Cob: Just can't leave the moisture issue alone^E

 On Thu, 24 Aug 2000 10:03:23 Russel Johnsen wrote:
>I had thought about straw bale cob hybrid with straw
 >bale between cob walls but I do see some
 >incompatablility here.  Straw needs to be very dry and
 >with the moisture going through a breathing wall I am
 >worried about moisture buldup in the straw thus
 >rotting it.

Actually (as Russel recently found out) strawbale is typically much better off surrounded by a "breathable" material that will allow moisture vapor to dissipate. The rule with strawbale, as it is with cob, is that the drying regime must exceed the wetting regime. If straw gets and stays wetter than its tolerance, it can rot; if cob gets and stays wetter than its tolerance, it can turn into mud.

To touch on "breathability":

Moisture and vapor barriers, whether they're sheet-type or painted or material-inherent applications, inevitably end up with punctures, rips, or imperfect seams from the construction process, or even just a nail in the wall for hanging a picture. In a heating climate, a tiny crack on an interior wall can allow a lot of vapor into a wall system. And in any climate, a tiny breach in the exterior surface can allow startling amounts of water into a wall system via wind-driven rain or simple gravity. Once in the wall system, vapor barriers will hold the moisture in the wall.  

Some independent study of permeance charts should verify to your satisfaction that there literally can't be enough vapor flow through a properly-applied earth plaster (let alone a cob wall) to rot bales. Think about it: if moisture vapor actually did flow freely through cob walls, they'd be a disaster in a climate with season-long freezing temperatures. Interior-sourced vapor would flow into the wall system (cob in this instance), condense somewhere near the outer part of the wall, freeze, and the walls themselves would experience frost heave - spalling, blow-outs, potentially catastrophic structural events...

So we can dispense with the concern over interior-sourced moisture - and the argument conveniently brings us to the topic of exterior-sourced liquid moisture.

Seems like a person wouldn't want enough liquid in any part of a cob wall that will experience hard freezes (namely, the exterior) so that it will frost heave. If the local weather is such that the exterior becomes saturated to any depth by rain or sleet, and then is subjected to deep freezing, the subsequent ice will potentially blow off parts of the face of the wall, or weaken the outer surface so that it will allow subsequent wettings to more easily "wick" deeper into the wall, creating ongoing and compounding difficulties. (Remember that I'm speaking to a climate-specific situation.) If the local weather dictates extra-wide overhangs and an extra-tall stemwall (it's my understanding that most of the surviving Devon structures are built on stone plinths up to 3' tall - can you English folks verify that for me?), it's unwise to ignore it.

Good hat, good boots, and a coat (often lime) that breathes. Goes for cob, goes for bales. If bales don't rot in ambient humidity conditions where you are, you should be able to design a safe wall system for their use.

Something else to consider is that straw and wood are nearly identical on a cellular level, and these ligno-cellulosic materials rot under the same conditions: approximately 20% moisture by weight (with variables for oxygen availability and ambient temperature and nitrogen content etc). If straw will rot embedded in a cob wall, so will wooden deadmen buried in the wall to accommodate roof attachments. Admittedly, the straw is like shredded wood, and it would rot more quickly - if it were going to rot - than would a solid log.

Earthen plasters, and cob, are a natural to use in tandem with straw, baled and loose. That guy who co-founded the Cob Cottage Company and wrote The Cobber's Companion (Michael Smith) will back me up on this contention. In the new (and highly recommended) college-level textbook "Alternative Construction: Contemporary Natural Building Methods" - brand-spankin' new from John Wiley & Sons (see http://www.wiley.com for their info) - Michael (who contributed to it along with a couple dozen other top-notch natural building folks) writes, "Other ways to improve insulation include imbedding straw-clay blocks in a cob wall or building a cob cavity wall filled with loose straw, straw-clay, wool, or other natural insulation materials." (Maybe I just should've quoted him and left it at that.)

Mark Piepkorn (mostly)
Sarah (a little bit)


Get your FREE Email and Voicemail at Lycos Communications at
http://comm.lycos.com