Need Some Advice on Insulation

[Mike Henderson]

I've been contracted to build a solar oven for a beekeeper. Beekeepers use these ovens to melt beeswax, which they later process into multiple products. They save money by using the sun instead of gas or electricity.

A solar oven consists of a box with a glass cover (to keep it simple and not go into too many details). The challenge is to keep the heat in the box and for that reason I need to insulate the box. To minimize the size of the box, I want to find some insulation that will provide a good R value for about 2" of space and can take a reasonable amount of heat. The interior of the box can - under the right conditions - get pretty hot, maybe as much as 300 degrees F.

While I won't use a lot of this insulation, I'd like something that's not crazy expensive.

I've included a pdf of a sample solar oven. The one I'll build will be sized differently but the general idea is there.

Do you have any suggestions for insulation material?

Mike

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[Mike Heidrick]

Mike,

Check with a local Restore/Habitat center or your local CLs and see if they have reclaimed extruded poly styrene - pink or blue foam sold at the home centers. Many times it is called XPS. Great R value for the thickness and comes dimentioned and flat. It can be easily cut to size on your saw. I used it as a thermal break under my concrete on my radiant floor heat setup. Mine came from a school that removed it when they got a new roof. Found it on Craigslist. If I could not get that I would use 1" ISO insulation. XPS is more resistant to moisture so try and get XPS.

[Ken Fitzgerald]

Mike,

What about using something like concrete or stone and use thermal mass to maintain the heat. You wouldn't have to worry about melting point temperatures.

[Bill McDermott]

Probably overkill for your purpose, but Googling this may provide useful information.
1" Ceramic Fiber Insulation Blanket

I think I would look for an old wool blanket if I were in your shoes.

[Mike Henderson]

Mike,

Check with a local Restore/Habitat center or your local CLs and see if they have reclaimed extruded poly styrene - pink or blue foam sold at the home centers. Many times it is called XPS. Great R value for the thickness and comes dimentioned and flat. It can be easily cut to size on your saw. I used it as a thermal break under my concrete on my radiant floor heat setup. Mine came from a school that removed it when they got a new roof. Found it on Craigslist. If I could not get that I would use 1" ISO insulation. XPS is more resistant to moisture so try and get XPS.

Thanks, Mike. I checked the data sheet for Owens Corning XPS and it has a maximum temperature of 165 degrees F. In fact they recommend installing it with the black print towards the house because the sun could heat the black letters to above 165 and damage the XPS. Even if I put a wood face on the insulation, I'm afraid the temp could exceed 165 and damage the XPS.

@Ken - I don't think rock would provide good insulation and the box needs to be movable. Seems that rock would make it too heavy, even if it provided insulation.

@Bill - That Ceramic blanket will certainly take the heat - it's rated to 2400 degrees F!. But it's probably more than I need and the description says make sure you don't inhale the dust from it.

What I need is a Goldilocks insulation between what Mike and Bill suggested. I'll keep looking.

Mike

[Right now, I might just go with that glass fiber insulation and live with a couple of inches of it. There's probably better R value products out there but that will probably work if I make sure there are no air leaks in the box.]

[Art Mann]

I built a solar wax melter very similar to the plan you linked to and used it for the 10 years or so I was an avid beekeeper. It had no insulation of any kind other than the double glass panes. It didn't make any difference. Even without insulation, the interior got far more than hot enough. I used to melt wax into bread loaf pans and it would take hours for it to cool enough to handle with the top completely removed. You are very smart to be concerned about the temperature tolerance of the insulation. I am sure I could have boiled water in mine.

[John TenEyck]

Mike, I used to work for a company that made that Ceramic Fiber Blanket, and a whole lot of other high temp. fiber based products. Heat transfer management is what we did. Anyway, you have at least a couple of options. Simple fiberglass wall insulation is the easiest option. It won't insulate as well as foam board, but it can take the heat just fine. If you want to maximize the insulation, then put a layer of 1/2" or 3/4" foam on the outside and 1-1/2" (or compress 2" stuff to 1-1/2") of fiberglass on the hot face. Or put a layer of aluminum foil bubble insulation on the cold face and then two inches of fiberglass towards the hot face. In both of the composite solutions the fiberglass will lower the temp. enough so that the interface temp. to the foam or foil bubble is below their degradation temp. In general terms, insulating value increases proportional to thickness and inversely with the coef. of thermal conductivity. That means both are equally important so the lower the conductivity and the thicker it is the lower the outside temp. will be. And the lower it's mass the faster the temp. will rise when the sun hits it, and the quicker it will fall when the sun goes down.

John

[Erik Christensen]

I like homosote - used it in a number of applications - class A fire rated and decent r value

[Shawn Pixley]

Short of refractory brick, the ceramic blanket sounds good to me.

[Mel Fulks]

I think Roxul would work.

[Mike Henderson]

Mike, I used to work for a company that made that Ceramic Fiber Blanket, and a whole lot of other high temp. fiber based products. Heat transfer management is what we did. Anyway, you have at least a couple of options. Simple fiberglass wall insulation is the easiest option. It won't insulate as well as foam board, but it can take the heat just fine. If you want to maximize the insulation, then put a layer of 1/2" or 3/4" foam on the outside and 1-1/2" (or compress 2" stuff to 1-1/2") of fiberglass on the hot face. Or put a layer of aluminum foil bubble insulation on the cold face and then two inches of fiberglass towards the hot face. In both of the composite solutions the fiberglass will lower the temp. enough so that the interface temp. to the foam or foil bubble is below their degradation temp. In general terms, insulating value increases proportional to thickness and inversely with the coef. of thermal conductivity. That means both are equally important so the lower the conductivity and the thicker it is the lower the outside temp. will be. And the lower it's mass the faster the temp. will rise when the sun hits it, and the quicker it will fall when the sun goes down.

John

Thanks, John. Your idea of putting a thin foam board towards the outside with fiberglass towards the inside sounds like the best solution.

@Art - I think you're right that just a well sealed wood box will be sufficient. I'll mock up something and see how hot it gets. I'm planning to put double glass on it, maybe with as much as 1/2" between the glass. Or maybe glass on the inside and some type of transparent plastic on the outside to protect against breakage if something falls on it.

Glad to meet another beekeeper. I have one hive, mostly for pollination, which is why I know other beekeepers. I don't have all the honey processing equipment so I get my honey extracted by one of the commercial beekeepers in the area. I try to trade labor things with them. If they need something built, I'll build it and not charge labor (just material). Then, they process my honey for free.

Mike

[Mel Fulks]

We are farther north than Art ,have a friend here who just uses an old steel appliance shell that is painted black inside. And a piece of old glass on top for the 'solar panel'. Wax is placed under the glass, works well. He is the most thrifty person I have ever known. Don't see him much now but he used to get IRS audit every year,always showed up with hundreds of receipts in paper grocery bags.

[Ted Calver]

I think Roxul would work.

Me too. ROXUL stone wool resists temperatures up to approximately 1177°C (2150°F).

[Art Mann]

Glad to meet another beekeeper. I have one hive, mostly for pollination, which is why I know other beekeepers. I don't have all the honey processing equipment so I get my honey extracted by one of the commercial beekeepers in the area. I try to trade labor things with them. If they need something built, I'll build it and not charge labor (just material). Then, they process my honey for free.

Mike

I started keeping bees as a hobby after taking a class in beekeeping at Auburn University. I bought 3 hives and put them at my retired Dad's house. Much to my surprise, he became fascinated with it and started helping me. As the years went by, we would catch swarms, buy strong hives from others and sometimes divide hives so eventually we had a total of 25 hives in 2 locations. We would have had more but that was all the time I could spare from my regular job. We acquired an extractor, several 30 gallon plastic drums for filtering and settling tanks and all the other stuff it took to run a small commercial apiary. That is where the solar melter came in. Wax is rather expensive and easy to sell to the foundation manufacturers. Wax that has gone through solar extraction and captured properly is almost always light premium grade even if the source comb looks pretty black.

My Dad packaged honey with a chunk of comb in pint fruit jars and it sold like crazy. We would have half our harvest sold before it was harvested. I never understood why that packaging was so popular. We kept that up for several years until my Dad got too old to work them and I moved 150 miles away. I am looking for a house and some land out in the country right now. One thing I want to do is build a big woodshop and another thing is acquire a few hives just for fun.

[John TenEyck]

Art, your story brought back a lot of good memories from my childhood. My grandfather had 30 or 40 hives that produced a LOT of honey. This was back in the 60's and he would sell honey as 1# glass jars up to 5 gal metal pails. But my favorite was the comb honey he sold in paper boxes with a cellophane window on the front. The comb was still in it's little wooden frame as he took it from the frame. I loved that stuff, or maybe I loved the time I would spend with my grandfather where he'd make toast and coffee and we'd sit at the kitchen table and eat. I think it was both because I still love comb honey. I remember a 1# jar cost about $1. I don't remember what he charged for the comb honey but I suspect it was less. Today, it's hard to even find honey still in the comb and if you do it's something like $7 and it's usually in a plastic container where half of it has already run out.

John

[Mike Henderson]

To add one more reminiscing story, my dad kept bees - but only a small number - 4 to 5 hives. I remember chewing honey comb as a kid.

But the bee world has changed a lot since then. There's more diseases and mites. And if you go to get a swarm, you have to be concerned if they are Africanized. I have avocado trees and keep the bees to pollinate the trees. The honey is just a bonus.

Mike

[I grew up in Louisiana where they grow a lot of sugar cane. We used to go into the neighbor's sugar cane field and take a few canes. Then peel them and chew the cane to get the sweet juice. Not as good as honey comb, however.]

[Lee Schierer]

I made a solar wax melter similar to your plans without insulation and a double layer of glass in the lid, several years ago and there was no need to insulate it. It melted the wax just fine here in NW PA. Beeswax melts at about 140-150 degrees so you don't need scorching hot temps to do the job.

Some people just put a metal pot full of cappings in an old cooler and lay a piece of glass over the top and set it in the sun.

[roger wiegand]

Service temperature for polyisocyanurate board goes up to 250 F. (Typically found in foil-faced 1" sheets at the borg). It has a higher R value per inch than XPS as well, so you can use thinner to achieve a desired insulation level.

[Mike Henderson]

Service temperature for polyisocyanurate board goes up to 250 F. (Typically found in foil-faced 1" sheets at the borg). It has a higher R value per inch than XPS as well, so you can use thinner to achieve a desired insulation level.

That looks great. Thanks Roger!

Mike