Insulation materials - a literature study on what matters to us

[Petter]

Many of you ready know all this, I wrote it to give guidance for those who just started to learn about building their own wood fired oven. If it is in the wrong section, please move this thread.


What is insulation?
Here is a very short literature study:

Insulation is a material whose purpose is to stop heat flux from one place to another. Heat is transported in three ways:

1. Conduction, Heat transport in a solid material. Less density increases insulation capacity.
2. Convection, Gas movement in the porosity within a material. Like air movement in a two-glass-window. Too wide and the window is non-insulating due to air movement between the glasses and too thin makes the insulating air gap not sufficient. About 16 mm distance was found optimum for windows.
3. Radiation, At elevated temperature (above about 600 C) – the IR radiation within the porous material is significant. Smaller pores increase insulation capacity.

In a WFO, the temperature is at most around 600 C and the dominating contribution to the insulation capacity is conduction. Being so, the type of material or pore size does not really make that much of a difference. The focus shall be on to get low density of your insulating material.

Reverse engineering of fire bricks, foam-crete, insulating fire bricks, perlite, Vermiculite concrete, CaSi, AlSi, fiber mats and construction insulating materials like EPS, glass wool and natural straw are shown in the following figure. Blue dots are Room Temperature data. Orange is at 400 C. Red is V-crete.



Below 400 kg / m3 – the scatter when thermal conductivity is plotted to density goes down. Why? Likely because the porosity in the materials are similar enough that the dominating contribution is conduction. See enlargement in next image:


The dot at 150 kg / m3 with very low TC is aero gel. You can read up on it if it is of interest.


Why is this important? Well, because it does not matter what insulating material you use if its density is low enough. The reason why CaSi insulates about twice as good as V-crete (vol. 5:1) is because V-crete has 50 % higher density.



What you should focus on is to get a material that is refractory, strong enough for your needs and water resistant, so it does not collapse if it gets wet. If you feel like experimenting, measure the density of your insulating material! It will tell if it is useful or not and how thick it needs to be. At 400 kg / m3, the material is a good insulator. At 200 kg / m3, the material is very good and below 150 kg / m3 is really not necessary to go.


Not all references are listed, but a lot of good reading is found here.

https://www.researchgate.net/publication/318702487_Mathematical_calculation_and_experimenta l_investigation_of_expanded_perlite_based_heat_ins ulation_materials'_thermal_conductivity_values

https://www.researchgate.net/publication/318702487_Mathematical_calculation_and_experimenta l_investigation_of_expanded_perlite_based_heat_ins ulation_materials'_thermal_conductivity_values

https://www.researchgate.net/publication/256163529_Heat_Transport_in_Evacuated_Perlite_Powd ers_for_Super-Insulated_Long-Term_Storages_up_to_300_C

https://www.matec-conferences.org/articles/matecconf/pdf/2015/07/matecconf_acmme2015_01002.pdf

https://books.google.se/books?id=qKFTDwAAQBAJ&pg=PA26&lpg=PA26&dq=Expanded +perlite+particle+size+mm+thermal+conductivity&sou rce=bl&ots=Z3R2aE6_FD&sig=ACfU3U31ijAOOBnSPmLtq-e2O92Eb_PnuQ&hl=sv&sa=X&ved=2ahUKEwjOucHTtfvoAhVGi qQKHViJAdgQ6AEwA3oECAcQAQ#v=onepage&q=Expanded%20p erlite%20particle%20size%20mm%20thermal%20conducti vity&f=false

https://www.promat-hpi.com/en-us/service/what-is-hti

https://www.google.com/search?rlz=1C1GCEU_enSE895SE897&q=High+temperature +insulation+pore+size&tbm=isch&source=univ&sa=X&ve d=2ahUKEwjAx-KLv_voAhWD5KQKHVFjAGsQsAR6BAgGEAE&biw=767&bih=744& dpr=1.25

https://reader.elsevier.com/reader/sd/pii/S1876610216302946?token=45A0C10C841110A3692A111E80 ABB276E0A3529ECAD0788A0FDA07E07788AAE2012EBFB66BD3 9D69ABB29A52A888961F

https://www.rsminerals.co.uk/perlite


Happy mixing!
/Petter

[DorothyGoff]

That's a great post. Thank you for sharing!

[DorothyGoff]

Why is this important? Well, because it does not matter what insulating material you use if its density is low enough. The reason why CaSi insulates about twice as good as V-crete (vol. 5:1) is because V-crete has 50 % higher density.



What you should focus on is to get a material that is refractory, strong enough for your needs and water resistant, so it does not collapse if it gets wet. If you feel like experimenting, measure the density of your insulating material! It will tell if it is useful or not and how thick it needs to be. At 400 kg / m3, the material is a good insulator. At 200 kg / m3, the material is very good and below 150 kg / m3 is really not necessary to go.

Gosh, why don't I find you a little earlier...

[SableSprings]

Welcome Dorothy ! Lots of very good explanations and words of wisdom (& great illustration/pics) from experienced WFO builders in our forum's threads & posts. Have you already built a WFO or are you just thinking about it at this point? Doesn't really matter as long as you enjoy your time wandering through the knowledge posted here.

[Pizzarotic]

Thanks Petter,
Great effort. What do you then recommend as the best materials to use based on your extensive research?

[Petter]

Thanks for reading. I would say the ceramic blanket is the best. Higher cost is compensated with higher insulation value.

Most then use perlcrete as rendering substrate. Read up on dome Davids posts.

Use twice insulation thickness (blanket + perlcrete) as dome thickness for best fuel economy.

Good luck :-)