drying rates thumb Drying Rates in Light Straw-Clay

We know that straw clay construction technology was in use in Germany (and other locales around the globe) over 700 years ago because there are buildings from that time still in service today. German architect Franz Volhard published his' Leichtlehmbau' in the 1983, the first modern scientific approach to evaluating and advancing this technology. Susequently Gernot Minke published his book 'Earth Construction Handbook' in 2000, which drew from Volhard's work and also contained some new investigation. In practice in the USA, this remained the extent of scientific straw clay development till 2002 when our team began field and scientific studies to deepen the understanding of straw clay insulation and advance this technology.

As it existed as of year 2000, the technology had drawbacks:

  • Low insulation values. With the wall thickness limted to 12 (30 cm) inches per Minke's investigation, the resultant insulating value was only 0.8 R per inch. This would not allow building code requirements to be met in colder USA climate zones, e.g. an Upper Midwestern 7,000 to 9,000 heating degree day annual conditions.

  • Density and wall thickness limits. Walls lighter than 40 pounds per cubic foot (pcf), or thicker than 10 to 12 inches (25 to 30 cm), were deemed doomed to degrade or even rot inside before drying.

  • Clay behavior vis-a-vis decomposition. Field experience but no experimental data existed to demonstrated clay's ability to protect straw from water-initiated decomposition.

  • Practical density control. No on-site manufacturing process existed to reliably reproduce a given wall density, especially at lower densities, because the effect of clay percentage was not well understood, nor had effective clay dispersion techniques been developed.

The graphs and tables below are the outcome of experimental work which addressed all of the above issues and has resulted in advancement of light straw clay as an effective insulation infill for colder climate zones.

We conducted Initial tests to find out if walls 12 inches (30 cm) thick — with the clay quality and at the densities that would yield the R values needed for an R-20 wall — would show the signs of degradation that Minke postulated. None were found however and our logged thermocouple data to monitor any "composting" within walls shows only a short temperature increase in the initial 2 weeks to a month depending on wall thickness.

  General Drying Rates 10 to 13 pcf Wall Densities, Midwest USA



midwest drying rates
  Number of Days

Drying rates were also monitored during these tests. Faster drying rates naturally occur when the walls are constructed during warmer summer months and slower rates in cooler fall months. Interestingly, when the drying rates are fast the chances for microbe activity are high while in cooler weather when the drying rates are low so is the chances for microbe activity. The faster the drying rate, the quicker microbe activity is reduced.

drying profile 2007

The above graph graphed as % of water loss looks like this. Water weight loss stops at one year in the Midwest USA. With a starting month of June with drying thru August, 50% or more of the water weight is typically lost in the first month.

cumulative moisture loss 2007

(below) Visual demonstration that there was no decomposition in a 12 pcf straw-clay wall, or even straw-only walls during drying:
14 in wall, 10 months dry straw-only, 12 month dry
Straw-clay Wall Section after 10 Months of Drying
12 pcf, 14 Inch Thick — 2010
Straw-only Wall section after 12 months Drying

Microbe Growth Within Drying Straw-Clay

Thermocouple data below shows a 2 week rise above ambient temperature for the center of the 10 lbs./cu.ft. wall with a one-day peak of 40 degrees F above ambient. A marked rise in termperature within a sample is an indication of microbe growth.

2007 thermocouple data

The 12 pcf wall data below shows a rise above ambient for 4 weeks but the peak is reduced to an increase of about 20 degrees over ambient at its peak.

MREA 14 inch wall data

The clay component of subsoil is used for sealing ponds or foundations because water penetration through it is extremely slow, even though it is highly hydrophilic (water loving). Its use in natural housing historically demonstrates its preservation of wood and straw. Experimental proof of this can be demonstrated in straw-clay, this time with clay protecting the straw from water and thus from decomposition by composting.

Clay's affinity for water is higher than that of straw, which can be seen in a drying rate comparison of a straw-only wall section vs. a straw-clay wall section seen below:

straw-clay vs. straw-only incremental drying graph
The straw-only sample dries faster at first. Note that once dry, however, the clay's imperviousness to water prevents the straw-clay sample from picking up moisture during a very humid 4 week period (July 2006) while the straw-only sample gains weight.

straw-clay drying D 2004 straw-only B2 2004

Using a Moisture Meter to Measure Straw-Clay

Using a Douglas Fir wood block buried in the middle of the wall and a Delmhorst BD-10 moisture meter we see the same phenomenon in these wall sections by moisture measurement instead of by weight loss.
Delmhort moisture meter
straw-clay drying profile D 2004-moisture meter straw-only B2 2004 drying profile - moisture meter

Clay Protects Straw from Re-wetting

Typical moisture levels in straw-clay walls settle down to about 12% after 2 years in the Midwest USA, as measured using a Douglas Fir sensor block.

To set up a re-wet experiment, we used dry samples from a 12 pcf straw-clay wall and one from a straw-only wall clearly demonstrate how clay protects straw from water damage. The samples below were laid out on plastic sheeting on a conrete driveway. There they were rained on 2 times and weighed after each rain the next day; then rained on a third time but not weighed until six days later.

Re-wet Experiment Before Rain
two samples on the driveway
Re-wet Experiment after Rain and 6 Days Drying
straw-clay after re-wetting straw-only after re-wetting
D 2004 —Straw-Clay
B 2004 —Straw Only
The result: the straw-clay sample quickly returns to nearly its original weight in six days drying while the straw-only one does not, and instead begins to decompose (the darkened portion of straw indicates microbe growth).
graph of re-weting experiment

© 2020 Douglas Piltingsrud with Lou Host-Jablonski
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