[GSBN] SB Lighthouse, CMHC moisture research (was Canadian research into straw bale swimming pools)

[Derek Roff]

To state the same concept a little differently, we are comparing the weight (actually, the mass) of the water in a sample to the weight/mass of the non-water.  If we want to state the moisture content of a strawbale, we compare the mass of all the moisture (water) in the bale to the mass of everything that remains after the moisture has been removed (the dried straw).  Since the two parts are being measured independently, they can have any ratio.  If a given 40 pound bale contains 22 pounds of water and 18 pounds of dry straw, then the moisture content is stated as 122%.   The simplified formula takes the mass of water divided by mass of dried straw, times 100 (to get percent).  (22/18) x 100 = 122%

This method of analysis is called the "dry basis", or sometimes "oven dry basis".   In practice, a small sample is weighed, then it is heated in an oven until all the moisture is driven off, and then the sample is weighed again.  The final weighing yields the weight/mass of the dried straw, and by subtracting that from the starting weight/mass, you can calculate the weight/mass of the water that was driven off in the oven.  

Some industries use the "wet basis" of calculation, which compares the weight/mass of the water to the weight/mass of the whole (damp) sample.  The moisture content of grains, such as rice, wheat, oats, etc, is sometimes stated using the wet basis.  The maximum moisture content possible would be 100%, using the wet basis method of calculation, as Martin mentioned.  But this approach is not used for wood or strawbales used in building.  If you have a bale moisture meter, you may find that it has a way to switch between the two methods, dry basis or wet basis.  SB builders should choose "dry basis".  

Neither of these calculation methods has anything to do with volume.  But volume does show up in the SB building codes.  If a code specifies that a bale must be at least 8 pounds per cubic foot (pcf), then volume is involved.  The advantage of measuring pcf is that you can get a quick, rough measurement in the field, with a tape measure and a bathroom scale.  The disadvantage is that the approach isn't a direct measure of anything meaningful.  It is supposed to guarantee that the bales are dense enough to be physically strong.  However, it may just be measuring that the bales are overly damp.  If you pour a quart of water into a bale, you won't make it stronger, but you will increase its measured pounds per cubic foot.  For this reason, the codes are likely to specify the minimum pounds per cubic foot within a certain moisture content range.  A bale with a 10% moisture content and a density of 8 pounds per cubic foot is likely to be compressed enough to be sturdy and good for building.  

Derelict "I'm not all wet" Roff

Derek Roff
derek at unm.edu


On Jan 29, 2012, at 8:02 PM, John Straube wrote:

> Most moisture content is measured by dry weight.  The glass weighs 1 ounces before, and 6 ounces after it is filled, means it has (6-1)/1 = 500%.
> A cubic foot of strawbale weights 8 pounds before wetting and 24 pounds after has (24-8)/8 = 200% MC.
> 
> You might be thinking of moisture content by volume. An empty glass is 0% and a full glass a little less than 100% (since some of the glass is glass, and so it cant be 100%)
> We dont use volume measures much because they are hard to measure that way.
> 
> 
> Dr John Straube, P.Eng.
> www.BuildingScience.com
> 
> On 12-01-29 8:39 PM, Bill Christensen wrote:
>> 
>> I'm also curious about these numbers.  What is the percentage of moisture being compared to?  The total weight of what it has absorbed into?  A glass of water, in my book, is 100% moisture content.  100% means "all water" in this case.  Relative humidity of 100% in air means it can't hold any more for the current temperature, and begins condensing.  
>> 
>> So are we comparing against the volume or weight of cellulose?   Or what?
>> 
>> Bill "90% of the game is half mental" Christensen
>> 
>> On 1/29/12 4:56 PM, John Straube wrote:
>>> 
>>> 300% is easy.  A glass of water can have 5000%.  
>>> Live wood regularly as 100-125% wood MC. Straw being lighter could be 300%, but I would only ever trust a gravimetric measure, not an instrument, to read this.
>>> 
>>> Some wood absorbs to 200% and will sink to the bottom of lakes before it makes it to the sawmill.
>>> 
>>>  Dr John Straube, P.Eng.
>>> www.BuildingScience.com
>>> 
>>> On 12-01-29 12:48 PM, martin hammer wrote:
>>>> 
>>>> 
>>>> 
>>>> Speaking of % moisture content, and at the other end of the spectrum (way beyond, actually), in the CMHC Straw Bale House Moisture Research paper regarding bales in floors, as Habib mentioned, it does say “Some were as wet as 300 per cent moisture content . . .”.
>>>> 
>>>> I love and respect the whole series of CMHC research papers and test reports.  But how can anything be 300% moisture content?
>> 
>> 

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