Introduction: Solar Kiln - With Solar Panel and Revised for Correct Air Circulation

First, kudos to Dr. Brian Bond, students, and staff at Virginia Tech (VT) Department of Wood Science and Forest Products for developing a solar kiln and providing the well-written plans at:

I, of course, modified the VT plans. The solar kiln is basically a box with a greenhouse roof that generates hot air with a internal solar collector. The hot air is blown through the wood with two (2) fans. A load of wood should take approximately one (1) month to dry.

Revision for correct air circulation:  I had initially installed the fans on the outside of the kiln, blowing cool, outside air into the kiln.  After visiting the sawmill which had two (2) solar kilns and rereading the VT plans, I realized that the fans should be installed inside the kiln to recirculate the heated air within the kiln.  Blowing in cool air would lower the kiln temperature too much and reduce the drying effectiveness.  I constructed and installed an interior baffle with the fans mounted on the baffle and cut holes in the solar collector to allow for recirculation.  It would have been easier to construct the baffle prior to installing the greenhouse roof but live and learn. Refer to Step 9 for baffle installation.

The kiln is also made completely solar with the addition of a photovoltaic solar panel shown in Step 10.

The wood is being dried to build an 18 foot Grand Banks dory which will used to haul picnic supplies and picnic princesses to islands offf the coast of Maine. Google the Maine Island Trail Association to get a sense of the place. Building a kiln before building the boat in order to haul picnic supplies is my typical, over-zealous approach.

Thanks to all who voted for my Instructable in the Green Contest.

List of materials:
Enough wood to build structure - I used a combination of new and salvaged materials so don't have a list; the longer pieces and treated lumber were store bought
plywood - 3/4 inch thick, exterior grade, 2 sheets
Polygal and brackets - 10 ft x 6 ft piece; cut in half
flashing - aluminum
styrofoam insulation - 2 in thick, 2 sheets
Reflectix bubble wrap insulation
DC fans - 2, 16 inch with ring frame
GRK screws - exterior grade; various lengths
paint - green for solar collector
oven thermometer
photovoltaic solar panel - 65 watts

Step 1: Building the Base or Floor

The first question when building anything is where are you going to position it.  A solar structure needs as much direct sun as possible and your neighbor may not be so keen on you cutting down their trees, even for a "green" project.  I also positioned it uphill and adjacent to our workshop/garage to allow future duct work from the kiln to heat the workshop when not drying wood or other items.  The position is a compromise between these functions.  

The "foundation" is limestone blocks on the uphill side and a treated 4 inch by 6 inch posts on the downhill side.  I set the floor level so that future duct work would intersect the adjacent workshop without hitting any wall supports.

Also my relatives on Fogo Island off the north coast of Newfoundland, had a sport of moving houses.  Refer to the excellent book titled "Tilting - house launching, slide hauling, potato trenching, and other tales from a Newfoundland fishing village" by Robert Mellin for more information on this "sport. "  I constructed the kiln so that it could be moved, if needed.

The next question is size.  The VT plans use a base with dimensions of 160 inch by 78 inch but I reduced the dimensions to 144inches by 48 inches or 12 feet by 4 feet.  I choose these dimensions to reduce building costs and the roof panels have a combined width of 12 feet and a plywood sheet is 4 feet wide.

The base is constructed of 4 inch by 6 inch treated lumber cut to a length of 11 feet 9 inches.  All wood near the ground is treated lumber due to termite issues in our area.  I used 2 inch by 6 inch by 4 feet long treated boards and screwed them the ends of the 11 feet 9 inches timbers to create a 12 foot by 4 foot box.  Measure twice and cut once is the old carpentry policy.  

Within this box, I installed cross-members.  The 2 inch by 6 inch boards are connected with Simpson Strong-Tie connectors using Simpson nails (  It won't be to building code without the Simpson nails.  (A neighbor's contractor had to remove and replace all the wrong nails on a project of theirs; the weight of the structure is borne by these nails.)    I also used leftover 4 inch by 6 inch material and attached with galvanized lag screws.  I counter-sunk the lag screws so that siding could be placed over the screws. 

Step 2: Insulating and Installing Floor

This probably excessive but I insulated the floor by making "shelves" to support blue board insulation.  The shelving is the light colored wood screwed to the inside of the frame.  The insulation is cut into the appropriate sized rectangles and laid on the shelves.  I prefer styrofoam to any fiberglass insulation because the fiberglass is itchy to work with.

Next I installed 3/4 inch exterior grade plywood over the floor frame.  This is where you get to see if you built the frame square (i.e., all the corners at 90 degrees) and it turned out pretty well; only about a 1/8 inch out of square.  I have been told that all carpenters make mistakes but a good carpenter knows how to hide them.

I painted the floor with some leftover stain to provide some protection.


Step 3: Building Wall Frames

The walls are basic, 2 inch by 4 inch stud walls with studs built on 16 inch centers.  There are headers built over each of the openings; the door opening on the back wall and two (2) side openings.  The side openings were constructed in case I wanted to dry boards longer than the kiln or to extend duct work to the work shop.  

I also framed in two (2) openings for the fans that measured 18 inches by 18 inches.

I also cut a 4 inch by 4 inch post diagonally in half and lengthwise to create a header for the front and back walls that would match the slope of the roof.  Regarding the slope of the roof, the VT plans state that for the best solar efficiency, your roof should slope at the same degrees as your degrees of latitude.  It would be approximately 39 degrees for us; however, 45 degrees is simpler to build, will work OK, and is what I constructed with.

Step 4: Wrapping and Siding the Structure

I am a big fan of wrapping any structure because it allows you to have better control of air flow and air flow is the whole point of this structure. 

The house wrap is stretched as tightly around the frame as possible and stapled in place.  Wrapping is a two (2) person job.  Guess which big box store I brought the wrap at?

Next, I installed siding over the wrap.  The siding is poplar boards salvaged from an old barn.  The barn had been re-oriented by a tornado to the point where it was unsafe.  Thanks to our friends for letting us repurpose the siding.  The top of the siding was cut to match the roof slope.

Step 5: Insulating the Walls

I used two (2) kinds of insulation on the walls.  Similar to the floor, I cut rectangles of styrofoam and placed them within the wall studs and pushed up against the exterior siding.  Next, I placed a layer of Reflectix bubble warp insulation around the entire interior of the frame.  I was attempting to create a dead air space between the styrofoam and bubble wrap.  The seams of the bubble wrap were sealed with aluminum-backed tape and stapled in place.  Excessive, Excessive, Excessive.

Since bubble wrap can be punctured, I installed some recycled cedar siding on the inside walls where the drying lumber is placed.  The cedar siding protects the insulation during wood handling because you will invariably bump the walls.

Step 6: Building the Solar Collector

The solar collector is a salvaged metal roof that was spray-painted dark green.  All you graffiti artists likely know about the handle sold by Rustoleum but it makes spray painting much easier.

Speaking of graffiti, I have been very impressed with the work of Banksy; almost seemed like a precursor to the London riots.

I cut the metal panels to length using a metal cutting wheel on a 4.5 in grinder with lots of sparks.  Wear your safety glasses...always.

I installed a wooden lip on the back wall to support the top of the metal panels and created a 2 inch by 4 inch "shelf" with a back stop to support the base of the metal panels.  The metal panels rests on these shelves and can be removed if need be.

I left a 8 inch gap between the panels and the front wall for air flow.

Step 7: Installing the Greenhouse Roof

The roof material is called Polygal; it is plastic with 1 cm by 0.5 cm channels.  There is even a double channel version available if you needed more insulation.  I ran the channels vertically but seems like you could run in any orientation.  There is an outside and inside to the panels, so pay attention when you remove the protective cover.  The outside has UV protection of some sort.  It is held in place by screws drilled through aluminum brackets.  The screws have a neoprene washer to provide a water seal.  Puckett Plants & Green Houses ( did a nice job of preparing the panels and brackets, although the shipping cost was almost as much as the material.  The brackets or holders are aluminum H-channel to join two (2) panels together and J-channel for the ends.   

There are other roof materials but I have been very impressed with how easy Polygal is to work with and how well it functions.  During kiln operation, you can feel the heat come through the insulated walls while the roof is cool to the touch.  It also seems very durable as it got whacked by a significant hail storm this spring with quarter-sized hail with no apparent damage.

I also put self-adhesive weather stripping between the frame and roof to reduce air leaks although the stripping did make it a little more difficult to slide the roofing panels into the brackets.  I beveled the edges very slightly with a utility knife and the panels slid into the brackets a little easier.

Step 8: Initial Operation

Revison:  Refer to the next step for installation of a baffle for correct air installation.

I initially installed a cheap box fan with an extension cord to push air through the kiln.  It eventually stopped working; maybe due to the heat or maybe insufficient electrical supply or maybe some other reason.  This setup is NOT recommended because of potential electrical issues like fire.  I do recommend getting professional electrical help.  In the final slide, I'll show version 2 of the fan system.

The ends of the wood were painted with leftover stain to reduce over-drying of the wood near the ends.  The wood is stacked in the kiln with spacers between the boards called stickers.  I stapled some Tyvek cloth between the base of the metal panels and laid it across the top of wood stack to direct air through the stack.  Concrete blocks were also placed on the wood to reduce any wood bending.

Based on the oven thermometer, the temperatures are typically run about 50-60 degrees F over the outside air temperature.  

The drying process seems to work very well but the real test will come when I purchase a moisture meter.  The VT plans also discuss a moisture measuring method using a scale.  


Step 9: Baffle Installation

Per the VT design, an interior baffle is needed so that the air can flow across the solar collector to be heated, flow through the stack of wood, and then recirculate to be reheated.  Some moist air is discharged through vents in lower, back side of the kiln.  I have read that the vents only need to be opened slightly and also observed this at the lumber mill's solar kiln.  There is a balance between enough venting to remove moisture but not lose too much heated air.

The baffle is a wooden structure that supports the fans and uses Reflectiix as the baffle material.  The sunny side was spray painted flat black to absorb solar radiation.  I built the baffle in two halves because it was easier to handle and then overlapped the Reflectix to create a continuous surface  I attached a framework to the inside of the kiln to support the baffle.

I also cut holes in the solar collector behind the fans, to allow air recirculate.  

It would have been much easier to construct the baffle prior to installing the greenhouse roof and prior to loading the kiln with wood.  Don't make my mistake.

Step 10: Photovoltaic Solar Panel Installation

I installed a 65 watt photovoltaic solar panel to power the two (2) DC motor fans.  The solar panel to run the fans and make the kiln completely solar.  The panel was installed on the front of the work shop to be part of a small, awning roof.  The angle of the panel from horizontal is 53 degrees which is our latitude of 38 degrees plus 15 degrees as recommended by the panel manufacturer.  I built the roof framework around the 53 degree angle. 

A grounding wire was installed fully across the framework for future panels. 

Wiring was run from the panel to the kiln in plastic conduit buried just below ground surface.  10 AWG stranded copper wire was recommended and it took some searching to find stranded wire; most electrical suppliers had only solid copper wire.  I used terminal strip to split power to the two (2) fans.  I could have used a "jumper spade" to send power to multiple terminals but this required a special order, so I fabricated a jumper with excess wiring. 

There is no battery or other controls in the solar electrical system.  The panel is directly connected to the fans through the terminal strip.  The fans only need to run when the sun is hitting the solar collector and heating the air.  The operation of the kiln is automatically and naturally controlled by the sun.  Bob at Kansas Wind Power recommended this wiring set up.  The fans spin with great speed on a bright, sunny day.  

I based fan size on following formula from "Woodweb - Selecting Fans for a Solar Kiln."    Air flow in cfm  = area of space between wood * 125 ft/min + 50% for leakage.  With a high estimate for the volume of wood, I calculated approximately 1000 cfm per fan.  The fans were purchased from Kansas Wind Power; the owner, Bob, is a great resource. 

PS.  I also installed a short metal roof to shelter the fan motors so that the motors could be placed outside the heated box.  I screwed together sections of metal roof soffet material to make the roof.  The Christmas ornament hanging from the short side is a visual reminder to not run into the roof.  Safety first even though this step is next-to-last in this Instructable.

Step 11: Solar Kiln Operation

I purchased white oak planks from Bonesteel Portable Sawmill & Molding, LLC (  The planks are 8 inches wide, either 3/4 or 1 inch thick, and 8 to 12 feet in length.  I obtained different thicknesses to reduce the amount of planing.  The dory plans uses different thicknesses in different locations.  I plan to use finger joints to join boards end to end for the eighteen foot dory.  Another experiment that will be another Instructable.  Highly recommend Bonesteel Sawmill if you are within striking distance of Paris Crossing, Indiana.  Roger Bonesteel cuts timber on family and adjoining property, uses a small, modern mill, and has two (2) solar kilns for drying wood;  a nice operation.

The ends of the wood need to be coated to reduce excessive drying and checking (i.e, cracking) of the wood.  VT recommended AnchorSeal shown in photo.  AnchorSeal creates a waxy coating on the pores at the ends of the boards.

I measure moisture content, temperature, and humidity on a daily basis to help understand the operation of the kiln.  Moisture content is measured with a Lignomat E/D pinless moisture meter.  The Lignomat meter measures to a depth of 3/4 inch.  The wood was milled to either 3/4 inch or 1 inch thick.  I marked the measurement location to be able to get repeatable measurements.  Also learned to use the HOLD function to get measurements in locations where it is not possible to see the meter.  I take three (3) readings and record the highest value.  A dashed trend line was placed through data to estimate when wood is sufficiently dried.

The semi-cute metal bug sculpture is used to hang an outside thermometer.

On sunny days, the kiln operates at 50 to 60 degrees F above outside temperatures.

As you can see from the above graph, moisture contents are dropping.  The kiln is working! 

Enjoy your project and
Take care all.

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