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Stephen's Column
The Affordable Energy Efficient House
The key to an affordable energy efficient building is to make both good choices and good compromises. One of the more clever compromises is to shuffle money out of HVAC (heating, ventilating and air conditioning) and put it into Envelope, (not an unmarked brown, but rather roofs, walls and floors).
Over time, HVAC systems have evolved to more effectively and efficiently counteract the heat loss from poorly insulated, drafty buildings. However, they are getting very expensive to buy. And even the most evolved still cost money to operate. Additionally, they also need to be replaced in whole or in part, every few decades.
If the truth were said, they are temporary patches for inadequate building envelopes.
Put another way, HVAC systems treat the symptoms, rather than the disease. A good Envelope, on the other hand, does the reverse. It minimizes the need to ameliorate symptoms.
Besides their fuel costs, HVAC systems also suffer from the requirement for regular maintenance, something an Envelope usually does not require. The bigger and more complicated the HVAC system, the greater the potential for problems.
As mentioned earlier, HVAC systems are also relatively short-lived. They or their key components will need to be replaced several times over the life of a building. The Envelope is more permanent. It usually lasts many generations without the need for any modification.
HVAC systems like ground source heat pumps and radiant floor heating systems can easily add tens of thousands of dollars to the cost of a house.
While these systems may use less fuel over their lifetime than other approaches, they still use fuel. An equivalent investment in the Envelope will typically produce even larger savings for many more years than these complex HVAC systems.
It is currently not realistic to completely eliminate HVAC systems. Controlled ventilation is not yet easily achievable by strictly passive means. So while they can't be eliminated, they can be simplified.
A good Envelope means perimeter heating systems are nearly always unnecessary. This can be a big savings in commercial buildings. In houses duct runs can be shortened, ending at inside, not outside walls. In fact, a building with a good Envelope should not require a dedicated furnace.
This is old news to some people. It was over 10 years ago we had our first customer build a house without a furnace. Their water heater satisfies his family's space heating requirements. Their HRV (Heat Recovery Ventilator) preheats the incoming fresh air with the outgoing stale air. If required, a coil connected to their water heater further heats the incoming air. (A coil is fluid to air heat exchanger and looks like the radiator in your car).
A more recently, another customer uses a 1000 W electric heater, downstream of their HRV to heat her house. (For the record 1000W is the equivalent of a hair dryer set on medium.)
It's not complicated to build a building with a simple HVAC system. The common threads among these houses are high levels of air tightness and insulation with some passive solar design.
- A high level of air tightness means tested air tightness of at least 2.5 air changes per hour (ach) @ 50Pa (the Canadian Energy Star standard), preferably 1.5 ach @50Pa (the Canadian R-2000 standard). Some houses have tested below 0.25 ach @ 50Pa. Consider giving your contractor a bonus based on the level of air tightness they achieve.
- A high level of insulation means levels of at least: RSI 9+ (R-50+) attic, RSI 5+ (R-28+) walls and : RSI 4+ (R-22+) below grade and, of course, - triple glazed windows.
- Passive solar design means biasing windows towards the south. Most sources suggest south facing window area equal to 6-8% of the floor area. Passive solar also means making some effort to store daytime gains in thermal mass like masonry elements or a double thickness of drywall. A simple way to increase thermal mass is to leave drywall scraps within internal walls.
The exact costs involved in the trade-offs between HVAC and Envelope vary. They vary not only from region to region, but also from house to house, so it's not possible to make blanket statements about how and where to make HVAC/Envelope compromises.
Never the less, since a good Envelope means HVAC systems can be downsized and simplified without compromising comfort, you'll almost always save money by spending more on Envelope and less on HVAC.
- Frame In? Frame Out?
- All Low e’s Are Not Created Equal
- Double Hung Inherently Flawed
- The Road Less Travelled
- The Affordable Energy Efficient House
- Green Priorities - Skin Deep Not Deep Enough
- Embodied Energy - As Important As Low Energy Design?
- Energy-Star's Solar Eclipse Ending?
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