1. What are the advantages of winter felled wood over summer felled?
In summer the Tree is in a Vegetative state and is working to grow its Annual growth ring and is saturated with Sap. At this point the moisture content is at its highest. As Autumn Leaves fall from the Deciduous species, the Coniferous trees also begin there cycle, but do not shed their needles, except for the Tamarack. This cycle is the preparation for the Winter Dormant State. The excess Sap is returned to the Earth. At this point the tree has its lowest moisture content and only enough is kept in the Root System to start the cycle of growth again in Spring.
When wood dries it shrinks, more Sap = more Radical shrinkage occurrence. When trees are made into Logs or Large cross section Timbers, the outer part of the Timber begins to dry while the inner heart wood is not. This creates a large amount of force on the outside part of the wood and begins to tear away from itself and creates a Crack for the inner part to begin drying. The higher the moisture content, the more active this cracking becomes, therefore by felling in Winter during the dormant state it produces the least amount of cracking and the Summer the most.
Cracking width and depth is directly proportionate to the season it has been felled in. In Horizontal Log construction this affects the log`s R value because Logs dissipate thermal energy in a deflective pattern and when the log mass is interrupted by a huge crack its ability to deflect becomes lower, therefore lowering its R-value, creates a place for water to infiltrate and sit causing pre-mature Rot decay.
In Timber Framing all of the connecting Joints are a derivative of the Mortise and Tenon and therefore rely on the integrity of the Tenon to carry and hold the timber in its joint. When the Timber is used while it is still Green (not dry), as it dries the wood shrinks and the joints begin to loosen as the Mortise moves away from the shoulder of the notch at every connection. If the timbers have been felled in Summer it will crack up ½” wide and to the heart of the timber. This now becomes a Structural concern.
2. Why do you use air dried Timbers or Logs instead of Kiln Dried Timbers or Logs?
The great builders of old always air dried their timbers before use. Some would even select the trees standing and cut a kerf at the base of the tree around the circumference to a depth of 4”-5”. The tree would die and dry standing and would be harvested at a later pre-determined time. Air drying is a more natural way of drying timbers and logs.
The most common kiln, today in North America are geared for drying lumber up to 2” thick and this type of process is not adequate for drying large timbers and logs. When this type of kiln is used, the timbers are usually dried to a moisture content of 19% at the end of a moisture meter probe which usually is 1 - 1.25” long. So on an 8”x 8” timber for example this means that the outside of the timber is at 19% and the 6” x 6 “core is still dead green. Also forced drying causes a large amount of cracking. Air dried timbers are usually at 12% +/- throughout the timber.
In the past few years there has been development in Radio Wave Frequency Vacuum drying of timbers. This a very costly way of drying timbers to a point of being prohibitive .Therefore there exists only a few in all of North America, combined with the cost of transportation it is not a viable way of procedure.