In this article...
A few weeks ago we shared our plans to build a solar kiln. In this series we'll document our entire build, starting in this article with what exactly is a solar kiln, why we decided to build one, the design we're using, and how we're going to build the floor assembly.
What is a solar kiln?
Let's get straight into it and look at what exactly a solar kiln is, and why we've decided to build one.
When you cut a tree down and buck it into logs, the wood is very wet - the exact moisture content depends on factors like the type of tree and the time of year. If you mill this log, you get what's known as green lumber.
When you buy lumber in a store, it's usually kiln dried - this means it's literally been cooked in a kiln to dry it out and reduce its moisture content.
A conventional commercial kiln is a big heated chamber - basically an oven - with an external heat source such as electric or gas. More modern kilns use vacuums to achieve a similar result without requiring as much heat.
A solar kiln works on the same principle as a conventional kiln, using heat to dry the lumber. But rather than using gas or electricity to generate the heat, it uses solar power instead - it's basically a big, south-facing greenhouse!
For our non-commercial purposes it has some big advantages - it's cheaper to build, cheaper to run, better for the environment (no burning fossil fuels) and is completely silent.
It's also considered by some to be a better way to dry the wood - if you dry the wood too quickly it can crack or warp, so conventional kilns actually add steam periodically to slow down the drying process. By contrast, a solar kiln cools down at night, giving it a natural heating and cooling cycle allowing the wood to regularly relax and relieve internal stresses.
Why dry wood in a solar kiln?
Drying wood is important because as lumber dries it not only has a tendency to warp, twist and cup, but will also shrink significantly - on average around 7% across the grain. So if you try to build with green lumber using conventional techniques you'll run into all sorts of issues as the lumber dries.
Once lumber has dried to an appropriate moisture content for its ambient environment, it's much more stable and can generally be used without much concern about further movement.
While you can simply stack the lumber and allow it to air dry naturally, this can take a very long time - up to a year or more per inch of thickness in some cases!
A solar kiln accelerates this process significantly - potentially drying an entire batch of lumber in as little as a month. But there are some other advantages too.
On a warm sunny day in summer, say around 80°F, the inside of the solar kiln can reach more than 140°F (60°C). And that's while it's full of lumber with the fans running for airflow and to expel the humidity. If the kiln is empty but closed up and the fans are turned off the temperatures can sore to over 200°F (93°C) - enough to damage plastic components in the fans!
These drying temperatures are enough to do two important things - kill any bugs and insects, and set the pitch. The first is self-explanatory, but important - we don't want any bugs in our lumber!
The second is particularly important for us though. Pitch is the sticky, sappy substance in the lumber that makes the wood feel wet, but will continue to seep out over time. Setting the pitch means turning it from a liquid to a resin-like solid, and that will stop the lumber from seeping over time.
Solar kiln design
We decided to base our solar kiln design on one by Virginia Tech. Their design has been around for years, and been through several iterations to improve it over time.
The basic concept is simple - it's a wood-framed building with a large south-facing sloped roof to act as a giant solar collector. Large doors on the north side allow for easy loading and unloading of the lumber.
Sunlight enters through the solar collector where it heats the dark-painted interior. Large fans run during the day to circulate the hot air past baffles to blow it through the stacked lumber and expel it out through vents in the back of the kiln. The fans can be powered by a regular 110V supply or solar panels.
Inspired by the Virginia Tech solar kiln design I built a model using SketchUp, with just a few tweaks to make it better suited for our needs.
The first modification was to change the dimensions. The original design is 13ft 4in wide by 6ft 6in deep. This limits the long lumber you can fit inside to around 11ft. Since our HM126 sawmill can mill logs up to 16ft 11in long, we opted to extend the kiln accordingly - in our case to 18ft 8in.
Second, we want our solar kiln to be a semi-portable structure so we can move it around if necessary. One way we can do this is to carefully load it onto our equipment trailer and then tow it with the truck or tractor.
While our equipment trailer deck is 20ft long (including the 18" dovetail at the back), it's only 6ft 5in wide - or 1 inch to narrow for the solar kiln! Since it was so close, we narrowed our design to 6ft 3in to give us an inch or so of clearance either side - a tight fit, but we should be able to squeeze it on.
So dimensionally the solar kiln should fit on the equipment trailer but how do we actually get it onto the trailer.
Our inspiration for this came from Diana's recent course at Yestermorrow where they built a couple of semi-portable buildings. Their solution was to build on skids - rails at either side that can be used to drag the building along the ground.
While you wouldn't want to drag the building very far, it's enough just to get it up onto the equipment trailer and back down again.
Skids & joists
With our plans clear, we took our equipment trailer down to a local lumber yard and picked up all the supplies we needed.
Incorporating the skids into our design, we built each skid out of two 2x12s nailed together to create an incredibly strong side rail. Since this would be directly in contact with the ground, these are made of pressure treated lumber.
Using the saw horses we had built from our lumber, we cut the 2x12s to length with our Dewalt 60V cordless circular saw which made short work of the cuts. We cut 3" off each bottom corner to create a bevel to help the skids slide without digging in on soft ground.
We then laminated two of the 2x12s together by nailing in at 16" on center on alternating sides. Last, we used off-cuts of the 2x12s to create two shoulders to widen the skids to triple thickness at each end, beveling both the inside and outside corners to prevent digging in.
Between the skids we chose to use 2x8 joists spaced at 16" on center, secured in place with joist hangers. To make things simple, we laid out all our hanger locations and nailed in one side of all the hangers while the skids were still on our saw horses.
Once the hangers were done, we lifted each skid onto the floor, then leveled and squared them ready to receive joists.
Diana was the appointed "cut person" and began cutting all 17 joists - one every 16" plus doubled up at each end. As she cut them, she passed them to me and I began nailing them in place, continually checking for square by measuring the diagonals. We started at the ends first, face nailing these in place and then moved to the inner joists on the joist hangers.
Last, we cut blocks and nailed these in place in a staggered pattern down the center of the floor. We were trying to minimize our wastage (pressure treated lumber is expensive!) so we had carefully planned out our cut list. We had bought 17x 8ft long 2x8s which let us cut each joist with enough left over for blocking with almost no waste.
Even with no subfloor installed, the structure felt rock solid! Even jumping up and down on the joists there was almost zero deflection. Given the enormous weight of wet, green lumber that it will need to support, this is paramount! Using 2x8 joists and blocking for a span of less than 6ft is probably overkill, but any deflection in the floor will transferred to any lumber we stack in there so a perfectly flat floor is essential.
After researching online and speaking to our lumber yard, we chose to make the subfloor out of 23/32" (nominal 3/4") Huber Advantech. This engineered composite resembles OSB but contains an advanced resin for extreme weather resistance. In fact, it even comes with a 500-day weather guarantee.
At almost $70 each, the five sheets of Advantech we needed made up almost a third of the cost of the floor structure but we think they were worth it. We double checked everything was square (it was to within 1/32" across the diagonals) and screwed in the Advantach sheathing.
We screwed in at 6" intervals along the outside edges and tongue & groove joints, and used 12" spacing in the field along the joists. Because of the dimensions of our floor we ran the boards across the floor which meant the markings on the boards didn't line up, but we easily solved that with a tape and chalk line.
Without a table saw we used our Dewalt circular saw - putting a couple of holding screws in place before cutting along the edge of the skid for a perfectly flush fit. Our Dewalt DCF850 impact driver made quick work of the screws.
Since the solar kiln is designed to get hot, it needs to be well insulated, and that includes the floor. The recommended method for insulating the floor is to use 1.5" foam insulation board.
We spent last winter in our RV in Vermont, and we used 1.5" foam insulation board to construct skirting around the RV to help insulate it.
XPS foam insulation board is a really poor choice in terms of environmental impact - it cannot be easily recycled and does not degrade. So when we removed the foam board this spring, we decided to put it all in our storage unit rather than landfill in case we could find a use for it later.
Well, we just did!
Using our Kubota L3901 tractor, our 5,000lb-rated ratchet straps and a chain we were able to raise the floor structure up onto one edge and tip it back slightly, exposing the bottom of the Advantech.
It took us about 2 hours to work through our pile of foam insulation board, cutting big pieces to fit and then filling the gaps. We didn't try to fill every tiny gap as that would be overkill for this project, but in the end we managed pretty good coverage!
We expected to run out of foam board and that we'd have to buy more, but we actually managed to do the entire solar kiln floor without running out. We don't have much left though, which is fine by us - we're just glad it didn't end up in the landfill.
We used a subfloor adhesive designed to adhere foam board to a subfloor, and gave it a few hours to dry before lowering the structure back down with the tractor. In reality, almost every piece was a tight pressure fit anyway so the adhesive is really just to keep things snug.
Our finished floor assembly is an absolutely rock solid, super stiff structure almost 19ft long and over 6ft wide. The pressure treated skids and joists, as well as the Advantech subfloor can be left exposed all winter until we're ready to work on the framing and complete the build next spring.
We did cover the floor with some tarps for a week or so but have since stopped bothering - the potential for mold growth under the tarps is a bigger concern than water on the surface.
Until next spring, the solar kiln is a perfect platform for us to stack our milled lumber - it's large, level and perfectly flat!
While there's nothing complicated about the structure we've built, this is our first time working on a construction project on our property together. Diana was able to put into effect some of the skills she had learned on her course at the Yestermorrow Design & Build School a couple of months ago. It was also a great opportunity for us to try out some our new tools.
They say that any successful project requires strong foundations, and we're really happy with how this solar kiln floor assembly has turned out. It'll be a little while until we work on the next stage of the build, so make sure to subscribe to our newsletter so you don't miss out!