Kiln insulation

A discussion of good insulations types for kilns and information on R values. February 7, 2001

Q.
Can anyone suggest the cheapest and most effective type of insulation for kiln walls and ceilings? Also, what is the recommended R factor or amount to be used?

Forum Responses
There are two reasons for insulating a kiln. Reducing energy consumption is the one most people think of, but of equal importance is preventing condensation.

This means that R20 is usually the minimum. R30 seems to be the best from an economic standpoint, but you can't have too much. After R30, the payback gets pretty tiny.

Avoid any open cell product like fiberglass, white polystyrene or cellulose. Urethane is good if the density is right. Blue or pink styrofoam is good if temperatures will not be over 160 degrees F. Polyisocyanurate is the most widely used. Mineral wool is okay but only if the structure is right and not for "homemade" kilns. If you use a board, be sure to allow for expansion and contraction by using multiple layers.


Be careful--you can over insulate! We built our kiln from freezer panels (which have a very high R value) and it overheats when the outside temperature rises above 70 degrees, with no heat source other than the compressor and fans.

There is something confusing here when one talks about a kiln over-heating. I wonder what the source of the heat is. In every DH kiln I have seen (except for some early Westaire units) the compressor is outside the kiln. The hot coils cannot get any hotter than the cold coils are cold, so these two balance each other. Also, the typical kiln schedules run at 110 degrees F (minimum) and often much hotter. Where is all the heat coming from? The insulation doesn't make heat.

Also, if there is extra heat, why isn't the lumber using this heat to evaporate moisture, thereby cooling the kiln off? The hotter a kiln is, the faster the wood dries, meaning it is using the heat.

Is the building perhaps painted dark black? Even so, with all the extra insulation, solar heating should be minimal.

I would be glad to have an explanation. As a technical point, it takes about 32,000 Btu's to evaporate 1% MC from one MBF of lumber (with no heat loss through the walls, floor, roof and no vent losses). This is equivalent to 64 kWh of electrical energy from an electrical heater.

Gene Wengert, forum technical advisor


The refrigeration system puts more heat into the air than the cooling coil takes out of the air, regardless of whether the compressor is inside the kiln or not.

For example, if a compressor is using 25 kWh, then 95% of that energy gets transferred to the refrigerant and thus to the kiln. In addition, all the electricity used by the fans and blowers gets translated into heat. In this case, adding the compressor heat, the fan heat and the blower heat, you would probably be adding in about 150,000 Btuh more than the cooling effect. Once the kiln is up to temperature, this heat is exhausted by the vents and you get rid of a pound of water for each 1500-3000 Btuh and this is water the dehumidifier doesn't have to take out.

Most DH kilns turn the heat off once the kiln reaches 85 degrees F and this "recovered" heat is what is used to finish the load. By insulating the kiln really well, you force yourself to vent to get rid of heat and this carries away water. If the walls are not well insulated, the heat leaks out by transmission and no water goes with it, so the cost of operation is higher.


A number of the smaller kilns locate the compressor, evaporator and condenser in one unit, and only place the controls outside the chambers. This makes them easier to install and I suspect cheaper to build. However, if the kiln is equipped with automatic over temperature (fan on a thermostat), overheating is not much of a problem. You may need to adjust the lower percentage timer for easily degrading species (oak), but that only means a cheaper power bill.

What happens to the pink foam above 160 degrees? I have 9 sheets of 4" stuff I had planned to incorporate into a small solar kiln. A 4' cube prototype I built had the 2" white styrofoam going over 146 (then the digital thermometer would go black) with no obvious problems.

Probably nothing will happen to the pink styrofoam up to 180 degrees F. We have lots of people use it with good results.

White EPS is chemically the same stuff, but is much more sensitive to temperature and is open cell, so it holds water. It varies a great deal by manufacturer, but the material tends to shrink starting around 150. We have had a number of bad experiences with it, and some good experiences. Because there is no way to know in advance, we stay away from it.


Is there a chance of off-gassing that might create a flammable/explosive situation (in a solar kiln with a small fan)?

I don't think there would be any problem with off-gassing at the temperatures you would be running. Normally, the temperature that this material sees in some attics or when insulating a flat roof under black asphalt, will be higher than it should see in a kiln. It is pretty safe stuff.

I am currently drying the first charge of larch after having added Krona insulation to the inside of my R20 framed kiln. I used roofing nails to install it.

I noticed a difference right away. The kiln is quieter, the preheat was on for less time, and it looks like Apollo 9 inside!

Krona is good to 300 degrees F or something, but the tape to seal it needs to be chosen carefully.