Net-Zero by Design – Tight Envelope

Net-Zero by Design – Tight Envelope & Smart Insulation – Part 4

In Part 3 of the Net-Zero Energy Home blog series, I addressed the importance of reducing the energy demand of the mechanical system, particularly in hot and humid climates. Before we started designing homes with the ducts in conditioned space, heating and cooling the home consumed two-thirds of the home’s energy bills. That ratio has been greatly reduced in homes built from Energy Smart Home Plans, to one-third, more than a 50% reduction in the energy consumption! It should be noted that without greatly reducing the mechanical system’s energy consumption, the cost of getting to Net-Zero is unaffordable (particularly in hot and humid climates) when considering the expense of the solar PV. The goal is to purchase the fewest number solar panels possible.

Tight Envelope and Smart Insulation

So the home must be properly designed and built with a mechanical system optimized and designed specific to the home’s location and orientation. This hi-performance system also considers the windows, insulation and the air tightness of the home among other things. Properly do all of these things and we can consider getting the ducts in conditioned space the “low hanging fruit,” towards a Net-Zero Energy Home. That is to say, a more effective HVAC system accounts for the lion share of the energy savings. For our latest example of how we do this, see the Tavernier in our blog A True Net-Zero Energy Home.

Optimal insulation is also a must when building a Net-Zero Energy Home. In our SW Florida, hot and humid climate, optimal insulation for our typical block exterior walls is R-8 with R-38 blown-in insulation on the attic ceiling as described in Part 3 in the Net-Zero Energy Home blog series. The R-8 Wall insulation is achieved with a 1” rigid insulation glued to the block wall and the taped seams for air-tightness. On top of the rigid insulation are 3/4” furring strips with 5/8” drywall to finish the wall yields our R-8 target. The typical slab on grade foundation is optimal here in the south, the 72 degree ground temperature here, means no foundation insulation required. A perfect example of a typical Florida Net-Zero is the Clearwater and Tavernier plans, see Part 1 of this series.

A Tight Envelope is Essential

So take the time to plug your leaks! It’s a dividend that repays you, year after year.

In colder climates, more R-Value is required. For our northern customers, we spec as high as R-30+ for the exterior walls, so we often call for a 1” rigid insulation on the outbound side of a 2×6 frame exterior wall insulated with R-24 blown-in insulation.

The optimal R-Value for northern vented attics, can also be as high as R-60. You’d be right to be thinking, “Not a lot of spray foam sold up there!” No pun intended but wrapping up a discussion of the thermal envelope, for both hot and cold climates a home’s air tightness remains the goal. We want as little “communication” as possible between conditioned and unconditioned space. We’re forever stressing the importance of plugging every hole and sealing every opportunity for air infiltration.

With such dramatic reductions in mechanical energy costs, the next “frontier” for hi-performance homes is the reduction of the “plug loads,” or all of the other household items that draw energy, now the larger share of the energy costs. Energy Star Appliances, surface mounted LED lighting, next generation thermostats, etc., all serve to reduce the home’s energy demand. The surface mounted LED lights replace can lights and help to greatly reducing ceiling penetrations, further minimizing communication between the vented attic and the conditioned space. Remember, the lower we can make the energy costs BEFORE adding solar PV, the less solar we’ll need to reach Net-Zero. Making Net-Zero more affordable, simply means it will happen more often. A great resource for detailed information about constructing a Tight Envelope can be found at www.greenbuildingadvisor.com.

In Part 5 of the Net-Zero Energy Homes blog series, we’ll discuss the importance of installing the best low-e windows the budget allows.