Extruded versus Expanded

I’m hearing from people on the list that concrete won’t stick to extruded polystyrene (XPS), but will to expanded polystyrene (EPS), a good tidbit to know. I don’t know how crucial that is when using push through connectors.

I found the other connectors for making insulated cement sandwiches. They’re made by Composite Technologies Corporation. I’ve put in a request for additional information. We’ll see what I get back.

I had an epiphany two days ago, but with all the weather and server issues I haven’t had a chance to sit down and talk through the process.

Rather than try and put the electrical boxes and conduit in place in the mold and pour around them I’ll add block outs to the mold, leaving channels and box shaped holes in the papercrete layer once the wall is pulled off the mold. When the walls are tilted into place I can easily install the boxes and run the conduit, filling the channels with papercrete once the wiring’s inspected and okayed. If I block in the power runs before adding the papercrete to the mold I don’t have to worry about cutting the runs and holes in after the wall has been set in place.

For the walls of our house I am leaning toward 6″ of papercrete on the inside (out of the weather), an expansion layer then my structural ferrocement layer. That all sounds pretty simple, and it is . . . until I start working on the formula for the papercrete layer.

If I mix the papercrete without sand, I’m going to have significant shrinkage issues, reduced thermal mass and a product that will smolder if exposed to open flame. But the layer will have better insulative values.

If I add sand to the mix I’m decreasing the shrinkage and flammability and increasing the thermal mass (good), but am reducing the insulative properties (bad) and making it more difficult to do alterations later. There’s a papercrete technical bulletin on mixes and testing results (strength and thermal properties) I’ll need before I can make a final decision.

Because the ferrocement layer will expand and contract at a different rate than the papercrete layer, it will be necessary to have a buffer between the layers, a 1″ layer of EPS. EPS has an R-value of 4 per inch. If I conservatively rate the papercrete layer at an R-value of 2 per inch, and I’m working with a 6″ layer, I’ll have a R-value of 16 with the EPS and a complete thermal break.

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One Response to Extruded versus Expanded

  1. WalterJ says:

    The concrete does stick to pink board insulation. Not super strongly but it does stick. Spraying with oil, like wooden forms, helps make it not stick. Roughing up the surface of the foam makes it stick better. My son did an experiment with plastering/parging down the side of the pink foam on the outside of our tiny cottage in one spot. That concrete is on tight. Made me wonder about doing the whole thing but we did not have time as the cold weather was setting in. I might have a photo of what he did. I’ll look around. Unfortunately it is now covered with other materials.

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