Building a solar kiln during this corona virus lockdown

[farmfromkansas]

  Have been planning to build a solar kiln, and already have a pipe skid to mount it on, and Saturday built the floor platform, need to turn it over and set it on the skid.  Have enough R-11 insulation on hand for this project, but will run into a few items it may be a pain to come up with to accomplish this project, including some 6 mill plastic sheeting.  Have some black 4 mill, and some roofing felt, need to lay it across the flooring after putting in the insulation before the flooring goes on.  Which would be better? or both?  Plan to use OSB for the boxing, have lots of the stuff on hand, was considering using black painted corrugated metal under the rafters, to keep the sun off my lumber, and using a bucket of white elastomeric coating to seal the interior.  Have some housewrap was considering putting on the outside,  any comments?  Trying to get by with supplies on hand.

[GeneWengert-WoodDoc]

A stack of lumber will not get any sun if you put a sheet of plywood painted black on top. No need for corrugated metal. Two clear covers.

 
4 mil is fine but make sure it is without leaks.  Really critical. 

House wrap is not needed. 

Follow VA Tech plan as closely as possible. 

[farmfromkansas]

 Gene, thanks for the reply.  Making the kiln 6.5' wide, and 16' long, planning about 14' of lumber.  Plan to lay out the 45 degree angle after getting the flooring laid. 30" tall short wall.  Want overhang on the back to protect vents from rain. Using the twin wall polycarbonate Menards sells.  They have the strips that go between sheets, and some edge strips. Do you use silicone caulk under the sheets to glue the sheets down, or depend on the screws?  

[btulloh]

There's thermal expansion to deal with using twinwall. The screws they make for it deal with that and work well. Don't glue it. I used their screws and their joining strips and it all worked well. Be sure to seal both ends as they direct. They make a special sealing strip, but i cheated and used the aluminized tape. It works, just need to put a small puncture every 8" or so at the bottom to let condensation out. Or just use the stuff they make to seal it. (Not in stock here, and it would have been a big delay to special order.)

[doc henderson]

I am thinking of doing the same, getting a start on my solar kiln.  I was thinking of an 8 x 20.  the floor will set on skids.  do I need to use treated timbers for the floor frame.  I read the plywood should be pressure treated.  and then rubber elastomeric coating.  I have a Dacron fabric that goes under roadways.  14 feet side and I thought about putting it under to keep out bugs, and let moisture out.. prob.  still cover with wood.  the wall frame can be regular wood I think?  the pressure treated is rated for ground contact and about the same price as regular 2 x 8s..  I have corrugated polycarb for the roof.  how much was the twin wall?  @GeneWengert-WoodDoc  @farmfromkansas 

[GeneWengert-WoodDoc]

Treated wood is not needed if there is good ventilation under the kiln.  Otherwise, you have ideal decay conditions.   Even with a house, we have a vented crawl space.  So, treated wood is a bit like insurance...it may pay off but I hope not.

A small overhang on the vents may be better than a large roof overhang.

[farmfromkansas]

Reason I used a pipe skid is it is welded solid and resists twisting.  Used stacked pipe on the long members to resist sagging.  Just 4 pieces of strap metal on each side to bolt the floor to the skid.  Oil well pipe is not expensive, so an inexpensive base. 106 feet of pipe at 1.00 dollar per foot.  The neighbors have a press, they smash the ends of the pipe being welded, to improve the weld joints.

[doc henderson]

@GeneWengert-WoodDoc does the plywood need to be pressure treated or just ext. grade so the glue holds up.  I may go with the pressure treated floor frame since it appears that it is the same price as regular.  so far Menards is open.  and I thought I could let them get the order ready. although I usually spend some time making sure I get the better lumber in the stack.  Gene, if I use the corrugated panel, is the uv plastic a good second layer.  I plan to rapid season some firewood and of course hardwood lumber.  I will just use fiberglass insulation, unless there is benefit to the foil backed stuff and or foam.  I do not want to get multiple moisture barriers with ratable stuff in between.

[GeneWengert-WoodDoc]

Fiberglass w/o foil is better.  Blue or pink Boards are even better if they can take the heat and are closed cell.  An intact plastic vapor barrier on the inside is required.

PT plywood is good insurance.

The flat sheet of uv stabilized is ok on the inside with corrugated fiberglass on the outside. I 

[farmfromkansas]

Doc, Menards has the twin wall poly for 40 some dollars per sheet, but it is only 6 mil.  Plan to pick it up on a 11% off day.  Have not purchased it yet.  Last thing to go on the kiln. Got the floor laid today, forgot how hard it is on an old coot to work.  All I can think of is laying out the walls and framing, but too tired to start.

[farmfromkansas]

When you go to build the baffle, how far from the back is it?  Picture from the side looks like approx a foot?  Following the VA Tech plan.

[doc henderson]

the closed cell foam would act like a vapor barrier it seems, so if I combined it with fiber glass, I assume it should be interior to the fiberglass batt.  then plastic, then plywood, then elastomeric compound?  @GeneWengert-WoodDoc
I guess the outside siding should keep rain out, but could have a ventilated airspace to allow vapor out if it makes it that far.

[farmfromkansas]

When you build a house, the vapor barrier goes on the inside.  Assume as this thing is heated, same way.

[doc henderson]

that is true, but I want to make sure I am not creating a problem with potentially having two vapor barriers next to each other.  the foam and the plastic.

[btulloh]

When you go to build the baffle, how far from the back is it?  Picture from the side looks like approx a foot?  Following the VA Tech plan.

I don't think it's specified or clear in the va tech plan.  Mine is more like 18".  I'd say 12" is a little too close to the back wall, but that's more of an opinion really.

[farmfromkansas]

Was looking at polycarbonate on the Menards site last night, and they only stock 4x8 twinwall, or 26"x8' single wall roof panels.  On the single wall, I could lap it over and put the clear all the way to the top, but what about the twinwall?  Can you use a H strip and add to the top, or is the H strip only for edges? Price of the twinwall is 38.26 this week.  There is a supposed 10 year warranty on the twinwall, complete warranty for 5 years, drops off the remaining 5 years.  Suppose you have to keep your receipt for the rest of your life. Have both the short and tall walls built, cut the roof framing yesterday and got the 2 end pieces up, along with the flat 2x4 under, so if it is not raining will get the studs in the angled walls today, then can start with the boxing. Kind of fun to build a building again.  

[btulloh]

I don't think the h-strips will work for joining the ends.  Seems to me that was addressed in the documentation by the manufacturer, but I can't remember for sure.  It's an important question though.  You may be able to special order longer sheets.  I solved the problem by changing the kiln dimensions a bit so that 8 ft sheets would work.  It means my kiln is a bit narrower than the original plan.  I don't think that's the best solution, but it worked for me.  The twinwall works so well, it's worth figuring this out.  Plus, that's good pricing you're getting at Menard's.

I'll look and see if I have the docs or the links for the twinwall.

[btulloh]

I found these in my stash of docs.

Seems like you can use silicon caulk to join ends of panels.

[farmfromkansas]

Thanks for the pdf's btulloh, had another brainstorm, what if I just cut some thin spacers above the first sheet, and overlapped the upper portion? The spacers would be the same thickness as the polycarbonate.  Assume the kiln would suffer if I just switched to roofing metal for the final 12"?

[btulloh]

Something like that could work.  The only time the kiln would suffer is if you block some of the sun that's going to the inside of the kiln.  The VT design as shown in their doc uses a simple method where the stuff extends beyond the kiln at the bottom and the top.  That is just the easiest way to get to the finish line, but the clear stuff that's outside the kiln collector area is not contributing anything.

I did my top a bit differently to gain a bit of panel-savings at the top.



 

This isn't the best view, but I stopped the panels just short of the top.  After I added the overhang, I added a bit of a ridge cap that extends far enough to lap over the top of the panel, but doesn't extend far enough to block the collector area (inside of kiln).  I'll try to get a better picture later that shows this.

If you do the spacer and overlap thing, just keep in mind that the air space in the panels need to be sealed to provide the benefit of double panels.  If you lap the panels over metal at the bottom you need to make sure that it's air tight at the lap.  All possible, just adds a few small details to resolve.

[farmfromkansas]

Like the overhang on yours.  Should help keep rain out of the vents.  Read the directions on the Menards site, said to support the panels every 24" horizontally.  Think I will put 2x4's on edge between the rafters. My rafters are 108" long, think I need the full length in plastic.  At the bottom edge, will have to put small pieces between rafters to fill the gap, and the rafter comes right to the wall at a 45 at the top, so about all that goes inside the kiln.

[btulloh]

That's why i put the overhang. Most don't, but it made me happy. 

Correct on the horizontal support. Essential. Mine's more like 28" oc. 

Also true on the blocking at the bottom. VT butted theirs into the top plate  oon the short wall so wasn't necessary. Some of these details just need to be figured out to suit you. Sounds like you've got a handle on it. 



 

[btulloh]

Actually, after checking just now, I did butt the rafters into the bottom plate, but I also ripped 2x4 pieces in half on a 45 and put them in for extra support.  FWIW, I staggered my horizontal supports, just because it was easier than toenailing.  Not as pleasing to the eye, but it pleased me nonetheless. YMMV

[farmfromkansas]

Got the studs all nailed in the end walls.  Started raining, so put everything away.  Too windy to handle any boxing today.  Next task. My air nailer makes it easy to toe nail, so think the horizontals will be even.  Old Hitachi, bought about 30 years ago, has yet to break down, although for the last 20 years, has not had a lot of use.

[btulloh]

I can't seem to do a good job on those angled rafters even using a nail gun. Just working in there after I installed the rafters was an adventure for me. Ouch!!  In line looks better for sure.