[GSBN] natural building in haiti
ejgeorge at riseup.net
ejgeorge at riseup.net
Thu Feb 18 07:46:50 CST 2010
There's been an interesting and informative discussion happening on the Natural Builders' Northeast list about building straw bale structures in Haiti ? a few of you are already in on it, but I thought others might be interested as well and possibly have some insight, especially anyone else who's worked in similar climates.
In a nutshell, NBNE member Andy Mueller of Greenspace Collaborative (who recently joined GSBN as well) will be traveling to Haiti very soon to do reconnaissance and build some prototypes to assist rebuilding efforts through Builders Without Borders. He had already exchanged some emails with a hygrothermal expert on potential moisture issues (included) and was looking for further feedback and thoughts.
Rather than forward all 30 some emails, I've compiled them into a few groups in roughly chronological order. I did summarize a few to keep things shorter.
NBNE's list is a closed list, so if you'd like to pass on a comment you could reply to this email on GSBN and I'll forward it over to NBNE.
Original NBNE email from Andy:
We are in the final logistical stages for our trip to Haiti and hoping to leave soon. For my own peace of mind I decided to contact Robert Riversong to help shed some light on my concerns regarding straw bale construction in a semi-tropical climate as a viable option to address the long term housing demands facing the Haitian population. Below is a thread of email exchanges we have had. If anybody has an interest in responding to the types of render that may be appropriate given the climate data results from Port Au Prince I would welcome them.
There's a 30° annual temperature range (64°-94°) with an average temperature of 84°.
>From this week's weather report, there seems to be a 15° average diurnal temperature swing.
Average monthly relative humidity varies from 57% to 70%, with an annual average of 62%.
That's a milder climate than summertime in Houston TX, which has a July average temperature of 95° with 71% relative humidity.
The CDD data were base 65°, so I converted to both base 70° and to the IECC Hygro-Thermal Zone standard of base 50° (very hot & humid climate zone 1 - including FL, New Orleans and Houston - is CDD50 > 9,000).
Most cellulosic materials don't reach the mold danger zone until persistent RHs greater than 70%, with 85%RH being the critical level for mold growth, particularly at higher ambient temperatures.
The wetting potential of a building is a balance of the climatic rainfall amount and the climatic drying potential (wind, sun & low RH), and the persistence of wetting conditions (rainfall & high RH) over time.
Haiti is in a very hot and humid climate zone with high annual rainfall, small diurnal temperature range and relative humidity that probably fluctuates between 50% and 80% (average US summer RH is 62%).
While I don't think that humidity level would be problematic for strawbale/earth plaster construction, the combination of constant high humidity and high rainfall (with high winds?) could be problematic since there is little drying potential other than direct sun. But, with wide overhangs and a high summer sun angle, the walls will be shaded much of the time.
Water vapor diffusion is driven by vapor pressure and temperature gradients, both of which should be quite low given that the interior spaces will be largely unconditioned. So there is little drying potential by that mechanism. But liquid water diffusion and capillary conduction are driven by concentration gradient and relative humidity differential, the latter being minimal but the former being potentially significant if the exterior walls are not well protected from wind-driven rain.
So I think the challenge will be to design overhangs that can withstand hurricane winds and to adequately seal the exterior plaster to minimize absorption. Given that there will be little vapor driving force to dry the bales to the exterior, it may make sense to use a relatively impermeable, low absorption exterior plaster such as cement/sand with no lime, and use a more permeable interior lime plaster.
What do you think?
That is encouraging news.
Given the potential for high rainfall with accompanying winds, if the design of this one story structure (gable roof with some overhang) were are able to minimize or largely deflect water deposition by rain or wind-driven rain on the exterior walls than we have in large part significantly reduced the need for worry. I imagine the lower to mid sections of the bale walls will be the most susceptible (depending on siting, proximity to other structures etc,) to higher concentrations of moisture loading. If we use a lime-based final render with a tight grain (burnishing), I assume that at some point the plaster will a reach saturation point and act much like an impervious material. However, total saturation is also problematic if there is no dry air to wick the moisture away. I am cautious about using a straight cement sand plaster for the exterior but not against it. Could adding a cementitious ingredient to the lime plaster reduce water absorption enough to make a difference and how much reduction in permeability might that be? Not having worked much at all with a cement/lime render, might there be a ratio of cement to lime that will strike a healthy balance between permeability (to allow for diffusion) and impermeability (to reduce liquid water absorption) that you would recommend given the climatic data? If we construct the house walls of Gore-Tex we might be all set. Although I like the idea of applying Siloxane or Waterglass to the exterior render as the first line of defense to mitigate the absorption of liquid water of the plaster surface, we are trying minimize the amount of imported materials. The bottom line is that we will need to do whatever it takes to respond to this data via detailing unless it appears that straw bale construction is simply inappropriate for this climate. My sense is that we should continue on.
Would you mind if I shared your findings with colleagues who are also considering mounting a natural build campaign in Haiti?
Thanks you very much for you input!
Response from Derek Roff:
It's great to read your data, and to learn that even in the Port au
Prince area, the climate is more favorable than Houston. I wouldn't
claim to be an authority, but I have some concern with the final
conclusion of advocating a stucco mix that is not moisture permeable.
The idea of protecting a building from moisture via a non-permeable
exterior plaster or stucco has been tried tens of thousands of times,
on different building types and substrates, in different ways and in
different climates. From my reading, the failure rates are very
high. From the cement stuccos over adobe in arid parts of New
Mexico, Arizona, and Northern Mexico, to the synthetic stuccos in
humid regions of Florida, Georgia, and the Carolinas, large-scale
failures seem to accompany vapor impermeable stuccos across many
climates. Water and moisture do get into the wall, contrary to the
desires and expectations of the builders. And the impermeable
surfaces do prevent sufficient drying to the outside. The results of
testing that I have seen, from Canada to Mexico, is that impermeable
exterior stuccos increase moisture levels inside the wall, rather
than decrease them. My memory of John Straube's articles in Bruce
King's strawbale building book is that he gives a lot of emphasis to
the folly of the "hold the moisture out" philosophy.
Direct sun on a wet wall is a mixed blessing, since it can increase
evaporation, but is also said to drive moisture through the stuccos
and into the wall. Tropical locations will have the sun shining on
all four walls each day for a good part of the year, excluding the
effects of clouds, overhangs, and surrounding shade. While the
mid-day sun has minimal contact with the walls, the powerful tropical
sun is hitting some part of the walls pretty strongly for six or more
hours a day. I don't know enough to say how that will effect
moisture transport, overall.
I believe drying to the outside is important. Given the average
relative humidity figures that you found, even the slightest breeze
will help dry out the plaster. I believe a vapor permeable plaster
is the prudent path. Trying to decrease plaster saturation and
absorption is great, if it doesn't hurt permeability. If we could
rely on coatings, such as silicate paints, to decrease water
absorption into the plaster, that would be great. I don't know if
that is realistic in Haiti.
Response to Derek from Robert
I don't pretend to have any expertise in SB building or natural plasters (though I've dabbled in them), but I know the theory of moisture mechanics quite well. I'm a strong advocate of breatheable buildings - natural or conventional - when the interior is conditioned space, but my advice about using a non-absorptive exterior render in Haiti was based on the observation that there will be no driving force to move moisture to the outside.
I think the vapor permeability of the exterior render is irrelevant in those conditions. Clay/sand/lime plaster with siloxane would be wonderful - breathable and non-absorbent - but that's not likely a locally available product in Haiti.
Last year's weather data showed 1 HDD65 (one day with a low temperature of 64°). In other words, those homes won't be heated but instead well-ventilated (the old-fashioned way). The only driving force besides the wind-driven rain will be solar radiant drive to the interior. So it would be important to have a vapor-permeable interior surface. Given that wide overhangs may not even be possible because of the hurricanes common to the region, it seems imperative to prevent absorption of rain into the exterior plaster. A permeable exterior will do nothing if there is no driving force to move water vapor outward, which means either vapor pressure or temperature differentials. If the interior space has a virtually identical climate to the exterior, then there will be no gradient to move water vapor.
I would be interested in hearing of experience with SB buildings in climates similar to Haiti's where no heating is required and windows will be open all year.
More detailed response from Robert:
Moisture moves by several mechanisms, but in each case
requires a driving force. That's Newton's first law of motion: a body at rest
will remain so unless a force acts upon it.
Bulk water moves by gravity, momentum or air pressure.
Water molecules move through porous media by surface
tension, called capillary suction, primarily from a reservoir source (such as
the ground). The theoretical capillary limit for concrete is six miles, and for
wood is 400' (which is the limit of tree height for that reason).
Liquid water molecules also move by surface diffusion
(adsorption) from a concentration gradient or relative humidity gradient.
In vapor form, water molecules move by temperature gradient
(including solar radiant flux) and vapor pressure gradient, which occurs only
when there are significantly different conditions on each side of a membrane -
either conditioned (altered) space or sunshine.
Moisture can actually move in both directions at once
through hygroscopic materials: by vapor pressure from inside to out in a heated
building which has higher absolute humidity inside and from liquid diffusion
from outside to in if the relative humidity is higher outside. This counterflow
can occur until the pores become saturated with water, at which point capillary
conduction becomes the primary drive.
So diffusion is not always happening. Liquid diffusion
requires a RH or concentration gradient and vapor diffusion requires a vapor
pressure or temperature gradient. Capillary suction requires pore saturation,
which usually requires a reservoir (which is why a capillary break is needed
between a foundation and above-grade stucco).
Most exterior water uptake, through plasters or any other
cladding, is through openings such as plaster cracks, flashings or unsealed
penetrations. Most rain on vertical surfaces runs off by gravity. What
determines exterior water penetration through uninterrupted surfaces exposed to
rain is the absorptivity of the surface.
A 1:3 cement:sand stucco has an absorptivity coefficient of
0.0127 oz/sf??sec with a perm of just over 1 per inch.
A 1:1:6 cement:lime:sand stucco is 0.0307, or 2½ times
faster rate, with a perm of 7.
A 1:3 lime:sand stucco is 0.0578, or 4½ times faster than
cement stucco, with a perm of 13.
[all data from Don Fugler's CMHC report]
I believe that the house that Rob Jolly found had north side
moisture problems had no overhang on that side, wicking from the ground, cracks
in the cement stucco, and snow piled high on the wall. That was not a stucco
problem, but a design and construction detailing problem.
In fact, among his conclusions were:
1) A primary design consideration must continue to be the
protection of the straw from exterior wetting.
3) Successful designs (in terms of moisture control) include
cement based parging used on exterior and interior.
And he also warned:
"It is still unclear how appropriate strawbale
construction is for high humidity / precipitation climates. At the very least, extreme caution should be
exercised when strawbale construction is used for walls with northern exposures
in these types of climates."
Straube's conclusions from his testing of SB plasters
included that lime washes, water glass and linseed oil had little effect on
water uptake, but siloxane almost eliminates absorption without altering
permeance. On that basis, he recommended siloxane on cement, lime and earthen
plasters. He also determined that an 18" bale has a vapor permeance of
only 2-4, so it will be moderately restrictive to cross-bale vapor diffusion.
An exterior coating with a lower perm than the bales is
problematic only if the primary drying direction is to the exterior and there
is condensation potential within the wall, which would be the case only in a
heated building in a cold climate.
The most important strategy for reducing the probability of
moisture problems in a SB wall is to prevent excessive wetting and bulk
penetration, whether by leaks, absorption or capillarity from the ground or standing
water or snow. In a heating climate, it's important that the exterior skin be
more permeable than the bales. In a hot, air conditioning climate, the inside
skin should be more permeable. In a sunny climate - with or without space
conditioning - the primary moisture drive will be to the interior and that skin
should be vapor permeable.
In Port-au-Prince, I believe keeping the exterior as
water-proof as possible, while leaving the inside vapor open, will be the key
to success. In the absence of a sealer like siloxane, I believe the outer skin
should be as non-absorbent as possible.
Reply from Derek:
Thanks for your response, Robert. I would be pleased to learn more of the theory of
moisture mechanics from you. I'm curious about your statement that there is no driving
force to move moisture to the outside, when the inside of the building is not conditioned
space. Can you tell me more about the forces that you see as being present when the
interior is conditioned, but absent in other cases?
Diffusion will always be present, and tend to equalize moisture levels. If the plaster
has received blown rain, then after the rain storm, air movement will tend to dry the
exterior of the plaster, and diffusion will move moisture from deeper in toward the
outside, until moisture levels are approximately equal. In this case, vapor permeability
of the exterior plaster is important. There are, of course, other factors, such as the
nature of all the involved materials.
I also think we've got something of a false dichotomy in discussing cement/sand plasters
and moisture transport. While they are rather vapor impermeable, they can transport
quite a bit of moisture through capillary action. Rob Jolly's reports for the Canadian
Mortgage Home Corporation found the highest moisture levels were under cement/sand
plasters on the north sides of buildings that had rain exposure. The stucco conducted
moisture in, and didn't let it out fast enough. South facing similar stucco didn't show
moisture levels as high, even though one might predict that sun driven moisture would be
an issue. This was in Canada, not Haiti, and with conditioned interior spaces. Some of
the studied buildings had other moisture factors.
My point is that I don't think that we can guarantee, a priori, that non-absorptive
plasters will keep the straw beneath them dry, nor drier than other plasters. Nor am I
sure that a cement/lime/sand plaster is significantly more absorptive than a cement/sand
plaster. It is significantly better in moisture permeability.
We don't have enough experience with strawbale buildings in warm, humid climates. The
oldest, well-known example is the museum in Huntsville, Alabama, built in 1935. Its
bales are said to be in fine shape, 75 years after construction. There have been a few
buildings in the Houston area, and other humid parts of Texas, but they haven't been
around that long.
I am not certain that we know enough to be confident about strawbale building in the Port
au Prince area. I appreciate everyone sharing their knowledge and experience. I am
confident that this sharing of information will increase our chances of success.
Reply from Mark Piepkorn:
An honest question: Given the general impoverishment and subsistence "lifestyle" of the
hardest hit in the area, and the inevitability of cracks and damage developing in the
finish (such as from hurricane-blown missiles), what exterior material would make the
highest (or lowest) contribution to the potential lifespan for the wall system over time?
Would a requirement for lime and cement ($) for a less absorptive and more inflexible
exterior plaster be enough of a barrier for performing later maintenance when it's
needed? Would a less stable, but free, material such as an earthen plaster be any more
likely to be maintained (and on a more frequent basis)? Would finding food and other
necessities of life be a bigger issue than home maintenance in either case?
FWIW, I think what Derek's saying - and I could be wrong, I often am - is that all things
being relatively equal (such as the indoor and outdoor temperatures and pressures),
moisture will spread itself throughout the wall system. I believe he's primarily
addressing vapor rather than liquid. If it can't as easily dry to the exterior as the
interior, drying will be a slower process. There's always a drive, there is no
equilibrium. If there are discontinuities in the exterior finish funneling liquid water
behind the render, I tend to think that something absorptive and open on the outside
would be good. But I don't know where the balance is, and haven't thought it all through
thoroughly. The answer may be on somebody else's fingertips.
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