Critical Elements to Achieving a Net Zero Home

Let’s take a deeper look at what it takes to construct a net zero home. While a fully integrated design-and-build approach is key to success, it also requires a construction crew that cares about the “what” and the “why” of high performance building. Here, critical elements include:

1. Air-Sealing the Building Envelope

When a building is sealed, interior warm air cannot escape to the outside, and exterior cold air cannot get inside the home. Living in a cold climate like New England, it’s essential to make sure that a home is air-sealed against winter weather. Typical 2006 and older homes leak between 25-50 percent of all energy used to heat them. Even in warmer weather, a sealed building envelope is important for keeping unwanted allergens, pollution, and dust out. Air leaks occur as a result of:

• Gaps in joints between building materials,
• Gaps around windows and doors,
• Pipe, wire, and duct penetrations between floors – basement to attic,
• Differences in pressure between inside and outside.

One of the best times to “leak test” the quality of the building envelope is before drywall is installed. This is done through a “blower door” fan (a specially designed high speed fan installed in one of the home’s doors). Prior to running the fan, all doors and windows are closed. Other penetrations and holes in the exterior shell are temporarily sealed during this first test in order to get a good sense of how tight the shell is constructed. Penetrations would include such items as bath exhaust fans and intake holes for things like an air exchanger or gas fireplace or kitchen stove fan. Once all penetrations are sealed, the fan is turned on and pressure gauge observed – establishing  how much resistance the house is offering against how much air the fan is trying to blow out. This creates a basis for comparison to readily available national data, and indicates  how well the house is performing. When a high speed fan is blowing air out of a house, science suggests that the house will always replace that air with more air. The question isn’t whether or not replacement air will find its way in, but: “How long will it take?” This is rated by “Ac/h” (air changes per hour).  A “blower door” test will expose leaks in the shell and allow the builder to seal such leaks before covering the areas with drywall. Sometimes it’s as simple as finding a wire that was drilled through the wall to install an exterior outlet or light. When the fan gauge is holding steady at -50 pascals pressure, one can literally hear a leak whistling for the builder to come and seal it. It’s fun to do this test and the typical crew will enjoy hearing test results, confirming that they have sealed everything. After the drywall is installed and the house is completely finished, we test again to see how much improvement has occurred. Usually, it’s significant.

A good builder will perform a qualified “leak test” to ensure the envelope is properly sealed. Just as you wouldn’t want any pipes in your plumbing to leak, it’s just as important that your net zero home doesn’t “leak” either.

2. Installing “Super Insulation”

An idea developed and refined since the 1973 oil embargo – the super-insulated house – made it possible to heat a home with minimal assistance, using traditional heating systems. Often, waste heat generated from appliances and body heat from home occupants adequately completed the mix. Super-insulated houses stay warm for much longer periods than minimally insulated houses, which is of critical importance during an unanticipated power outage. Super insulation is achieved with close attention to details, such as:

Use of higher R-rated materials. [An R-value is a measure of thermal resistance for a particular material or assembly of materials.] With super insulation, thicker walls are insulated to minimum R40, and roofs to R60. Houses built before 1973 had 3½” thick walls (the width of a 2×4), which were insulated with R11 fiberglass and roofs that used R19 fiberglass insulation. Both were standard construction at that time. It’s accurate to say that a super-insulated home back in the 1970s used 2×6 (R19) walls with 9-12” of insulation in the attic (R30). It’s likely that none of these homes had insulation under the slabs or around the foundation. A lot has changed since then. High performance homes built from 2010 onward have wood-framed walls with thickness that sometimes exceeds 12”.(R45). Attic ceilings are sometimes insulated up to R90 and foundations have at least 2” of foam on both sides, and underneath each slab is a minimum of 2″ insulation (with sometimes as much as 12” of foam underneath).

It is critical to ensure that no gaps exist in the insulation, especially where walls meet the roof, foundation, or other walls. Everything must be air-sealed. There’s quite a science behind the “how” of air sealing, and this is where an inexperienced builder can really foul things up both for his client and also for himself. Not too many years ago, a three-year-old house in New Hampshire had to be torn down because the low bidder didn’t pay any attention to the proper placement and installation of air and vapor barriers. He didn’t understand the difference between closed cell and open cell construction or air tight construction in general. He was an older builder nearing retirement and only knew what he had always done since the 1950s. Consequently, within three years the house was full of mold and there was no easy fix. Condensation had accumulated in the cathedral ceilings and brought down the sheetrock. What was left hanging had turned black from mold. The walls showed high water marks with the same black splotches. The carpet had mushrooms growing in it. It was a beautiful house from 100 yards away, but the only way to fix it was to tear it down and rebuild.

3. Use of Insulated Windows and Doors

Windows and doors can leak heat to the outside, from their perimeters and directly through them. In fact, windows and doors are the primary source of lost heat. Net zero homes use doors and windows that offer the highest insulation values. Strategic placement of windows and doors is also important to energy efficiency through passive solar design.  With passive solar design, the windows of the home will be oriented to maximize solar heat gain in the colder months and to minimize heat gain in summer. Again, it’s important to achieve a balance between passive solar gains while not sacrificing the potential of a stunning view. A qualified net zero builder can help you work through these and other choices.

4. Reducing “Thermal Bridging”

Thermal bridges are connections between the inside of the house to the outside, without any break between them. Most of the time they are just studs or rafters or sills and rim joists, and so forth. That may not sound like much, but when you calculate the number of studs and rafters and the lineal footage of sill plates and rim joists that are telegraphing your heat directly through a stud and plywood to the outside, it can add up to a lot of flowing heat loss. This is called “conductive heat loss” because the heat that is lost is “conducted” through the material and not through the air. To remedy this issue, it’s very important to separate the inside walls of the home from the outside with a “thermal break.”

Many people think that insulation makes things warm. But in reality insulation doesn’t make anything warm. The performance of insulation is a measure of how well it resists the transference of heat or cold through it. Refrigerators are insulated to keep the cold from getting out and the warmth from getting in. Houses are designed in a similar way, only we rarely think of a house fighting to keep the cold in … unless it’s really hot out, of course. But in Northeast climates we usually think of heat as our dominant energy source and cooling as secondary. So the design and heating systems need to be geared for really cold climates.

Even with the highest value of insulation and best-insulated doors and windows, a home will lose heat if it is not built in such a way as to eliminate as much thermal bridging as possible. One way to do that is to space the studs further apart. Stud spacing at 24” apart will minimize thermal conductivity.

Because this facet of construction really does save energy, some houses are built with stud and rafter spacing at 32” on center and the cavity is then completely filled with closed-cell foam insulation. When this material hardens, it’s often stronger than the lumber.

 One good way to overcome thermal bridging and minimize conductive heat loss is to add a layer of Styrofoam insulation over the studs on the entire outside of the house, covering over all the studs and their direct contact points with the outside. Another way to overcome conductive heat loss is to build a double stud wall. This is where the inside wall is completely disconnected from the outside wall. It is usually about 12” in total thickness and filled with dense-pack cellulose insulation. When this system is used, there is no need for Styrofoam insulation. This is a favorite system for those who are uncomfortable with using any foam products in their house as it gives a thermal performance of R45 and has no thermal conductivity through the framing members except at critical structural support points, which are very few.

5. Installing Energy-Efficient Lighting and Appliances

Technologies for energy efficiency in lighting and appliances have been rapidly advancing and costs have been lowered. LED lighting is now mainstream and cost-effective. When shopping for appliances, ENERGY STAR labels allow energy use to be compared; although some of the claims on those blue labels can’t always be trusted. An ability to turn off energy consuming features, like heaters that dry dishes in dishwashers, can result in significant savings.

In small and mid-size houses, it is recommended that the primary water heater be a 50-gallon hybrid “heat pump” electric unit with a secondary standby 40-gallon electric unit, both of which should be able to be controlled manually. This has become a favorite option for homeowners, especially those who are rarely home during the day. A lighted switch may be located by the homeowner’s primary door and be turned on and off when entering and leaving the home. Most water heaters will take only fifteen to twenty minutes to reheat when turned back on and the secondary unit can be activated when needed for additional users, or for filling a larger tub. At other times, the lighted switch is a reminder to turn off the water heaters when leaving home for extended periods of time; providing an opportunity for savings that can add up to hundreds of dollars a year. Vacation homeowners especially value this feature.