Tuesday, October 4, 2011

Windows

After a few too many weeks hiatus....!

General Principles

In retrospect, selecting windows was my biggest headache. They have to perform well, both in keeping the cold/hot out and in visibility. Plus, they will impact the aesthetic of a home tremendously. We have a lot of them in our house--52 counting the unconditioned garage and entrance breezeway--with the goal of maximizing daylight and winter solar gain on our south side. In our rental, a good chunk of our electric bill was spent on illuminating dark spaces, and it did nothing for my mood over the long winter months in CNY. But what you save in lighting, you obviously loose in wall performance, since windows can't compare to good insulation.

We have “eyebrows” over the first and second floor windows on the south side because the double pane windows do less well in keeping the heat out during the summer months. During the winter, the angle of the sun will be lower, and the gain will be maximized. The kids rooms, also on this side, will have insulated shades to do the same (and to allow them to sleep until noon as teenagers). A pagoda on the west side should help us mitigate high summer sun gain through these windows, but these are also the triple glazed units, and they allow less solar heat through. We actually experienced this first-hand, during a 90-degree day this August when you could feel how much cooler the window was than it's double pane counterpart. We have very few on north and east side, both to improve R-value of total wall and because we don’t need them there for more than light.

How to Choose 
Martin Holladay, a Green Building Associate Advisor, is probably one of my favorite bloggers on building materials and principles, and is privy to all sorts of green building cutting edge trends. Here are two links that might be useful, with the second one being most informative re: what to look for in windows when designing an energy efficient house.

Here's a link to a new window manufacturer that Martin writes about--manufactured in Maine and comparable to the European Passivhaus windows: http://www.greenbuildingadvisor.com/blogs/dept/musings/new-green-building-products-0

He also posted a complete review of high performance windows on his blog in 2010:

Some window lingo, before I continue (courtesy of the Efficient Windows Collaborative):
U-factor: the rate of heat loss is indicated in terms of a U-factor for windows. The lower the U-factor, the greater the window's resistance to heat flow and the better its insulating properties. Many manufacturer's now convert U values to R values because the general public is more familiar with the latter.
Solar Heat Gain Coefficient (SHGC): The SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed and subsequently released inward. It is expressed in a number between 0 and 1. The lower the SHGC, the less heat transmitted. Energy Star provides guidelines for selecting the best SHG numbers for different regions of the country.
Visible Transmittance (VT): An optical property that indicates the amount of visible light transmitted. A VT of .40 is considered clear glass, with lower numbers representative to glass that may appear colored or tinted due to various films applied to the glass. Most windows fall between .30 and .70: the higher the VT the more light and clearer the view! Other things that can lower VT: divided "lights" of any kind and screens!
Air Leakage: Heat loss or gain that can occur through cracks or gaps in the window assembly. Obviously, the tighter the window assembly the better, as any air leakage will allow for loss of heat. Air leakage accounts for the different performance values between a fixed (non-operable) window and casement/awnings. The National Fenestration Rating Council reviews window assemblies and these attributes, and is a good place to start shopping. You can enter in your location, and it will give you a range of possibilities.

What We Ended Up With
Earthwise Windows from PVC Industries, just a few hours east of us in Clifton Park, NY. It is a vinyl window, something I really didn't want because of it's very eco-unfriendly composition, but I couldn't come close to the builder's price for a comparable product: Less than 12k for all 52 units! Plus, we'll never need to paint them. Here's a quick summary: all the numbers can be found in the rough spreadsheet (thanks to PVC for collecting this information) at the end of this post:

  1. All units meet or exceed Energy Star Standards for our climate.
  2. Triple glazed windows on the West, East, and North walls. These units have pretty aggressive U values of .21-.22, and are offered in double hung units only. We changed the glass from the company's standard Solarban 70 to Solarban 60 (both PPG products), in order to improve the VT for the units from .28 to .35. While this doesn't hit the optimal VT of .40, it helped a little, without compensating the overall efficiency of the unit.
  3. Double glazed units on the South wall, main house in order to maximize the SHGC. We were able to get the numbers up to .38/.39 (target for optimal SHGC is closer to .6). Again, we'll use the eyebrows and shades to control light on this side. These are all single hung units.
  4. Argon gas in all the spacing. Here's the scoop: you can pay for more "efficient" krypton gas, but ultimately, it doesn't boost the performance as much as you would hope for the price differential. Also, the sad fact is that over time, all this gas will eventually leak out, so you'll end up with a unit that simply relies on air anyway.
  5. They don't manufacture sliders in triple glazing (too heavy for the frame), so our three west side sliders have a slightly different look as their VT is almost twice as high as the fixed/awning assemblies adjacent to them. This REALLY bothered me when I first saw it, but I'm used to it now. 
  6. All windows have "foam filled spacers" to reduce thermal bridging between glazing, as well as a butyl seal around the glass assembly. They also spray foam around the unit before trimming it, in order to stop leaks, and in the case of vinyl, to add some structural rigidity so that the plastic doesn't twist or warp over time.

The problem with smaller manufacturer’s like PVC is that they tend to only carry one or two glass products and they can’t “special order” glass to meet customers needs. I'm sure as the sustainable building movement continues, more companies in this country will be able to tailor windows to meet specific project needs and will offer better performing units, but I didn’t find them in time for my project. However, I'm sure there will be a premium for this.

Remember: all 52 of our windows cost just under 12k. As a comparison, our Pella replacement windows in our former house (Architect series, double glaze) cost more than twice as much, for less total glass! I did get one other quote for our project from SeriousWindows in CA, but for their vinyl window with comparable specs, the price was nearly 18k. Serious does, however, offer a 725 series that offers U factors that equate to R values as high as 7....for many multiples of the vinyl window packages.

Fiberglass is also a new option, which is a greener choice than PVC, and even wood, if you consider the embodied energy and durability of the product. It won't warp or twist, and can come in different colors. 

In short, we chose the best option for the best price, and if we happen to be living in this house in 20 years and we are dissatisfied with their performance, well then we can invest in replacements while finding somebody further south to donate our vinyl windows to. Like all decisions effecting the envelope of the house, make sure you think about your window’s performance and the look you want to achieve early on as it is a decision that needs to be made in the very early stages. Especially if you are working with the companies that create more custom packages: lead time can be a couple of months!

A Summary of Window Specifications for Our House:


Type Location U-Factor SHGC VT

Patio Door 0.28 0.32 0.59

Single Hung Basement S 0.3 0.38 0.58

Picture Window Garage 0.29 0.39 0.6

Awnings Garage 0.29 0.38 0.5
PW/Awning Foyer S 0.2 0.24 0.38
0.21 0.15 0.32

Picture Window Foyer N 0.2 0.24 0.38

Picture Window Kitchen 0.2 0.24 0.38

Casement Kitchen 0.21 0.15 0.32

Double Hung Dining 0.22 0.23 0.35

PW/Awning Living 0.2 0.24 0.38
0.21 0.15 0.32

Single Hung Living S 0.3 0.38 0.58

Picture Window Living S 0.29 0.39 0.6

Double Hung Workshop 0.22 0.23 0.35

Awnings Garage Gable 0.29 0.38 0.5

Single Hung Bed 2,3,4, S 0.3 0.38 0.58

Double Hung Bed 2 Utility 0.22 0.23 0.35

Double Hung Hall 0.22 0.23 0.35

Awnings Master 0.21 0.15 0.32

Double Hung Master bd 3 0.22 0.23 0.35