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Building with Wood chip and Light-clay Infill Systems

An Introduction

by Frank Andresen

From JQ#35, Fall 1997. Copyright © Fox Maple Press, Inc. All rights reserved

Exterior woodchip/clay projectClay is an excellent building material. Found in most places in the world, clay is the result of a slow efflorescence of feldspar, quartz and mica. It's affordable and recyclable, an excellent heat absorber, and regulates indoor temperature variations. Mixed with fibers, it provides insulation. The clay preserves and protects the fibers from insects, mice, fire, and absorbs odors, while lending itself to architectural creativity and flexibility.

One of the most recent innovations in the development of modern German clay-building techniques is the use of wood chips, bound together with clay as an insulation infill for exterior and interior walls. Since its introduction during the latter part of the 1980's, contractors are now able to offer another naturally healthy, monolithic clay infill system. Wood-chip light-clay is comparable to straw-light clay in terms of its excellent physical qualities, but the production and manufacturing process is much easier and quicker, with less drying time, settling, and tamping. It's a more efficient and less labor intensive construction method.

By using an ordinary cement mixer, wood chips in different sizes are mixed together with a clay slip (other machinery may be required to produce "clay slip"). The size of the chips vary between rough sawdust and chunks up to 2" in diameter, depending on the chipper. The chips can be dry or green, but they should be bark-free, especially when poured into thick wall sections. Generally, the mixture is 3-4 buckets of woodchips to one bucket of clay; depending on the required strength & weight, the quality of the clay, and the size of the chips. Let it blend in the mixer for a minute or two, until all the chips are coated with "clay slip", then it is ready to pour into the wall forms. Just spread the mix into place with a board or stick to fill all voids--there's hardly any tamping involved.

Depending on the system you're using, the weight ranges from 1000-1500 lbs/cubic yard of dry mix. The insulative value for a 12" exterior wall with plaster can be up to R-25, depending on the quality of the woodchips, clay, and the density with which the mixture is packed. Vapor barriers are unnecessary bacause of clay's inherent property as a diffusing water reservoir and the capacity for moisture dispersal.

A variety of forming systems and materials can be used. When using reed mats as an infrastructure, studs should be no wider than 12-16 inches on-center, depending on the quality of the reeds. By using wooden laths or bamboo as a light framework to cage the infill, these distances can be nearly doubled. Because the infill is anchored when using laths or matting, the whole system is stiffer, stronger, and shrinkage is virtually nil. Reed mats can be applied quickly with a staple gun by connecting the wiring to the framework. Wooden laths take more time to install, however, they may be more available in certain areas and in the end, they work just as well. Slip forms can also be used to form walls that require no mats or lathing to act as a cage. In this system, vertical reinforcing, such as saplings or 2 by material, should be placed in the middle of the wall to add lateral strength. Many types of plaster can then be applied easily to any of these systems. Exposed beams should be covered.

A 12" thick exterior wall requires up to 8 weeks of drying time during a normal summer here in southern Maine. Thicker walls can be used if there is a sufficient drying season and the additional insulation value is required. It is possible to make bricks, blocks or panels with clay and fiber which can be used in a pre-dried state. Using these dry materials, the range of construction techniques is even greater and the building season can be extended. Hollow wall systems can be laid up and filled with cellulose fiber and unfired bricks can be used on the interior to create thermal-mass. When using wet systems, everything must be dry by the first frost, so don't start too late or too early--work with the seasons. Prepare everything carefully and make it hassle-free.

Wood chip /clay sample Clay construction must be properly protected against the weather influences with appropriate construction details. Large overhangs are helpful, lime plaster will protect the clay on exterior walls, and also, paint, clapboards or shingles can be used. Every precaution must be taken to insure that the clay is protected from the elements.

Get advice, when necessary, involve friends and family...then besides building a healthy home, you can save money, too! Working with clay is labor intensive and requires patience, but you will be using materials from your own property, like clay, wood, stone, straw, reed and other fibers, instead of forking your money over to the bank or industry in return for artificial, potentially toxic, (not to mention expensive) products.

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