Writing in 1987 in New England Builder (the Journal of Light Construction before it launched as a national publication), Joe Lstiburek contributed what continues to be a highly relevant and important article, "How Insulation Can Peel Your Paint." Here he addressed the specific problem of capillary rise between the overlaps in horizontal siding. For all those who advocate rain screen siding, this is a must-read article, as it's one of the first to identify the problems of increased moisture content in siding applied directly over walls with a low drying potential (a.k.a. insulated walls). Back in the late 1980's when this article came out, a lot of walls were being dense-packed with cellulose insulation and a lot of people were discouraged by how this accelerated the deterioration of their painted siding. It still happens all the time, maybe more now with new homes insulated to current code standards when the siding has no rainscreen or effective drainable house wrap behind it.
Per usual, Lstiburek summed up the issue and the solution succinctly: "The major problem caused by retrofit insulation and insulating sheathing is a reduced drying potential. It turns out not to matter whether the moisture source is interior or exterior. The treatment is the same: Increase the drying potential of the siding by increased drainage, air circulation, and capillary breaks in the siding."
This treatment is now common; we call it a "drainable assembly" or "rainscreen" or "drainable houswrap" - terms, methods and products that have Lstiburek's indelible fingerprints all over them.
In Capillarity Sucks, the latest entry in his Building Science Insights series, Joe Lstiburek dives much deeper into capillarity, or capillary suction. It's a moisture transport mechanism that's familiar to almost everyone: "a paper towel, with one end in contact with liquid water, draws water into itself against the force of gravity," Lstiburek explains. Yet in building construction it's often overlooked, despite its capacity to create moisture problems not only in the siding but in every part of the building shell. In this latest article, we are treated to a thorough explanation of the physics and its many effects on buildings. Most importantly, we are given the details for avoiding dire capillarity in foundations, walls (including the siding he covered so long ago) and roofs.
