[Strawbale] Convection in SB walls
strawnet at aol...
strawnet at aol...
Wed Dec 8 22:54:30 CET 2010
I've been periodically skimming these messages and was thinking about chiming in when Rob Tom mentioned what I was contemplating pointing out. And since Andre's comment isn't quite accurate I thought I had best add a bit more about this. I think that the gypsum board was pressed as close to the straw as could be easily managed - though the tested wall assembly (at Oak Ridge National Laboratory) was built by high school science teachers in a summer education program for such teachers. My understanding of the modeling done afterwards to try to understand how the assembly could have tested with a worse R-value than anyone could have imagined possible was that it doesn't take very large spaces to allow a convective loop to form and thereby pump energy through the assembly.
My position has always been that there are several reasons not to place bare bales against other sheet materials without coating them with at least a clay slip or, preferably, a coat of earthen plaster. This is fire protection, a level of moisture protection for the straw, and if done thoroughly, an added level of air flow resistance that will not greatly impede the passage of water vapor out of the bales.
As mentioned by others, I have also consistently been concerned about the notion that moisture can be sealed out of an assembly and thus continue to be concerned about not inhibiting the drying potential of assemblies. If we can't keep moisture from getting into double glazed window units, we can't keep it from getting into other far more susceptible assemblies such as those being discussed here. And once inside, if there isn't a good drying potential we will likely see the accumulation of moisture within the assembly over time.
I think RT had it about right in his message (we Murricans hate to admit it, but he often does...).
Desert (but I have been to places where it actually rains) David
From: forum at lamaisonenpaille... <forum at lamaisonenpaille...>
To: European strawbale building discussions <strawbale at amper....muni.cz>
Sent: Wed, Dec 8, 2010 1:58 pm
Subject: [Strawbale] Convection in SB walls
Some of us EuroBaleHeads are indeed aware of the test you mention ;-) . We talked about it with 'Murrican' David Eisenberg during the last European gathering in Belgium as he was involved in that particular testing. I understood that there was a considerable space between the bales and the gypsum board and that he believes that this air-space increases tremendously the convection that is common in fibrous insulation materials. If I understood him correctly he does not consider it a problem to put a bale tightly agains a panel but strongly advices against an airspace between the bales and a panel.
Herbert, has any testing been done on your finished panels or is 'your' R value based on a theoretical accumulation of the different matérials you use?
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