Passive Solar Design From a Passive House Perspective

Intermediate

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Good air-sealing practices, as well as ample insulation, are keys to developing an energy-efficient building envelope.
Spray-foam offers good insulation value.
Structural insulated panels (SIPs) are one of the best-insulating wall systems, providing good R-value, limiting air infiltration, and minimizing thermal bridging.
Except in cooling-dominated climates, thermal mass floors should have a thermal break—be insulated—from the ground.
In cooling climates, using higher levels of thermal mass can help reduce the need for mechanical cooling equipment.
In this home, high-mass paperless cement board was used instead of traditional drywall to provide additional thermal mass.
High-performance windows are now achieving U-values as low as 0.09 (R-11), while still maintaining reasonable SHGCs.
Properly shaded south-facing glazing may take many forms, from overhangs to porches to pergolas, but the end goal is the same—preventing excess heat gain during warmer months.
Properly shaded south-facing glazing may take many forms, from overhangs to porches to pergolas, but the end goal is the same—preventing excess heat gain during warmer months.
Properly shaded south-facing glazing may take many forms, from overhangs to porches to pergolas, but the end goal is the same—preventing excess heat gain during warmer months.

The Passive House approach to passive solar home design optimizes a home’s energy and economic performance with the three tenets of passive design—insulation, windows, and mass.

The Passive House (PH) high-performance design has two primary goals: minimize energy losses and utilize passive energy gains. A PH uses up to 95% less energy for space heating and cooling than a conventional house by using highly insulating materials; high-performance glazing; reducing thermal bridging; creating an airtight envelope; and heat/energy recovery ventilation. These strategies can keep a house warm passively with internal heat sources (people, lights, and appliances) and solar energy through the windows. Even the incoming fresh air can be warmed by an earth tube, for example.

The Passive House Institute US (PHIUS) technical committee and Building Science Corporation have researched economical passive design for North American climate zones. The results of that Department of Energy study can be found in the “Climate-Specific Passive Building Standards” report and will be incorporated into the PHIUS certification program in early 2015.

PHIUS emphasizes Passive House methodology, rather than just passive solar. Designing for modest winter solar gain is part of the picture, but not the overriding feature as with homes that are classically passive-solar designed. The three tenets of passive solar design—a well-insulated envelope, optimal thermal mass, and high-performance and well-placed windows—are still important.

Insulation

For Passive Houses, air-sealing and insulation are key, as these homes rely less on passive solar gain and more on limiting heat losses or gains due to air leakage. Dealing with air leakage produces straightforward results—the tighter the home, the less energy is lost or gained. Determining “ideal” insulation levels is more difficult, as it is highly climate-dependent and the right balance must be found between heating and cooling needs.

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Comments (10)

Peter Gruendeman_2's picture

Heat loss error, p45:
Heat loss is calculated by this equation:
loss= area x delta_T x time
where area equals your sample area in square feet,
delta_T equals the temperature difference across a wall,
time, measured in hours.
example using heating degree data:
loss= 100 square feet x 4400 HDD x 24 hours in a day
divided by R-value.
= (100 x 4400 x 24) / 10
= 1,056,000 BTU/year
The chart shows 10 MMBTU which, if the author meant 10 million BTU/ year, is off by a factor of 10. The trend shown by the chart is correct, a trend you can explore yourself by plugging in different R values of insulation. And you can plug in different costs of insulation, with prices you can get from your home center or contractor. Here, costs range from 2 cents per R-square foot for fiberglass batts to 16 cents per R-square foot for spray foam-- materials only. Thicker walls require more wood so be sure to add that as well.
You'll also need to factor in your energy cost. You can use today's costs as a starting point though who is to say what energy will cost even five years from now? With a 5:1 variation in price between today's historically cheap natural gas and electric resistance heat (not an unusual choice for a modest, superinsulated house with annual heating costs on electricity of less than $200.), the choice of insulation values is less about budget and more about materials versus energy costs. Your goal is to design a wall with the lowest sum total of mortgage payment + heating bill. Why pay more to live in a skinny-wall house, with its higher exposure to energy prices, higher noise levels and greater room to room variation in temperature? As mentioned, there are other costs, some that are hard to put a dollar value on, but dollars are a good starting point.
Pete Gruendeman

David Bitter's picture

PHIUS, in cooperation with the Building Science Corporation, has been developing climate specific standards which attempt to find the "sweet spot" (based on project location) where the envelope performance requirements for PH certification make economic sense relative to options such as PV. The new standard is hot off the press. Check out the following link:
http://www.phius.org/phius-2015-new...

Peter Gruendeman_2's picture

My cost on PV is in error, though my starting point was that a local installer showed me that his wholesale cost for made in USA panels including racking was $1.50/ watt. Add to that installation but not inverters as they are Not needed for PV-->heat.
Very few if any Grid-ties are being approved in WI at this time. PV is still very much an option here for heating water and space, both of which can and need to be DC powered at this time. I'll concede that grid-tieing and using heat pumps though requiring more $$ investment, is a good deal. That's not an option here, not right now at least.
What is an option is to wire the panels so their peak power volts and amps is proportional to the ohms resistance of an electric baseboard heater and/ or a water heater element for DHW in summer. Funny enough, I cover sizing these elements in my post on diversion loads (HP166.52). The balance of system in this PV--> heat configuration is cables and one DC rated swtich. No grid-tieing fees are part of the equation. AFIK this is the fairest comparison between $$ spent on passive solar vs the same $$ spent on PV. That the PV can be switched off or switched over to domestic hot water in the summer is a bonus that passive solar can't provide.
Another bonus is that with proper load switching, PV can provide some heat on an overcast day while PS is losing more heat than it is gaining. Even with a load resistance more suited for a sunny day, the PV produces some watts of heat when PS is a net loss.
As I see it, PV is something close to the performance of PS for the same $$ on sunny days and never ever adds to the heating load of a building no matter how uncooperative the weather or how dark the night.
Pete Gruendeman

Lancatronics's picture

Just to let you know Peter, that a California company is now selling Solar PV inverters that take the solar DC power and converts it to wild AC, so it works effiecently on a large range of resistive loads. No need to switch loads! Heat hot water, air or cook with it.
Google cyboenergy.

Andrew Lau's picture

Peter I agree with you that simpler less expensive windows, combined with the homeowner moving insulating window treatments, are better. Your calculation below for windows is $62/(1000 w/m2 / 10.76 ft2/m2) = $0.67/W. But that is not comparable to PV cost at $2/W for several reasons. The biggest one is that the PV cost is for output, not input. At best a window will transmit about 60% of what hits it, I guess you used the SHGF of 0.43. That means the cost for delivered heat is $1.55/W. Furthermore, if heat is provided by a heat pump with a COP of 2.5, then the heat saved is equivalent to less electricity by that factor. That in effect increases the cost of the electricity displaced by the window by 2.5 or $3.9/W. Lastly, a window receives only about 70% of the solar a roof will receive, so to compare with PV that further increases the window cost to $5.5/W. Of course windows have many other benefits than a PV panel does. So good design might be to put in windows as desired for views, ventilation, and aesthetics, and to orient the house so that a lot of the windows are on the south side. But with a superinsulated airtight passive house you don't need more. And if you can get people to interact with their windows, you can use less expensive windows. The cost of better window treatments should be considered along with the windows, and the PV cost should include all of the other system costs which is near $5/W installed.

Peter Gruendeman_2's picture

The author mentioned uPVC windows from Intus, which lead me to:
http://www.intuswindows.com/product...
The averages of the ranges stated there, combined with the prices mentions works out to:
Solar heat gain (0.25 to 0.62)/2 = 0.43;
R-6.3
cost ($50 to $75)/ 2 = $62;
There is 10.76 square feet per square meter
and sunshine is 1,000 watts per square meter on a clear day.
If you grind this through, these windows are providing heat for $1.55/ watt-- Not cheap at all. This is at noon with the sun perpendicular to the glazing. At all other times, the cost per watt is higher. Night time happens around here as does cloudy weather. Then the R-6.3 windows or same plus window quilts/ thermal shutters are supposed to keep the cold out. Windows oriented other than true south are R-6.3 holes in what otherwise might be a R-20 or better structure.
Solar hot air panels produce heat for around 50 cents a watt and even PV is less than $2./ watt. I like to look out of my windows, but my feet and knees don't need to see out. It would be good if people could discover solar hot air, PV or at least discover walls though none of these look as flashy in design books or magazines. Let's stop pretending that all this glass is efficient. It's a luxury, which is fine for those who accept that it's nothing more than that.
Pete Gruendeman, La Crosse, WI

Scott Turner's picture

The house in the photos is scary bad design. Excessive exterior wall surface area causes increased heat loss and gain. Excessive glass pointing every which way. Flat roofs are bad period. I could go on.

Mass and glass is indeed a poor choice. Lots of glass is good in some climates, but not directly in the living area.

Andrew Lau, you are right. Plus angle of incidence of solar radiation on vertical glass in summer is so poor that reflectance eliminates much heat gain. Diffuse IR radiation is the real problem in summer and is not affected by over-hangs.

Andrew Lau's picture

Fixed overhangs don't work well on south windows either. They don't block much of the summer sun that comes mainly from the entire sky and reflected from the ground. They can however block sun in the swing seasons when you may want it for heat.

David Bitter's picture

The least expensive I have found are the UPVC windows from Intus…either their Arcade (least expensive) or Eforte models. If you work with the rep you can avoid the $4,000 shipping charge for smaller orders if they can combine multiple orders so they are shipped together. Have seen $75/SF to under $50/SF.
Would like to hear of any less expensive options that offer same performance.
David - AIA, CPHC

Peter Gruendeman_2's picture

I suspect these U-0.125/ R-8 windows are not cheap. At one passive house near here, they paid $100per square foot for windows. At the other one, a hand motion suggested they paid more than that. What prices and cost info can you provide?
Here in WI, fiberglass batt insulation costs about 2 cents per R-square foot, and in 30 pound containers spray foam costs 16 cents per R-square foot, materials only in both cases. I assume you are dispensing the foam of big drums, at what cost per R-square foot? How essential is it to use spray foam? I can air seal and provide an excellent vapor seal with 6mil LDPE which costs 12 cents/ square foot.
Pete Gruendeman, La Crosse, Wisconsin
(Around here, a 4,400 HDD climate is regarded as the tropics)

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