Net-Zero by Design – Low-E Windows
Net-Zero by Design – Know Your Low-E Windows SHGC – Part 5
In Part 4 of the Net-Zero Energy Home blog series, we talked about the great advancements we made in the last 10 years to dramatically lower the energy usage of the mechanical system, particularly in hot and humid climates. We also covered creating an optimally insulated thermal, air tight envelope and a quick review of plug load reduction.
Potentially responsible for over 20% of a home’s energy loss, windows are a big part of the hi-performance equation. Today’s home designs have more and larger windows than homes of old, generally speaking, magnifying the potential issues should the wrong windows be selected. Avoiding the many details, I’ll generally cover what we look for in windows in our very sunny hot and humid climate.
Educated homeowners are choosing impact, Low-E Windows, particularly for coastal communities. Photo by Andersen Windows
In the south, it’s largely about a low Solar Heat Gain Co-efficient, or S.H.G.C. The Solar Heat Gain Coefficient is the fraction of solar radiation admitted through a window, door, or skylight, either transmitted directly and/or absorbed, and released as heat inside a home. The lower the SHGC, the less solar heat the window transmits. Low SHGC windows are frequently referred to as Low-E Windows. We want SHGC numbers to be near .20, with the smaller number the better.
Vinyl windows are better than aluminum in our hot and humid climates, offering less heat transfer at less cost. A big fan of lower and also the higher priced Andersen Windows, I’ve often said that placing less than Andersen’s great hi-performance, Low-E Windows in a new home is like leaving your windows open 24/7/365. They make that much difference to a home’s performance!
A SHGC of 0.17 is about as good as it gets! Energy Smart Home Plans recommends 0.20 SHGC or lower in hot and humid climates.
I just happened to read an expert’s blog about building in hot and humid climates today and the writer stated that good Low-E Windows aren’t worth the cost and manufacturer’s efficiency claims usually exceed reality anyway. He may have a point for replacement windows, but he’d be dead wrong when it comes to new construction. For our Net-Zero Energy Home customers, the best Low-E Windows for the price point are a must and worth every penny. After all, any deficit in the windows’ performance will result in more expense in solar panels when reaching for that Zero HERS rating.
We look for that same low SHGC rating with doors with lights and sliding glass doors. However, if the sliders are under deep cover, we can do with less performance but the impact rating here is really useful. Because we live in Hurricane Alley in SW Florida, (think Irma) customers are increasingly opting for hurricane impact glass and not shutters. Even at the lower price points this is true. The impact windows and sliding glass doors offer increased security especially important to our part time residents.
The truth is for new construction, once the ducts are in conditioned space, the next big bang for the buck is investing in good hi-performance Andersen Low-E Windows. Save a few dollars initially on installing low-performing windows and risk suffering unnecessary discomfort, reduced longevity and higher energy bills and mechanical equipment costs in the long term. If you live in the South and a Net-Zero Energy Home is your goal, installing windows with very low SHGC numbers (preferably close to .20) is a must!
Coming up in Part 6, the last of the Net-Zero Energy Home blog series, I’ll wrap up with suggestions about moving forward with your own Net-Zero Energy Home. Understanding what questions to ask will be a great advantage when seeking out a competent architect or home designer as well as the builder. Photos courtesy of Andersen Windows.