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Rondout
Natural
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The Building Envelope
The building envelope, which includes the walls, windows, roof, and foundation, forms the primary thermal barrier between the interior and exterior environments. The envelope must control airflow, temperature, and allow materials to dry. The combination of thermal mass (the clay or lime) and thermal insulation (the fiber) is called “dynamic insulation” because in combination these qualities create very comfortable interior climate and a high level of energy efficiency. This thermal mass/insulation combination is especially effective in conjunction with natural passive climate controls​.​
Let it Breathe
​The materials should ideally have high thermal mass on the inside and be progressively more insulating towards the outside of the wall, providing what is called “out” sulation. Walls constructed from primarily natural materials alone can provide the full range of desired properties as well as health-promoting subtle energies. These walls are often referred to as breathable. The structures they form allow gaseous vapor to diffuse freely through them. They are the lungs of our buildings. If we let them breathe, the house stays dry and warm. If we strangle them, the house dies - moisture is trapped, walls become cold and damp, timber starts to rot, mold and mildew start to creep into every corner.​
​​The thermal envelope of a building should have similar properties to the human body. Our skin is both airtight and watertight from the outside, but permeable as we sweat or ‘transpire’ In traditional (natural) mass wall construction the materials are hygroscopic and have the desired degree of capillarity creating a vapor permeable wall.
Natural cottons, wools and linens feel good when we wear them because they breathe. In contrast synthetic materials tend to trap moisture and odor on our skin surface creating a clammy feel. Similarly conventional synthetic materials with little vapor permeability such as latex paints and many of the foam insulations common in high tech homes trap moisture in the building. Traditional clay lime natural plasters, act like natural clothing for the clay/fiber walls creating a beautiful protective finish that allows the free flow of vapor through the wall.
Breathability is not random air infiltration. A leaky home is drafty less comfortable and consumes lots energy to heat and cool. There is much care taken in sealing potential air movement in and out of any gaps and cracks in building envelope.
In Conventional light frame building, infiltration of moisture laden air is common and will cause condensation and moisture problems within the wall cavity. The clay and lime we use in a naturally built mass wall has a very high capacity to take on vapor when conditions are humid, to store it without any damage to the wall system and then to re-release it when the indoor or outdoor climate has changed. It is this ability that has enabled the historic buildings made of clay and fiber to endure for hundreds of years all over Europe and Asia
Straw Bale
​​Straw is the stems of cereal crops, left over after grains such as rice, wheat, barley and oats have been harvested. When packed and tied into tightly compressed bales, this material turns into excellent building blocks, fire-resistant and highly insulating. Straw bale construction takes two main forms: “load-bearing,” in which the bale walls support the weight of the roof, and “infill,” in which a post-and-beam framework does most of the structural work. Infill bale systems work best in the northeastern climate. The bales are plastered on the interior and exterior surfaces with earth/clay render typically 1 ½ inch thick mixed with chopped straw and sand providing permeable moisture barrier as well as fire protection.
Bale buildings first appeared in the Great Plains over 100 years ago, and have been rapidly gaining popularity since the late 1980’s. Many states and jurisdictions have adopted straw bale building codes, making this technique relatively easy to permit and finance.
Straw Clay
If you coat loose straw with a little bit of clay slip, like dressing a salad, it becomes sticky enough to hold its form when it dries. Straw clay can be used as an insulating infill in any kind of wood-framed wall, such as a timber frame or a conventional stud frame. There are special framing techniques to maximize the insulation value of the wall while reducing the amount of wood required. The mixture is packed between temporary plywood forms that are screwed onto the wall frame, then removed to let the straw clay dry.
When dry, the wall can be coated with clay plaster inside and out to make a breathable, insulating wall. This technique should only be used in dry weather, or under an existing roof, and care needs to be taken with the design to avoid water infiltration into the wall, or mold problems may result.
The density of a slip straw wall can vary a lot depending on your recipe proportions and installation technique. The lightest mixes have an R-value comparable to straw bales, between R-1.5 to R-2 per inch of thickness. That makes straw clay appropriate as exterior walls for homes in almost every climate. Exterior walls are generally between 8” and 12” thick, but interior partitions can be as thin as 2”.
Chip Clay
​This technique is very similar to straw clay, only using wood chips or shavings instead of straw. The chips are lightly coated in clay slip and then packed into a wall cavity, providing thermal and acoustic insulation. Because the resulting wall is a bit weaker than other infill systems, we often use permanent lath of thin wooden strips, branches, or reed matt. Slip and chip is most suitable in forested regions where wood is a locally available resource but straw has to be imported.
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Timber framing and "post-and-beam" construction are traditional methods of building with heavy timbers, creating structures using squared-off and carefully fitted and joined timbers with joints secured by large wooden pegs. In timber framing, builders work with natural logs and trees, rather than lumber pre-cut to common dimensions (like 2x4s). This framing method fell out of fashion because timbers were hand-hewn, and the advent of machine-sawn lumber allowed builders to construct homes more quickly with standard-sized (dimensional) lumber.
Hempcrete
Hempcrete is unique among the plant-fiber insulation in its ability to maintain integrity in humid conditions.. This storage capacity is very helpful in allowing the material to take on moisture when it exists and to release it when conditions allow. Approximately 1300 pounds of water vapor could be stored in 35 cubic feet of hempcrete, providing storage capacity for a sustained elevated relative humidity of 93 percent without overwhelming the capacity of the material to adsorb moisture. A major advantage of hempcrete is that the Lime binder has a high pH and is inherently antimicrobial and antifungal, and the lime coating around each piece of hemp hurd in the mix creates a surface that resists the development of mold. This resilience in the presence of humidity or even liquid moisture makes hempcrete unique among insulation materials and a desirable choice in both cold and hot climates
Hempcrete insulation doesn’t have the structural capacity to fully support roof loads, but cast around conventional wall framing or double-stud framing, it can help restrain the studs from bending or buckling under loads, thereby increasing the load that can be carried by each framing member. A 2 x 6 wood stud with 19.5 pounds per cubic foot of hempcrete infill could support three to four times the compressive loading of a standard stud wall due to the support the hempcrete provides to the wood stud in weak axis bending. The rigidity of hempcrete insulation and the textured surface it presents on the face of the wall makes an excellent substrate for plaster finishes without any need for mesh or other bonding agents.
Hemp is an agricultural crop that has particularly high yields between 2.5 to 8.7 tons per acre compared to yields for wheat straw of 1.25 to 2.5 tons per acre. In terms of the amount of biomass available for use from a single crop, no other plant provides as much volume as hemp . The hemp plant is typically grown for either the strong fiber it produces or for seed. In either type of hemp production, the hurd isn’t the primary use and is considered a by-product.