This article reviews several common alternative building materials, evaluating their strengths and weaknesses.
Cross Laminated Timber (CLT) is a panel system where each panel consists of multiple timber boards glued together in a perpendicular pattern. The result is a structural product that doesn’t bend and is very strong, rather like plywood.
The panels are large, custom-made and prefabricated off-site. Assembly on-site is then a fast process of slotting pieces together. Additionally, the panels can be manufactured with pre-finished interior wood surfaces, although this does complicate things and requires careful planning and installation. CLT and other engineered wood products are becoming a popular recourse for construction.
Unfortunately, the renewability aspect of wood isn’t as ‘clear-cut’ and simple as re-planting trees. Wood harvest must be done in accordance with strict sustainable forestry practices, which is rarely the case. Ultimately, the Achilles heel of CLT is the unnecessarily large amount of timber it uses. Timber is an excellent yet precious resource, best used sparingly with its full structural potential in combination with other natural materials, if possible. CLT also needs to be insulated, so the environmental impact of wall construction depends considerably on the insulation material and the wall build up as a whole.
Advantages:
- Strong and durable.
- Aesthetically pleasing.
- Fair fire resistance.
- Fast and dry construction.
- Renewable.
Disadvantages:
- Needs extra insulation when used for exterior walls.
- Overuse of wood.
- Complex services installation.
- Use of carbon intensive glue.
While lacking in load-bearing capacity, hempcrete holds its own as a good insulation material. It is made of hemp hurds (contents of hemp stalks) mixed with lime, which binds and preserves the plant material. The mixture is either wet-cast into a supporting frame or prefabricated into dry blocks or even whole walls.
Hempcrete is natural, renewable, non-toxic and widely considered to be sustainable. Lime has the ability to reabsorb carbon dioxide through carbonation, while hemp absorbs it via photosynthesis during growth. Sadly, the carbon-storing effect is mitigated to a degree by the energy required for lime production.
Hempcrete has proven to be resistant to fire as well as mold, insects and rodents. Hempcrete is also resistant to moisture, although protection against weather conditions and moisture is important regardless of how resistant a material might be.
On the flip side, the traditional construction method is a wet and labor-intensive process. The casting method requires a pan mixer on site for mixing hempcrete on the spot, and strict safety precautions, as lime can be aggressive to skin and eyes. Wet-cast hempcrete or lime mortar (in case of hempcrete masonry) needs time to harden.
Advantages:
- Natural.
- Robust and homogeneous.
- Renewable.
- Good thermal performance, if sufficiently thick.
- Resistant to moisture, fire, insects and rodents.
- Can be directly plastered over.
Disadvantages:
- Wet and labor-intensive process, if cast on site.
- Comparatively longer construction time.
- Cannot be worked with at cold or very warm temperatures.
- Working with lime requires strict safety precautions.
Rammed earth, adobe, cob and wattle are ancient construction techniques that use raw materials, such as earth, in many different ways to create mostly robust monolithic or masonry walls or floors. One of its main advantages is low embodied carbon, making it highly sustainable. Production takes significantly less energy compared to concrete or burned bricks, and construction produces only minimal waste.
Another aspect where earth material shines is its ability to absorb and release heat (or remain cool) due to thermal mass and air moisture due to clay minerals. Therefore, earth can contribute to a healthier and stable indoor climate, if used in the interior.
On the other side of the coin, even thick rammed earth walls have rather poor thermal insulation, earning a spot on the disadvantages list. As a result, it needs additional insulation in cold climates, which can influence environmental impact. Earth is better suited for internal walls and surfaces, or for houses in warm climates where it excels.
Earthen construction is resistant to fire and pests, but is susceptible to moisture-related issues. Adequate protection from exposure to rain is essential to prevent deterioration. That is especially true during construction, which is also a labor-intensive process with technical brakes for drying. Still, its distinctive appearance, abundance, low embodied carbon, thermal mass and air moisture micro-regulation make earth attractive.
Advantages:
- Unique aesthetic.
- Thermal mass and hygroscopic properties.
- Locally sourced and widely available.
- Highly sustainable.
Disadvantages:
- Needs insulation when used for exterior wall.
- Weather sensitive when building.
- Labor and time intensive.
The main component of Straw is dried plant stalks. When pressed together into bales or panels, straw becomes very sturdy and insulating, surpassing even hempcrete. This is because straw has a unique cell structure full of tiny air pockets, and doesn’t require lime as a binding agent.
Like CLT, both production and construction of prefab straw panels are dry and waste free processes. But it takes less than one year to grow wheat or hemp, but trees take decades.
Compared to earth, straw may be more sustainable thanks to its impressive CO2-storing ability and insulating properties. Neither earth nor straw need any kind of chemical treatment, thus the positive climate impact of both is uninhibited. Even without additives, straw is still fire resistant. Applying clay plaster on the walls seals the deal, putting any house fire fears to rest. That said, an unfortunate trait straw shares with earth material is its vulnerability to moisture. Both are vulnerable during construction and need to be protected from rain.
Another disadvantage is wall thickness. To achieve the best thermal properties, straw walls tend to be on the thicker side – 400 mm or more – making them more suited for exterior wall applications. Straw can come in the form of prefabricated panels with timber frames. Such a panel system simplifies construction, confers additional load-bearing capacity and ensures certified quality.
Advantages:
- Great insulation.
- Carbon-storage.
- Renewable and widely available.
- Can be directly plastered over.
- Good fire resistance when covered with finishes
- Fast construction if prefabricated in panels or modules
Disadvantages:
- Can be labor intensive depending on building technique
- Vulnerable to moisture
- Weather sensitive when building
- Quality of bales may differ, unless they are certified products
As is often the case, we are forced to wrap up our search for the best option with a somewhat boringly predictable conclusion — it depends. Each has its strong points as well as weaknesses, and is better suited in some scenarios than others. A building can have straw exterior walls filled into a load-bearing wooden frame with interior clay plaster, earth or wooden floors, hempcrete or earth interior walls. And, of course, each of these alternatives are likely far better than the carbon-intensive conventional options still dominating the industry.
You can read the original article at ecococon.eu