Straw Bale - What's the R-Value

The reported R-value of straw bale walls varies from R 2.38 per inch to R 0.94 per inch. Reputable labs arrived at these measurements; however I have not seen an adequate explanation of the varying results. A major part of this variation may not be due to the straw itself but to the distribution and redistribution of moisture within the wall.

What Goes on within a Wall
The air within a straw bale wall has a constant moisture content per unit of volume, or constant absolute humidity. If there is a temperature gradient within the wall, the relative humidity will be lower near the warm wall and higher near the cold wall. If the outer wall dips below the dew point, moisture will condense on its surface and possibly freeze if the wall is below freezing. The moisture content of straw primarily depends on the relative humidity, and to a much smaller extent, the temperature of the straw. A straw bale left in a 70º F room with the relative humidity of 75% will have equilibrium moisture content (EMC) of 14.5%. Wood and straw have similar moisture-absorbing characteristics; moisture data generated for wood can thus be used to determine the moisture content of straw.

If bales at 70º F and 14.5% EMC are placed in a wall with a vapor barrier on the inner and outer surfaces and the outside wall is dropped to 0º F the moisture in the bales will be redistributed. When equilibrium conditions are established, the absolute humidity within the bales will be constant as it is in a wall made of porous insulating material, and moisture will condense on the cold wall. The relative humidity near the cold wall will be close to 100%. The condensed moisture will lower the absolute humidity within the bales. When equilibrium conditions are established, the relative humidity will vary from 8% on the warm side to 100% on the cold side. The change in relative humidity within the bale will result in a redistribution of the moisture within the straw. When the moisture is completely redistributed, the moisture content of the straw will be about 2% near the warm wall, and about 13% near the cold wall. The average moisture content of the wall will be reduced from 14½ % to 6%. A further explanation of the principles governing moisture in insulated walls can be found in the “ASHRAE Handbook of Fundamentals.”

For a three-string bale, the straw in one square foot of wall weighs about 16 pounds. As a result of the drop in the EMC from 14½ % to 6%, the straw in one square foot of wall will contain 1¼ pounds less water. This water when condensed on the cold outer wall will form an ice layer about ¼” thick. This much moisture will not collect on the outer wall in a home. In the fall, as the weather becomes colder, moisture will begin to condense on the inside of the outer wall. If the outer wall is porous, water will wick to the outside where it can evaporate. In practice this gradual seasonal change in temperature will allow water to be wicked through the outer wall as it is formed, minimizing the water collected on the inner surface of the outer wall. The temperature at which water freezes in a porous medium is reduced to a value less than 32º F. The smaller the pores, the lower the freezing point. As a consequence, when the temperature of the plaster is below 32º F moisture can still migrate through the wall. When temperatures become much colder and the water inside the plaster walls freezes, this process will stop.

If the outside air is below freezing and conditions are just right, water migrating through the outer layer of plaster may freeze as it emerges. This process is similar to the way ice lenses are formed. As the water freezes on the surface, it produces a pressure difference that sucks more moisture through the plaster. The freezing water then liberates its latent heat, keeping the boundary between the ice lens and the wall from completely freezing, so the process can continue.

I recently looked at some ten-year-old conversations on straw bale R-values on the Crest (not the toothpaste) web site. In these conversations, only the sensible heat (energy associated with heat capacity or thermal mass) was considered when looking at how moisture redistribution affects heat flow. The energy required to evaporate a pound of water is 14 times larger than the energy needed to raise the temperature of a pound of water to 70ºF. If only sensible heat is considered, the extent to which moisture redistribution affects the R-value will be substantially underestimated.

A research paper on the effect of moisture flow in a sawdust-insulated cavity2 showed that at the onset of an applied temperature differential, heat flow could be increased by as much as 170% due to redistribution of moisture. The tested cavity was only 2 inches thick. A straw bale wall has a considerably higher R-value and also is capable of holding much more moisture. As a result of the higher R-value of the straw bale wall, the heat flow through the wall will be reduced, leaving less energy available for the redistribution of moisture. The R-value for a straw bale wall will then be reduced by a higher percentage at the onset of heat flow, thus the time to reach equilibrium will be much longer. In other words, the time required to reach equilibrium conditions is proportional to the quantity of moisture in the wall and the wall’s R-value.

Wicking water away from a cold, wet outer wall towards the warm wall can significantly decrease the R-value of a wall because the moisture transported towards the inner wall must be re-evaporated. The phenomenon was observed in sawdust-insulated cavities. Dry straw has a limited ability to wick, so this phenomenon is probably not significant. If straw is cut green and not allowed to dry, the ability to wick moisture will be much greater and the R-value of the straw will be significantly decreased. I initially thought that straw would readily wick water. However, after soaking straw in colored water for a month the water barely rose up the straw.

Equilibrium Time Constant for a Straw Bale Wall
Evaporation of 1¼ lb. of water requires 1250 Btu’s while only 30 Btu’s flow though a perfectly dry straw bale wall each day. If all the heat flowing into the wall contributed to water evaporating, it would take forty-one days to evaporate 1¼ lb. of water at the rate of 30 Btu’s/day. If only part of the heat flowing into the wall contributed to the redistribution of moisture, it would take months for the wall to reach equilibrium. During this time period the measured R-value will be continuously increasing as equilibrium conditions are approached and less water is being transferred to the outer wall.

Test results for R-values of straw bale walls vary considerably, from R 0.94 per inch to R 2.38 per inch. Moisture flow within the bales could cause a large part of this variation. A paper published by the California Energy Commission claims the most accurate test data obtained was for a straw wall that allowed only two weeks to reach equilibrium. The warm side of the wall was at 70ºF and the cold side was at 0ºF. The measurements made may have been accurate, but if the wall had been given the several months needed to reach equilibrium, the measured R-value may have been considerably higher. In that report the test results with the highest R-values were measured in the Southwest where the bales are relatively dry.

The effect of moisture on the R-value of a straw bale wall is more significant than with typical insulating materials because the energy required to redistribute moisture within the insulation is large compared to the energy flowing through the wall each day. As a consequence of the slow redistribution of moisture it could take months to reach equilibrium conditions. Unlike other more common building materials, the R-value of a straw bale wall can probably not be simply defined by a single number. It would probably be beneficial to do additional experimental work to determine exactly how the redistribution of moisture in a straw bale wall affects the wall’s R-value.

References

1. ASHRAE. .2005. ASHRAE Handbook of Fundamentals, Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

2. Paxton, J. A. and N B. Hutcheon. 1952. Moisture. Migration in a Closed Guarded Hot Plate. Transactions, ASHVE. 58:301-320.

3. Commins and Stone. 1998. Tested R-value for Straw Bale Walls and Performance Modeling for Straw Bale Homes. California Energy Commission. 1998 ACEEE Summer Study on Energy Efficiency in Buildings Proceedings.

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