How to Build a Foam-Free Passive House (And Why You Want To)

©. Andrew Michler

Passive Houses in colder climates are super-insulated, usually with a thick layer of plastic foam under the concrete slab on grade. But a lot of builders want to get away from the use of foamed plastic; it has a high embodied energy, is made with hazardous chemicals and is full of toxic flame retardants. When designer and builder Andrew Michler was designing his own passive house, he wanted to go totally plastic foam free.


© Andrew Michler

He came up with a clever and cost-effective solution: forget the slab altogether and do an old-fashioned ventilated crawl space, and fill the floor with insulation instead. By doing this he was able to wrap the whole house in a thick blanket of Roxul rock wool and cellulose insulation.

south elevation

© Andrew Michler

The 1200 square foot off-grid building near Fort Collins, Colorado is his house, office and shop but is designed for easy reconfiguration. It's designed according to cradle-to-cradle principles:

Only naturally regenerative materials or fully recyclable products will be used, and using Cradle to Cradle methodology materials it will be easy to separate at the end of their use cycle. Most of the materials will have the capacity to be reabsorbed into the mountain environment when the building reaches its lifespan. No foams or other petroleum based products will be used below grade besides drain pipes. In fact we don’t have to use foam anywhere. Remaining materials can be reabsorbed into current industrial technical cycles.

In fact, it's all pretty low tech stuff, using traditional materials and techniques; he "borrows from technologies developed before synthetic materials and HVAC became the norm: organic felt, mineral wool, cellulose. We discovered looking to the past was an invaluable guide in developing the future of sustainable design."

larson truss

© Andrew Michler

The walls are in fact deep trusses turned on end, to leave enough space to fill with two feet of cellulose insulation. The only major compromise Andrew made was in his choice of windows, where he specified U-PVC for cost reasons.

interior under constrcution

© Andrew Michler

It's a very simple and open plan, with a "unique wedge shape is informed and inspired by the local hogback mountains in the area." It's also carefully placed to maximize solar gain and natural ventilation.


Lloyd Alter/ Andrew Michler at PHNW/CC BY 2.0

Andrew presented his house at the Passive House Northwest convention in Seattle, just before Ken Levenson of 475 High Performance Building Supply made a persuasive case for detoxifying our construction industry.

detox slide

Lloyd Alter/CC BY 2.0

Andrew and Ken's concerns about the use of foams and plastics are dismissed by many in the construction industry but it is becoming a movement all of its own, with designers and manufacturers beginning to take it very seriously.

window with cork

Lloyd Alter/ Synergist window/CC BY 2.0

Even the window manufacturers are catching on; I was very impressed to see these passive-house quality windows from Synergist Window Company are actually insulated with cork instead of the usual foam required to hit Passive House specifications. We are going to see a lot more of this kind of thinking.