ASHRAE Journal - February 2009 - (Page 56) Photo 1 (left): Extreme enclosure. “Superinsulated” does not do justice to this house constructed by the Hydaburg Tribe in Alaska that is ultra-airtight and ultra-vapor tight (less than 0.5 ach at 50 Pa [0.20 in. w.c.] and less than 0.5 perms). Photo 2 (right): Exterior insulation. Multiple layers of rigid insulation on the exterior of a wood-frame structure held in place with wood furring strips. Note the inset location of the windows allows the water management layer of the window to connect to the water management layer of the exterior wall (the fully adhered membrane). Building in Extreme Cold By Joseph W. Lstiburek, Ph.D., P.Eng., Fellow ASHRAE N orth of the Arctic Circle there are only two seasons— this winter and last winter. Who would ever want to live there? Being human, we can’t help ourselves. We’ve been there a long time. And, we are going there in increasing numbers because of resources—that’s where a lot of them are. We’ve learned a few things. We use snow to create temporary structures. The igloo is the ultimate sustainable structure. We use frozen rivers in the winter as truck interstates to provide fodder for cable TV shows. We never shut machinery off because in the extreme cold we can never start it again. So, how do you build permanent structures in such a miserable place? The physics are straightforward, but the engineering is not. Moisture flow by air leakage and vapor diffusion from the inside to the outside is a huge concern. Even tiny gaps leaking air can lead to substantial icicles and frost boles. There is no argument that an actual air barrier is essential. The vapor drive when it is 40 degrees below zero* outside is formidable. This is one climate that needs a real vapor barrier, not a vapor retarder. There is no argument that an actual vapor barrier is essential (less than 0.1 perm). Seemingly insignificant thermal bridges that would be laughable in other climates are a big deal.† Triple glazing is not an option; it is essential. There is little room for error. We need, in essence, a “perfect” air barrier and a “perfect” vapor barrier. That means using a fully adhered sheet membrane that looks more like a refugee from the roofing industry than a building enclosure housewrap and vapor barrier.‡ The engineering problem entails constructing a six-sided cube wrapped with fully adhered sheet membrane with all of the insulation exterior to the membrane. Not just a little bit of insulation, but a lot of insulation. Did I mention a lot of insulation? No penetrations, none at all, are allowed in this sheet membrane. All services must be kept to the inside of the membrane (wiring, plumbing, communications, and HVAC). How this cube touches the ground is a big deal. You don’t actually want the cube to touch the ground. The ground is frozen all the time—permanently—that’s why it’s called “permafrost.” It gets really ugly if heat from the building melts the frozen ground. That’s why buildings on permafrost are elevated with ventilated airspaces between them and the ground to control heat loss from the building to the ground. An elegant solution *At minus 40 it does not matter whether it is Fahrenheit or Celsius as both scales cross at minus 40. Minus 40°F is equal to minus 40°C. Folks up north call it minus 40°FC for “freaking cold.” †I remember asking why a blow drier is often hung outside the front door north of 66, and a second blow drier is hung inside the front door? Exfiltrating interior air carrying moisture leaking through the door hardware causes it to freeze solid. The exterior blow drier is used to melt the ice in the lock to get in. The interior blow drier allows you to get out. ‡After too many years of rips, tears, blow-offs and other annoyances, I will not specify 6 mil polyethylene in a hostile climate to serve a critical function. I took my lumps early on my own projects. Never again. 56 ASHRAE Journal ashrae.org February 2009 http://www.ashrae.org
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