[GSBN] GSBN] Modeling or measuring mass effect of interior plaster

[Frank Tettemer]

Hi Laura,

I have considered this question on a number of bale home designs. My 
concern with the effect of this thermal mass was around the sizing of 
the South facing windows.
I didn't want to oversize the amount of window area because of concerns 
with overheating during the "shoulder seasons", when the solar 
insolation would still be active, due to the roof overhangs and the 
awnings still allowing about half of the sun inside before the Summer 
shading period. So I needed to know how much heat these thermal 
materials would absorb over the course of a day.

Not having a very good head for computer modeling, I've always crunched 
numbers by hand, for heat loss calculations, and mass/window area/floor 
area relationships.

My reference for calculations has been from Dan Chiras's book, The Solar 
House. Around pages 100 to 106, Dan offers information around 
relationships of types of thermal mass, with good ratios to use for 
number crunching. I've been referring to these ratios for years, and 
every home that we've designed and built since then, has worked quite 
well for our region-specific designs. And I think you are correct to 
consider the direct vs. indirect gain on these wall surface areas. I've 
used Dan's ratios, and fudged them a bit, for lit and unlit areas.
Dan refers to ratios of glass-to-mass areas, and specifies using the 
ratios of 1:5.5 for sunlit floor areas, and 1:40 for unlit floor areas.
He specifies 1:8.3 for unlit wall areas, and I've used 1:6 for sunlit 
wall areas.  That ratio of 1:6 is from my own imagination, so don't 
blame Dan for that. Blame my "intuitive approach" to engineering.  After 
observing the various passive-solar homes that we've built over the 
years, the numbers seem to work.

Frank

Frank Tettemer
Living Sol ~ Building and Design
www.livingsol.com
613 756 3884

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Laura Bartels writes:

I'm writing to ask if anyone has had experience with modeling or 
measuring the mass effect of interior plaster of bale walls versus other 
interior finishes. This has come up on a straw bale project in design 
phase I've involved in which has a net zero energy goal. The project is 
large, about 6000 sf. The owners are interested in  barnwood interior 
wall surfaces (over plaster) on all or some walls. With the net zero 
goal, the question is what we might lose in having wood rather than 
exposed plaster. There will be adobe floors which will already provide 
direct and indirect gain mass.

Anyone tackled this topic or have a guess about how to look at this? Our 
team has talked about estimating direct vs. indirect gain wall surfaces 
through sun studies in ArchiCAD as a starting point.

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