Pilot Study Results and First Comments
Photos of some of the most meaningful samples are shown, with brief notes on condition, apparent moisture exposure history, and first comments. Recall that we look hard for "worst case". Photo codes refer to field notes, S means sample. Task 1's exploratory cases proved valuable:
Figs. 1 & 2, photos X3-3-1 & X5-5-1: 6-yr-old Biocrete experimental structure described above in Task 1.
Fig 1: The east facing wall — most exposed to driving rain — is sound and dry under its concrete cap with proper drip edge; while the south facing wall to the right, mostly under an amply wide concrete cap but the reverse of drip edged, is badly eroded. (The hole and two horizontal cuts are from our first trials of sample extraction.)
Fig. 2: The latter wall has just been knocked down for inspection, exposing the bottom of its straw bale core (straw used here only) where it had rested on soil-cement mortar on the ground. The straw is super-saturated and rotting, fibres very weak. Moisture from below as well as above clearly exceeds the drying regime despite the generous water vapour "breathability" of the deeply eroded soil-cement skins.
Figs 1 & 2, all inspectable points: The cement-rich skins and caps have not caused extra deterioration of fibre at the interface, and indeed appear to have preserved the fibre bright and sound where it's encased within the skins and caps. (Mostly shredded wood is visible in Fig. 1, straw in Fig 2.)
Other exploratory case (photos on file): The greenhouse noted earlier also proved valuable, although we did not obtain clear permission to cut into it. Truck damage to corners and along one wall made some of the fibre-soil-cement and raw straw core visible and inspectable. Some such spots were exposed to frequent wetting from rain and snow but generally could drain and dry fairly freely. Only those spots where water could pool or linger revealed considerable deterioration of the shredded wood and straw. Here too, the cement-rich mortar and stuccoes seemed to preserve the fibre they encased, and no more deterioration was seen at their interface with the fibrous core than further into the fibre.
Figs. 3 & 4, P6-S1-13 & P5-S2-12: 6-year-old straw bale foundation footed on thin layer of shredded straw-soil-cement mortar 6 in. below final grade, which in turn was borne by rubble stone drain/capillary break in deep trench, Frank Lloyd Wright style. No polyethylene film or other dampproof course separated the straw from the trench below. The trench is well sloped and drained to daylight, and must handle a lot of water much of the year since the (wood frame) summer cottage is on a hillside with considerable hill above it.
Selected samplings are well under a fully closed porch addition. Temperature 8 C. The skins are composed of the shredded straw-soil-cement slurry applied to the bales about 1-1/2 in thick and faced with a formed-in-situ finish of cement-rich mortar, at least equally thick, exterior and interior. This dense finish would be rather impermeable to water vapour, like the good concrete it essentially is, and it appears free of cracks; the crawl space is not lined but runs cold: it must contribute little to the moisture in the straw but does block one drying route.
What we have, though, is an upside-down concrete bottle with its open mouth a few inches above water or vapour at 100 % RH — a humidifier that's warmer than the outer zone of the bottle's interior much of the winter. And it's filled with straw.
Fig. 3: Sampling at 3 ft above grade, into a bale which is separated from the 100% RH trench by at least two horizontal layers of cement-rich mortar between the bales; S1. (Gagne style construction, but built by others; structure is ok, not relying on bond to straw.) Cement-rich outer finish broke away from inner slurry, which itself broke up so no sample obtained with straw bonded into slurry. Straw is moist and deteriorating as can be seen, and is weak in tension. Moisture meter reading (MMR) 37-42 through the bale: about 38-43 % MC corrected for temperature. Saturated. Smells just slightly musty.
Fig. 4: Sampling just above grade, below the first spot; S2. Lacy white mould indicates advanced rotting; straw very weak. But note bright, sound straw in the upper left corner of the shredded straw-soil-cement slurry sample, which this time stayed intact. MMR off scale, straw supersaturated. Figure 3
A 2-storey house case:
Figs. 5-10, respectively P3-S3-15, P10-S4-17, P15-S6 over S4-22, P12-S6-18, P14-S6-21 and P17-S8-24: 10-year-old Gagne type on slab.
South face shows clearest telltales beneath apparently unflashed windows with no drip edge, but this faces driveway and highway so we left it unprobed. (Such observations and sampling results are pointed out to the householder, with guidance on how to fix.) Less obvious moisture paths and some telltales were noted on the east and north facing walls, respectively the most and perhaps least exposed to driving rain. Light rain this day, and just 4 C.
Fig. 5: East wall, one storey up, under a spaced-board deck supported on cantilevered floor joists: S3. Unreinforced stucco already cracked due to joists, which was one reason this spot was chosen. But there was no sign of water entry, and the open cracks would aid the drying action. The straw, placed between the joists in flakes oriented with the wall plane rather than transversely, is bright and sound throughout, and well bonded where the stucco encased it despite its parallel lie. Lime rich stucco mix. MMR below scale right through the straw core area, i.e. less than 13% MC.
Fig. 6: North wall, under same wrap-around deck, sampling just above slab and, as it happens, just under a mortar bed between this bale and the one above; S4. Discovered that the bales had been limed before construction: the white in the straw is not mould. (The loose flake straw of Fig. 5 had not been limed.) And apparently all the stucco and mortar is lime rich, whereas we have read that the vertical and horizontal structural mortar in this type of wall construction is normally a cement-sand mix.
We think that the slab is not underlain by polyethylene nor is there such a barrier between it and the straw above. There are no obvious cracks in the stucco up to the roof overhang (3 ft.), and the deck above this north face sample area does not appear to induce much backsplash. But the straw is moist and musty-lime smelling, deteriorating. The lime dust seemed to nullify straw-stucco bond in some spots. MMR of 18 straight through bale, and 20-21 angling down to slab; adding 2 to correct for temperature, MCs are 20 to 23%. We return to this zone after a little more probing in the east wall. Figure 5
2-storey house continued, back to east face: Sample S5 cut under the lower south window on east wall, with little roof protection on this exposed gable end, and with no drip edge on window sill - more or less flush with stucco. (But no visible telltale of trouble within.) Sample 1 ft. up from slab. Cutting through 3-1/2 inches of solid lime-rich stucco proved troublesome. (The skins vary greatly in thickness; the earlier samples had 2 in thick skins.) The layered skin broke up with no clear sample with straw bonded into it, and in the soaking rain we forgot to photograph the apparently sound, sweet smelling straw within. But we did take MMRs right through to the 11-in. length of the probe: 19 near newly exposed surface, 13 through to the middle of the bale; correct to 21 and 15%, the latter probably a truer measure, being free of rain and wet probe effects.
Then returning to the north face to explore further a suspected slab effect (ground moisture, "rising damp" as its called in Britain):
Fig. 7: A further sample S6 is cut just above the Fig. 5 north wall sample S4; the photo shows both. (In between lies a thick layer of mortar separating the bales, Quebec style; that was cut into and later pried into as a further sample, showing that it is also a lime-rich mortar in this case. Again the straw impinging onto and into it was not affected, nor was there any clear indication that these transverse mortar layers in this type of construction act to wick water into the bales, as is a concern.) The photo records that the top sample, S6, is brighter and sounder in appearance than the one so close below, S4.
Fig. 8: Looking into S6. The lime in the straw shows as a grey powder; otherwise the straw looks, pulls and smells sound. The lime powder does seem to weaken or even prevent its adherence where the stucco encases it, but a mechanical bond is still present.
Fig. 9. MMR 13 near surface, rising to 15 a couple of inches in and staying at that reading; temperature corrected to 15% and 17% respectively.
The straw is indeed drier and rot-free just a little further up from the slab, and with a thick layer of mortar intervening. In the absence of any visible flaw or source, we must suspect "rising damp" wicking up from an unprotected slab, perhaps placed on gravel which worked down into the mud or the mud up through it. Figure 7
Fig. 10, below: S8, north wall, under corner of window sill flush with stucco surface, about 1-1/2 ft above slab; a faint dustmark smudge led us here. The inner bond coats of the 3 in. thick stucco broke up as shown, with little to tell of straw bond. Musty smelling, weak, moist straw in this 10 yr. old wall. MMR 25-25-22-20% probing straight through to full 11 in. probe length; angling downward toward slab, the MMRs became 25-30-35-30%. (Warmer today, 12 C: MMR=MC%.) Figure 10