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re: building an eco house in the uk?
2 sep 2000
news wrote:
>nick pine wrote in message
>> > 1) superinsulate the house. this will save a ton of heating money...
no argument there, but let's focus on how much heat the house uses,
vs how much it "saves." that's independent of the alternatives, and
it's harder to fudge, especially if there is no "backup heater."
>with the house looking like any other...
>> arithmetic indicates that if it's only heated by a few people
>> and their compact fluorescent lights, it will be more like
>> a sleeping bag than a house.
now we seem to have a difference of opinion. does a sleeping bag look
like "any other house"? :-) one of my profs used to say that a chair
is a lot like a beach ball, in the sense that wrapping it with enough
cotton wool makes it look just like a beach ball...
perhaps we need to clarify what we mean by a "superinsulated house."
is it 1) a very-well insulated house with few air leaks and a very low
heating bill, compared to other houses, or 2) a house which is entirely
heated by the occupants and their frugal electrical usage, which has no
"backup heating system," by definition. perhaps we should call these
"hyperinsulated houses." we've done several calculations that say they
need to be very small. perhaps we need to do more, to banish this myth.
or... find a genuine example. but i doubt they exist.
let's try again: it's 20 c indoors and 0 c outdoors, at night. enid and
wolfgang each contribute 100 watts, and they use 300 kwh/mo of electrical
energy in the house, which adds 417 watts. they get 15 l/s (30 cfm) of
fresh air from a 100%-efficient air-air heat exchanger, and the house
is absolutely airtight. it would float forever in a flood. the maximum
possible total house thermal conductance is 617w/(20c-0c) = 31 w/c (58
btu/h-f.) four 1 m^2 metric r1 (us r5.68) windows lower the max wall
conductance to 27 w/c. say it's 10 m square and 2 m tall. with 176 m^2
of exterior surface, the walls and ceiling need an r-value of 176/27
= 6.5 c/w (us r37) with no thermal bridging. well maybe, with 10 inch
sip walls.
but the best sip builders only guarantee 0.2 air changes per hour, ie
7.2 l/s (15.2 cfm) of air leaks in this case, vs. "airtight," which makes
the walls 12 inches thick. and what if the house is larger or taller, or
the air-to-air heat exchanger is not 100% efficient, or the occupants use
less electricity or go to work during the day?
a 10x10x5m tall house with 0.2 ach (28 l/s or 59 cfm) and no air-air
heat exchanger and 8 w/c of windows and an average of 100 watts of body
heat and 180 kwh/mo of electrical energy usage needs g=350/20=17.5 w/c.
subtracting the windows gives 9.5. subtracting the air leaks makes this
minus 0.9 w/c, so it can't be heated this way, even with walls that are
10 meters thick. otoh, it can be 100% solar heated.
>> > superinsulated houses cost little, or nothing,
>> > over non-superinsulated to build.
>>
>> especially if they are a lot smaller, with fewer windows.
>
>which tends to be the trend in the us - moving away from the overlarge hulks
>and using superior interior design to maximise space and make the houses
>more cosy.
that makes sense, but it isn't the trend where i live. real estate
columnist alan j. heavens (aheavens@phillynews.com) writes: "we are
building larger homes. in 1950, the typical new home was 983 square
feet, according to the national association of home builders. in 1999,
it was 2,225 square feet. in some markets and price ranges, houses
of 4,000 or 5,000 square feet are standard." this 1.6% yearly growth
rate can't continue with limited land and energy resources. if it did,
the average american family would be living in a house with 10 acres
of floorspace by the year 2323.
>> > it eliminates a heating system, with only a backup required.
>>
>> what's the difference between a heating system and "a backup"?
>
>about £900 a year in heating bills. superinsulating can give
>heating bills of £30-50, dropping from £1,000 per year
nice, but that wasn't the question. superinsulation is wonderful, but
let's tell the truth: there's still a heating bill, and superinsulated
houses are not entirely heated by the occupants and their frugal
electrical usage...
>eliminates a "full" heating system.
oh. it eliminates a "full" heating system...
>a backup may only be a copper coiled fan convector in the hallway or the
>living area of an open plan, to bring the house up to temp asap and the
>people and lights etc take over.
sounds like a "full" heating system to me. what heats the fan-coil?
and here's that myth again, that people and lights can keep a house
warm all by themselves, after the house is "brought up to temp..."
>> i wonder if any uk houses have "frost protected warm foundations,"
>> a newish technique that puts the foundations over (vs in) ground
>> that never freezes because it is horizontally insulated...
>...this is not mainstream - either is it in the us.
it seems to be growing in the us, with help from government studies.
>...only 1" of insulation under the foundations slab and heavy insulation
>on the sides of the foundation walls.
"foundation walls" that are in vs. on the ground aren't the same thing.
>> > the deep 300mm void can be packed with insulation (breathing wall)...
>>
>> this sounds different from scandinavian "breathing walls" and ceilings
>> with slowly-moving wavefronts of air flowing into the house to make
>> the r-values infinite. any of those in the uk?
>yes. the breathing wall is slowing gaining acceptance. i believe it isn't
>in the us. the breathing wall breaths "vapour", not air. that is the idea
>behind it. to allow moisture to leave the building and not cause
>interintestinal (sp?) condensation.
could be a serious medical problem. scandinavian breathing walls are
different. they breathe outdoor air into the house. the idea is this:
if air is only moving from a cold object (eg a wall) towards a warm
object (eg a person), the warm object can't lose any heat to the cold
object by convection. (the person still loses heat to the wall by
radiation, unless it's a mirror.) for this to work well, the air must
move out from every cold surface, all of the walls and the ceiling.
and it needs to be warmish air (a stack-effect-powered air-air heat
exchanger can help) and moving with a low velocity that isn't perceived
as a draft.
>> > 8) use air-locks (porches) on all external doors.
>>
>> not worth the energy saved, unless you live in a department store.
>
>they also serve as cloakrooms etc, that add value to a house.
perhaps they add charm as well, but we are talking about energy...
>...in a house with a small backup heating system you don't want warm air
>constantly pouring outside with children constantly in and out.
say the house thermal conductance is 50 w/c, and each door opening admits
35 l/s (1000 cfm) of outdoor air for 5 seconds, increasing the heat loss
of the house by about 16 wh (55 btu) for each opening. the children would
have to open the doors 150 times per day to raise the heat loss 10%, and
an air lock with any significant volume wouldn't save much energy anyway.
>> > also in scotland:
>> >
>> > architect builds affordable zero heating house
>> >
>> > architect gokay deveci has build an affordable rural
>> > home in scotland that needs no heating. the building,
>> > near aberdeen, was designed to be heated solely by
>> > the occupants' body heat and electrical equipment...
>> > they have used a small wood burner occasionally,
>> > but only for a maximum of half-an-hour on any
>> > day, he said...
>...these houses are an attempt to cajole a conservative
>construction industry and population that these are feasible.
the cajoling might be more successful with fewer false claims
and obvious contradictions.
nick
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