[GSBN] Modeling or measuring mass effect of interior plaster

Tue May 7 23:59:28 UTC 2013

Hi All:

Building on Desert Dave's note here, we've measured bale walls as having a 12-hour time lag of thermal transfer from the surfaces to the middle of the bale, thus creating a perfect foil to most diurnal temperature swings.  Attached is page 4 from a report a researcher and I prepared on the Real Goods Showroom, with discussion of the bale walls and graph showing a beautiful purple line of the center of bale temperature directly opposing the other ones.  We concluded that the straw bales are performing not only as insulation but also as an element having heat capacity, aka thermal mass.  

On the days of this test, in autumn, the outdoor temperatures ranged from 36°C (97°F) during the day to 10°C (50°F) at night, while indoor temperatures at the surface of the bale walls ranged from a comfortable 26°C (79°F) during the day to 19°C (66°F) at night.  Meanwhile, the center of the bale wall measured 21°C (70°F) during the day to 24°C (75°F) at night.

Worthy of note is that this building features a 50mm to 75mm (2" to 3") interior finish coating of PISE (a spray-applied form of stabilized earth) and up to 100mm (4") on the exterior, and both surfaces have relatively rough textures.  The building also has high ceilings so the warmest air stratifies near the ceiling, and at night clerestory windows are opened to flush the space with cool air.  You'll note in the graph that interior temperatures at a 3m (9') height, just above the bale walls, had wider temperature swings.  

In any event, I think it just furthers the agreement that ample thermal mass within a good insulating envelope is a good thing, and straw bale walls with any kind of finish do this better than just about any other wall system.  

Ride On!

David A.

On May 7, 2013, at 2:50 PM, David Eisenberg <strawnet at aol.com> wrote:

> Many years ago I had a low-level argument with a couple of rammed earth builder friends who made the claim that you could never have too much thermal mass. They were both building rammed earth buildings with no insulation in places like Phoenix, Arizona where in the summer the daytime temperatures are in the 100s and 110s for months and the lows may not go below 90 or the high 80s (F) for most if not all of that period. The temperature of the mass is going to increase toward the average temperature which is way above something comfortable and when you have that much mass ten or fifteen or twenty degrees above the desired temperature, you can definitely have too much mass. 
> 
> Thermal mass works well when there is adequate diurnal temperatures swings and not so well when it doesn't cool off enough at night to keep the mass from heating up in the summer. And of course, similarly in really cold climates, uninsulated mass colder than you want it can be a problem. The points made about straw bale and the mass in the plaster being isolated from the exterior by the bales and responsive to indoor temperatures and very effective is right on, in my experience. 
> 
> Desert Dave Eisenberg
> 
> 
> On Tue, May 7, 2013 at 1:25 PM, John Swearingen <jswearingen at skillful-means.com> wrote:
> Well, it's hard to take seriously anyone who thinks the sun shines into north-facing windows, but outside of that, the results are very much in line with our observations--more mass equals temperature moderation and comfort.  The paper stresses (over and over) that this is for mass in visual contact with the sun, because the buildings are designed as solar collectors.  Similar advantages in temperature moderation apply to mass that does not have direct solar contact. (Also for wood-stove heated buildings, when the mass can absorb excess heat in the burning cycle). 
> 
> The question I have is whether you can have too MUCH thermal mass, and at what point does that occur.  Any thick mass walls in a building with eventually reach an average ambient temperature, and will only release (or absorb) heat very slowly, in the long term.  In my experience, plastered bale walls hover pretty close to room temperature, and like mass walls, aren't experienced as cold.  A slab floor, by location, thickness and generally low insulation values, can tend to be experienced as cold if it isn't heated by sun or other means.
> 
> Sometimes single-level homes with a collector slab will, at night, tend to stratify.  The efficiency of heat transfer can be improved by just a slight level change to drive the convection loop. Even one step between the bedrooms and living area is enough to make a significant increase in air circulation during the night.
> 
> 
> On Mon, May 6, 2013 at 6:05 PM, Bohdan Dorniak <bohdan at bdcoarchitects.com.au> wrote:
> Hi All
> 
> This is a note that has been published by the Australian Institute of Architects regarding Thermal Mass.
> 
> I thought that you may find this interesting? Any comments John?? Laura??
> 
> Regards
> 
> Bohdan Dorniak
> 
>  
> 
> From: GSBN-bounces at sustainablesources.com [mailto:GSBN-bounces at sustainablesources.com] On Behalf Of John Swearingen
> Sent: Tuesday, 7 May 2013 9:38 AM
> To: Global Straw Building Network
> Subject: Re: [GSBN] Modeling or measuring mass effect of interior plaster
> 
>  
> 
> 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
> 
>  
> 
> 
> <image001.jpg>
> 
>  
> 
> 
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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
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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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