[Greenbuilding] Passivhaus propaganda

[nick pine]

"Shrink That Footprint" (a Passivhaus Astroturf lobby?) at 
http://cleantechnica.com/2013/11/26/value-well-insulated-house/#BvfyIGJDp0mUmBxC.99 
says:

>The leaky house has solid walls, poor loft insulation, an uninsulated 
>floor, single glazed windows, and lots of draughts. Because of this, it 
>needs 300 kilowatt-hours of heating for each square meter of space per year 
>(kWh/m2a) just to stay warm.

How large a house? How warm? In what climate? Maybe the UK, with lots of 
draughts. It is unwise to believe everything one reads on the internet.

>The modern house has insulation in the wall cavity and loft, an insulated 
>floor, double glazing, and some draught excluders. Because of its better 
>insulation, it needs just half the heating of the leaky house, 150 kWh/m2a, 
>to maintain a similar internal temperature.

How much insulation in the wall cavity? Similar to what internal 
temperature?

>The passive house has superb insulation in all materials, triple-glazed 
>windows that face the equator to maximize solar gains, and is so air tight 
>that it uses a ventilation system to keep the air fresh. It needs just 15 
>kWh/m2a of heating.

How superb? :-)

>What does this mean in simple terms? The leaky home's heating bill might be 
>$1,500 a year, $750 for the modern, and $100 for the passive house.

Might? What's the cost per kWh? Why not  less  than 15 kWh/m^2 per year?

Would a 2400 ft^2 (223 m^2) Passivhaus near Philadelphia require 223x15 = 
3346 kWh/year or $100/($0.168/kWh) = 595 kWh/year of $0.168/kWh PECO 
electricity?

Germany has no sun. Why do we have to build houses like they do? In my 
opinion, this anonymous research group spent too much money insulating and 
making this mythical Passivhaus airtight, and not enough on solar air 
heaters.

Where I live near Phila (4954 HDD with 65 F base), an average-sized 
40'x60'x8' tall pre-McMansion with 4000 ft^2 of wall and roof surface with 
no windows and no air leaks and no heat loss down through the floor could be 
heated to 65 F with 595 kWh/year (about 2.03 million Btu/year, like 200 
gallons of oil) with a 4954x24x4000/2.03M = R234 wall and roof R-value, eg 
47 inches of R5 per inch Styrofoam :-)

A few inexpensive PVs and a minisplit heat pump with a COP of 5 would allow 
a zero-energy house with R47 walls and roof, but Passiv purists (eg Hudson 
at http://hudsonpassiveproject.com/about.html ) refuse to use such "external 
technologies."

Let's assume that all of the 2.03 million Btu/year backup fuel helps heat a 
direct gain house with no night or cloudy-day window insulation in December 
and January with 2.03M/62 = 32.7K Btu/day, and the house solar heats itself 
with no fuel during the rest of the year. And let's cram lots of room 
temperature thermal mass into the house: a 20K Btu/F 4" slab + 
3000x8"/12"x25 = 50K Btu/F 8" solid concrete walls + 10K Btu/F of 
furnishings, totaling 80K Btu/F. With this huge thermal mass, how much 
insulation do we need to heat the house for $100/year?

An average January day in Phila is 30 F. A house with a G Btu/h-F 
conductance would have an RC = C/G hour time constant. C = 80K Btu/F and G = 
200 Btu/h-F makes RC = 400 hours. The house would cool from 70 to 60 F after 
Dc = -RCln((60-30)/(70-30))/24h = 0.012RC cloudy days, eg Dc = 0.012x400 = 
4.8 days, using the numbers above. With a coin-flip cloudy day model, this 
makes the average January solar heating fraction f = 1-2^-Dc, eg 0.964 
above. On a cloudy day, with no stored heat, the house needs B = 24h(65-30)G 
= 840G Btu/day of fuel, eg 840x200 = 168K Btu, using the numbers above. So 
the house above needs Y = 62(1-f)168K = 375K vs 2.03M Btu/year of fuel. 
Oops. This Passivhaus is way too good. We spent way too much money on 
thermal mass and insulation and airtightness.

Let's try G = 350. Then Dc = 0.012RC = 959/G = 2.74 days and Y = 62(1-f)840G 
= 52KGx2^(-959/G) = 3.32M vs 2.03M Btu/year. Oops. This Haus is too bad.

Y = 2.03M = 52KGx2^(-959/G) makes G = -959ln(2)/ln(39/G) = -665/ln((39/G). 
Plugging in G = 300 on the right makes G = 326 on the left. Repeating makes 
G = 313, 319, 316, 318, and 317. This Haus is just right.

It needs 24h(70-30)317 = 304.3K Btu to stay 70 F on an average January day, 
when 1000 Btu/ft^2 of sun falls on a south wall in Phila... 304.3K/500 = 609 
ft^2 of R4 argon triple-glazed direct gain south windows with 50% solar 
transmission (what do they cost?) 
http://www.greenbuildingadvisor.com/blogs/dept/musings/choosing-triple-glazed-windows 
and no night insulation could provide that, altho that's 27% larger then the 
south wall area :-)

The windows would have a 609/4 = 152 Btu/h-F conductance, leaving 317-152 = 
165 for the rest of the house, or 165-2400/40 = 105 for the walls, given an 
R40 ceiling. So the walls need a (1600-609)/105 = 9.4 R-value, which seems 
oddly reasonable for a Passivhaus, until we recall the huge thermal mass and 
the assumptions that it is absolutely airtight and there is no heat loss 
through the floor.

Removing the windows and their conductance would make G = 165 Btu/h-F, so 
the house would need 24h(70-30)165 = 158.4K Btu to stay 70 F on an average 
January day. A 1 ft^2 80 F air heater with R2 glazing with 80% solar 
transmission would gain about 0.8x1000-6h(80-30)1ft^2/R2 = 650 Btu/day, so 
we could heat this Haus with 158.4K/650 = 244 ft^2 of air heaters over the 
480 ft^2 south wall.

With an R40 ceiling and R30 wall insulation, G = 2400/40+1600/30 = 113 
Btu/h-F and f = 0.9998 and Y = 1213 Btu/year (way too good :-) This house 
could have 24(70-30)113/650 = 167 ft^2 of air heaters.

A less expensive house could have less concrete. G = 113 makes Y = 
5.89Mx2^-Dc = 2.03M Btu, which makes 2^-DC = 0.345 and Dc = -ln(0.345)/ln(2) 
= 1.54 days, ie 36.9 hours, with RC = -36.9/ln(0.75) = 128 hours = C/113, so 
C = 14.5K Btu/F. IIRC, an average house contains about 7K Btu/F of building 
materials and furnishings. We could add a slab, or a concrete wall, or 
(14.5K-7K)/5 = 1500 concrete blocks and boards for a bookcase, or 2 shallow 
pebble beds above and below a few 55 gallon water drums, which could help 
heat water for showers.

Nick