Updated: Feb 5, 2021
Building a house (from straw or not) can be generally split into its base components:
Foundation is traditionally thought of as a concrete slab serving as a base to erect the walls. To date that seems to be the most common type of foundation, which, if done right, can ensure a stable and thermally insulated walls and flooring base. In some countries it is even possible to build off the ground - using earth-anchors to secure the base floor elevated from the ground. In that case it should be insulated even better than an "earthed" concrete slab (as freezing or hot winds could make way beneath the entire house). Advancements in technology could make this method even more viable in future.
This element is inevitable for any house - conventional or straw. The cost would therefore be the same and could only go down by luring friends and helping hands on-site with promise of a good meal and couple beers. Task which is not easily accomplished in modern days.
Entering Straw walls territory.
A well-built simple straw wall can easily compare to a high-end conventional wall in terms of thermal insulation parameters.
We are not going to go into details about high-end conventional wall cost - it varies from city to city and there is an even greater variety between different countries and there are many specifics.
Let's discuss straw wall types as this is more our thing - Straw house can be built in many ways:
- Load-bearing or "Nebraska-style" technique - it is constructed with whole bales.
- Structural-frame (post and beam or double timber frame) walls which carry the load and are fitted with tensioned bales - also constructed with whole bales.
Both above methods require very long days building on-site which is spent less doing carpentry work and more straw-bale fitting and arrangement. It is important to compact straw around/between bales when aiming for a quality wall. That is a very labor-intensive and risky endeavor - bales must also be protected from rain at all times until the roof is up.
There is the modern way as well:
- Prefabricated Straw modules
The Straw wall is composed of individual load-bearing double-drame wall-panels which are produced in a factory and transported to be erected on site during a short window of stable weather. The walls can then be easily wrapped with weather membrane until the roof completion.
What would a straw wall cost? Let's try to break it down:
- the walls need structure - here we account for all the timber used
- the walls need insulation - that would be the straw
- the walls would be protected - from the inside with clay plaster and a façade
Looks like it should be dirt cheap to end up living inside some cozy straw walls when we look at the materials - some straw, earth clay and wood. So what's the deal?
As it turns out - building naturally requires much more effort than using conventional means. Achieving a good straw wall is a matter of attention to details - filling all the gaps and holes or rather - filling them to the same density (on-site, hmm...)?
What a straw house really costs is the energy and focus of the builders, their attention and consistency than the "house ingredients" themselves - Let's dig a little deeper.
We mentioned density and then consistency - these are key properties of the straw when we want the perfect wall. Straw must be compressed to restrict airflow and thus capture precious air:
Compress it less - it is susceptible to burning, hard to keep in place and not really insulating;
Compress it more - it becomes sturdy. So sturdy in fact that it would get closer to wood than straw characteristics and it loses rapidly its insulation properties with density increase. Such a wall will never catch on fire and will have excellent sound proofing but need more energy.
Hitting the sweet spot and press the straw just right is then quite an important operation - something really hard to achieve when using a whole straw-bale.
Straw bales vary in size and density - bales produced by the same baler and same field can be different when harvested from different regions of the field. It is not unheard of to find small objects and even animals inside the straw bale compressed by the baler.
We believe fitting whole bales is therefore diminishing the qualities of the wall as the straw-bale itself counters equal distribution to achieve uniform insulation.
Building with whole bales leaves no way to remove the dust and debris from the straw compacted on the field, not to mention the risk of sealing something hidden in the bale which can later cause issues while decomposing.
Naturally, we would recommend using Straw modules for your project and let us take care of all the fiddling so you can align the construction on-site according to the weather forecast. It only takes a couple or three days with a few people, having simple tools, to assemble and wrap the walls.
This way you need not worry for the process since:
- the frames dimensions will always be according to the plan (built with high-grade construction lumber)
- the straw we use is dry, unbaled and de-dusted
- the modules are filled with straw to the best compression - we're aiming at 100kg/m3
- the excess straw would be cut flat to facilitate transportation, fixing and consequent finishing
A straw-bale panel is made quicker and thus can turn out at a lower cost but it would not be as uniform and have other issues too. For instance - it is quite hard to plaster a whole straw-bale. Its straws are arranged by the baler leaving some parallel to the surface so the clay won't stick.
Our modules with unbaled straw lack order and the surface has plenty of straw-ends to stick clay to. No wire nets required - straight on the flatly cut straw surface.
Hopefully we can already see that driving costs on straw walls down would actually mean to drive labor cost down while maintaining a quality job.
Wall finishing plays an important factor too. Inside is always clay on straw. It can be quite costly to build up a thick clay plaster - it takes about 40-50kg/m2 for about 2cm layer. Clay plaster can be bought as a ready mix or sourced locally. Mixing your own clay plaster is a completely different story we may develop another time - it is a hard task to achieve a consistent result by mixing the plaster this way. Clays differ in contents and ratios which makes it difficult to hit the right proportions of sand and fiber. Applying the plaster to the wall is easy and it is also possible to use a machine to spray the clay on larger surfaces.
Outside can be finished with a ventilated façade (leaving an air gap between the siding and wall module) or lime plaster. We are reaching best results with an additional protection/insulation layer on the outside wall - on the straw panel, then a ventilation gap and siding to suit your taste.
That extra insulation layer is what helps to "heat" the straw module's outside surface and thus "pull" the condensation point outside the straw.
The siding itself can be compared to outside lime plastering in terms of price but we would advise ventilation even if it turns out a bit more costly.
Straw walls are not cheap to build - for all the listed reasons.
What Straw walls demand in attention - they pay back in dividends for years to come.
Roof on a straw house is also not much different from that of a conventional house. What a straw house needs is a reliable roofing that can withstand the worst weather.
The tiles you pick must be able to withstand hail and not crack! The roofing membrane is also a must to ensure that no cracked tiles let water drip down and make way into the straw. Straw houses can be healed in that case too but it may take a long time to discover the leak which could have already done damage.
Cost cannot be saved on this stage as the warm walls would only be warm if the roof is also well insulated.
As we can see - both a straw house and a conventional one require a solid foundation and well-built roof.
What we are left with are the walls. We are confident that our straw modules provide quality solutions to many issues faced on-site and can help drive project costs down by reducing installation time. This way you can plan the construction with shorter time-frames and enable more flexible schedules.
Consider the following - a house must have a roof, walls and foundation.
We would advise caution in selecting the right technology for you and building your dream straw house.
Would you save on rebar in your foundation or use thinner or less beans on your roof? I truly hope - not!
Then why not get the best possible outcome... only quicker?