The Last Straw
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A Threat To Strawbale Housing
Issue #22, 1998
by Bob Platts, P.E. - Ottawa, Ontario, Canada
Moisture warning! The promise of strawbale housing is being shot at by the age-old nemesis of cellulose fibre: excessive moisture and resulting rot. We should make it a more difficult target.
Strawbale construction proponents must not take false comfort from the cellulose similarities with well-researched wood-frame housing. Straw may rot no worse than wood when equally wet and warm, but stuccoed strawbale is not as "moisture load" tolerant: the drainability and breathability of most wood-frame walls is perhaps a whole order better. That's one strong reason why we have two-hundred-year-old wood-frame houses, even wrongly-insulated, looking to shelter the next and still next generations.
Even the much-forgiving wood-frame house can be brought down by trouble-prone detailing, high wetting/poor drying climates, and hard-driving usage (excessive indoor relative Humidity [RH], and flueless). Add in stucco exterior finish to that kind of mix, and rot is what you've got. Wood-frame housing can easily be built and run to last, but it does take some thought; stuccoed strawbale takes redoubled thought.
Not too worried, comforted perhaps by the century-old stamina of the Nebraska pioneer houses? They are in comparatively dry country, facing mainly indoor-source moisture—not often a problem. Think hard of what is happening now, far away from Nebraska: strawbale enthusiasts are facing walls with that ancient blotter, stucco, sometimes not even meshed; marrying that blotter intimately to thousands of wicks—straws—embedded in it and leading inwards, tightly packed, poorly draining; and plunking that construction on a coast or other region with sustained slanting rains and weak springtime drying conditions. And sometimes not bothering with extra-wide overhangs, water-shedding window sill detailing, and avoidance of deck backsplash, etc. The usual source is outdoor moisture penetration, and then entrapment simply guarantees trouble: the straw is wetted, dries too slowly, stays too wet into the warm weather, and rots in a few years.
The worst enemy is not usually indoors. Let us waste no more time tilting at the old straw man of interstitial condensation from indoor-source moisture diffusing through the "vapour retarder" interior finish. We learned in the Arctic in the early 1960s that vapour diffusion through interior finishes, good vapour retarder or not, just can not transfer and drop excessive condensate in the wall. Excessive wall moisture from indoors, when it is found, can always be tracked back to gross air leaks exfiltrating through the whole wall, driven by sustained pressure differentials.
Here in Ontario, our east and south walls are wetted due to prevailing winter winds, much worsened if the house is electrically heated (flueless) so the outward lee-side wind pressure differential is not countered by indoor depressurization. And the moisture piggy-backing on the exfiltrating air is increased because the house is overloaded: it may have poor ground cover, unvented everything, and be rather airtight—and fluelessness also reduces air change. That's where we get most of our indoor-source troubled walls. The important exception: excessive RH also fogs the indoor side of the window glass in the fall and winter, and lousy window details invite the resulting drips down into the wall. The deep-set windows inherent in strawbale buildings cause the glass to run cooler, fogging up at even moderate indoor RH and worsening this threat.
Wet-plastered houses don't generally, or certainly need not, offer gross leakage points anywhere (unlike drywall, which is often gappy under the trim details). Well-insulated, flued older houses in Canada, including blown-cellulose jobs, very rarely suffer rotting walls from indoor-source moisture, lack of "vapour barrier" be damned. (We can make dogmatic statements like this because we've investigated about 16,000 houses across our country, including a 14,000-house cross-section done with infrared thermography.) Now, if the fledgling strawbale industry can begin to investigate its rich field history, our Nebraska heritage will also confirm that, though some will have signs of excessive indoor RH, few will have gross air leaks and even fewer will exhibit wet wall spots. No vapour barrier, no need for one; let's move on.
The enemy outdoors: overhead and underfoot. We have long advocated proper study of strawbale wall moisture performance, but have found no funds except for a small pilot study, thanks to Don Fugler at Canada Mortgage and Housing Corporation. The results are posted on the internet at http://www.cmhc-schl.gc.ca./Research/Reports/straw.html . Here's a much-abridged summary:
The design of a short-term laboratory study to predict long term performance and "threshold" moisture limits would be a daunting task. Fortunately the long term study is in place and has decades of results ready to be revealed: "the field is the laboratory." Since we know too little to design anything like a statistically-representative field study, we set out to conduct a pilot "worst-case" study. Field experience tells us what likely comprises "worst-case," and even the telltale signs to look for, of trouble within. We don't need infrared thermography and certainly don't need dowsers: we know where to look for wet and where for dry, and can learn from both.
The pilot study was conducted in the Outaouais region of Quebec, a cold, wet climate. We developed means of moisture probing, cutting and removing wall core samples, assessment and recording—including evidences of main moisture sources, paths, transfer mechanisms—as well as repair and make-good. We did not try to determine the types of fungi, but a full study should be conducted. The study dug into strawbale wall portions of two experimental structures and two houses, ranging from six to ten years old.
Areas high in the wall, under roof eaves or like protection, were found sound and dry in late winter—before spring drying could have removed moisture. This included spots above lee-side windows where indoor-source moisture can be a problem; there was none. Cracked and gapped stucco also revealed bright, sound, dry straw behind: rain penetration would be rare under the overhangs in these particular spots, and drying ability would be excellent. The straw was not only dry as found in all these higher, externally-protected areas, but it revealed a history of never being wet and warm for significant times.
Slowly-rotting straw: A rain-exposed test wall had been built six years ago with a concrete cap which led water directly onto the face and behind the top of the stucco. The cement stucco had eroded and broken away from the deteriorating straw some time ago. Considering only the upper area, the fibre was apparently wetted much too generously and too often, winter and summer. Despite the obvious breathability at that face, it was mouldy and degrading well within the wall; less so near the quick-drying surface. Conversely, an equally-exposed arm of the test wall had been fitted with a proper concrete cap with 2-inch drip-edged "eaves": everything fine, in the upper part of the wall.
In an occupied house, zones of damp, slowly-rotting straw were found under poorly-built window sills, even under a 3-foot roof overhang (but it was two stories above the spot). Some water may have been seeping down from fogging of the indoor glass surface, but the telltales of water entry were on the exterior only. Conversely, on the more-exposed east gable end, similarly-positioned first-storey windows seemed very well-built with tight sills drip-edged over excellent cement-rich stucco: the straw is sound and dry under these. In the same walls, zones just one foot above the slab-on-grade were found damp, dark and musty, slowly rotting in a 10-year old wall.
Fast-rotting straw: We couldn't cut into the walls everywhere, but the telltale signs were becoming more familiar, including smell. Fast-rotting straw was found or inferred under leaky, no-drip-edge window sills on exposed faces. Perhaps more discouragingly, where protected from ground moisture "rising damp" only by the concrete slab—no polyethylene or other effective moisture barrier—the straw was rotting from the slab upward, to a foot or so above. The wetting regime exceeds the drying regime. (Pompeii, built B.C., featured slate damp-proof courses near the base of all masonry walls; are we super-slow learners?)
In these test walls, and in a very well-stuccoed strawbale foundation wall, the straw close to the ground had composted. The straw was separated from the ground only by a thin layer of cement-rich mortar atop a well-drained Frank Lloyd Wright-style trench foundation. Each such wall was in effect an upside-down stucco bottle, open mouth practically against the soil, with much of its interior running at 100% RH much of the year.
Do we know enough to build stuccoed strawbale houses practically anywhere? We think not; certainly we shouldn't yet build where we would have any concerns at all about building stuccoed wood-frame. But we can probably learn to build stuccoed strawbale even there: big overhangs; breathable, good quality stucco tension-meshed against shrinkage cracking; proper sills and flashing of windows and openings; damp-proof course and foot-drained wall detail; minimal backsplash exposure; no gross air leaks through the interior face anywhere; operate the house at moderate RH in winter. In the South, don't make the interior finish into anything like a vapour barrier; elsewhere, don't worry about it either way.
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