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 fiberglass vs. polyurethane foam 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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Comments: 24
It's much easier to do superinsulation with new construction. In a retrofit you either have to build out and put new siding on, or tear the plaster off the inside walls and build in, losing some floor area. It can be done, but it's a major undertaking.
If the plaster were in bad shape but you couldn't spare any floor area, even just filling between the studs with 3-1/2" of high density spray-in-place polyurethane foam would be a major improvement because of the reduced air infiltration. An additional inch or two of polyisocyanurate sheet foam before the drywall goes on would reduce thermal bridging through the studs without making the interior much smaller.
The other evening the dome's interior was 50 degrees at 5:00 p.m. Overnight, the outside temperature dipped below zero. At 9:00 a.m. the interior temperature was 43.7 degrees. It had dropped 6.3 degrees in 16 hours, which is not bad for a totally unheated structure on a cold night.
I'm very optimistic that the BTUs from a few lights and appliances will be enough to counteract such slow heat loss.
Is that enough to use electric space heating as the primary heat source and not break the bank? We want to use tankless hot water, with mini on demand units at each sink. We would like to avoid putting any fossil fuels on the property. We had intended on putting highly compacted cellulose insulation into the walls and ceiling with a foam board layer in order to get a true R30 in 6 inch walls. We'd love radiant flooring but it seems uneconomical.
Keep us posted on what you are doing. Thanks.
R-30 walls may be a little skimpy to go with electric heat in a low-solar situation. I'd recommend R-40 walls and spray foam insulation rather than cellulose. With any insulation other than spray-in-place foam, your main losses will be through air infiltration.
PEX is not too expensive, you could put it in the slab yourself if it hasn't been poured yet.
But concrete has a low R-value, so it doesn't do as much good in climates where winter means many months of cold days and even colder nights. The main thing you need then is insulation, though thermal mass in there could still help. The sun heats your concrete for free, then the concrete releases its heat to your rooms through the night.
A couple of questions:
-Has the question of "how much insulation is enough?" really been studied in any systematic way? I've seen the government-produced map for recommended insulation levels in various climate zones, but it looked pretty much as though anywhere from New York to Nome called for R30 walls and R60 roofs. I see that Steve has gone much higher than that in the walls (maybe I missed the roof specification), to R60, but why stop there? Why not R100? R200?
I know that at least part of the answer would be "because the walls would be 6 feet thick", but would if it were technically possible to make a 12-inch wall that was R200, would it be better than R60? Does anyone really know?
-My other question: Steve mentioned that this kind of insulation is good for cold climates, but wouldn't it also work in hot climates (either way, the goal is to keep heat from crossing the wall)?
Lastly just a comment about passive solar: I don't think that passive solar homes necessarily have to overheat, if the windows and the thermal mass are sized correctly. It sounds like with Steve's approach, one would have to be pretty careful about those calculations, but possibly by combining passive solar and superinsulation, maybe even the space heater could be eliminated. The job of thermal mass in particular is to resist changes in temperature, so it seems like it could be made to go hand-in-hand with superinsulation which has the same goal.
Anyway, we live in SE PA and want to employ a consultant to design the insulation of our planned new home. Anybody in mind? We are wood heat enthusiasts, and this new home is planned to be superinsulated with electric under floor heat grid as wood stove back up. And, it will be passive solar. Thanks! Eleanor
From June 2008 to June 2009, we spent $1196 for 10,930 kilowatt hours of electricity at just under 11 cents per kwh. That averages out to $99.66/month for 911 kwh, which is almost exactly the average U.S. electric bill. Of course, most people also have a gas bill. We didn't burn any natural gas, oil, wood, propane, corn, or anything else for heat, it was all electric. I suspect that most of this electricity was used by the water heater.
The coldest six months averaged $128 for 1308 kwh/month, and the warmest six months averaged $71 for 514 kwh/month. If you compare the heat-using half of the year with the no-heat half by cost, it suggests that we spent $343 on electrical space heating for the year.
While $343 is not bad to heat a 1770 SF house with 20' ceilings over an unusually chilly Iowa winter, I was hoping it would be even less. I suspect two main reasons:
1) In the design phase, I was very worried about summer overheating so I undersized the south windows. Apparently passive solar gain is a bigger part than I realized of why superinsulated houses work so well in the winter. The rule of thumb is to have south glass equal to 8% of your floor area, and ours is under 2%. The south glass should have insulated shades, well-designed overhangs, and not use the kind of low-e coating that reduces solar gain.
2) The foam contractor ripped me off. I ordered ten inches of 1.75 lb foam, which would have given us R-60 everywhere. What neither I nor the foam contractor knew was that you need to build it up in 3" layers. Each layer should have a day to expand before you shoot the next three inches. He instead shot the whole ten inches at once for the top half of the dome. The foam couldn't expand properly, so his chemicals didn't go as far as they should have. Rather than make less money, he waited until I had to be out of town and then quickly covered the bottom half of the dome with much-cheaper fluffy 0.5 lb foam, followed by one deceptive inch of the rigid 1.75 lb stuff that I was supposed to be getting throughout. I discovered this after paying him, unfortunately. So we have the dome equivalent of R-40 walls and an R-60 ceiling. Which is still good, but not as good as the R-60 everywhere I paid for.
The other consequence of him shooting 10" of foam all at once is that it couldn't cure properly and the foam REEKED for months. Even now, 2-1/2 years on, we'll still sometimes catch a whiff of foam smell when it's hot and humid out. If only he had sprayed the foam in 3" layers, the foam smell would probably have been gone in a month. So if you use more than 3" of spray-in-place polyurethane foam, get it in writing that they won't spray more than a 3" thickness in a day, then watch 'em like a hawk.
This next year I am adding more sg footage and redoing the previous existing 2x4 walls. I was thinking of using some spray foam and after steve's experience with it shall have to carefully consider who I use for that. I live in spokane wash and we don't have the extreme cold of iowa but do have nights at zero or below.