Solar Dried Lumber Moisture Concerns – Solar Kiln

Solar kilns are gaining popularity among hobbyists and professional woodworkers seeking to dry their own lumber. With the high cost of dried wood at lumberyards, solar kilns offer an economical, eco-friendly alternative to air-drying, which can take months and result in warped or cracked wood.

In this article, we’ll cover topics such as:

• The Advantages of Solar Kilns
• Understanding Key Solar Kiln Design Features
• Are Solar Kilns Worth It?
• How Long Does it Take to Dry Wood in a Solar Kiln?
• Monitoring Moisture Levels in Solar-Dried Lumber
• Moisture Meters
• Optimal Moisture Content Levels for Various Applications
• The Benefits of Solar Kilns for Woodworkers

Greenwood, on the other hand, costs much less. However, air-drying lumber can take several months, leading to warping and cracking before it can be used. Drying wood in a controlled environment requires much less time and allows the wood to contract without warping.

The Advantages of Solar Kilns

A solar kiln dries green wood quickly and safely, minimizing defects and generating zero CO² emissions. This makes them an environmentally responsible choice for wood drying. Dr. Eugene Wengert, an extension forest products specialist at the University of Wisconsin-Madison, highlights their ease of use: simply load the wood, adjust the vents, and monitor the moisture content (MC) near the end of the drying process.

Solar-drying kilns are the simplest, cheapest, and safest way to dry green wood quickly, and they are good for the environment since they don’t generate CO² emissions. A solar kiln will dry the wood with a smaller chance of defects than air drying.

“And they won’t dry wood too fast,” says Dr. Eugene Wengert, an extension forest products specialist at the University of Wisconsin-Madison. “It’s pretty much a ‘set it and forget it’ system. You can just put the wood in there and almost forget it. It’s only near the end of the drying when you need to measure moisture content (MC) to determine if it’s time to stop the process and pull the wood out.”

Understanding Key Solar Kiln Design Features

Dr. Wengert developed a popular and simple solar kiln design in 1978. It is often referred to as the Virginia Tech Solar Kiln because Dr. Wengert developed it while working at Virginia Tech. Although there are different solar designs today, the Virginia Tech design is the kiln model considered the standard by which other kilns are built. Woodworkers can find instructions for this model and other similar designs online.

Are Solar Kilns Worth It?

The initial investment for a solar kiln depends on its size and building materials, but it typically pays for itself in the long run. Solar kilns come in various sizes, with small kilns drying 800-1,000 board feet and large kilns drying 1,500-2,000 or more board feet.

How Long Does it Take to Dry Wood in a Solar Kiln?

Drying wood in a solar kiln, like the Virginia Tech model, takes approximately one month in moderately sunny conditions in the mid-latitudes of the United States. The kiln absorbs sunlight through transparent glazing and heating the black-painted interior surfaces. Factors such as the size of the kiln, solar panel area, and roof angle play critical roles in the drying process.

Circulating air through the stack of wood requires an electric fan, and larger kilns may need multiple fans for optimal airflow.

As air is heated in the collector space, one or more fans circulate it through the lumber, enabling it to absorb moisture from the lumber’s surface. When the evaporated moisture increases the relative humidity to where it gets too high in the chamber, it releases it through vents in the back of the kiln.

To alter the size of a kiln, one needs to keep in mind an important ratio: For every ten board feet of capacity, one square foot of solar panel or roof area is needed. If the kiln has too much solar panel area, one risks the wood drying too fast, causing the wood to check and split.

However, if there’s not enough solar panel area, there’s the risk of never being able to dry the wood below 15% MC.

In order to circulate air through the stack of wood, it’s necessary to install an electric fan. If a larger kiln is built, two or more fans will be needed for optimum airflow.

Roof angle also is important. To figure out the proper angle, one needs to determine the latitude of the kiln. The latitude number is used for the pitch of the kiln roof.

For example, in Minnesota, the latitude is about 45 degrees north. In that case, the solar panel would be mounted at a 45-degree angle facing south. If used during winter, the angle should be increased by 10 degrees (in this case, to 55 degrees) since the sun’s angle changes from summer to winter.

Interested in building a solar kiln? Review the following magazine article about building a solar kiln.

Monitoring Moisture Levels in Solar-Dried Lumber

Solar drying time varies depending on factors like season and wood thickness. Woodworkers new to solar kilns or drying wood thicker than one inch should monitor MC daily to avoid drying too rapidly. Comparing daily moisture loss rates with safe rates for different wood species is essential to prevent defects in the lumber.

Some tables list the safe rates for drying 1- or 2-inch thick lumber from different species. The safe rate refers to the loss of moisture in one day. Safe rates must be followed until lumber MC drops to 20%.

Although moisture meters don’t work well when the MC is above 30%, the design of the kiln is such that it normally won’t allow the wood to dry too fast. Still, when just learning to dry lumber in a solar kiln, it’s best to monitor the process to avoid any unforeseen problems.

Exceeding the safe rate drying speed for a given species can cause defects in the lumber. If drying is too fast, part of the roof should be covered, the fans turned off, and the vents closed during the hottest part of the day. Although the temperature will likely rise inside the kiln, the trapped air will quickly reach 100% of its capacity to hold water, and the lumber will stop drying.

Identifying sample boards at the start is important to determine the MC loss rate. These boards may have started out with a higher MC than the rest of the load, were cut more recently than the others, or are a little thicker.

“If you check the sample pieces and they’re too wet, you’re going to continue drying,” says Dr. Wengert. “You want to make sure your sample pieces are dry enough before you stop drying. It doesn’t matter if half the load is ready to be pulled out.”

“So don’t pull the load based on the average. We usually pull the load based on the wettest pieces. The wettest has to be dry enough,” he adds.

Dr. Wengert notes that the Virginia Tech solar kiln was designed not to allow the wood to dry too fast. However, if the kiln is only half full and the lumber inside is prone to cracking, such as oak is, it would be necessary to cover half the collector on the roof. Otherwise, there would be too much solar input for the small amount of wood in the kiln.

Moisture Meters

Moisture meters for kiln driers should meet all the specific needs you might encounter. A pin meter measures the MC of wood by running an electrical signal between the tips of two narrow metal probes inserted into the wood.

The major disadvantages of pin meters are the relatively small area tested with each insertion, their sensitivity to wood temperature, and the damage the pins do to the wood surface. The pins can also break or give inaccurate readings when improperly inserted.

Pinless meters use an electromagnetic signal to penetrate the wood surface. Their design lets users “scan” wood quickly. Because they don’t have pins, they do not damage the wood surface. They take readings at a fixed depth but require sufficient pressure to give a correct reading.

“The best meter, however, is one that’s used,” says Dr. Wengert.

“If you’re drying wood and making something of it, how much would you pay for insurance that it will work right? A moisture meter will help you prevent it from cracking, warping, or splitting after you’ve made it. What’s that worth to you? Other than the initial investment, you only have to pay for the batteries,” he remarks.

Optimal Moisture Content Levels for Various Applications

According to Dr. Wengert, the optimal MC reading for softwoods used indoors is 10%, while hardwoods should have an MC of around 7%. Both softwoods and hardwoods should be dried to approximately 12% MC for outdoor use. However, the ideal MC may vary depending on factors like climate and wood exposure.

The Benefits of Solar Kilns for Woodworkers

Solar kilns offer numerous advantages for hobbyists and professional woodworkers, including cost savings, simplicity, and eco-friendliness. By monitoring moisture levels during drying, woodworkers can prevent defects in their solar-dried lumber and create high-quality wood products without warping, cracking, or splitting.

Although solar kilns are designed to keep the wood from drying too rapidly, it is wise to check the MC level, especially near the end of the drying cycle. It’s also recommended that woodworkers new to drying lumber in a solar kiln or are drying wood thicker than one inch frequently check the moisture levels to prevent defects later on.

A moisture meter is a woodworker’s best insurance for solar-dried lumber to prevent the finished wood product from cracking, warping, or splitting.

Learn more about Kiln dried wood and why moisture readings are important.

Troy Edwards

Troy Edwards is Technical Service Supervisor for Wagner Meters, Inc., where he oversees manufacturing, quality control and IT service for their electronic measurement products for the building and construction industry. Troy holds an AAS in Electronics Technology and has over 20 years’ experience in various electronic manufacturing and production positions.

Last updated on April 23rd, 2024