After I asked Can we get rid of plastic foam in our buildings?, the tweet came through in response: "YES! We’re growing high-performance insulation materials that are renewable and far safer than EPS or XPS!" It was from the gang at Ecovative, known to TreeHuggers as the inventors of myco-foam technology, where they use fungi to bind agricultural waste into a substitute for stryofoam. Up until now they have been mainly selling packaging materials, but the green building material world is a much bigger market that is screaming for this kind of thing.

© Mushroom Tiny House wall

In this tiny house demonstration project, a form is built of interior and exterior tongue and groove pine siding, and the walls are filled, a foot at a time, with the mix of mycelium and agricultural waste, "adding a foot every two days. The time in between allows each layer to fully grow out and not get suffocated."

© Mushroom Tiny House roof

The roof is grown the same way. The mushroom mix adheres to the pine formwork, turning the whole thing into a type of structural insulated panel. What a wonderful idea; non toxic, non-flammable, no fossil fuels, grow your own insulation. They are really on to something here. Nice little design too, it will be a hit with the Tiny House fans.

© Ice and water shield being applied

And then it all goes awry. They are covering the whole thing in moisture-impermeable ice and water shield; it's adhesive and glues right onto the wood. As an architect, I think this is a big problem.

© Shingles appear to be attached directly to wall without strapping

There is a lot of debate these days about where you put the vapor or moisture or air barrier, but the consensus is, in cold climates, the moisture in a wall is driven from the warm side out and the exterior wall has to breathe. It's good practice to put shingles on strapping and create a rain screen. Here, they appear to nail the shingles right onto the outside, no strapping, no air space. Ice and water shield is grippy around nails, but there are now a whole pile of nice little cold spikes going right into the insulation that moisture can condense on.

There is also the issue of plastics. They write that the whole thing is almost completely plastic free, saying "we’ve only cheated in one area: the electrical wiring." Then they wrap the whole thing in a thick layer of petrochemical product, defined by Grace as "an aggressive rubberized asphalt adhesive backed by a layer of high density crosslaminated polyethylene."

The problem with this is that this whole thing is an experiment, the first time they have tried building a house this way. If the wall or more likely, the roof fails, they will have no way of knowing whether it was because of their actual mushroom product, or if it is because of the design of the wall and roof assembly.

Mushroom insulation is an absolutely wonderful product. I look forward to being able to write about mushroom-based structural insulated panels and all of the other products they are dreaming about. The mushroom tiny house is a beautiful thing. But for a century, good building practice has been to treat the exterior siding as a rain screen, to design for drainage and ventilation. I hope on subsequent prototypes they mind the gap and let it breathe.

Read all about it at Mushroom Tiny House, and here is a PDF of a great old article by building science expert Joe Lstiburek about how (and why) to build a wall.