Finishing my Kiln - Two Questions

[Everest123]

I have a few last questions about my kiln as I finish it up.  Today I ran solar 120v out to the kiln so I am fully powered and my fans are now running on a speed control and a thermostat. I I have two questions.

1) I put down a row of 2x4s as the base for any lumber I will stack in the kiln.  My plan is to put down stickers on top of each two-by-four to create a nice level and stable base for any lumber stacked in the kiln.  I will have anchors on the bottom and use a 2 x 4 on the top and ratchet straps to compress the whole stack with quite a bit of force.  Hopefully that will prevent warping and cupping as much as possible.  I did screw down the base level two by fours.  Was that necessary? Do most people just put stickers on the very bottom and arranged as needed for each stack? It just felt right to have permanently screwed down two by fours so I went ahead and did it.

2) I have a large black tarp that I will be attaching to the bottom of the fan baffle. I also have leftover 2-inch foam insulation that I will use to seal the sides of each stack so that there is forced air flow towards the front. My question is on the length and positioning of the tarp.  I have seen images where folks just have the tarp goes straight down to the closest board in the stack. That does not cover the entire lumberjack stack and allows air to filter down from the top as well as from the front and work towards the back of the kiln.  I have also seen pictures where people pretty much cover the entire stack of lumber with a tarp which forces air down the very front of the stack.  What is the best way? from a convenience perspective a shorter smaller tarp would be better but I want to do what is best here.

I really appreciate your input and experience as I finish up this project.  Thanks!

-Jeff



 

[scsmith42]

Jeff, that's a nice looking solar kiln build!

A couple of comments..... it appears that your 2x4's go all the way to the back wall of the kiln.  You need to leave adequate clearance between the wood stack and the front and rear kiln walls for air flow.

In my solar kilns I do not strap the loads, but if I'm worried about cupping I'll stack some 4" thick concrete slabs on top to help keep everything flat.

The wood will shrink as it dries, so if you use straps you'll need to take  click up on them ever few days.

Regarding the tarp, in a perfect world you would have the lumber stack fully sealed on the top, bottom and both ends, so that all air is forced through the lumber from the front side of the kiln to the back side.

Do you have a forklift?  If so many miller/kiln operators build pallets to stack their lumber on and then just fork the pallets into the kiln.  Usually this is much easier stacking rather than having to lift every board into place in the kiln.

[YellowHammer]

Weights work much better than strapping. The never get slack and always put even force downward.

My tarp baffle drops to the wood stack near the door, so that the top of the main wood stack, which is covered in black corrugated metal, has the maximum coverage.

If you want to baffle the sides, the 2 inch thick pink foam board from Lowes works great.  Just cut a few reactangular pieces and jam them between the sides of the wood pack and the wall of the kiln.  

[GeneWengert-WoodDoc]

1.  The 2x4 idea is fine and may prove helpful when putting down stickers.  Are the 2x4s already fastened down in the picture?  The lumber stack should have equal clearance between the wall and the edge of the stack, so it would seem that the 2x4s should also have that spacing rather than touch the one wall.  The idea of a sticker on top of them is great, as this minimizes contact area...3-1/2" of contact with a 2x4 is too much.  Some people would say, and I agree, that you need just one sticker space on the bottom.  With the 2x4s you will have a spacing around 2-1/4" high.  So, I suggest putting one short 2x4 between each of the ones you already have.  Location is not too important, but maybe toward the front is better.  This will stop all the excess and unneeded air flow underneath, so more will go through the stack.

Incidentally, the straps are not likely to help a lot, especially if you air dry first.  If you think about a strap, the main amount of force will be on the two top edge pieces.  The strap will not put much restraint in the center of the pile, even with a fancy jig.  So, cupping will not be controlled.  

Analogy.  Have you ever seen straps on a truck that are really tight but they flutter halfway along their length?  They control the corners.

As far as controlling other warp, such as side bend, hardwood lumber has varying widths so the width of each layer varies enough so that very few layers have contact with the strap.

2.  Regarding the fan baffle, many people put down a layer of sticks on the top layer of lumber and the a sheet of plywood or other durable sheet.  Then the baffle, flexible or rigid, is fastened from the fan shroud down to the plywood sheet with weights or fancier jigs.  Actually, the distance to the top layer will vary from load to load, so allow extra length and then allow enough length to fasten the end down with heavy bricks, or whatever.  If you paint the plywood black, it will absorb solar too. The baffle can be black too.

[Everest123]

Awesome feedback. I did but the 2x4s all the way to the wall, but won't do that with the lumber. I'll leave 6" or so I was thinking. I do have forks on my tractor, but I did not build a huge door on the back of this kiln.  So I will have to load by hand from the side. It is what it is. The doors would have been massively heavy and I wasn't confident I could make it work.

We actually used 2" pink foam to insulate the whole structure and I have 2 extra sheets. That's exactly what I plan to use to block off the sides of the stack.

Somehow in all my research I missed the section on the black plywood on top of the stack. It's right there in the VT instructions. No idea how I missed it!  I'll use that or just a piece of my black tarp. Seems like either would work ok. 

The addition of small 2x4 piece to block airflow underneath is brilliant. Thanks so much.