[GSBN] Modeling or measuring mass effect of interior plaster

[John Swearingen]

Laura,

Modeling thermal mass is generally difficult because of the large variable
conditions of heat transfer related to air circulation. We've done this in
Energy-10 with some success, and usually our projections have come out on
the conservative side--the temperature swings have been less than we
calculated.

The other very large variable is climate--temperatures and sunshine at
different times of the year. Everyone I know who does this successfully had
dialed it in from years of experience in one particular climate, with which
they are familiar.

As to whether the walls make a difference,  the short answer is, I think it
makes a big difference, and that many of the lauded characteristics of
thermal comfort in straw bale buildings may have as much to do with the
thermal mass on the walls as with the insulation.  Modulated temperature
swings can influence occupant behavior positively, reducing reliance on
mechanical systems, over and above straight Btu calculations.

The ultimate efficiency of thermal mass is tied to the heat-transfer
mechanism for exchanging heat between the mass and the rest of the building
(air).  A floor slab is thick and of limited surface area; bale walls are
thin with a much larger surface area. So floor slabs are longer term
storage, and walls  function very effectively to modulate temperature
swings on a short term (diurnal) cycle which can reduce loads on mechanical
systems and increase comfort in passive buildings.  I don't think there is
too much danger of over-massing, and haven't seen it in our buildings,
because the relatively thin mass of the walls, backed by insulation and
actively transferring heat, stays close to room temperature and so isn't
felt as too cold or hot.

Temperature modulation can result in significant changes in how mechanical
heating and cooling are used by the occupants: if the building is slow to
cool off at night, for instance, the occupants don't call for heat early in
the evening.  The key here is responsiveness, which is related to surface
area. Mass walls also help to distribute Btu's somewhat between warmer and
cooler areas of the building: cooler walls will absorb heat more readily
than warm walls, so they are somewhat of a magnet for warm air when located
in cooler areas of the building.

Well, hope this helps!

John.




On Mon, May 6, 2013 at 12:39 PM, Laura Bartels <laura at greenweaver.com>wrote:

> Hello All,
>
> 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.
>
> Looking forward to hearing any thoughts on this.
>
> Laura
>
>
> *
>  *
>  *
>  Laura Bartels
> GreenWeaver Inc.
> 520 S. Third St., Suite 5
> Carbondale, CO 81623
> 970-379-6779
> www.greenweaverinc.com
>
> *
>
>
>
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>


-- 
John Swearingen
Skillful Means Design & Construction
2550 9th Street   Suite 209A
Berkeley, CA   94710
510.849.1800 phone
510.849.1900 fax

Web Site:  http://www.skillful-means.com
Blog:         https://skillfulmeansdesign.wordpress.com
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