The Surprising Benefits Of Combining Siding With Strawbale Construction

Adding siding to a straw bale house is possible, but it requires careful consideration to ensure the structure remains protected from moisture and fire. While plaster is the standard finish for straw bale walls, some people opt for siding due to aesthetic preferences or high rainfall and humidity in their region.

If you choose to add siding to a straw bale house, it is crucial to first apply at least a slip coat of plaster to the bales. This initial layer of plaster serves two purposes: reducing the risk of fire by covering the hairy surface of the bales, and minimising the condensation that may form between the metal siding and the bales. Additionally, a product like a mortar net can be used to separate the bales from the metal siding, providing a drainable skin that prevents moisture from becoming trapped.

Another important aspect to consider is ventilation. It is essential to ensure that there is adequate airflow behind the siding to prevent moisture buildup. This can be achieved through various methods, but it is crucial to avoid creating gaps that allow wind-driven rain to enter the structure.

In summary, while it is possible to add siding to a straw bale house, it requires careful planning and execution to ensure the integrity and longevity of the structure.

Siding over bale walls

The technical term for siding over a bale wall is a "rain screen" or "multiple defence assembly". This is in contrast to the standard way of finishing a straw bale wall with plaster, which is known as a "faceseal" wall.

A rain screen is an exo-barrier that catches rainwater and sheds it before it can hit the structure behind it. The main goal is to minimize any pressure differences so that water is not accidentally driven into the wall assembly. This can be achieved through different methods, but it is important to ensure that there is no gap for wind to blow rain behind the cladding.

Before installing siding over a straw bale wall, it is necessary to apply at least a slip coat of plaster to the bales to protect them from flame spread. This is especially important if using metal siding, as there may be a tendency for the metal to condense and drop water into the bales. It is also crucial to leave a gap between the back of the siding and the plaster to allow for ventilation and drainage.

One way to install siding over a bale wall is to first install nailers for the siding. These can be in the form of 2-inch by 2-inch wood strips attached to the sill plate and beam at the top of the bale wall. After the nailers and bales are in place, one coat of plaster is applied between the nailers. A rough coat of plaster over the bales is sufficient, as no one will ever see the results. Once the plastering is complete, building paper is stapled to the nailers, and the siding is installed, leaving a gap behind the paper for ventilation and drainage.

It is important to note that gaps can occur between the plaster and nailers as the nailer wood shrinks over time, allowing airflow in and out of the bale wall and creating a path for insects and/or rodents. These gaps can be addressed with extra care and/or the application of caulk.

Overall, the use of a rain screen over a straw bale wall can provide significant protection against rainwater and backsplash. However, it is crucial to pay attention to the details and understand the forces acting on water that comes into contact with the walls.

Vapor barriers

In most cases, straw bale walls are finished with a vapor-permeable plaster made from earth, clay, or lime. These plasters allow any water vapour in the bales to escape and the bales to dry out when the weather permits. Synthetic or cement-based plasters, on the other hand, have limited ability to transfer moisture away from the bales and can cause decay in the walls.

House wraps, such as Tyvek and Typar, are typically not used in straw bale construction as they can cause more damage than they prevent by trapping moisture inside the walls. However, they may be necessary in certain situations, such as when bales are exposed to rain splash or snow drifts. In these cases, a house wrap can be used in conjunction with proper flashing and ventilation to protect the bales from moisture damage.

When using a vapor-permeable plaster, it is important to ensure that the plaster is properly applied and protected with smart design details, such as large overhangs and proper flashing around openings. This will help prevent liquid moisture from finding its way into the bales and allow any moisture that does enter the walls to escape.

In addition to vapor barriers, proper flashing and ventilation are essential to protect straw bale walls from moisture damage. Flashing should be installed around windows and doors to direct water away from openings in the wall. A toe-up or curb can also be used to raise the bales off the ground and provide a capillary break, preventing moisture from wicking into the bales from the floor.

Overall, the key to preventing moisture damage in a straw bale house is to allow any moisture that enters the walls to escape while preventing condensation. This can be achieved through the use of vapor-permeable plasters, proper flashing and ventilation, and smart design details.

Rain screens

A rainscreen is a system of materials used to create a gap between the siding and the water-resistant barrier (WRB). It helps keep the entire wall assembly dry by allowing for both water drainage and airflow.

Rainscreens are especially useful in climates with high rainfall and humidity, as they enable water to drain away from the structure. They are also useful in cold climates, as they allow walls to dry without compromising the siding or its finish, and in hot climates, they can assist with cooling.

Rainscreens can be as thin as 1/16 of an inch, but the thicker the rainscreen, the more effective it is in wet and cold climates. In these climates, a gap of at least 1/4 of an inch is recommended to break capillarity and prevent water from moving into the wall.

There are two types of rainscreens: vented and ventilated. Vented rainscreens are open at the bottom to allow for airflow, which helps accelerate drying. Ventilated rainscreens have openings at both the top and bottom of the wall, which creates a chimney effect for exhausting hot air before it enters the building.

When installing a rainscreen, it is important to ensure that flashing extends behind the WRB and not to the outer face of the rainscreen strips. In fire-prone areas, thicker rainscreen gaps may not be allowed due to their chimney effect, and combustible exterior insulation may be forbidden. It is also important to use window screening or a corrugated plastic product to keep pests out.

Rainscreens can be made of various materials, including wood, plastic, or metal. When installing horizontal siding, use vertical rainscreen strips underneath, and when installing vertical board siding, use horizontal rainscreen strips.

Breathability

• Vapor Barriers: Straw bale walls need to breathe, so it is essential to avoid using vapor barriers or impermeable wall treatments such as vinyl siding. Moisture must be able to pass through the walls to prevent rot and decay.
• Plaster Finish: It is recommended to apply at least a scratch coat or slip coat of plaster to the straw bales before adding siding. This helps protect against fire and reduces the risk of flame spread. The plaster also helps regulate moisture and air quality.
• Ventilation: When adding siding, it is crucial to provide a ventilation gap between the straw bales and the siding. This allows any moisture that moves through the walls to escape and prevents it from becoming trapped behind the siding.
• Materials: The choice of materials for siding is important. Wood siding, metal siding, or other breathable options are preferable to impermeable materials. The siding should allow moisture to pass through and not trap it within the wall assembly.
• Climate Considerations: The climate of the location should be taken into account when considering breathability. In dry climates, the ability for the building to release moisture is more important than trying to keep it out. In humid climates, additional measures such as Energy Recovery Ventilation systems may be necessary to manage moisture effectively.
• House Wrap: House wrap is commonly used in conventional homes, but it can cause more harm than good in straw bale houses. It can separate the plaster from the straw, weakening the connection and trapping moisture. In most cases, a vapor-permeable finish like earth-based plaster (clay or lime) is preferable as it allows the bales to dry out.

Foundations

When retrofitting an existing structure with straw bales, the foundation may need to be modified to support the added thickness of the bales. This could involve extending the existing foundation or creating a new foundation that spans both the bales and the original structure.

It is important to ensure that the bales are protected from moisture, as this can lead to rot and decay. One way to achieve this is by providing large overhangs to shield the walls from direct rain. Additionally, windows and doors should be positioned outside the bale wall to avoid creating deep wells that can collect water.

When adding straw bales to an existing structure, it is crucial to maintain separation between the bales and the siding to prevent moisture buildup. Breathable netting and air spaces can be used to direct moisture away from the bales.

The foundation design should also consider the overall weight of the straw bale structure. Proper engineering and reinforcement may be required to support the added load.

In summary, when building a straw bale house or retrofitting an existing structure, it is important to ensure that the foundation is adequately sized, provides protection from moisture, and can support the weight of the straw bale walls.

Frequently asked questions