Mortar instead of vermiculite on top of CF blanket?

[Omaha]

I just finished putting on my CF blanket and covering with chicken wire. I still have plenty of refractory mortar and want to know if I can just use that on top of the chicken wire instead of vermicrete which is mentioned here more often. Eventually I’ll be adding stucco as the final layer.

[david s]

The refractory mortar is a dense material and likely to compress the blanket. In addition it is designed to withstand high temperatures not act as a thin stucco layer. Most refractory mortars also recommend that the mortar thickness should not exceed around 6mm. I think this is because they do not contain burnout fibres that provide a measure of steam spalling prevention. Adding thermal mass above the blanket with a conductive material will do nothing to enhance the oven performance. The purpose of a lean10:1 vermicrete layer is to provide a firm substrate for a render layer, as well as providing the equivalent insulation/ thickness as the blanket. It also rested the perfect hemisphere reducing the thickness required of the stucco/render layer.

[Omaha]

The refractory mortar is a dense material and likely to compress the blanket. In addition it is designed to withstand high temperatures not act as a thin stucco layer. Most refractory mortars also recommend that the mortar thickness should not exceed around 6mm. I think this is because they do not contain burnout fibres that provide a measure of steam spalling prevention. Adding thermal mass above the blanket with a conductive material will do nothing to enhance the oven performance. The purpose of a lean10:1 vermicrete layer is to provide a firm substrate for a render layer, as well as providing the equivalent insulation/ thickness as the blanket. It also rested the perfect hemisphere reducing the thickness required of the stucco/render layer.

Thank you for the detailed response. I picked up vermiculite and Portland cement this morning and put on a 10:1 layer. I had some issues keeping the chicken wire tight, but it seems to be holding. I think I’ll need to do a touch up layer to fill in some gaps but pretty pleased with the progress.

[david s]

I don’t bother with chicken wire over the blanket because it takes ages and the vermicrete holds it in place anyway. You can easily carve away any high areas of the 10:1 vermicrete as well as fill any low spots once it has set.

[Omaha]

I don’t bother with chicken wire over the blanket because it takes ages and the vermicrete holds it in place anyway. You can easily carve away any high areas of the 10:1 vermicrete as well as fill any low spots once it has set.

Thanks. Based on other posts I need to let the vermicrete dry out before starting any curing fires, correct? I'd assume at least a few days. Should I cover it while it dries? We should have sun here for most of the week.

[Sandieego]

Hope you don't mind me jumping in on this as I am at the same stage of my build. Is vermicrete better than perlite at sticking together? My bottom insulation layer was 5:1 perlite with portland and it was falling apart. I dry mixed first, then added water. Maybe it was too wet? however, the final texture I was applying was kind of like rice krispies without the milk. in about a month, parts were sticking together.
Was wondering why not just add a couple layers of brown coat to make it smooth and not deal with the extra vermicrete if there is already enough insulating blanket?

[david s]

I find a 50/50 mix of perlite and vermiculite makes a better mix than either of them alone. Too much or too little water does not make a good mix. Too much water washes the cement off the grains and too little makes it crumbly/ The correct amount is when it just starts to pool a little in the bottom of the barrow, This is usually 3 litres of water for every 10 of vermiculite, but it depends on the grade you use. A finer grade requires more water. A handful of powdered clay for every litre of cement makes the brew stickier.

Layering mortar directly over the blanket is a bit of a problem because the blanket does not provide a firm substrate and being rather lumpy means you end up having to make the render layer much thicker to restore the true hemispherical form.

Also a 10:1 vermicrete has around the same insulation value as blanket so provides additional insulation possibly saving a blanket layer.

[david s]

Thanks. Based on other posts I need to let the vermicrete dry out before starting any curing fires, correct? I'd assume at least a few days. Should I cover it while it dries? We should have sun here for most of the week.

Leave it uncovered. I usually leave it for a week. One I'm currently building was drying nicely then literally out of the blue we had an overnight downpour of 45mm. Fearing that the water may have penetrated right through to the blanket I have tested it with a garden moisture meter after a few more days of drying. It is now almost dry.