[GSBN] Modelling or measuring mass effect of interior plaster

Wed May 8 00:32:47 UTC 2013

If John "Earth has two hemispheres?" Swearingen got up early enough in the morning to see the sunrise, he would notice that some sun comes in the north windows of the houses that he builds in California, for about three months a year.  But in fact, he just has a good sense of humor.  John was probably a consultant on that John Wayne war movie, which showed the sun setting in the east, too.

Derek

On May 7, 2013, at 5:21 PM, Bohdan Dorniak wrote:

Bear in mind that we are talking about the Southern Hemisphere.
We haven’t fallen off the earth as yet!!!
Bohdan Dorniak
“Architect still hanging onto the earth”

From: GSBN-bounces at sustainablesources.com<mailto:GSBN-bounces at sustainablesources.com> [mailto:GSBN-bounces at sustainablesources.com] On Behalf Of John Swearingen
Sent: Wednesday, 8 May 2013 5:55 AM
To: Global Straw Building Network
Subject: Re: [GSBN] Modeling or measuring mass effect of interior plaster

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<mailto: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> [mailto: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<mailto: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<tel:970-379-6779>
www.greenweaverinc.com<http://www.greenweaverinc.com>

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

Web Site:  http://www.skillful-means.com
Blog:         https://skillfulmeansdesign.wordpress.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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Derek Roff
derek at unm.edu<mailto:derek at unm.edu>

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