The Superinsulated House

Filed in Uncategorized by on February 3, 2006 0 Comments

I used to live in Wisconsin in a house with no furnace.  It didn't have a wood stove or solar panels, either.  What my house had was really thick walls filled with lots and lots of insulation.

A typical solar house has panels on the roof and a wall of south-facing windows, and it is certainly warm when the sun is shining.  It can roast you on a sunny winter afternoon.  An advanced model might store solar heat in a massive masonry wall to carry the house through a couple cloudy days.  The problem comes after three or four cloudy days when this wall of thermal mass has released all its heat. Suddenly the solar house needs a full-sized furnace to stay warm.  Until the clouds go away, it is consuming as much fuel as any conventional house — sometimes more, because of all that glass. 

A superinsulated house takes a different approach.  It doesn't have solar panels or dedicated thermal mass, just unusually large amounts of insulation and a building envelope that has been made as airtight as possible.  It can be built in any style, with its only visible distinguishing trait being thicker walls.  It might feature an airlock entry.  An effort may be made to put most of the windows on the south side for solar gain, but the total area of glass is modest.  The superinsulated house has one big goal, and that is to lose heat as slowly as possible.

Houses are full of electric lights, appliances, electronics, people, pets — all of them throwing off heat. South windows let in warmth from the sun. In a conventional house, these incidental heat sources are trivial compared to how much heat is pouring out the walls. A superinsulated house reduces that massive outflow of heat to a trickle, so all these incidental ambient heat sources are not so trivial any more.  In fact, they're enough to keep the house comfortable. When additional heat is needed, it's not very often and not very much.  In a superinsulated house you can meet all your supplemental heat needs with a cheap electric space heater for less money than it would cost to maintain (and periodically replace) a furnace.  Normally electricity is the most expensive way to heat, but, when the amount of heat needed is so small, it can make sense.

How much insulation is needed to achieve this kind of performance? The small superinsulated house I built in Madison, Wisconsin in the early 1980s had 13" of fiberglass in the walls and 18" in the ceiling, roughly R-40 and R-60. There were insulated shades on the triple-glazed windows (no low-e or argon in those days), and all the windows faced south, which was atypical. Inside the walls was a very thoroughly sealed polyethelene vapor barrier. An air-to-air heat exchanger used the outgoing stale air to take the chill off the incoming fresh air. Four inches of extruded polystyrene foam provided R-20 under the concrete slab, with 2" around the perimeter for R-10. In retrospect, the relatively skimpy perimeter insulation was the design's weak link.

Still, with no added heat, this house would stay between 64 and 68 degrees most of the winter. After long cloudy spells in the depths of February, it once or twice got as low as 58. When the sun came out on a below zero day, my little furnace-free house would get up to 70. These indoor temperatures were achieved with ambient heat sources only (lights, stereo, refrigerator, water heater, sunlight, occasional warm-bodied visitors). I was pretty diligent about closing the shades at night and opening them in the morning. No matter how cold and cloudy the weather, I could make it warm and toasty inside by running a 1500 watt space heater for a couple hours. It was rare that I actually did this, though. My electric bills were slightly over what you'd expect to pay just to operate the electric water heater. There was no gas bill.

So why aren't all cold-climate houses being built this way? Twenty years of artificially cheap oil probably has something to do with it. Also, insulation doesn't sell houses. It's not even on the radar screen for most homebuyers, so builders do the minimum permitted by code. For a given footprint, a house with thick outer walls will have smaller rooms, and who's going to pay extra for that? Who's going to buy a house with no furnace, for that matter?  It's counterintuitive.  I've come to accept that superinsulation will not become the norm anytime soon.

But just because it never became popular doesn't mean it's not a good idea. I'm currently building my second superinsulated house, and I'd urge anyone planning a new home in a cold climate to seriously consider going this route. The one I'm working on will be insulated with spray-in-place polyurethane foam instead of fiberglass batts. For my reasons on that, see David B. South's excellent article comparing <!–LINK REMOVED –>fiberglass vs. polyurethane foam<!–END LINK REMOVED –> in real-world conditions. I don't accept his contention that three to five inches of foam is the most anyone would ever need, however. His designs have furnaces and a lot of thermal mass. I'm planning to use ten inches of foam in the walls, approximately R-60.  

Construction costs for a superinsulated house will be higher. Some have calculated that their savings from not having to install a furnace actually paid for their extra insulation, but I personally think that's a little optimistic. It's definitely not possible with an expensive insulation like foam. I would stress that while a superinsulated house will cost you more up front, your combined mortgage and heating bill will almost certainly be lower. The higher natural gas prices climb, the happier you'll be with a house that uses little or no natural gas.

Our new house will not have a natural gas connection. There's been a trend in recent years for utility companies to charge higher "connection fees" for gas to cover the cost of maintaining the infrastructure. In the past, your gas bill with everything shut off might be four or five dollars. With the new rate structure, it might be more like fifteen or twenty dollars. The cost per therm is supposed to be lower to compensate, and if you use conventional amounts of natural gas it works out about the same. However, if you burn substantially less gas than average you'll be paying substantially more per therm than your neighbors. It's like a tax on conservation. I calculated the effect a proposed rate change like this would have on a superinsulated house I was thinking of building in Minnesota, and it actually made it cheaper to heat with electricity.  This would only be true for a house with very low heat needs, but that's what I was thinking of building.

Environmentalists tend to frown on heating with electricity. If you burn natural gas in a furnace in your basement, you're extracting 80 or 90 percent of its heat. If you burn natural gas to make electricity, send that electricity through miles of wires, and use it to run a space heater, you're only getting about 30 percent of the energy that was in the original natural gas. But, looking long term, electricity can be generated from wind power without burning anything. There would still be inefficiencies in transmission, but at least the original source would be clean and renewable.

There are similar reservations about the environmental impact of urethane foam insulation. It has been reformulated to no longer damage the ozone layer, but it is a petroleum product and it has a lot of embodied energy. But if a house superinsulated with foam requires almost no heat for the rest of its useful life, at some point that house will have saved more energy than went into making the foam. I believe that using petroleum to make a really superior insulation material is a better use of a limited resource than just burning it for heat.

No one knows for sure where heating fuel prices are going, but it's probably higher. You already hear about retired people whose winter utility bills consume most of their social security checks.  Living in a superinsulated house would let you avoid that scenario in your own future.  It's a design approach that's good for your peace of mind, and good for the planet.

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