[GSBN] Modeling or measuring mass effect of interior plaster
martin hammer
mfhammer at pacbell.net
Wed May 8 10:29:49 UTC 2013
Great discussion on this subject.
I often characterize plastered straw bale walls as “a perfect balance of
mass and insulation”. A generalization, to be sure, but not far off, for
almost any climatic condition. I also often refer to them as “natural
SIPs”. Structural Insulated Panels, are usually thought of as having an
insulating polymer foam core between layers of plywood or OSB, but can also
include a similar core between layers of cement plaster (applied in situ).
These are sometimes called “composite panel building systems” or “structural
insulated reinforced concrete panels”. Structurally, plastered SB walls
(can) function like any of the SIPs, but thermally they perform only like
SIPs with the cement plaster skins. (Though I suppose gypsum board is
sometimes applied to the interior and cement plaster to the exterior of OSB
SIPs, which thermally gets them closer to SB walls.)
The systems with cement plaster skins usually use a welded steel wire space
frame that ties the skins together through the insulating foam core. If you
squint hard and ignore petro-chemical insulations and the Portland cement
based plaster (I know that’s impossible), then in terms of structural and
thermal performance its a great system. This might be like saying “except
for the fact that it’s terrible, it’s great!”, but there’s a valid point in
there somewhere. And one could presumably use lime plaster or soil-cement
(or even clay plaster?) with one of these foam-wire-core systems (anyone
done that?) that would significantly reduce the environmental downside but
maintain the thermal performance and structural capacity (with careful
consideration of the weaker plasters for the particular application).
Of course instead of messing with any of that one could just build SB walls.
I’m just making a comparison to “similar” industrialized systems, that to be
fair, have certain advantages. One advantage is SIPs (with wood panel or
reinforced cement plaster skins) can be used structurally for floor and roof
elements and plastered SB cannot (notwithstanding SB vaults).
One observation on the very good article Bohdan shared, is that the mid-mass
building gets you far in terms of thermal performance (compared to the
low-mass building), whereas the high-mass building then demonstrates a more
modest improvement. Like anything else, the first steps in the right
direction have the largest payoff, with still-valuable but diminishing
return after that.
One other thought is that the mass on the interior of a plastered SB wall is
more important than the mass on the exterior in terms of thermal
performance. I wonder if anyone has therefore used a thicker interior
plaster than exterior.
Martin
On 5/7/13 5:23 PM, "Derek 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 <tel:970-379-6779>
>> www.greenweaverinc.com <http://www.greenweaverinc.com>
>>
>>
>> <image001.jpg>
>>
>>
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>>
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