Render questions

[Darrell]

Hello,
I've gotten to the point in my oven construction where I'm looking at render mixes for the dome. The dome is cast refractory and will be covered with 2" of Forno Bravo's version of Kaowool with either chicken wire or construction cloth over that.

1a) I see people using perlcrete as a base render. What does perlcrete provide that a straight render does not?
1b) What is the mixing ratio for perlcrete as a rendering layer?
2) What is the mixing ratio that is used for the non-perlcrete render?
3) I live in northwest Wyoming where humidity is almost always low and we receive little snow in the winter (generally). What kind of waterproofing is or isn't recommended either as an addition to the render or as a finish coating?

I did a lot of searching on the forums to answer these questions and found a variety of answers and am hoping to be able to narrow the answers down to a few definitive ones. I have to drive 100 miles to get to a Home Depot or larger suppliers of masonry materials so making one run with all of the materials I need will be beneficial lol.

TIA

[david s]

The answer to your questions depend firstly on the style of oven you intend. If you build a dog kennel enclosure type then the hard sides can be built directly against the blanket. If building an igloo style with a rendered hard shell a firm substrate is required to render against. My solution to this problem is to cover the springy lumpy blanket surface with a layer of vermicrete about an inch and a half thick, which not only sets up firmly enough to act as a suitable render substrate , but enables adjustment to be made to the form and surface to restore the perfect hemisphere again. I don't add a chickenwire layer over the blanket as it's way too time consuming. If a lean brew is used (10:1) the insulation value is about the same as the blanket, so it also creates an additional insulation layer which can save on blanket cost. Some folk find the lean mix difficult to apply, but if the correct amount of water is used as well as some powdered clay added to the mix to give it some stickiness you should have no trouble. Adding additional cement in order to make the mix more workable will reduce its insulating capacity enormously. I find a 50/50 mix of medium grade vermiculite and perlite works better than either of them alone. For every 10 parts by volume use 1 part cement and 3 of water and around a handful of powdered clay for every litre of cement added.

However you choose to go the water in the inner dome and insulation layer(s) should be driven off before a render layer is applied over it.

There are other approaches, some folk have managed to successfully render against the blanket in a number of layers, but this requires extra material and labour to get back the perfect hemisphere.

Commercial renders contain a waterproofing ingredient which makes them partially waterproof but still breathable. This has advantages in that it allows some passage of moisture both in and out. If you choose to fully waterproof the exterior it can also be treated with an additional waterproofing coat or simply painted, which will create an impervious layer.

[Darrell]

David,
First, I would like to thank you for spending the time to answer my questions in depth. I really appreciate it. The vermicrete mixing ratios and reasoning behind it's use are crystal clear. You mention water in the inner dome and insulation layer(s) should be driven off before a render layer is applied over it. How is this done? Is it just allowing it to cure slowly over time? Also, with the final rendering layer... Is a mix of 1:1:6 (1 cement: 1 lime: 6 sand) accurate? Should there be an acrylic additive added to increase flexibility? Sorry to be so clueless but this is uncharted territory for me.

[david s]

David,
First, I would like to thank you for spending the time to answer my questions in depth. I really appreciate it. The vermicrete mixing ratios and reasoning behind it's use are crystal clear. You mention water in the inner dome and insulation layer(s) should be driven off before a render layer is applied over it. How is this done? Is it just allowing it to cure slowly over time? Also, with the final rendering layer... Is a mix of 1:1:6 (1 cement: 1 lime: 6 sand) accurate? Should there be an acrylic additive added to increase flexibility? Sorry to be so clueless but this is uncharted territory for me.

You can use that recipe. I use 1:1:4 The addition of an acrylic additive will improve the mix, usually 50/50 acrylic and water. An alternative is to use a proprietary render mix, most of which contain some waterproofing compound I previously detailed. Some are marked PM (polymer modified), just add water.