Adding something on a topic I saw on another thread, but mentioned in Jay's post, is the concern about whether a straw bale building is suited to the wet, damp Pacific North West. It is! However, as in any building where moisture could cause a problem, care must be taken to keep the bales dry. There are many straw bale buildings performing well in parts of the North West that receive in excess of 60" of rain each year. If the building has been designed for the site and built well, it'll last for a very long time. The general rule is to make sure the structure has a good hat (large protective roof overhangs), good
boots (
water proof sills and elevated above grade to prevent rain splash from soaking the walls) and a coat that breaths (vapor permeable plaster skins).
The exterior of a plastered straw bale wall is a reservoir system. Wind-driven rain that strikes and soaks into the wall won't cause any harm so long as that moisture can come back out on the next dry, sunny day. That works well in my part of Southern Oregon where wind-driven rain is common as a storm front moves in, but tapers off after an hour or two. Most of our rain tends to fall straight down for the rest of the storm--a few hours to a day--followed by a week of sunny, dry weather. But where wind-driven rain can be constant for days and weeks, and dry days are far and few between, there's little opportunity for the moisture to evaporate out. The wall system could be overwhelmed and the liquid water can begin soaking into the bales, raising the moisture level to a point where microbes thrive and start to
compost the bales!
Knowing the year-round weather conditions of a building site informs the design. Where long periods of wind driven rain can be expected, all is not lost. Wrapping the vulnerable sides of a bale building with a full depth porch (minimum 8') affords great protection and relatively low-cost outdoor living space. Landscape solutions like a
trellis, hedge, or screen of
trees can offer protection. Many straw bale buildings have siding on them, with a proper rain screen. The bale walls receive a scratch and brown plaster coat, then siding is attached to furring strips that have been planned for during the house framing and bale stack. The flatter the bale walls are, the flatter the furring strips, the flatter the siding. An air gap between the siding and plaster is screened at the top and bottom to protect against insect intrusion; any liquid water that makes it through the siding drips harmlessly to the base of the wall, while water vapor migrating through the wall from inside can still escape. I understand that yet another option is mineral paint, though I haven't worked with it yet. These long-lasting, highly vapor permeable paints are reportedly much more water repellent than lime plasters, and they're also more durable than a clay plastered exterior wall that hasn't been treated to make it more water repellent. You don't want to confuse mineral paint with the kinds of paint usually used on conventional siding, e.g. latex paint, which is considered much less permeable, if not a vapor barrier after a few coats.