Anybody who has milled lumber before likely understands the importance and difficulty of properly drying it for finish carpentry use. We have had several projects in our past that we thought were dry enough to build with, but after time, they continued to shrink and warp. Utilizing and understanding proper wood moisture reading instruments and wood kiln systems is a very important step to make trees into lumber. We will discuss the types of wood kilns, why we chose the one we did, and the process to build it.

Types of Kilns: There are many different ways to dry lumber from very basic air dryers to very complicated kiln systems. Air drying lumber either outside or inside a building can take substantial amounts of time to dry, typically about 1 year per inch thick for many hardwoods. This can be sped up a bit by using fans or forced air through the stickered lumber stacks. But even when the wood is almost completely dry, it is important to control the exact humidity and temperature of the environment the wood is in for the couple of weeks prior to using the lumber in order to ensure the wood has acclimated to the equilibrium moisture content for the location it will be installed. The best way to do this is utilizing a kiln drying method. There are 4 main types of kilns that can properly dry lumber including solar kilns, convection heated kilns, vacuum kilns, and dehumidification kilns.

Solar kilns are a more budget-friendly kiln system that utilizes solar heat to help evaporate the water from the lumber. They typically consist of a basic structure with a roof/wall made of Lexan sheets that capture and insulate solar heat. They also use a venting system to purge the air inside the kiln if it is either too hot or too humid. A lot of solar kilns also have an air circulation system on the inside using fans and air baffles to move air between the stickered lumber. The biggest issue with a solar kiln is that it is weather-dependent, which makes drying times unpredictable. In the summer, the lumber could dry too quickly, which can degrade the lumber and cause drying issues. In the winter, it could take excessive drying times due to limited sunlight and snow accumulation. Overall, solar is okay if you have small amounts of lumber to dry, but they are not accurate for continuous lumber drying, especially if you are in a climate zone that changes rapidly.
Convection kilns utilize heat typically from hydronic coils or a furnace in order to extract moisture out of the lumber. The kiln chamber is cycled by heating up to a certain temperature, which evaporates the moisture from the wood, and then the moist, hot air is exhausted out of the kiln and replaced with new ambient air. This cycle repeats itself until the lumber has reached an appropriate moisture content. These kiln systems are controlled accurately, which can result in high-quality lumber, but the downfall is they are not very efficient because the air in the kiln has to be repeatedly heated and exhausted, which results in high operating costs.
Vacuum kilns are another type of kiln, but instead of controlling temperature like convectional kilns, they control the pressure inside the kiln chamber. The reason for this is because the temperature at which water boils/evaporates depends on the pressure, and by decreasing the pressure, that boiling temperature also decreases. Vacuum kilns utilize this relationship to dry lumber at very fast rates by using a sealed chamber, lowering the pressure, and using heat to extract the moisture from the lumber. The downfall to vacuum kilns is that they require special chambers that are completely sealed, and this usually limits the size of the kiln and therefore the amount of lumber you can dry in each load. The other disadvantage is the initial cost and operating cost for these systems is extremely expensive compared to other kilns, but if you are looking to turnover kiln-dried lumber in a short time frame, the vacuum kiln is the best option.
The fourth type of kiln is a dehumidification kiln, which controls both the temperature and humidity within a kiln chamber. The system consists of a dehumidifier, auxiliary heat of some sort (electric, gas, hydronic, etc.), and a fan circulation system. These kilns are a closed system that cycles the air in the chamber to dry and heat it at the same time, and this requires very little maintenance and is a very efficient system, which makes it cost-effective. If the air in the kiln does become too hot or humid, the air can be exhausted out and replaced with ambient air. After researching all the different types of kilns, the dehumidification kiln had the most reasonable startup cost with relatively low operating costs and offered consistent control of the drying process.

The size of the kiln affects the price substantially and depends mainly on the size and quantity of the lumber being produced. Making a kiln larger than what is needed leads to excessive operating costs. The majority of the lumber we mill is about 8’-12’ long, so we planned on building it with a length of around 14’. The height and width typically are around 7’-8’ but can be increased if larger lumber loads need to be dried, but it's also important to make sure the kiln system kits or equipment you are looking at can handle the kiln size.
The location of the kiln is important because once it is built, it is not moving! It’s important to ensure there is plenty of room around the kiln’s doors to maneuver the loads of lumber. If you are looking to dry large quantities of lumber, installing a track or rail system will increase efficiency by allowing mills to stage lumber loads and quickly load and unload the kilns, but they often require a separate inlet and outlet for the kiln. The climate you are located in should also affect where you place your kiln. Warmer climates may want to avoid direct sunlight to prevent overheating. Whereas colder climates may want more direct sunlight to help with heating but avoid wide-open areas as wind can drastically increase heat loss from the kiln. Wisconsin’s climate is VERY unpredictable with hot summers and cold winters, so we experienced both issues with our kiln. We decided to utilize part of an existing barn to build our kiln into to save on material cost and space. We have noticed most wood kilns are standalone structures, and if that’s the case for you, I would say the most important aspect to account for is plenty of space for maneuverability .
The design of a dehumidification kiln is quite simple. The three main components are fans for air circulation, a compressor for dehumidification, and a heater for temperature control. A central controller will then operate these components as needed to dry the wood by using dry bulb and wet bulb sensors. There are other additional features that can be added as well, such as spray humidifiers and vents, which help prevent denaturing the wood by controlling the environment inside the kiln if they reach extreme temperatures or humidities but are not necessarily required. The layout of these inside the kiln revolves around the baffling system, which is just a fancy term for tarps or panels that direct the air through the stickered lumber. Nyle offers many great kiln options from complete commercial kilns for high output to small DIY kiln components for hobbyists. We went with the Nyle L53 unit due to the size of the kiln we were planning on building, which was limited by the existing structure we were working with. If we had built a new structure, we would likely have gone with the L200 for slightly larger capacity and drying capabilities. The L53 kit came complete with the compressor, heater, and fans in one unit and a controller, which is mounted outside the kiln in an enclosed space. It also came with an intake vent, exhausting vent fan, and a dry bulb and wet bulb sensor to monitor the environment inside the kiln. Nyle also offers diagrams on how to build your kiln to the correct specs and exact placement of all the components. The only other main component to a wood kiln system is to monitor the moisture inside the wood. We utilized Delmhorst’s Kil-mo-trol system, which utilized a moisture reader and a system of banana plug wires and pins to monitor the moisture of the surface and core of the lumber. Delmhorst has great customer service that will walk you through how it works and make sure you have all the correct components for your kiln.
The actual construction of the kiln will be different for each person depending on location and climate. We decided to design our kiln in a unique way because there was a support beam in the middle of the barn overhang, so on the left side would be a solar kiln to pre-dry the lumber, and the right side of the beam would be a dehumidification kiln. But we wanted to add a door between the kilns that we could open to create a large dehumidification kiln, which we refer to as MegaKiln. We started construction by demoing the existing cracked concrete so that we could put foam beneath the new slab to properly insulate it and also ensure the base of our kiln was perfectly flat when we poured the new slab. Because it was an old barn, we also had to redo some beams and joists to ensure the framing was square, level, and plumb. Studding up the walls went fairly quick and then came insulation. This is probably the most important part of the kiln because it can drastically impact how well your kiln performs and how much it costs to operate. Nyle recommends a minimum of R-30, which we found 3” foam panels from a commercial cooler online, and then we added an additional 2” pink foam sheet to reach our R-30 value. The hardest part about building our kiln was making sure the doors that swing out, had an R-30 rating, and spanned the 14’ opening for the dehumidification kiln. The doors were 7’x7’ and 6” thick and incredibly heavy, so we used 6 industrial-grade hinges for each side, and they have been holding up very well. It has been about 18 months since we built it. The greenhouse doors were built from polycarbonate greenhouse panels and treated 2x4 framing. There was an existing sliding door on the barn, which we just placed 1.5” insulation on the inside of and slid it over the solar kiln at night to prevent heat loss. All the interior walls were wrapped in 1/2” plywood and sealed with a reflective aluminum paint typically used for metal roofs, and this acts like a moisture barrier. For the exterior we used corrugated metal siding to match the rest of the barn. The last piece of the puzzle was to install the Nyle Kiln system, which was very simple as the factory provides all the necessary components, as well as a clear step-by-step installment instruction manual. We built the air baffles per their spec and used a reinforced poly tarp as an adjustable air baffle to conform to the load of lumber. Our kiln butted up against our woodshop, which is where we conveniently located the controller. The only component we did not install on our kiln was the intake vent and exhaust fan vent, which we felt our cold climate did not require and figured we could always install it later on. Our YouTube channel has a video series which shows how we built our kiln in detail; check it out if additional direction is needed!

It has been about 18 months since we built our kiln, and we broke it in by drying wood for one of our biggest projects so far, the Silo renovation. We dried out all the timbers for the roof and entryway. The timbers are very tricky to monitor and dry because of their thickness. Proper stickering and airflow are a must to efficiently dry any lumber, so we ended up installing some additional fans to increase the air circulation. We haven’t had any major issues with the kiln so far. The solar kiln is great for staging the next load of lumber and also pre-drying it out before it goes into the dehumidification kiln. We have had some issues with moisture build-up in the solar kiln, which we installed a humidistat-controlled exhaust fan to help regulate, but in the winter, the residual heat from the dehumidification kiln causes a lot of condensation along the walls and floors, which we are working on a solution for still. Let us know your thoughts or if you have any specific quotations on a kiln you are working on; we are happy to share our experiences, both good and bad!

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