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No. 69, Winter 2016

In Defense of Straw

By Kyle HolzhueterApril 25, 2017May 9th, 2022No Comments
by Kyle Holzhueter

There are many challenges to ecological building in Japan. For example, building codes and officials limiting the use of some natural materials and architects and clients primarily concerned with appearances without an understanding of the materials. But as ecological building, or low carbon building, continues to grow in Japan, a new challenge has been encountered: the traditional craftsman.

In Japan, most traditional artisans using natural building materials did not pick up their craft due of ecological sympathies. A love of tradition and beauty and maintaining a family business are probably the primary reasons a craftsperson takes up their trade. Being rather attached to centuries of tradition, it is understandable there would be some resistance to change.

It is interesting to note that ecological building grows at a much faster pace in South Korea, where much of its traditional building culture was lost due to pre-WWII Japanese occupation, the Korean War, and then rushed economic development.

Structure of a traditional bamboo lattice and earthen plaster wall finished with shikkui lime plaster.

Structure of a traditional bamboo lattice and earthen plaster wall finished with shikkui lime plaster.

Traditional Japanese timber framed homes did not incorporate insulation. In fact, the first residential thermal efficiency (insulation) requirements will go into effect in 2020.

As straw bale, light straw clay and other ecological building methods continue to grow in popularity, fearing the loss of their own tradition, craftsmen in Japan are quick to assume these new building methods are unsuitable for Japan and will fail.

Through these interactions, I’ve noticed that as a simple survival mechanism, we draw conclusions based on our observations of the world, like, the sun orbits the earth, the earth is flat, and straw molds. But if we change our perspective or the position of the object we’re observing, sometimes we see a very different reality.

The issue in question here is moisture and the durability of buildings, and specifically the decomposition of rice straw. But these questions are also related to the hygrothermal environment of homes, both interior and interstitial (in the wall), heating, insulation and our general desire to modify our environment using things like clothes, buildings and campfires.

I lived in Japan for many winters with no supplemental heating other than hot tea. In my 20’s, I loved it! In the morning, I knew exactly what temperature it was outside, because it was the same temperature inside. It was very similar to camping and I felt very close to nature. Unfortunately, I’ve become soft and weak, and my family has forced me to address the issue if indoor thermal comfort. The thermal tolerance of most of my peers is relatively narrow compared to our grandparents, and culturally we now demand a completely modified environment.

To achieve a modified interior temperature, we can use supplemental heat and measures to prevent that heat from escaping. I think in many areas of Japan, completely or mostly passive homes are possible. Requirements: sufficient solar gain in winter, insulation, thermal mass and diurnal temperature swings for cooling in summer. Not possible everywhere but definitely possible some places.

When relying on supplemental heat, the options are many. The principle method in probably 99% of Japanese buildings is electric. Not a good idea. In addition to a passive solar base, active solar, heat pumps, geothermal (in hot-spring areas), biomass and good old wood burning stoves are good options.

One strategy traditionally used in Japan is to rely completely on supplemental heat and ignore heat escape. If we’re talking about natural hot springs, ok. If we’re talking about electric air conditioners or dreaming about 100 million Japanese relying on firewood, I think we’re kidding ourselves. We need to urgently expand the responsible use of Japan’s biomass, but this isn’t going to be the sole solution for the entire population.

A beautiful timber frame structure with traditional bamboo lattice and earthen plaster walls

A beautiful timber frame structure with traditional bamboo lattice and earthen plaster walls

Take, for example, a beautiful timber frame structure with traditional bamboo lattice and earthen plaster walls. The building is kept comfortable in winter with three wood burning stoves running full time. Although incredibly beautiful and representing centuries of tradition, we must consider in what measure these buildings are ecological and sustainable.

So if homes require insulation and thermal mass, in what measure and what options are there? Insulation requirements will depend on the local climate. Inorganic materials are less susceptible to decay, but less plentiful and a limited resource by nature. What organic options are there? I think straw is an excellent option: thermal conductance similar to fiberglass insulation; low embodied energy; renewable. So what’s the catch? moisture!

Moisture IS the number one factor in the durability of buildings. When we see a home with moisture damage, where is the damage located?

  1. Base of posts
  2. Leaking roof
  3. Water intrusion around windows
  4. Sinks and baths

….just to name a few.

Building science talks about four sources of moisture:

  • Exterior (rain, snow, fog, humidity)
  • Ground
  • Interior (from cooking, showers, etc.)
  • Embodied (like the water in concrete or moisture in green wood)

(It’s been a while since I thought about this. A better explanation is most definitely in my PhD thesis:・・・)

Of these four, exterior moisture, namely rain, is the greatest threat to building durability.

Because of the relatively high absorption rates of earth and lime plasters, I prefer not to use these as the final exterior finish on straw bale walls. My research has found ventilated rain screens are an excellent strategy to protect organic insulation (straw) from exterior moisture.

The following image provides a basic section of a ventilated rain screen.

Siding over bale walls

Siding over bale walls (courtesy Jeff Ruppert)

As a slight amendment to the image, I would recommend two coats of earth plaster and/or possibly some other air barrier fin behind the nailers.

Finishing exterior strawbale and light straw clay walls with earth and lime plasters is the simplest and least expensive option, but I think the suitability is a question of microclimate, wind driven rain, and opportunity to dry. There is also an issue of earth plaster and water resistance.

Most exposed exterior walls finished with clay in Japan will need to be re-plastered every 5-10 years. Depending on exposure, shikkui lime plaster over earth will last 20 or more years. Of course this can be much longer with less exposure. The most populated areas in Japan have over 2000mm rain per year and the intensity of rainfall is also increasing. And when walls, clothes and futons do experience wetting, there is relatively little opportunity to dry, which I think helps explain the Japanese love of umbrellas. When dealing with traditional Japanese earthen walls, I’m not so concerned about water absorption and erosion. When working with walls insulated with organic materials in Japan, without having the benefit of the doubt, I have to recommend deep eaves or ventilated rain screens. I think this would be different in a more forgiving climate. Lastly, my research has found that even when a straw bale wall may look dry, because of the hygroscopic nature of straw, it can still contain an unbelievable amount of moisture. Walls finished with lime can be more deceiving and require much more time to dry.

High performance straw bale and cellulose insulation with exterior rain screen

High performance straw bale and cellulose insulation with exterior rain screen

In dealing with humidity and vapor drive, it is important to seal insulated walls with a permeable air barrier to prevent gross air leaks. In this case, moisture and other gases can still travel through diffusion. The greater the interior and exterior pressure differential (often understood as a temperature difference), the more attention to detail is required.

After exterior moisture (rain), ground moisture is the second greatest threat to building durability. How did our grandfathers protect the foundations of their homes from frost heave? The foundation of foundations is that footings rest below the frost line. And to ensure there is no moisture to freeze, in cold climates, perimeter drainage is generally used to carry moisture away from buildings. More so than a post and beam structure on a pier foundation, frost heave is a dire threat to load bearing masonry walls. Given the severity, building codes and common sense require perimeter drainage around footings.

Interesting to note that our great-grandfathers used basically the same system to turn much of the US into arable land. Also note that the primary crops (potatoes, corn and wheat) require good drainage.

In Japan, the close proximity of homes, rice paddies and other water elements in Japanese gardens suggest a very different symbiosis of moisture and buildings. And as beautiful as those images are in Japan, we have the understanding and ability to greatly reduce the ground moisture that enters our homes and the damage it causes to our buildings.

Tea house built over pond

Tea house built over pond

In conclusion, I’m fairly certain that with proper planning, detailing and construction, straw can successfully be used as an insulation material in Japan. Likewise, I’m also certain 16th century Europeans thought Galileo was nuts.

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