There was a bit of a stir lately within the building science online communities when a well-known and respected building scientist published a review of the Passive House standard.

John Straube published a review of the Passive House standard on BuildingScience.com compared to standards and practices applicable in the U.S. and Canada, available at http://www.buildingscience.com/documents/insights/bsi-025-the-passivhaus-passive-house-standard. In it Straube takes a close look at Passive House from a North American context, comparing it to other low-energy building systems for cold climates.

Katrin Klingenberg of Passive House Institute U.S. posted a lengthy response to John Straube’s article at http://www.passivehouse.us/bulletinBoard/viewtopic.php?f=5&t=208, correcting a few misunderstandings.

On GreenBuildingAdvisor.com, there is an initial discussion and reaction to Straube’s article at http://www.greenbuildingadvisor.com/community/forum/passive-house/14647/very-recent-passivhaus-article. Later, Marc Rosenbaum and David White wrote a point-by-point clarification of why the Passive House Standard sets a worthy goal for North America at http://www.greenbuildingadvisor.com/blogs/dept/green-building-blog/defense-passive-house-standard

Also on GreenBuildingAdvisor.com, Martin Holladay and John Straube discuss the lowest cost approach, which differs from Passive House. The lowest cost approach improves the building envelope until the incremental cost of further improvements would be more expensive than photovoltaic technology. Passive House on the other hand looks at absolute energy consumption of the building envelope, so in a sense it is more “future-proof” than a house with more technology, and is likely the best choice for a future with constrained energy supply. Plus, it is much less expensive to add PV later than to retrofit additional underslab insulation. See http://www.greenbuildingadvisor.com/blogs/dept/musings/can-foam-insulation-be-too-thick

And finally, John Straube clarified his position in the whole discussion at http://www.buildingscience.com/documents/insights/bsi-026-passivhaus-becomes-active-further-commentary-on-passivhaus.

If you are interested in helping bring Passive House to Canada, visit http://www.passivebuilding.ca and join the email discussion list.

This situation can occur when the capital cost of installing a greener heating system, such as a “geothermal” ground source heat pump, depends on the design heat loss, or required capacity. A geothermal system can be so expensive that on a lifecycle cost basis it’s one of the few heating systems where it’s good practice to equipment for less than what’s needed under design day conditions, by up to 30% less.

First, you need to make sure that you have completed a full building energy model. You should also be familiar with the design heat loss concept and how it is calculated. Beware the programmer’s motto: garbage in = garbage out. For existing buildings if at all possible, verify the model against actual energy bills and historical weather data. To use historical weather data, overwrite it in the climate data section on the Start worksheet.

On the climate data screen, make a note of the annual degree day information:

Annual heating degree days in RETScreen

If you have specific degree days to use here, by all means enter them. RETScreen uses 18 C for heating degree day values. Simply erase the whole column and put the annual degree days all into January:

Using a custom annual degree days value in RETScreen

Next, on the Energy Model worksheet make the following changes:

• Set all schedules for 24/7 and occupancy rate at 24 hours a day all year
• Set Temperature – space heating to 21.0 C
• Set Heating/cooling changeover temperature to 18.0 C
• Select Energy – base case in the Show field. Use GJ as the reporting number, as this will give you the best accuracy.

Preparing RETScreen Energy Model worksheet for design day heating calculation

Now you’ll need the reported heating energy required due to the building envelope.  Use the sum of Heating GJ numbers, or better still, add up only those that relate to space heat. Typically this means the Building Envelope, Ventilation and Lights. Exclude Hot Water because it goes down the drain.

Check to see if your Electrical Equipment is marked as having an effect on space heat or not. It’s up to you. Since I place lighting loads in the Electrical Equipment section, I include it as having an effect on space heat.

Next, open up a regular spreadsheet. Obtain a whole-building UA factor by multiplying your heating GJ by 1E9, and then dividing by: (heating degree days * 24 * 3600).

Multiply the result by your desired delta T in degrees Celsius. That is, your desired indoor design temperature minus desired outdoor design temperature. Use whatever you want or are required to under code.

The result is the instantaneous (steady-state) design heat loss in Watts. Divide by 1000 to get kW, and then multiply by 3412 to get the design heat loss in BTU/h.

That’s it in words, here it is with numbers.

Given: 4194 Heating Degree Days, heat energy 522 GJ, outdoor design temperature of -23 C, and indoor design temperature of 21 C.

Heat loss = (522 * 1E9 / (4194 * 24 * 3600)) * (21 – (-23)) / 1000

Heat loss = 63.4 kW

Multiply by 3412 to get BTU/h:

Heat loss = 216,267 BTU/h

Now take this number to your mechanical contractor and find out how much it would roughly cost to install this capacity to complete your financial analysis. If you are considering capital-intensive equipment at less than full capacity, you can use a Heating model in RETScreen to specify an undersized heat pump that still provides for over 90% of the annual heat load.

Beware, RETScreen rounds its values to the nearest GJ, which for small buildings can make the design heat loss number very uncertain.

Finally, keep in mind that RETScreen is based on metric calculations, and accuracy drops in some unit conversions.

Two houses of equal area but different shape

Architects designing custom homes often seem to go overboard on the complexity of the shape, seemingly unaware of the impacts on construction cost and operational cost. Let’s look at the walls of two custom homes of equal floor area but different shape.

Both homes are 5400 square feet on one level, nice sprawling ranch homes for a well-to-do business person. The first thought is that the house is too big, and it is, but this not an unusual size in the area of custom homes. It’s still well within Part 9 of the Ontario Building Code, which deals with houses and other small buildings.

House A was designed with a highly irregular shape to add “architectural interest”. It follows the room layout and offers more views of the local scenery. From the plans we count 56 corners.  This is not unusual for this size of house. If you walked around the perimeter of house A with a tape measure it would be 600 feet.

House B was designed by a home designer-builder who doesn’t know a lick about heat loss either, but didn’t want to do all the design work of a complicated home, especially the roof, and wanted a simple L-shape so that it would be faster to construct. House B has a perimeter of 370 feet and just 6 corners.

Assume the walls are 9 feet high. Let’s also assume an insulation R-value of R-20 for the walls and R-2.00 for windows. In reality the effective R-value for house A will be lower because of the extra wood framing material used to create the more complicated shape, but let’s hold as many things constant as we can for this exercise. Also assume that the windows are 40% of the wall area. Each of the two homes will be built in Toronto, which has 6426 heating degree days (in degrees Fahrenheit).

The way to calculate heat loss for a year is with the equation heat = 1/R * area * heating degree days * 24 hours per day.

When this is all multiplied out, the energy lost during the heating season is 192 million BTU’s for house A and 118 million BTU’s for house B.

Let’s put some dollars to that. Converting to kilowatt-hours the amount of heat is 56140 kWh for house A and 34620 kWh for house B.  For simplicity’s sake let’s say you’re paying $0.10 per kilowatt-hour after taking into account the distribution fee, debt retirement charge, taxes, and so on, and you decide to use electric baseboard heat that consumes 10% more energy than actually required, because of delays in responding to the thermostat’s call for heat. The annual heating cost of just the walls and windows is $6,175 for house A and $3,808 for house B.

That’s an added cost of of $2,367 per year, every year, for having a complicated shape instead of a simpler one. The same floor area, same wall insulation, and same window quality. If you prefer the environmental route, that’s 4.6 more metric tonnes of CO2 per year, about the emissions of an average passenger vehicle.

What about equipment costs? If it was heated with a furnace or hot water boiler the capacity of the equipment would need to be correspondingly larger and thus more expensive. That wouldn’t be too much—but move to a geothermal system and look out! It could be $12,000 more in ground loop installation costs and larger equipment.

I’ve only looked at conductive heat loss. The most diligent builder is going to have a great deal of difficulty providing a continuous air barrier with a more complicated housing plan. If the builder managed to pay very close attention to detail, even if the normalized leakage area (expressed in square inches of hole per square foot of wall) were the same between the two houses, the equivalent leakage area would be proportionally larger. Given that infiltration commonly accounts for 30% of heat loss, the impact is significant.

Then there’s the foundation required to support the wall. Ben Polley of Evolve Builders Group in Guelph, Ontario, tells me that in his estimation, every corner of a foundation wall adds the equivalent of 5 linear feet to the cost due to formwork and labour. Assuming 10 inch thick walls, 7 feet high and $80 per cubic yard of poured foundation wall, it would cost $15,210 for the house A wall foundation and $6,913 for the house B wall foundation, a capital cost increase of $8,297.

Don’t even get me started on the complicated roof design that goes on these complicated houses. Ask your local roof framer.

The take away from this evaluation is that “corners cost energy” and “corners cost money”. Lots of money.

Other free energy modeling with a 3-D interface is eQuest and a beta Google Sketchup interface for EnergyPlus.

Of course, the idea of the free back end and plugin is to get you hooked enough to buy the expensive modeling packages. How expensive? I don’t know, you have to call or write for details. Usually this means “If you have to ask, it’s too expensive”. But if the tools are worthwhile I’m not that concerned about the price. Both Carrier and Trane publish prices for their hourly building simulation software, which can result in some sticker shock. But at least the price is known.