Boards Vs Plywood / Solar kiln

[ironstumper]

I am in the process of building a simple solar kiln that calls for plywood on floors and walls. I don't have the extra funds right now for plywood. But I do have about 40 pine logs to saw and a sawmill....Ya!! If i use the boards will I be sorry? Will that not be tight enough? Will tyveck wrap help? Anybody gone that route?   Thanx ahead of time...Doug

[pineywoods]

Ironstumper, my solar kiln has pine 1X10 on the outside, fiberglass bat insulation, and nothing on the inside. Works fine, temps will hit 160 deg plus. I highly recommend placing a sheet metal collector on the bottom side of the rafters. I use old roofing tin painted black. Use the search button to do a search on solar kilns.  Lots of info here.

[Fla._Deadheader]

 

I highly recommend placing a sheet metal collector on the bottom side of the rafters.

  Kin ya 'splain this a lil better  : : ;D

[Radar67]

Here is a picture of Piney's kiln looking from the inside, up toward the collector.



The glazing is on the outside of the rafters and the metal in on the underside of the rafters. The sun facing side of the metal is painted flat black.

[pineywoods]

 

I highly recommend placing a sheet metal collector on the bottom side of the rafters.

  Kin ya 'splain this a lil better  : : ;D

The ideas behind these kilns (I've built two of them) is a bit different from generally accepted  solar kiln theories. Conventional wisdom says  a solar kiln is an enclosure with a south-facing opening covered with some sort of glazing material. Sun shines on the stack of stickered lumber heating it and thereby drying it. Maybe  fan to circulate the air and some method of venting hot humid air out the top.
There's several limitations to this approach.
1. A piece of lumber is a lousy solar collector, doesn't capture much infrared radiation.
2. Only the top layer is heated, un-even drying of the stack
3. air vented from the enclosure gets replaced with outside air, at 90% moisture content.
     Consequently, most solar kilns won't get over about 120 F and I had several experts tell         me that a solar kiln won't dry lumber much below 12%

Our approach is different. We choose to heat air by passing it over a metal collector and then using a fan to force that hot air through the lumber stack. Sun never shines on the lumber, instead on the metal collector.( roofing tin painted flat black and secured to the under side of the rafters with the glazing on the top side.) Fans are down-draft, pulling hot air from the top of the building and forcing it through the lumber stack, where it picks up moisture from the wood and cools, then passes upward along the metal collector to be heated again. By nightfall the enclosure will be full of hot humid air. As it cools down, much of the moisture will condense out and by sunrise, we have a kiln full of cool dry air, ready to start the cycle over. I run a small cheap room de-humidifier at night to further dry the air.
With  500 bd ft load of wet cypress, the de-humidifier will pull 2 gallons of water a day er night, for the first few days. In spite of the experts, we can pull wet cypress down to 6% and the internal temp will frequently exceed 160 F. That is supposed to be too hot for hardwoods, but we have dried red oak for flooring with no visible checking. I'm thinking that cooling down at night allows the wood to stabilize enough to prevent most rejects.

[ironstumper]

Thank You Pineywoods and Radar. If you're ever around Greensboro in a few months stop on by and see your twin solar kiln  8)

[Fla._Deadheader]

Thanks, Guys.

[brdmkr]

I don't have a kiln, but I really like the way Piney has done with his kilns.  The tin just under the glazing makes a lot of sense to me.  I figure that has something to do with the high temps.  I wonder if the fiberglass batting and pine siding works because of the DH unit.  I always thought that plywood and waterproofing were done to keep the wood from rotting due to high humidity.   Maybe the DH unit helps with that?  Regardless, when I build a solar kiln, I'mm gonna try to mimic his.

[ironstumper]

Pineywoods, How tall is your kiln?

[Radar67]

Here is a picture of Piney's kiln for size reference...



I have several other pictures in my galley of our trip to Pineywood's place to get a kiln class from him.

[pineywoods]

Couple of minor details that might be worth remembering. In the pic Radar posted, behind the bunch of handsome sawyers, you can see that the back side is sheet metal. That's a full width set of double doors.  So I can load a stack of stickered lumber with the fork lift. I hate stacking lumber ;D More important, someone mentioned rot. So far, no problems with that, BUT. If you don't use the DH unit to pull out the moisture at night, water will condense on the underside of the clear plastic glazing and trickle down to the bottom. The glazing needs to overhang the wall by an inch or so, and leave a small gap between the glazing and the top of the wall, else the water will trickle down the wall and you will get rot. For what they cost, ($120 bucks or so) the DH is highly recommended. It completely solves the condensation problem. Run a hose from the DH through the floor to a container outside. Member Arlcon put a dual sensor Radio Shack digital thermometer in his to monitor the internal temps. One sensor is mounted up near the top above the fans. The other where the air exits from the stack. The difference between the two temps is in direct correlation to the amount of moisture being pulled from the stack. After 2 years of operation, we have found that conventional kiln operation techniques DO NOT APPLY. Consider this unit as experimental. YOUR MILEAGE MAY VARY.

[ironstumper]

Pineywoods, There is no need to worry about my conventional kiln operation techniques. As all I know about drying has come from here and a couple of articles in magazines. I'm thinking your idea is feasable. And I won't call any lawyers on ya if'n things don't work out just right K?

[scgargoyle]

Ummm- how does all that moisture get out? Only during the nighttime cool down/condensation, or is there a way for it to get out during the day?

[Handy Andy]

  I'm also getting ready to start on a kiln.  My main reason is the ash here is just loaded with borers, and if I air dry it, most is just drilled by the time it is dry.  Here in Kansas, air will dry the boards below 10%, so it is more of a borer thing.  Nice to get the boards down to 7 or 8%, but they do that in the shop with a little heating.  I'm going to build my kiln from scraps picked up off job sites.  Have short lumber, waferboard, wf siding, trim, and 5 pieces of patio door glass.  Have to buy the insulation and nails, but have the other stuff.  Might use shingles to go above the glass, as I'm going to go 6' wide x 16' long, and the glass on a 45 degree angle reaches less than 5 feet.  Like Piney's doors on the back side, plan to do that too, only I was going to try to use siding on them.

[pineywoods]

Ummm- how does all that moisture get out? Only during the nighttime cool down/condensation, or is there a way for it to get out during the day?

The little room DH seems to do a good job of that. With a fresh load of lumber, I leave the fans and DH running around the clock. Without the DH, humidity will build up inside the kiln until the air becomes saturated, then condense out when the glazing cools off after sunset. The DH usually cuts drying time at least in half.

[Handy Andy]

  The last solar kiln plan I saw posted from one of the colleges had the inside painted with basement waterproofing tar.  Was planning to use visqueen over the studs, then waferboard, and then paint the inside black with that stuff.  Maybe it isn't so necessary?  Waferboard is pretty water resistant, maybe I could just line it with that and forget the visqueen and the waterproofing.  Just use galvanized nails.  They say that the moisture from oak is pretty corosive.  Noticed in the picture that the floor wasn't black, so they must not have painted it.  THat tin under the glass looks like it takes care of lots of the steps the state colleges went through.  Thanks Piney

[Hokiemill]

Hey ironstumper, I'm currently building the VT solar kiln.  It looks a lot like Pineys but with a slightly different collector set up.  For up front cost savings I would go with the Piney method of using nothing on the interior walls.  I used some black plastic stapled on the interior in preparation for the treated plywood that will go on next, but it became clear to me that as long as you're careful of not puncturing it, the plastic by itself would work out pretty well.  I did have a problem with some of the plastic starting to melt at the very top of the kiln because I had it closed up and no air circulation added yet.

I do think plywood on the floor would be a good idea to minimize any condensation from getting down into your floor insulation.

Where does the moisture go?  The all solar kiln (no dehumidifier) relies on multiple vents (inlet at the top, outlet at the bottom) on the back of the kiln.  However, when I took the VT kiln class last spring, it was clear that vents are a bit of overkill and aren't adjusted hardly at all and are kept almost closed.  Regardless of how tight you try to build it, there will be plenty of leaks for air and moisture to escape.

Due to the lack of electricity at my sawmill lot, I will be forced to go with solar attic fans to move the air in my kiln.  Not the ideal situation, but its the best I've got.  Hopefully construction will be complete in a month or so and I can start testing operation.  Updates to follow.

[pineywoods]

Hey ironstumper, I'm currently building the VT solar kiln.  It looks a lot like Pineys but with a   I did have a problem with some of the plastic starting to melt at the very top of the kiln because I had it closed up and no air circulation added yet.

Hey, it do get hot in thar.  Arlcon used plastic window fans from walmart---the blades melted
and fell off ;D