Musings of an Energy Nerd

During the winter, when indoor air is usually warm and humid, most wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. is cold. Under these conditions, we really don’t want water vapor to move from the interior of our homes toward the exterior. That’s why builders in the 1980s installed polyethylene on the interior side of walls.

If your house has an exhaust-only ventilationMechanical ventilation system in which one or more fans are used to exhaust air from a house and make-up air is supplied passively. Exhaust-only ventilation creates slight depressurization of the home; its impact on vented gas appliances should be considered. system, does it need passive air inlets — that is, holes in the wall to let in outdoor air? In most cases, the answer is no. Unless your house is very, very tight — close to the PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. standard of 0.6 ach50 — your building’s envelope is almost certainly leaky enough to allow for the smooth operation of a bathroom exhaust fan rated at 50 cfm or 100 cfm.

If it is designed well, the thermal envelope of your home should control the flow of heat, air, and moisture. Unfortunately, the floors, walls, and ceilings of older buildings are often leaky: they leak heat, they leak air, and they leak moisture.

If you are building a new house, you have the opportunity to control the flow of heat, air, and moisture through your home’s building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials.. The result will be a durable, comfortable building that doesn’t cost much to heat and cool.

Most air conditioners and heat pumps sold in the U.S. — including most split-system air conditioners and ductless minisplits — are air-to-air heat pumps. During the winter, these appliances extract heat from the outdoor air and deliver warm air to a house through ducts or small fan-coil units. During the summer, these appliances deliver cool air to a house and dump unwanted heat into the outdoor air.

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Homeowners’ interest in energy efficiency measures waxes and wanes. During the 1970s, when oil prices repeatedly spiked upwards, everyone wanted to save energy. However, in the 1980s, when oil prices collapsed, Americans forgot about saving energy, and most of us reverted to our usual wasteful habits.

By 2008, oil prices were high again, and green builders were receiving lots of phone calls from homeowners who wanted lower energy bills. But between June 2014 and now, oil prices have collapsed again, tumbling from $115 to between $37 and $40 a barrel. This raises several questions:

If you go to one of the many web sites that sell house plans, you can use filtering software to sort through hundreds of available plans by a variety of criteria: square feet, number of bedrooms, number of bathrooms, number of stories, or even “number of garage bays.”

On most of these sites, however, you can’t sort by climate zone. Why? Because most house plan companies ignore climate. They’re happy to sell customers in Minnesota the same house plan that they sell customers in Florida.

If you own an older Cape Cod home, you have my sympathy. If you’re the type of homeowner who regularly tackles DIY projects, you’ve probably spent weeks chasing air leaks with a foam gun, lying on your back in a cramped attic. And there's a good chance that, in spite of your efforts, your house still suffers from ice dams.

I’m sorry for your troubles. You deserve better.

If you are thinking of building a new Cape, it’s not too late to get the details right — as long as you’re still at the planning stage.

Residential construction practices in the U.S. have a certain Wild West flavor. Quality standards vary widely from one area of the country to another. For example, while builders in some regions pay meticulous attention when lapping a water-resistant barrier (WRB) or installing wall flashing, builders in other regions ignore best practice recommendations and code requirements.

Compared to a photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. system, a solar hot water system yields very little energy per dollar invested. I presented that argument in a 2012 article called “Solar Thermal Is Dead.” Two years later, in 2014, an economic comparison between these two solar technologies showed a stronger tilt than ever before in favor of PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow., leading me to write a follow-up article called “Solar Thermal Is Really, Really Dead.”