[GSBN] Modeling or measuring mass effect of interior plaster

Wed May 8 00:23:17 UTC 2013

Thanks for posting the article on thermal mass, Bohdan.  I found it very interesting.  Based on the data it contains, I have a few comments relevant to Laura's situation.

As John mentioned, the article emphasizes that it is addressing thermal mass with direct solar exposure/gain.  However, I didn't see anything quantifying what amount of time the sun needs to shine on each bit of thermal mass floor and wall each day, in order to attain the listed results.  As the sun moves across the sky, some parts of the floor and walls will receive direct sunlight for only a few minutes, while other locations will be in the sun for several hours per day.  The amount of furniture, area rugs, and wall decorations will also affect how much sun shines on thermal mass.  I'm thinking that as the amount of time in the sun decreases, the needed surface area of the thermal mass needs to increase, to get the same thermal mass effect.  The total mass of the thermal mass might need to increase a bit, too.

At 6000 sq ft/560m3, Laŭra's project will likely have a higher floor area to wall area ratio than smaller houses.  By itself, this might decrease the importance of the thermal mass in the walls.  However, depending on the number of stories and the floor plan, a large project might have a smaller percentage of floor and wall area accessible to direct sun.  That would increase the importance of maximizing the surface area of the thermal mass.

Laura didn't say where this project will be built, but I'm guessing it might have a climate closer to the "cold European climate" listed in the article, rather than to the alternative Melbourne figures.  The article quotes Vale and Vale as suggesting 1,200kg of thermal mass for each square meter of floor area.  It's worth noting that all this thermal mass could not be in the floor.  If it were, the thermal mass floor would need to be about .5m/20" thick.  Not only is that absurd from a construction perspective, but much of the mass in a slab that thick would not respond to daily temperature variation.  My reading is that walls have to be involved in the thermal mass equation, to get the results described.

At the top right of page 7, the article says that temperature modeling for the three reference building variations (low-mass, medium-mass, and high-mass) are predicted to have similar maximum temperatures.  However, the numbers in the chart paint a different picture, as I see it.  Table 3 shows the maximum temperature at the hottest part of the summer as 32 degrees C/90 degrees F for the low mass house, contrasted with 25 C/77 F for the high-mass house.  To me, that's the difference between the average American demanding air conditioning (or feeling that they are suffering), versus a temperature that many would find acceptable.  23 C/77 F is pretty comfortable, when the radiant surfaces (ceiling, walls and floors) are at a similar temperature or lower, as they would be in a well-insulated home.  That temperature can feel uncomfortable, if the radiant surfaces have heated up to 30 C/86 F or more.

Table 3 also shows surprising figures for the morning lows in the hottest month.  The low-mass building is predicted to be at 16 C/61 F on a summer morning.  Some residents would be turning on the heat in the morning, and the air conditioning in the afternoon.  The high-mass building will get down to a comfortable 20 C/68 F on the same summer morning.  To the extent that covering the walls with barn wood will diminish the effective thermal mass, as Laura describes, it could have a significant impact on comfort in the summer.  The winter figures indicate substantially greater differences between high-mass and low-mass temperature variations.  I also conclude that Hobart has a pretty pleasant temperature range.

I agree with David, that uninsulated thermal mass can easily be a liability.  I'm not sure the same is true for well-insulated thermal mass.  A large amount of thermal mass combined with a well-insulated building envelope will result in small daily temperature variations.  At some point, adding more thermal mass will have little effect, because of the low temperature swing and the resulting low delta T across the thermal mass.  After this point, adding more thermal mass wouldn't help much, but I'm not seeing how it would have a negative effect, either.  Unless the residents found consistent temperatures monotonous.

Derek

On May 6, 2013, at 7:05 PM, Bohdan Dorniak 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] 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
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Web Site:  http://www.skillful-means.com
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Derek Roff
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