Moisture is every bit the very real concern in straw-bale construction that it is in standard wood-frame construction, and many of the same considerations that need to be given to other super-insulation techniques need to also be given to straw-bale construction. A properly designed and constructed straw-bale house will last as long as any “standard” house; and an improperly executed straw-bale structure will fail just as miserably as an improperly executed stick house.
The thicker walls and increased amounts of insulative material in super-insulated buildings make it more difficult for any moisture that enters the wall system to escape. Sustained elevated levels of moisture can disrupt and cause failure in just about any system—including strawbale. In moist and heating climates particularly, thorough research and thoughtful consideration before and during design and construction are very important, along with a recognition of the distinct differences in moisture behaviors within different wall systems. A plywood-clad super-insulated stud-frame wall will not exhibit the same tendencies in that regard as a super-insulated straw-bale wall clad with plaster, nor will they have identical (or even similar) ambient wetting and drying regimes.
Often overlooked is the fact that straw and wood are similar in composition, and both will rot if the right conditions are present. The breaking point of the moisture tolerance of straw is typically said to be a sustained 20% moisture-by-weight (equivalent to about 80% relative humidity), which is the necessary level of moisture to support fungus and rot. (Some certain measure of heat is also part of the equation.) Note that this moisture content must be sustained; experience shows that occasional high moisture levels are acceptable provided the bales are afforded an adequate drying regime.
A striking difference between wood and straw was illustrated by John Straube of the Building Engineering Department of the University of Waterloo, Canada, in an article on page 5 of the Spring 1998 issue of The Last Straw Journal, in which he wrote:
Straw will store almost the same amount of water as wood on a weight basis (2 to 3 pounds of water per 10 pounds of material at 98% relative humidity), but less in relationship to volume, since most species of wood weigh 25 to 40 pounds per cubic foot (pcf), and baled straw weighs only 6 to 12 pcf. For example, a square foot of 18-inch-thick straw-bale wall is 1.5 cubic feet, weighing 15 pounds per square foot, assuming a density of 10 pcf for the baled straw; this wall can store 3 to 4.5 pounds of water per square foot. Lower density straw would store proportionately less water. On the other hand, a wall of 1-inch-thick pine weighs about 2 pounds per square foot; it can store 0.4 to 0.6 pounds of water per square foot. (If we add wood studs to the wall, it can store more water.) The difference in moisture storage capacity between the two walls in this example is about 2.5 to 4 pounds of water per square foot.
A recent study by Fibrehouse Limited, published by Canada Housing and Mortgage Corporation, looked into the first wave of “Quebec Style” (or “Gagne Style”) straw-bale houses built about ten years ago north of Ottawa. This report is well worth acquiring by anybody concerned about the long-term effects of moisture on straw-bale constructions; it gives clues provided from actual structures about what and what not to do.
The vast majority of failures in houses of any type are directly related to moisture intrusion; it is imperative that we understand how best to prevent moisture from entering the wall system, yet realistically allow for adequate drying regimes in the event of minor failures. We must know how moisture behaves, reacts, and interacts with this material and its applications… and recognize that each set of climate, micro-climate, and site conditions is unique and needs to be considered. Kelly Lerner wrote in a post to the Global Straw Builder’s Network:
Here in Ulaan Baatar, Mongolia (8″ rain/year), I detail for moisture much more casually than in California (20″ rain/year) or Oregon (40″-60″ rain/year). As in any type of building, detailing for SB needs to be climate specific.
A major part of the function of any wall is to separate the interior from the exterior. As part of this job, walls must control heat, air, and moisture flow. Specialized layers and materials, sometimes several of them, are typically needed to control air flow and moisture flow in all of its forms: vapor, rain, ground water, etc. Standard building practice dictates the use of Moisture Barriers (which are intended to stop liquid moisture, generally in the form of rain), Vapor Diffusion Retarders (more simply known as vapor barriers, intended to stop moisture vapor, which in heating climates is generally humidity in the interior air that’s trying to get out), and Air Barrier Systems (which are intended to stop air leakage).
Liquid Moisture
Primary consideration needs to be given to preventing liquid moisture from finding a way into the bales: ensure appropriate overhangs, splash protection, window and door detailing, etc., and don’t forget to remember adequate protection for the bales during the actual construction process.
Taking care in detailing window and door openings by design and implementation is very important—providing a drip edge at the tops and bottoms of the openings, making sure that any “ledges” (such as at the bottom of a window opening) are sloped appropriately, and installing a sheet-type Moisture Barrier such as tar paper, or felt, over areas above, beside, and below openings in the wall system before stuccoing in consideration of any potential water intrusion that might occur at junctions of dissimilar materials. Some straw-bale builders in wetter climates feel that tar,paper isn’t sufficient, that it’s too easily punctured by the building elements (or builders)—in which condition the tar,paper can potentially create a bigger problem than it solves—and recommend more flexible and self-healing membranes such as Bituthene or Ice & Water Shield (which is not intended to be an endorsement for WR Grace, which happens to be the manufacturer of both of those products).
It should be pointed out here that there is a profound difference between vertical water and horizontal water where straw bales are concerned. Vertical water—such as driving rain hitting the sides of a straw-bale wall—is shown by experience to not penetrate very far. Horizontal water, on the other hand, will seep from above into the middle of the bale wall, which is precisely the worst place for it to be.
A Moisture Barrier need not be a sheet-type product. Many paints will create an effective moisture barrier; however, this option should be approached with careful deliberation, in consideration of the entire house as a system, and in light of the entirety of the discussions in this paper.
Another Moisture Barrier option which appeals to many people’s esthetics is the use of a vented rain screen, generally implemented in the form of a siding material installed such that an air gap exists between it and the stuccoed straw bales. Again, the effects of each design decision must be weighed, and the house-as-a-system must be considered on the whole. The general consensus of experienced & knowledgeable straw-bale builders indicates that external cladding will perform better than stucco alone in nearly all circumstances, if properly implemented. It’s important that the bales be sealed with a directly-applied wet render, rather than leaving an air gap between the raw bales and the cladding. That sealing-coat, whether cementitious or earthen, provides an air barrier in consideration of the possibility of thermal performance degradation; seals against potential infestations; and reduces the concern over the creation of a chimney effect in case of fire (which needs to be a concern despite the highly-touted fire-resistance of SB).
Beyond physical design and implementation for the prevention of liquid moisture intrusion into straw-bale wall assemblies, things can get tricky… and sometimes contentious.
Vapor Diffusion Retarders and Air Barrier Systems
While they can (and perhaps should) be considered as distinct devices, the practical functions and applications of Vapor Diffusion Retarders (vapor barriers) and Air Barrier Systems are often confused and combined; and so the two devices will be discussed here together.
A common rule of thumb for the implementation of vapor barriers states that the warm & humid side of the wall should be five times less permeable then the cool & dry side; also, two-thirds or more of the effective R-value of the assembly—the straw, in this case—should be on the cold side of the vapor barrier. One of the apparent difficulties with this rule of thumb, in many climates, is that the cool/dry and warm/humid sides are effectively 100% reversed over the course of the seasons. However, the biggest vapor-intrusion risk in these climates usually exists during the heating season, during which a much greater difference in temperature, humidity, and air pressure exists between the inside and the outside of the structure… and so, the Vapor Diffusion Retarder should always be placed to the interior in climates with cold winters.
It has become typical in wood frame construction to apply a sheet-type vapor barrier, such as 6-mil poly, beneath the finish material of the wall system on the warm/moist side. However, this practice appears to be becoming increasingly understood as unnecessary, if not suspect.
David Eisenberg, co-director of the Development Center for Appropriate Technology (DCAT), wrote in a post to the CREST-sponsored straw-bale construction email listserve on the internet:
Now what all of the “experts” I’ve talked with are in absolute agreement about is that the permeability of typical wall finish materials on the interior is almost insignificant in relation to the amount of moisture that will pass through the material. The critical source of water vapor migration into walls in heating climates is gross air leakage, through holes in the finishing system such as at electrical boxes and fixtures, around windows and doors, at the connections of the walls and the ceiling and floor, etc., which George Tsongas [a mechanical engineering professor at Portland State University in Oregon] said would carry a hundred times more moisture into the wall than can pass through even a very permeable plaster. Bob Platts [founder and a principal at the consulting firm Scanada Ltd, now with Fibrehouse Ltd] said in Canada in dozens of studies that were done, they proved that, although the guideline of having the exterior finish system 5 times more permeable than the interior finish system is a good idea, in a building that has all the air leaks sealed, it is virtually impossible to get enough moisture migrating through the wall finish to cause moisture failure of a wall system… This means that you could have an earthen plaster on the interior walls that was carefully applied and had no unsealed cracks or holes, and cement stucco on the outside, and not develop a wall moisture problem from interior water vapor sources. I think this is critical to understand—we are talking about two different things here: breathability/ permeability for water vapor—the ability for water vapor to pass through a material such as plaster, and air leakage—holes through which relatively large quantities of warm moist air can enter the wall from the warm side.
It would seem from these statements that nearly any stucco or finish material with some amount of air-impermeability would be adequate protection from moisture vapor, so long as it was detailed meticulously against air leakage.
In a slightly more cautious take on this topic, Rob Tom of Erewhon Design Group/Atelier OCTO wrote to the listserve:
… there is a difference between the “breathability of surfaces” and the infiltration (and exfiltration) associated with air leakage. Air leakage occurs through cracks usually found around junctions between dissimilar materials & components and penetrations of the building envelope… losses due to infiltration/exfiltration can account for ~50% of the space heating & cooling energy consumed by a well-insulated building. One of the many deleterious consequences of this leakage is that in heated buildings, condensation of exfiltrated moisture can occur in the envelope materials. In cold climates, this problem can be severe, often resulting in deterioration of the wall and/or roof materials, not to mention increased energy usage as a result of the lowered R-values; and as a result of the exfiltration losses; and that required to condition infiltrated air.
As to whether this deterioration is as big a problem with straw-bale walls as it is with other superinsulated walls is a contentious issue. The homeowner moisture monitoring program that all SB owners are strongly encouraged to participate in [coordinated by Don Fugler of Canada Mortgage and Housing Corporation, a division of the Canadian Federal Government] will be vital to providing some much-needed answers where a void now exists…
Heating and Ventilating magazine indicates that the modern life of a family of four can easily generate 150 pounds, or more than 18 gallons, of water per week into the household air. The following chart is from “Builders’ Guide to Energy Efficiency in New Housing, 2nd Edition” published by Canadian Homebuilders Association/Ontario Ministry of Energy/HUDAC:
Source |
Amount of Water Vapor Generated |
Cooking (3 meals) |
1 kg per day (2.2 lb) |
Dishwashing (3 meals) |
0.5 kg/day (1.1 lb) |
People (family of 4) |
5 kg/day (11 lb) |
Bathing (shower) |
0.25 kg each time (.55 lb) |
Bathing (tub) |
0.05 kg each time (1.76 oz) |
Clothes Washing |
2 kg each time (4.4 lb) |
Clothes Drying |
12 kg each time (26.46 lb) |
This publication also indicates that a very small break in the Air Barrier System can allow a startling amount of moisture to find its way into the walls. Under typical Canadian winter temperature and humidity conditions, assuming an interior temperature of 21C (70F) and 40% relative humidity, in a 10-square-meter (108-square-foot) room with 9mm (3/8-inch) ceiling board painted with two coats of enamel paint, the water vapor moved by diffusion over the course of 100 days would be 3 kilograms, or 6.6 pounds. If, however, there were a crack in that ceiling 1.5 millimeters wide (6/100th of an inch) and 1.2 meters long (about 4 feet), the water vapor moved by air leakage over the same 100 days would be 20 kilograms, or 40.4 pounds. That one tiny crack creates a six-fold increase in water vapor intrusion.
It becomes an extremely persuasive argument that the most important Barrier of concern in straw-bale construction with regard to moisture is the Air Barrier System. The Air Barrier is not a single piece of material, but a system which may consist of several common building elements: interior floor/wall/ceiling materials; weather-stripping; caulking; gaskets; etc… all working together to prevent the leakage of the moisture-laden air inside the house into the walls. Gaps in the interior finishes around electrical outlets, overhead fixtures, window and door frames, plumbing, floors and ceilings, etc., should be sealed and work together as a whole system to prevent vapor movement from the interior of the house into the walls. It’s also common and good practice to use small active ventilation systems in humid areas such as bathrooms and kitchens to remove much of this concentrated vapor at the source, for self-evident reasons.
The practice of attempting to create a “fail safe” redundant air/vapor barrier system, such as installing 6-mil poly sheeting behind the interior stucco/finish to act as both an Air Barrier and a Vapor Diffusion Retarder became widely-popular in the 1970s, and seems to largely be a false security. The same areas that have discontinuities in the finish materials are the same areas that will have discontinuities in the sheet barrier. By creating the internal Air Barrier System of the interior finishing components themselves, the system can be designed to be visually inspected over the entirety of its surface.
On Sheet-Type Barriers
The biggest difficulty with the use of sheet-type barriers (Moisture, Vapor, or Air) in straw-bale construction is not figuring out how to affix them to the walls before stuccoing without poking them full of holes, but rather the fact that these sheet barriers will eliminate the stucco-to-straw bond, which is indicated by most sources to be an important part of the overall physical strength of this building technique, particularly in a load-bearing (Nebraska-style) capacity.
It’s occasionally suggested that any discontinuity in a barrier, such as a rip or puncture, can serve as a moisture funnel, concentrating moisture at locations in the wall where the breach exists… and that the “increased likelihood” of localized failure due to flawed installation is the best argument for not using one. While this can be true in the case of Moisture Barriers under saturation conditions, and is undeniably true in the case of the Air Barrier System, it is not the case with Vapor Diffusion Retarders. In a heated structure, a one-inch square puncture in a Vapor Diffusion Retarder per ten square feet will only increase vapor permeance by 1/1440. The same scenario, in the case of Air Barriers, produces a rather different result: because air has momentum (due to higher air pressure within the house during the heating season), it will tend to concentrate moisture on the other side of the hole in the Air Barrier System… and the volume of moisture will be anywhere from ten to 100 times more at that location.
The argument against the use of specific-component Moisture Barriers and Vapor Diffusion Retarders in straw-bale construction is compelling. It’s strongly felt by many that the bales should be allowed to “breathe,” (which is a bit of a misnomer: common sense indicates that air-exchanges through properly-stuccoed bales just wouldn’t occur under normal circumstances—normal being a house with things like doors and windows). The idea is that any moisture that gets in the wall should be allowed to leave it again, or it will just end up causing trouble. In a post to the Global Straw Builder’s Network, Kelly Lerner wrote:
In heating climates, water vapor travels through the wall (pushed outward by positive air pressure indoors) and will condense on the cold surface of a Moisture Barrier [such as Tyvek-like materials] at the inside surface of the outside stucco. If the moisture were still vapor, it could travel out through the Moisture Barrier—but being liquid water, it is trapped. Without the Moisture Barrier, the water vapor could condense on the cold back of the exterior stucco and would escape via capillary action through the stucco. Stucco and other plasters seem to me a perfect combination with straw-bale; they are good air barriers and they transport moisture well through capillary action allowing the wall to “perspire.”
The use of whole-house specific-function Vapor Diffusion Retarders and Moisture Barriers (such as lining the entire structure,inside and/or out, with plastic sheeting for the sole purpose of doing what the finish components should do if properly implemented) in straw-bale construction is under debate. The original Nebraska houses didn’t use any sort of barrier besides the stucco itself, and as we know, they’ve done quite well. At the same time, we must also consider the differences in lifestyle (no indoor plumbing, less bathing, non-airtight wood stove heating, etc) and building technologies (leaky windows and doors, etc)… and while Nebraska is no stranger to rain and snow, there are certainly harsher climates than that.
The big questions seem to be:
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Are the other measures that can be taken as a precaution against liquid and vapor moisture intrusions, such as continuous finish plasters inside and out, large overhangs, etc., enough… and if not, will sheet materials help? The most serious concern about sheet-type barriers is that if moisture does somehow get behind them, it has no way of getting out. A moisture barrier won’t let moisture travel in either direction, and a vapor barrier won’t let vapor travel in either direction. (That simple concept is of great importance.) Since it is sustained moisture that causes damage to bales, there is less need for concern about moisture that gets in if we are confident that it will get out reasonably soon. The ideal, of course, is that the moisture level never reach a critical stage.
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Will using a plastic sheet barrier weaken the structure needlessly (and possibly significantly) by eliminating the stucco-to-straw bond, creating the need for structural members where none would otherwise have been needed, and which may be inferior to the structural system which was deposed?
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Will the potential degradation of the wall’s thermal-resistance characteristics be acceptable? (Computer models indicate up to a 50% loss of thermal resistance if air pockets between the bales and the cladding permit convective airflow.)