What are ‘natural’ building materials?

Natural building materials are things such as wood, stone, straw, hemp or cellulose. They are minimally processed and ideally travel only a short distance from where they are harvested to where they will be used. Since they are processed so little these materials can generally be said to be ‘carbon-storing’, meaning a house built with natural materials could store more carbon than was used in the construction.

What is a ‘natural’ house?

Any building that seeks to reduce its carbon footprint or improve indoor air quality by using natural materials is thought of as a natural house. Very few projects will ever achieve the goal of being 100% natural. The challenge is to look for alternatives and figure out what makes sense for your project given the constraints of budget, timelines and the availability of local trades.

How much do natural homes cost, compared to conventional homes?

The bulk material cost of straw bales, rammed earth, light straw clay, earthen block is very low and comparable in price (per cubic ft.) to blown cellulose or fiberglass. The material cost of clay plaster can be extremely low (cheap as dirt, basically), while prices for lime and cement-lime plasters will be roughly equivalent to other similar stucco finish materials. As for “installed cost”, i.e. the price of the whole wall, fully built, this will of course depend on what contractor(s) you use and how much you do yourself, among other things. Generally savings are gleaned from lower energy costs required to heat/cool a natural home.

Bear in mind, as you begin this journey that you are setting out to build a custom home. At this point there is no such thing as a cookie-cutter, off the shelf natural house. Every natural home is crafted by the designers, homeowners and tradespeople who work on it. This does not necessarily make it more expensive than a run of the mill suburban house, but it will definitely take more thought and involvement.

A run down of Natural Materials:

Strawbale

Straw is the leftover fiber after the grain has been harvested from the top of the plant. In Ontario this is usually wheat, oat or barley straw. In strawbale building this straw is bound together into square bales. These tight bales are stacked (sort of like LEGO) to form a wall. Once this wall is sealed (usually with a plaster coat) it provides good insulation.

Light-Straw Clay (LSC)

In this approach loose straw (not in bales) is mixed with a clay slip (a mixture of clay and water) and packed into forms. When it is dry this mixture becomes a light masonry unit, basically a resilient blend of insulation and clay.

Cob

Cob is a mixture of straw and clay, but with much more clay in the mixture than used in Light-straw clay. As such it has much less insulation than L.S.C, but is much more structural and sculptural.

Rammed earth

Is the process of ‘ramming’ clay rich soil into forms. This, when combined with a small amount of cement, essentially changes the earth to ‘stone’, making it impossible to erode or wash away. This is a labour intensive process that produces a unique and beautiful wall system.

Hempcrete

When hemp is harvested the seed is used as a protein source and the fiber is used to make clothes, the last bit is called the hurd. This is the exterior husk of the plant stalk. It is broken up into short 1”ish bits and mixed with lime and water. This mixture is called hempcrete or hemp-lime and is packed into forms to make a light masonry wall.

Cellulose

Cellulose is perhaps the most commonly used natural building material. It is widely used as a blown in, loose fill insulation in attic spaces. It is made from ground up recycled paper with borates added to make it more fire resistant, mold resistant and to deter insects. It can also be blown into wall cavities (dense packed). The recycled content will vary from one manufacturer to another.

Timberframe

A traditional approach to supporting a roof, timberframes use use large timbers (like 8”x8”) to hold up roof trusses or rafters. If this is done in a heritage context no metal fasteners would be used. All connections would be done using (sometimes complicated) wooden joinery and pegs to hold them together.

Post and beam

A more common approach to ‘timberframing’ this uses metal brackets, engineered fasteners and simplified joinery. This might be buried in the wall system where aesthetics are not an issue.

Plaster

Many natural wall systems are finished using either an earthen or lime plaster. This is often the ‘look’ people think of when talking about natural buildings. Plasters have been used for thousands of years as a way to seal and finish walls and make a lot of sense in the context of natural buildings. They provide a good barrier to the elements and air seal, but allow the wall to be permeable (water vapour can escape). Plaster might also be referred to as a parge coat or render. These are all (sort of) synonymous, but a modern off the shelf product might contain portland and other ingredients not suitable for a natural building.

Clay

Clay is a highly versatile and widely used natural material. It is possible to use locally sourced clay or clay rich soil, or it can be bought as a bagged (and more processed) product.

*If you want to learn more about all these materials check out this great Free Encyclopedia of Sustainable Building Materials!

Popular sustainable building terms:

Net-Zero

Basically this means that your house produces as much power as it uses.

Off-grid

A house built with no connection to the electrical grid. This is usually because power lines are far away and it would be expensive to make the connection. The decreasing cost of renewables, improved batteries and improved tech makes this a much easier and appealing decision.

Carbon Sequestration

If you build using a material that is high in cellulose and minimally processed you will store more carbon in your walls than were used to build them. A strawbale house built with local straw will sequester more carbon than it will take to build it.

Passive Solar

A passive solar design means that (in the Northern hemisphere) a house is mostly oriented to the south to take advantage of heat from the sun. In a well built house with good glazing a large proportion of the heat load can be supplied by the sun. This should not be confused with:

Passive House or PassiveHaus

This is a building standard that promotes strict air tightness levels and high levels of insulation. In some parts of the world this means that no heating systems are needed (hence the ‘passive’ thing).

Breathable Walls

Natural walls are often called ‘breathable’, which is mis-leading since it implies that air is moving through them. A natural home (like all modern homes) should be built air tight. A plaster finish will be permeable - meaning that water vapour has the ability to diffuse through it- and escape harmlessly to the inside or to the outside.

Won’t mice and bugs get into the walls?

Pests will try to live in any type of house, no matter what the wall system is. Good detailing that’s appropriate for your climate zone and geographical area will always help eliminate pest issues. In a straw-bale home, the straw is sealed on all sides by either plaster or wood substrates, making it difficult for a pest to access. And remember, straw is not typically a source of food for animals… Hay is for horses, and straw is for houses, we like to say.

How do you keep straw bale walls from decomposing?

Straw is made of cellulose, just like wood. When exposed to moisture, cellulosic materials will decompose. To keep wood-framed houses dry, we protect them with a good roof and a good foundation that will keep moisture away from the wood. The same is done with a straw-bale house. When kept dry, straw (and wood) can last hundreds if not thousands of years. In 2012, members of COSBA were able to obtain a straw bale that was taken out of a historic 1920’s bale house in Nebraska (to make way for an addition). The bale appears as though it could have been installed a year ago. Straw-bale walls also have the quality of being vapor permeable, which allows them to dry out to the interior and exterior more easily than a conventional wall system.

What about fire?

Contrary to popular belief, straw bales will not burn up spectacularly when lit. They are too dense. Full-scale fire tests on straw-bale wall assemblies have been performed in accordance with ASTM Method E119-05a: Fire Tests of Building Construction and Materials. An earth-plastered wall successfully obtained a 1-hour fire rating, and a cement-plastered wall obtained a 2-hour rating. More information about the tests can be found at the below links:

1-Hour Fire Resistance of a Non-Loadbearing Wall w/ Earth Plaster

2-Hour Fire Resistance of a Non-Loadbearing Wall w/ Cement-Stucco 

ASTM E84 Test Report: Surface Burning Characteristics of Straw Bales

How do you incorporate electrical and plumbing?

In straw bale homes, electrical cables can be run in between bale courses or in small chases below/above the wall, depending on your wall design. Some contractors also chose to use conduit. Electrical boxes can be nailed to small wood posts or stakes that are fastened to or embedded in the bales. In almost every climate zone, it’s generally not a good idea to run plumbing in exterior walls (to avoid freezing issues), and should be avoided in straw-bale walls as well. Where it needs to be done, using extra casing and air sealing/insulation is not a bad idea. In “busy” wall areas like at a breaker box or mechanical room, consider switching to a more conventional framed wall system to improve access to cables and mounting equipment, etc. Also, note that locating showers next to exterior/straw-bale walls should be avoided.

Will my building official allow it?

The short answer is yes, they should, especially since straw-bale and light straw-clay con

The short answer is yes, they should, especially since straw-bale and light straw-clay construction systems are both now fully codified in the International Residential Code (IRC). There shouldn’t be any type of “special” permit process, although some officials may request an ‘Alternative Solutions’ package to review. Links to the IRC code sections for straw-bale and light straw-clay.

I like straw bale homes, but I don’t like the look of stucco, what do I do?

Recent innovations in straw-bale wall assemblies include various pre-fabricated systems as well as rain-screen and “straw-cell” assemblies. With these systems, it’s possible to have a variety of different exterior and even interior wall finishes.

What maintenance is required for rammed earth walls?

There is no maintenance required for rammed earth walls and they are fireproof, waterproof and contain nothing for mould to grow on.

Is straw-clay suitable for all climates?

Straw-clay is suitable for climates that have both hot summers and cold winters. As with any building process, timing the building for warmer months is advisable so that straw-clay walls have a chance to completely dry and cure.