[Strawbale] Translating Relative Humidity into Moisture Content of Bale Walls

[Mark Bigland-Pritchard]

Just realised, having read Rob's message, that I misread your message, 
Andrew.  The chapter I sent you deals with the sorption isotherm, i.e. 
what bale moisture content at a given point in the bale corresponds to 
the r.h. in its vicinity.
I certainly don't think it's useless information, but it only goes part 
way to answering the question that you asked.  I can go a bit further 
when I've got my pdf conversions all done - which, owing to the failure 
of a supplier to supply the stuff he was supposed to supply today, will 
be earlier than I thought it would be.
Mark


Rob Tom wrote:

>On Wed, 04 Oct 2006 16:11:46 -0400, Andrew Morrison <andrew at strawbale.com>  
>wrote:
>
>  
>
>> Any thoughts on how to directly relate relative
>>humidity of a home or environment to moisture content percentages within
>>bale walls?
>>    
>>
>
>
>Andrew;
>
>I don't think that your request is reasonable or do-able.
>
>While it is possible to do a direct read from psychrometric charts to get  
>moisture contents for given temperatures and relative humidities, the  
>humdity readings taken on the house interior (ie conditioned air, on one  
>side of what I hope is a relatively good air barrier) will have no  
>correlation at all to the humidity in the SB walls, which are on the other  
>side of the wall air barrier.
>
>As has been mentioned numerous times on these Lists recently, the plasters  
>used on SB walls , at the thicknesses which they are typically applied,  
>are sufficiently vapour impermeable and non-air permeable to make moisture  
>movement through the plasters via vapour diffusion a non-issue.
>
>Bulk moisture transport via leaks OTOH is another matter entirely and has  
>nothing to do with relative humidities.
>
>If anything, the moisture content of the wall materials as a result of  
>hundity conditions would be closer to the ambient seasonal levels of the  
>outdoors and an idea of what those may be can be obtained by looking at  
>the seasonal equilibrium moisture contents (EMC) for wood (since wood is  
>similar to straw) in a particular climatic area.
>
>For instance, in Central Canada, for indoor wood, the  winter EMC is 5%  
>and 8% for summer... and the outdoor EMC is 17% for winter and 10% for  
>summer. (source: Canada Wood Council Datafile CWC-1).
>
>A good place to start in trying to understand this relative  
>humidity/moisture content stuff would be the section entitled  
>"Psychrometrics" in references like the ASHRAE Handbook of Fundamentals.
>
>
>===* ===
>Rob Tom
>Kanata, Ontario, Canada
><archilogic at chaffyahoo dot ca>
>winnow the chaff from my edress in your reply
>
>
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>