Tiny Tuesday: Net Zero, Affordably

Dave Posluszny designed and built a net-zero house in Massachusetts from an existing foundation. No fossil fuels are used to power the house: HVAC and electric all come from solar panels on site. Posluszny aimed to make the house inexpensive (not commonly associated with net zero, at least up front) and easy to build, which led to some unusual details. Green Building Advisor shares this article he wrote about the design, and it’s worth a read to understand his decision-making process.

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Vapor barrier, WRB, and thermal break at the top of the leveled foundation wall.

One detail actually generated a disclaimer on the article and much discussion in the comments. For his the water-resistant barrier (WRB) on the wall sheathing, Posluszny used Ice & Water Shield, which is also impermeable to vapor. This detail results in what some builders call a wrong-side vapor barrier, located outside the insulation instead of inside, which threatens to trap condensation in the walls and cause mold. Posluszny claims there won’t be any vapor to trap because the house is airtight and properly flashed, but a lot of builders feel uneasy.

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The entire house is wrapped in Ice & Water Shield.

Posluszny describes several other unusual but logical decisions he made. Instead of making the insulation better (at a cost of square footage and budget), Posluszny opts to just install more solar panels instead. A roof vent above the scissor truss was accomplished with two layers of sheathing in order to place the WRB on the underside. The house has few windows, which is very smart from an insulation perspective; a semi-gloss white paint on ultra-smooth plaster ceilings brightens the interior.

I applied this thinking to all my decisions, and found that the least expensive way to be net-zero is not always the most energy-efficient way.
–Dave Posluszny

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Open floor plan, showing one of the two lofts.

BONUS! Here is a glossary of the four barriers a house requires to maintain the temperature and humidity inside – in other words, to keep you comfortable.

Air barrier: The layer that prevents ambient air from entering the building, or conditioned air from leaving it. The sheathing usually serves as the air barrier, but builders need to be religious about filling in accidental holes (with spray foam, perhaps) in order to make a house completely airtight.

Thermal barrier, aka insulation: The layer that prevents heat from moving in or out of the building. The thermal barrier defines the building envelope. A conducting material that passes through the thermal barrier is called a thermal bridge, which wastes energy and should be avoided.

Vapor barrier: The layer that prevents water vapor in the air from entering the building. Exterior-grade plywood, plastic or aluminum sheeting, or Ice & Water Shield may be used as a vapor barrier. (If the house has a perfect air barrier with no holes, then the air barrier also serves as a vapor barrier.) Installed incorrectly, a “wrong way” vapor barrier may trap water vapor in the walls, leading to mold.

Water-resistant barrier (WRB), aka drainage plane: The layer that keeps rain and other liquid water from entering the building. Most houses use felt paper, also known as housewrap, but this house uses peel-and-stick Ice & Water Shield, which doubles as a vapor barrier as described above.

Tiny Tuesday: Consider Aerated Concrete

recent Boston Globe feature describes a luxury home built with autoclaved aerated concrete (AAC), the first in New England. The material comes in a lightweight masonry block, giving it the flexibility to create unique wall shapes and angles. AAC is a boon for indoor air quality, as it contains no volatile organic compounds (VOCs) and provides excellent air tightness. It’s also fire- and mold-resistant. So why hasn’t AAC caught on in America to match its popularity as a high-performance building material in northern Europe? In short, why have you never heard of it?

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The first New England home built with autoclaved aerated concrete.

Before we answer that question, let’s look at what AAC actually is. AAC is manufactured by mixing sand with a binding agent (such as cement, fly ash, lime, or some combination), water, and a tiny amount of aluminum powder. Unlike regular concrete, it contains no large aggregate like gravel. Instead, the aluminum powder reacts with the other ingredients to form hydrogen bubbles which greatly increases the volume of the mix. That’s the “aerated” part. The mix is still soft at this point, so it’s cut into blocks and placed in an autoclave, or pressure chamber, until it achieves its full strength.

The result is a block that looks like pumice and weighs about the same – much lighter than regular concrete by volume. And unlike a CMU, this block is solid all the way through. In theory this property makes the block self-insulating.

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Building a wall of AAC blocks.

AAC’s insulating properties are disputed, though. A Green Building Advisor article states that the R-value of an 8-inch block is only R-8 to R-11. (Compare 8 inches of dense-packed cellulose which is about R-24.) Other issues include moisture and water vapor – both can readily permeate AAC – and structural integrity – there’s no space for reinforcing bars which could resist extreme wind.

Add that to a 15% cost premium over stick-frame construction, and you can understand why American builders show little interest in the material. But consider AAC for yourself and decide whether the health benefits are worth it.

Tiny Tuesday: Vacuum Panel, King of Insulation

Small-living firm Leaf House designs houses on wheels for very cold climates. Their latest creation, the 97-square-foot Leaf Version.3, withstands temperatures as low as -50°C. And it weighs only 5000 pounds, which makes towing a breeze.

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Leaf Version.3 on the road in the Yukon.

What’s the secret? Panasonic Vacuum Insulated Panels (VIPs) built into the house’s shell have an R-value of R60 per inch. For comparison: an inch of dense-packed cellulose or fiberglass batt typically rates R3, while an inch of closed-cell foam might go as high as R7. Most high-efficiency houses enable these conventional insulations with extremely thick walls, but that’s not an option for a lightweight house on wheels. The Version.3 has R68 insulation in the floors and roofs (just over an inch thick), and R38 in the walls (5/8 inch).

Tight VIPs lead to another problem, moisture control… which Leaf House alleviates through the world’s smallest heat recovery ventilator, the Lunos E2 HRV. Inside there’s an open floor plan, a small kitchen, and a murphy bed (the lack of a lofted bed reduces road height and enables a stylish curved roofline). A bathroom with composting toilet might be the only flaw in this house’s extreme-weather plan, since the microbes that enable decomposition can’t survive when temperatures go way below freezing.

Leaf House sells its VIP Version.3 house for $40,000 – a bargain for the portability and cold-weather performance. Laird Herbert, the owner of the company, currently lives in a Version.3 in the Yukon Territory.

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Inside the Leaf.