Madeira seca em estufa – Por que as leituras de umidade são importantes

Freshly sawn lumber is “green,” meaning it hasn’t been dried. Green lumber is very wet, and once its moisture content (MC) drops below the fiber saturation point, it shrinks with every percent of moisture it loses and can warp, no matter how it’s dried.

• A secagem em estufa não é permanente
• Secando sua própria madeira em estufa
• Leituras de umidade são importantes
• Dois tipos de medidores de umidade
• Como secar madeira em estufa

When lumber companies kiln dry their wood, they control the temperature and humidity carefully to avoid over-drying or under-drying. That lets the wood dry to the correct MC for its intended use, so it stays stable once it’s in service.

Under-dried wood keeps shrinking after it’s put to use, and over-dried wood swells as it picks up moisture. Either way, it can warp and won’t have the correct MC for its intended use.

Kiln-dried wood is dried to a specific MC range depending on its final use. According to Virginia Tech, lumber for interior uses, such as furniture, should be dried to 6% to 8% MC, lumber for exterior uses is usually dried to 12% to 15%, and softwood framing lumber is usually dried to 19%.

A secagem em estufa não é permanente

Algumas pessoas acreditam erroneamente que a secagem em estufa é permanente e que uma pilha de madeira com o selo "certificado como seco em estufa" sempre estará boa para ser usada como está.

Wood is hygroscopic, which means it absorbs and releases moisture. The moisture might come from a direct source such as rain, from an adjacent material, or from humidity in the air.

That applies to any wood, including kiln-dried wood.

So when woodworkers buy kiln-dried wood, all they can be sure of is that the lumber went through a kiln. They have no assurance that it was dried properly without drying stresses, that it was dried to the MC they need, or that it hasn’t regained moisture since.

This last point is very important to understand. Once kiln-dried wood leaves the lumber company, its MC can change at any time: during shipping, during manufacturing, or while it’s stored in a woodworker’s garage or shed.

Secando sua própria madeira em estufa

Kiln-dried lumber can be expensive, so some woodworkers dry their own wood, which also lets them control its final MC.

One option is to air dry green wood. But air drying can take several months, and in most places the humidity keeps air-dried lumber from reaching the low MC needed for interior use without the extra step of secagem em estufa.

Another option is a backyard solar kiln. Solar kilns are relatively inexpensive to build and easy to operate, and Virginia Tech’s design can dry a load of 1-inch lumber in about a month of moderately sunny weather.

Dr. Gene Wengert of The Wood Doctor’s Rx, LLC, a former extension wood products specialist at the University of Wisconsin-Madison, developed a popular solar kiln design while at Virginia Tech. In his words, “Solar kilns won’t dry wood too fast. It’s pretty much a ‘set it and forget it’ system.”

“É somente perto do final da secagem que você precisa medir o MC para determinar se é hora de interromper o processo e retirar a madeira”, acrescenta o Dr. Wengert.

Leituras de umidade são importantes

When you’re first learning to dry lumber, especially lumber thicker than 1 inch, or drying a new species, monitor the MC daily to make sure the wood isn’t drying too fast.

Virginia Tech recommends doing this with sample boards: short pieces cut from the load that you weigh each day to calculate their MC. Compare each day’s moisture loss with the “safe drying rate” for that species, which is the maximum MC loss in one day. Electric moisture meters aren’t accurate above about 30% MC, so the sample board method is the way to go early in drying. Once the wood drops below 30% MC, a moisture meter is a fast way to check it.

Safe drying rates for 1- and 2-inch hardwood lumber are listed in tables, such as the one in Virginia Tech’s solar kiln publication. Exceeding the safe rate can cause cracks, checks, and other defects. The risk is highest early in drying, and once lumber is below about 22% MC, the risk of new cracks and splits is low. If the wood is drying too fast, you can cover part of the kiln’s solar collector to slow it down.

Dr. Wengert recommends choosing sample boards at the start that will dry slowest, such as boards that started out wetter than the rest of the load, were cut more recently, or are a little thicker.

"Se você verificar as amostras e elas estiverem muito úmidas, você continuará secando", diz o Dr. Wengert. "Você precisa ter certeza de que as amostras estão secas o suficiente antes de parar de secar. Não importa se metade da carga está pronta para ser retirada.

"Portanto, não puxe a carga com base na média. Geralmente puxamos a carga com base nas peças mais úmidas. As mais úmidas precisam estar suficientemente secas", diz ele.

Dois tipos de medidores de umidade

Woodworkers have a choice when it comes to selecting a moisture meter. They can choose a pin meter or a pinless meter, and each has advantages and disadvantages.

One advantage of pin meters (only those that use longer pins with insulated shafts) is that wood can be tested for moisture at different depths. The main disadvantages are the small area tested with each insertion, the need to correct readings for wood temperature, and the holes the pins leave in the wood. Pins can also break or give inaccurate readings when they’re inserted improperly.

Pinless meters, on the other hand, don’t penetrate the wood’s surface, so they won’t damage the wood. They also let you scan many board feet of wood quickly.

Pinless meters read an average MC over a set depth, typically up to about ¾ inch, and over a larger area than pin meters. They do need to be held flat against the wood with firm pressure to give a correct reading.

Orion dos medidores Wagner^® pinless meters use IntelliSense™ technology, which helps keep surface moisture from skewing readings.

O Órion 930, 940 e 950 have two depth modes: a shallow mode that reads ¼ inch deep and a deep mode that reads ¾ inch deep.

Orion meters have settings for a wide range of species, from common softwoods and hardwoods to tropical woods.

Como secar madeira em estufa

Several resources provide plans for building a backyard kiln and go into more detail about kiln operation. Here are some to consider:

First is the design mentioned above, developed by Dr. Wengert. It’s often called the “Virginia Tech solar kiln” because he developed it while working at Virginia Tech, and many other solar kilns are based on it.

Dr. Brian Bond, Professor and Extension Specialist at Virginia Tech, has since published an updated version adapted from Dr. Wengert’s work, Design and Operation of a Solar-Heated Dry Kiln. It’s free and covers a kiln that holds 750 to 1,000 board feet, with a materials list, operating instructions, and safe drying rates for common hardwoods.

Virginia Tech has also offered hands-on workshops on drying lumber in a solar kiln in Blacksburg, Va. Check with the university for current dates.

MADEIRA magazine also published an article on building a solar kiln, “WOOD Magazine Builds a Solar Kiln”(PDF).

Woodworkers can choose from several types of kilns, but the resources above are for kilns that are inexpensive to build and simple to operate.

Resumo

For hobbyists and professional woodworkers who want to avoid the cost of commercially kiln-dried lumber, a forno solar caseiro pode ser a melhor opção.

Solar kilns are relatively inexpensive to build and easy to operate, and beyond the electricity to run the fans, they cost little to run.

To make sure a solar kiln doesn’t dry wood too fast, which can cause costly defects, track the MC regularly with sample boards and a medidor de umidade. That helps keep the finished wood from cracking, warping, or splitting.

Saiba mais sobre o secagem em estufa and what happens when it goes wrong.

Compre medidores de umidade Orion

Última atualização em 8 de outubro de 2026