home, on the coast of central Massachusetts. The home’s original horsehair plaster is in decent shape on the majority of the exterior walls. I am planning on insulating the attic spaces and sealing the basement sills, as well as doing a thorough job of sealing air gaps around doors, windows, and any other seems I can find. I have read a few concerning articles about the prospect of doing blown-in dense-pack cellulose behind plaster lathe walls due to a lack of vapor barrier. Assuming that the condition of the plaster is good enough to go ahead with the dense-pack cellulose, should I be concerned about vapor issues given that the home is stucco-sided rather than wood-sided? Does the combination of stucco exterior and plaster lathe interior exacerbate the potential for mold, etc?
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Most stucco houses that vintage in MA applied the stucco to wooden lath nailed directly to the studs. Without an air gap to dry into the lath will soon begin to rot.
This is NOT a vapor barrier issue, it's a an exterior moisture drive problem due to direct-wetting of the stucco which wicks and stores the moisture, soaking the lath if it can't dry into the stud bay cavity. This is a serious problem for stucco clad homes with or without vapor barriers installed.
If the wall assembly has exterior plank sheathing & tar paper with an air gap between the tar paper and lath for the stucco it might be safely insulated. It helps if there are ample roof overhangs (at least a foot of overhang per story), and there would need to be special attention paid to window flashing & other bulk moisture handling issues before moving ahead.
It's expensive, but leaving the stucco in place and adding rainscreened siding (or vinyl siding) directly over the stucco can work without interior vapor retarders. (The interior paint on 100 year old plaster is usually a Class-I or Class-II vapor retarder due to multiple layers of alkyd paint, sometimes leaded paint.) As long as the stucco isn't being chronically rain & dew wetted and can dry in to the rainscreen gap (or behind the vinyl siding) it's moisture content stays reasonably low. The window flashing details still have to be up to snuff first. The additional thermal mass of the stucco also yields a modest improvement in thermal efficiency (more in the cooling season than the heating season).