Solar Kiln Insulation R-Value

[Pepe_Silvia]

My in-progress mill is based off the Virginia Tech design.  Their document says "...install paper-backed fiberglass batt insulation between the framing members (Figure 2). Cover the top of the batt insulation with a sheet of 0.6-mm plastic to prevent condensation."  Doesn't give any recommended r-value.  My searches in the forum haven't been too fruitful.

My wallet says to go for the R-13 (about $0.70/ft for 15" x 3.5") but I want to do it right, so if that will not suffice, I'll go with the R-30 which is about twice as much.

Also, how important is it to insulate the floor?  It presents some complications because I'm building it on a trailer.  I've put down 3/4" plywood so far, and am wondering if I should lay down some 2x4" joists so that I can put insulation, then lay some 1/2" plywood on top of that.

[doc henderson]

more is usually better, but at some point the bang for your buck begins to diminish. very important to keep moisture out of the insulation or allow and exit.  also a moisture tight box is also important so you can regulate the rate of drying and accumulate heat.  @GeneWengert-WoodDoc

[btulloh]

insulate the floor

[Pepe_Silvia]

more is usually better, but at some point the bang for your buck begins to diminish. very important to keep moisture out of the insulation or allow and exit.  also a moisture tight box is also important so you can regulate the rate of drying and accumulate heat.  @GeneWengert-WoodDoc

I was looking at the specs and the R-30 vs. R-13 was just a difference in thickness.  I framed with 2x4 so I think I'll go with the R-13 as it's 3.5".  I'm planning on using 6 mil plastic as a moisture barrier per the instructions in the VT document.

insulate the floor

Ok, will do.

[K-Guy]

Hola Pepe!!

I'm not an expert on solar kilns since we sell DH kilns but being in Texas means too much insulation can work against you. Here the north that isn't usually a problem.

Even with only R-13 insulation you may find it difficult to dry some woods like white oak in the summer because it gets too hot and over dries it.

[GeneWengert-WoodDoc]

If you have 2x4 studs for framing the four walls, then use 3.5" insulation.  The floor does indeed need the same insulation amount.  Likewise for doors.  Note that this is for a solar kiln.  

For a DH kiln, saving electrical energy is worth the extra insulation, so 2x8 is better....the extra cost will be paid by electrical energy saving.

[YellowHammer]

Also, it helps to paint both the inside and outside flat black.  Use every bit of solar energy that comes your way.  

You know what they say, "A photon earned is a photon saved" or is it a "photon saved is a photon earned?"  Well, something like that.  

[Mike W]

R value is based on total thickness of insulation without compressing such.  a 2x4 wall will net you an R-13, trying to add additional insulation thickness will only compress the insulation and reduce the R value as it needs to trap air between the fibers to function correctly.  with a 2x6 wall an R19 value is reached, this is of course all about Batt insulation and not ridged board as that is self explanatory.  Need to insulate the floor as well.  If using paper backed or other backed or encased insulation batts, do not add additional vapor barrier as it will trap moisture and degrade the insulation quickly, use un-faced batt insulation if adding vapor barrier on the inside section of the kiln, otherwise put the paper backed part of the insulation toward the inside of the kiln and don't add additional vapor barrier to the exterior side without compensating for vapor release to the exterior by other means and methods.  higher insulation value does cost more up front, however one should figure on operational costs over the lifespan of the unit to off set the lower R value and higher costs to maintain temps or in the case of a solar kiln loss of heat and longer run times the material is needed to reside within the kiln before the next load can be cycled through.

[doc henderson]

@YellowHammer   must be your radiant personality and sunny disposition!!!

[YellowHammer]

Yep, that's me. :D  Just like in Charlotte's web.   :D
Who's old enough to get that reference?

[Jack Lilley]

You could also consider using Roxul insulation. In a 3-1/2" wall you would be able to get R-15 and it will hold up to moisture if that ever become an issue.

[PA_Walnut]

What about polyiso insulation? I think 2" is R13 and 3" is R20 and will not absorb moisture at all.

Fiberglass batt insulation is risk business in a kiln, telling you from experience.

I bought a used Northland kiln, which was built with standard fir 2x's insulated with R13 batts, covered with plastic, sheet metal, etc.
Moisture got trapped under the floor and rotted out the entire bottom--the 2x's, the plywood, etc.











I replaced all of the sleepers with PT 4x4's, put polyiso in between covered with plastic, then PT ply and a coating of silver roofing pitch.
The original construction seemed a poor design but remedied now...runs 24x7. Great unit for smaller loads (800bf).

Don't be tempted by fiberglass insulation. Moisture will get into it, get trapped and rot your kiln.
Next I have to redesign the CURSED UNHOLY track/cart that came with it, which gets hung up ALL THE TIME.  :-[

[GeneWengert-WoodDoc]

The previous post is a good example of why we always want the wall, roof, or floor built so that any moisture that gets into the wall has an easy way to get out.  We want to avoid a double vapor barrier that would trap moisture between the two vapor barriers, one of which could be polyethylene sheets in the interior or an impermeable kiln coating on the interior, and the other could be a non-permeable coating on the exterior or fiberglass batts with a built-in vapor barrier.

The other concern with insulation is that it must be able to perform well at your kiln's peak temperature, which often is 150 F or so.  Many insulation products designed for homes cannot withstand higher temperatures.  Oftentimes, you will be unable to find a maximum temperature rating on a product, so avoid that product for a kiln.

[PA_Walnut]

The other concern with insulation is that it must be able to perform well at your kiln's peak temperature, which often is 150 F or so.  Many insulation products designed for homes cannot withstand higher temperatures.  Oftentimes, you will be unable to find a maximum temperature rating on a product, so avoid that product for a kiln.

Good point Doc! My mill shed has standard fiberglass batts on ceiling and STINKS REALLY BAD, when the sun is beating down on it hard, and its hot outside. HORRIBLE!
Just looked up PolyIso...it's -100° to 250°, so should be good! 8)

[Don P]

I think polystyrene has the lowest allowable in service max temp at 165°F.