Understanding windows is a daunting task for homeowners as well as building professionals. Information is usually provided by window manufacturers or their supply chains clouding its independence in favor of marketing.
- A source for independent, credible information can be found on the U.S. Department of Energy's website here.
- This is a science-based technical information site to further your investigation.
Holes in your home's thermal envelope
Doors and windows are part of the thermal envelope, often the most inefficient component of the overall assembly.
Your selection of SIPS construction is a great choice since it reduces thermal bridging (by reducing framing) and eliminates convective heat loss.
The type and number of fenestrations (doors and windows) you install in the thermal assembly will have a negative impact on the performance of the thermal envelope. The more holes you put in the walls as windows the more you reduce the thermal performance of the thermal envelope.
The following factors will impact the overall performance of the thermal envelope with regards to windows and doors:
- Percentage of windows to floor area and wall assembly. The greater the percentage the more importance given to the performance of the window.
- Type of window installed. Stationary, casement, single hung, double hung, awning, and sliders all perform differently. Basically the more moving parts the lower the overall performance.
- Construction material. Fiberglass, wood, metal-clad, and vinyl assemblies all perform differently. Vinyl windows are the most popular when taking initial cost, ease of maintenance and performance into consideration. I would warn you that not all vinyl windows are created equally. Pay careful attention to vinyl window construction to make sure that you are purchasing a well built, energy efficient vinyl window. (Strassburger and other Canadian window manufacturers lead the industry in window performance but the US is catching up).
- Installation. If the windows are not properly installed they may not perform properly. I would encourage you to purchase windows from a supplier that also installs their windows so there is not finger pointing between manufacturer and installer if there is a window performance failure.
- Window orientation. Sun-facing windows can lead to unwanted solar heat gain thus increasing cooling loads.
NFRC labeling of windows
The National Fenestration Rating Council (NFRC) operates a voluntary program that tests, certifies and labels windows, doors and skylights based on their energy performance ratings.
- The NFRC label provides an easy, reliable way to determine a window's energy performance.
- The NFRC label can be found on all Energy Star qualified windows, doors and skylights.
U-value and SHGC
Energy Star qualification is based on U-value and solar heat gain coefficient (SHGC) ratings.
In your climate zone you should focus on the lowest U-factor.
- U-values can be based on center of glass (not the preferred method of testing) or overall window assembly. All Energy Star certified windows are whole window, overall assembly ratings.
- The lower the U-value the more energy efficient the window (heat loss).
Improved U-values are dependent on a variety of factors including, numbers of panes, film coatings, air gaps, gasket material, thermal breaks, inert gases and material.
- Triple pane windows may offer lower U-values but not in every case and the cost increase may not be cost effective.
- Triple pane may offer a greater reduction in noise transmittance which could be important in urban or high traffic areas.
Solar heat gain coefficient (SHGC) is the rating of the window's ability to reduce solar heat gain. A low SHGC indicates less heat gain, and lower cooling loads. In your climate zone SHGC is less important unless you have a high percentage of sun facing windows.
Recommendations for your project
Since energy efficiency is a major concern in your project I encourage you to pursue windows with a U-value below 0.25 (single-hung or casement).
SHGC is not that important as long as you can control heat gain with window shading or treatments.
Return on investment could be determined against life cycle costs by an independent energy auditor to help you make a purchasing decision.
I suggest either a high quality vinyl or fiberglass window for ease of maintenance and durability. Vinyl has the potential for excessive expansion/contraction in mixed climates so a quality brand like Straussburger is highly recommended over a local (small) window shop.
I would also encourage you to make sure that your window to wall ratio is under 15% and that most windows face South with summer shading.