[GSBN] Modeling or measuring mass effect of interior plaster
John Swearingen
jswearingen at skillful-means.com
Wed May 8 01:25:26 UTC 2013
>
> 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.
We've recently completed a pretty high mass house. Strawbale with
lime/cement plaster, and an Air Floor <http://www.airfloor.com/> system
which consists of 6" of tunnels sitting on a 4" slab and drenched in
concrete with a 2" topping slab. Conditioned air is sent first through the
floor, where it drops a large portion, about 2/3, of its heat into the
slab, which becomes a radiant heater/cooler, and the rest is blown into the
room. The building temperature is remarkably stable. We took measurements
this winter when the temperature went from a maximum 58F to about 30F at
night. The owners turned off the heating system at 9-10pm when the bedroom
was 68F. In the morning it had dropped only one degree, even though the
room has an abundance of glazing: French doors and a large window seat.
The high mass floor system has one complication, noted last fall: the
system is slow to respond in the short term. The owners have not shaded
their glazing, so when rooms get a lot of direct gain, the HVAC system does
not immediately cool the room, as a conventional system would do. However,
the rooms also don't get very hot, because the mass absorbs the solar
gain--and we're only talking about a 3-4 F rise.
John "Hot Air" Swearingen
On Tue, May 7, 2013 at 5:23 PM, Derek Stearns Roff <derek at unm.edu> wrote:
> 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] *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
> Blog: https://skillfulmeansdesign.wordpress.com****
> <EDG_76_AuSES-2_Tas_ThermaL_Mass.pdf>
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>
> Derek Roff
> derek at unm.edu
>
>
>
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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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