Choosing an Exterior Plaster for Your Earthen Home

Keeping Your Building Dry

Water is clever. Despite all of our best attempts to keep it out of buildings it seems that it invariably gets in. Conventional builders try to thwart water infiltration by wrapping homes in plastic, inside and out. Moisture exits a building through windows, or more likely through some mechanical, electricity consuming air exchanger. These buildings are tight, often stuffy and generally unpleasant to live in. Earthen buildings, be they cob, light clay-straw, or straw bale, take a different approach.  Rather than relying on mechanical means (e.g. an air exchanger), the walls and their coatings are designed to “breathe” or, more appropriately, configured to allow vapour to pass directly through the walls. For this to work vapour barriers are omitted with these natural wall systems. If they are not and water does get in, you are bound to get compost in your walls!

Water vs. Vapour

Earthen Plaster With Moderate Overhang

For a natural wall system to perform and stay healthy, two things need to happen:

1.  Water must stay out of the walls, which generally means water must stay off the walls.

2.  Water vapour must be able to travel through the wall, typically moving from inside to out in temperate climates such as we find on the prairies.

Now before we get into more deals about these two points, I think it’s important to have a basic understanding of what earthen homes are generally finished with. And no, it’s not vinyl siding! Generally speaking, natural wall systems are finished with one of three things, listed here in order of most environmental and health sensitive to the least: 1) earthen plaster made from sand, clay and straw, 2) lime, or 3) cement stucco. In some cases earth and lime are combined to create what is called a stabilized earthen-plaster, which has the benefit of added durability (this will be covered in a future article).

There are volumes written on the relative benefits and drawbacks of each these materials, so I’m not going to attempt to summarize this grand discussion into several paragraphs. What I’ve done instead is provided a few worthwhile quotes pertaining to each material and their associated benefits and drawbacks. This way, when it comes time for you to make a decision on how you might finish your walls, you come at it from an informed place.

Earthen Plaster

straw bale house earthen plasters dirt craft natural building“I love earth plasters. I want all new straw bale homes to be wearing them. They make the most beautiful walls I have ever seen. The colors are gorgeous and incredibly varied. The walls feel soft and homey. Earth plasters can be rustic and undulate with the bales, or create smooth walls with even curves and a polished finish. The materials are cheap, easy to come by, and healthy for the environment. Earth-plastered walls breathe like living beings, protecting bales from moisture damage by exhaling moisture instead of locking it inside. Mud is a blast to work with, can be extremely durable, and is easy to patch and repair, when necessary. I highly recommend it.”

– Keely Meagan. Excerpt from Mud: Magic, Fun and Sometimes Quarrelsome, which appeared in the Issue #43 of The Last Straw – The International Journal of Straw Bale and Natural Building.

“Clay will erode, pure and simple. Make no mistake about it. Plasters with higher amounts of straw or carefully graded aggregates will be more resistant. Some stabilizers will give a degree of improvement, however, they are often overrated and not understood in terms of how they may affect vapor permeability and moisture flow. The best strategy for plasters used on the exterior of a building in moderate to severe climates is to simply keep water from hitting the walls. The most common way of doing this is a porch.”

– Bill and Athena Steen. Excerpt from The Clay Straw-Bale House: Complete Clay Integration, which appeared in the Issue #43 of The Last Straw – The International Journal of Straw Bale and Natural Building.

Lime Plaster

A Lime Plastered Wall

“Lime-based renders work very well in cold climates. Except in dry regions, we believe that lime is a superior material to cement, for use over [straw] bales. First of all, lime render is more vapor permeable than cement stucco. It is also less prone to cracking, and has some ability to heal its own cracks. Lime is much easier to work than cement, and is also easier to repair. Its manufacture is less energy intensive. And, because it is very alkaline, lime plaster is quite hygienic. Finally, lime also forms a beautiful finish.”

– Paul Lacinski and Michel Bergeron. Except from Serious Straw Bale: A Home Construction Guide for All Climates.

“What time of the year [lime plaster] is applied becomes another vital issue. Newly applied lime plasters most likely need at least three to four months to ensure sufficient carbonation to take place before it is allowed to get wet in freezing temperatures. Also of major importance is keeping newly applied plaster damp and not allowing it to dry out too quickly.”

– Bill and Athena Steen. Excerpt from their Guest Editor’s Notes from Issue #29 of The Last Straw – The International Journal of Straw Bale and Natural Building.

Cement Stucco

A Hard Cement Stucco

“Normal cement stucco is capillary-active, and wicks water through to the inside. If a capillary-active material like wood [or straw or earth – ed.] is on the other side, wetting can occur even if no unbound liquid water is available… Steel, glass, and vinyl are also waterproof materials, but roof and walls made of these materials leak all the time – there is in fact a thriving industry that diagnoses and repairs these leaks. So the fact remains that cement stucco walls can only be considered waterproof in the rarest circumstances; in practice they should be considered to be quite “leaky” and the wall designed accordingly.”

– Dr. John Straube. Excerpt from Choosing A Plaster by Catherine Wanek from Issue #33 of The Last Straw – The International Journal of Straw Bale and Natural Building.

With these comments in mind, let’s get back to the original discussion, which was related to keeping walls dry. To reiterate, for a natural wall system to perform and stay healthy, two things need to happen:

1.  Water must stay out of the walls, which generally means water must stay off the walls.

2.  Water vapour must be able to travel through the wall, typically moving from inside to out in temperate climates such as we find on the prairies.

A Porch Protects The Earthen PlasterDesign to Keep Water Out

Concerning the first point, all manner of attempts have been made to keep earthen walls dry, ranging from applying chemical stew-slathered stucco, to impregnating them with vegetable-based oils, to painting them with products like water-glass. Lime has been used for added durability and “water-proofness” but it too suffers from people having unrealistically high expectations regarding what it can do. Despite the best of attempts, if water has access to walls, it’s bound to create problems for natural materials. The only way to really keep walls from getting wet is to provide: 1) generous roof overhangs, even a wrap-around porch or 2) cover the walls with a rain screen (usually siding suspended away from an underlying plaster to allow for vapour permeability while keeping driving rain and snow off the wall), especially in areas and on exposures that receive wind-driven, almost-horizontal rain or snow. It’s important to know your site, how weather may affect it, and plan your home accordingly.

Making Friends with Moisture

Moving to our second point, it’s important to understand how water moves in and out of a building. It occurs through several mechanisms – air leakage and vapour diffusion. With air leakage, it comes in two forms – 1) infiltration, whereby air leaks into a building such is the case during the heating season when we feel a draft, and 2) exfiltration, whereby air exits through the fabric of the building. It is this exfiltrating air that generally produces moisture problems in a building, especially in the winter. When warm moisture-containing air moves through the fabric of the building it cools and if it drops below the dewpoint, liquid water begins to form. To deal with air leakage, one must pay proper attention to sealing the home such that air doesn’t enter or leave the home unless you want it to (like through a window).

Vapour Barrier With Pink Fiberglass Insulation

When it comes to vapour diffusion, this is another matter altogether. When you hear that a natural home is “breathable”, this is referring to the ability of the wall to pass moisture in a vapour state. To understand how this works we need to know that warm air holds more water vapour than cold air, thereby excreting a greater vapour-pressure. Water vapour (or any gas for that matter) will always move from an area of high concentration to an area of lower concentration. This means that in the winter, warm interior air contains a higher vapour pressure than cold exterior air and is therefore forced toward the outside. The conventional approach to keep this vapour from penetrating the wall is to put plastic up, thereby blocking the moisture. This doesn’t so much ‘deal’ with the moisture as seal against it. In an earthen home, this vapour is allowed to pass directly through the wall through proper plastering techniques. To facilitate this movement, it is understood that an exterior plaster be at least as vapour permeable as the interior plaster. With clay being the most vapour permeable and cement being the least, and lime somewhere in between, it follows that it is unacceptable to put cement stucco on an exterior if clay is used on the interior. It is always best to pair like with like, though an exterior lime plaster may successfully be paired with an interior earthen plaster.

earthen plaster straw bale house dirt craft natural building

Conclusion: Design Details + Vapour Permeability = A Healthy Building

There’s a lot here to process, I know. If I can summarize a few key takeaways, it’s that pairing the right design details – overhangs, rain screens, etc. – with the right plaster for the site is vitally important to the health of the building and its occupants. It’s also extremely important to seal your home properly such that you are not gaining unwanted air or losing hard earned heat. Finally, ensuring that your exterior plaster is at least as vapour permeable as your interior plaster will go a long way to ensure you’re building is functioning as well as it can. Keep these things in mind and you won’t run into problems down the road, saving you time, money, and frustration.


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