Principles for materials used indoors

The basic premise of the materials used in PAC housing is that they contribute to health, are safe, and are ecological. In this section, I will not mention the basic premise as a matter of course, but will consider it from a slightly different perspective.

Materials used indoors are specifically the materials used for floors, walls, and ceilings.

The common performance required of all these materials is moisture control, or the ability to absorb and desorb moisture.

In the humid climate of Japan, this is an absolutely essential element. In the olden days, this was achieved as a matter of course with earth and boards, but the mainstream of post-war house building has become predicated on materials that are completely upside down.

Nowadays, the main focus is on materials that do not have moisture control functions.

Plywood flooring, plasterboard and vinyl cloth, printed veneer ceilings, occasional painted walls, and fake ※Juraaku walls hardened with glue.

※Juraaku walls are a type of earthen wall. Earthen walls are made by kneading earth, straw, hemp, soot, sand and water. No non-natural materials are used.

They are flimsy materials with no texture. They have been used for the past 20 to 30 years under the illusion that they are cheap, shiny and clean, and look like a hotel.

Combined with these non-humidity absorbing and non-desorbing materials, insulation and airtightness, Japanese houses continue to suffer from moisture damage such as condensation, mite mold, and rot. These measures against moisture require more fundamental measures such as floor plans and construction methods, but at the same time, the materials used in the room cannot be avoided. For example, no matter how well insulated a pair of sashes are, it does not mean that condensation on the window surface can be prevented. If there is a large amount of water vapor in the air inside the room, there is inevitably a large possibility of condensation on the glass surface. In order to prevent this condensation on the window glass, it is necessary not only to improve the insulation performance but also to reduce the amount of water vapor in the indoor air.

The most common countermeasures so far have been a parade of mechanical devices such as exhaust fans, dehumidifiers, air conditioners, and futon dryers.

These days, we are increasingly relying on machines, such as 24-hour mechanical ventilation.

The starting point of PAC housing is to do as much as we can with architectural methods before immediately relying on machines.                          In reality, if you use materials with excellent humidity control for ceilings, walls, floors, etc., condensation on window surfaces will be greatly reduced.

Let’s take a look at what kind of materials are suitable for this purpose.

First, it is wood. It must be solid and not treated with paint.

It is more desirable if it is domestic the wood. Glued laminated wood will not perform well, and chemically applied coatings such as urethane will be completely useless.                          

If you are going to paint it, only apply a thin coat of natural oil or wax so as not to interfere with the breathing of the wood. Essentially, solid wood that does nothing is best.
For ceiling panels and wall panels, unpainted is best.

The next theme is ※NURIKABE.

※ NURIKABE is a plaster or mortar covered wall.  

Recently, diatomaceous earth has become a popular material, but unfortunately, most of it is hardened with chemical adhesives.
Still, there are materials for NURIKABE that use only natural materials, such as real plaster.

In addition, recently, thankfully, materials that are safe and have moisture control functions have emerged in industrialized products.

One traditional material is silica board, which has a slightly lower moisture control function than cedar board.

Cement-based boards with greater moisture absorption and desorption properties and new gypsum boards have been released one after another.

With them as a base, moisture permeable cloths and paints that do not block the passage of moisture are also available on the market. Of course, as a safe material.

With such a combination, it became possible to construct floors, walls, and ceilings.

Lastly, however, I would like to mention some concerns about moisture-absorbent materials.

In order to use these materials, there is a premise that must be followed. They should be used in places with good ventilation and sunshine.

As for moisture absorption and desorption, the function of absorbing moisture is only good if there is a suitable material for it, but ventilation and heat are needed to expel the moisture once it has been absorbed.

In a confined, poorly ventilated place, or a place where the sun does not shine, the humidity regulating property will only last for a moment, and will not be able to function for a long time. Such an environment is the reason for the unfortunate story of mold growing on charcoal placed under the floor.

Yoshiaki Tanaka
From the book, “The Real House I Finally Met


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