Foil

[MikeD55]

I wanted to revisit this topic. I know in the past the majority of people on this group were against foil but I’ve spoken to and looked at builds from some top oven builders like Alex Chernov, David Neufeld (both retired) Jon Santiago and Patrick Manley. Collectively they have over 250 builds. All use(d) foil. I’ve also spoken to one mason who repaired an oven and when he opened the foil he said he got hit with a blast of hot air such that they had to leave the room. It’s still used by quite a few oven manufacturers around the world.

Im wondering if maybe the foil needs to be used as a system. For example in applications where the amount of water in the build is kept to a minimum. Where there is NO vermiculite or perlite mix. Tight mortar joints etc.

Also what about perforating the foil or using foil that is already perforated to allow any moisture that is present to escape.

It should be noted that they all use multiple layers of foil BETWEEN insulation layers. Not directly on the dome. In this regard I’ve done the math and although yes foil directly on the dome is conductive, it’s so thin I feel it would have a minimal impact on increasing conductivity.

Would love to hear some opinions.

[david s]

As well as the insulating quality of foil, which only reflects radiant heat, some builders like to use foil as a slip layer to prevent moisture loss from one layer to another when applying eg moisture loss from vermicrete to cal sil insulating board applied as underfloor insulation, or outer cement render to vermicrete when applied over the dome.

Unfortunately as well as preventing moisture transfer during the build it also acts as a moisture barrier in operation. Some reduction in this effect might be obtained if perforated foil is used, but I prefer to skip it entirely, especially after using it over my vermicrete layer on the dome, in order to eliminate excess water loss from the external render. After applying the foil I was concerned about its moisture barrier qualities, so perforated the foil in two placesaround 5 sq in each. Now, when the insulation layer becomes moist after prolonged torrential rain and a drying fire is started, the outer dome becomes warm after around 1/2 hr, but holding a hane to the outer dome where the two spots of perforated foil sit under the outer render, they are hot to touch. This indicates to me that steam is passing through the foil at those points. Under normal operation the outer shell is only ever cosy warm.

Now, to counter loss of water in the outer render, I simply wrap the whole oven in cling-wrap as soon as the render has been completed and the surface wet sponged, leaving it in place for at least a week. This method is highly successful, sealing moisture in the outer layer, evidenced by the visible beads of water under the plastic film.