Did we just make a mistake by insulating no-paper stucco walls?
severaltypesofnerd
| Posted in Energy Efficiency and Durability on
Did we just make a big mistake? The house has stucco walls with various hairline cracks. We had cellulose insulation blown in. The house was built in 1938, and the building paper is completely gone… we can only find scraps of the stuff. The sheathing is fir boards.
Will the cellulose grab water that otherwise would have dried, resulting in wetter wall cavities?
Climate: California fog coastal. Dry summers. Rain in winter. Fog frequently.
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Replies
Bryce,
Q. "Did we just make a big mistake?"
A. Probably not, but the situation bears monitoring. Where do you live?
Look on the bright side: at least you have board sheathing, not OSB. The board sheathing allows for drying to the exterior.
It should be said that when stucco is installed on a wood-framed building, it is the most problematic of all siding types. However, most wet-wall problems in stucco houses are happening on newer OSB-sheathed homes, not older board-sheathed homes like yours.
Climate: California fog coastal. Dry summers. Rain in winter. Fog frequently.
Elastomeric paint has been mentioned as a solution, but that will no doubt inhibit drying even more.
Bryce,
At this point, I assume that you have no evidence that you have a problem. I would relax, but keep my eye open for any signs of a moisture problem.
A one-story house with wide roof overhangs will have fewer problems than a two-story house with stingy roof overhangs.
Are these 2x4 walls?
These are 2x4 walls, 2 story Mediterranean style house with no overhangs whatsoever. There's plenty of evidence of past sheathing water damage (the Realtor participated in covering most of that up). Still, the sheathing in the past could dry. With the insulation I can imagine it getting humid, even if the total volume is not sufficient to leak to the interior stucco.
The old paper, where we have found scraps, is white and brittle. In places the termites ate it. Everywhere else it just seems to have evaporated.
If there is at least 3/8" of air space between the sheathing and stucco, and the stucco is vented both at the bottom (at least a few inches above grade) and at the top it will probably have sufficient drying capacity to purge bulk moisture. If it's only vented with at the bottom, or not at all, it's a problem.
Elastomeric paints may limit the amount of bulk moisture getting in, but would of course limit the ability of the stucco to release stored moisure directly to the exterior, making the back-venting of the stucco even more important. Stucco will absorb and store a lot of dew/rain moisture, then release it as water vapor at a very high rate when heated by the sun, raising the humidity level in the cavity between stucco & sheathing sky-high. If vented the heated stucco would induce a convection current, cycling the hyper-humid air to the outside. Unvented the humidity reaches levels where it's dew point can be well above the temperature of the interior finish wall, and both the finish wall and the cellulose can end up taking on that moisture.
It's possible to retrofit venting into stucco siding, if you think it needs more venting than it has.
BTW: The borate fire retardents in the cellulose may inhibit the termite issue somewhat, but isn't a solution to a termite problem. Hopefully that issue has been addressed separately. (The borates are better at limiting wood-boring ants, bees & wasps, since they don't stay completely within the wood, and are more likely to consume the borates if they tunnel through the insulation.)
Oh man, I did make a mistake. I'm now on the third area of pulling papier-mâché out of these walls. What would have just dried with minimal damage now just stays wet.
One example: some nails used to hold a downspout installed 1938 rusted, leaving a small channel for water. This was apparently not much of a problem for many years until I insulated. The paint fell off the interior plaster. This is in the San Francisco Bay area (7-9 month dry climate).
Consider carefully before insulating an old stucco house!
Bryce,
I'm sorry to hear about your discoveries. The right details for stucco, as I wrote in my 2010 article, To Install Stucco Right, Include an Air Gap, include a drainage gap behind the stucco, as well as two layers of WRB and sheathing. These elements are often lacking on older homes.
In my comment on this thread (August 8 comment), I noted that a one-story house with wide roof overhangs might be OK. Unfortunately, as you wrote later, you have a "two-story Mediterranean-style house with no overhangs whatsoever."
It doesn't sound like a good situation.