Q: Hello Paul,

I read your previous column mentioning a Passive House in Syracuse, NY. What are the best insulation choices to build a Passive House? I’ve heard about aerogel insulation. What can you tell me about that?

—Kim, Colorado

A: Dear Kim,

If you can afford the upfront cost, absolutely use a "blown" insulation. It’s much better to spend more money on this than other insulation types and it works in tandem with highly efficient windows, super-efficient HVAC, and renewable energy systems to lower your energy bills. Properly installed blown insulation fills cavities and stops air leakage, essential in creating airtight building envelopes.

The Passive House in Syracuse (The R House) used blown cellulose, which is normally somewhat less expensive than foam, but not always, as it depends on the contractor. Most spray cellulose uses recycled paper such as the Greenfiber product. Cellulose is blown either wet or dry. Choose a contractor who knows what he or she is doing, because dry cellulose settles if not done properly and wet cellulose needs to dry properly before being covered up with sheetrock.

Foam insulation choices include the product "Biobased," made partially with plant-based material instead of petroleum-based elements. (Most "green" or "bio" foams are only partially bio products, despite their claims of being totally "green.") Icynene has a partially bio-derived foam now. Within foam choices there’s open-cell and closed-cell. Closed cell is impervious to water and generally more expensive.

Aerogel is an amazing futuristic material originally developed by NASA. Aerogel is billed as the most efficient insulation ever, with an R value of almost 11 per inch, which is two to four times better than blown cellulose or foam.

Cornell University’s 2009 Solar Decathlon entry, the "Silo House," used Thermablock aerogel to seal connections between modules of the house and on the face of steel-frame columns to prevent thermal bridging. Chris Werner, Cornell’s team leader, said, "The primary benefit was that we were able to prevent thermal bridges and have high R-value in a very thin material." According to Aspen Aerogel: "In a typical building, the framing (25 percent of the building envelope) is uninsulated, resulting in heat loss through the studs." Their Spaceloft insulation reportedly "stops this thermal bridging and improves thermal performance up to 40 percent in steel studs and up to 15 percent in wood studs."

Aerogel is not used much yet, partly because of cost. I asked Werner if aerogel might become more popular. He reasoned: "If price comes down, yes. Right now, in cold climates, double-stud walls are becoming increasingly common for a highly insulated, energy-efficient house, but one of the drawbacks of this kind of wall assembly is that it is quite thick. With aerogel, a builder could conceivably build a much thinner wall with thermal breaks and high insulation value. Ideally, this could open up interior floor space, cut down on construction time, and create a well-insulated envelope."

Check out the amazing ingenuity of the Silo House: cusd.cornell.edu—Paul

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