Wednesday, July 15, 2015

Windows - Efficiency

The Anatomy of a Passive House Window
The question I hear most often is - Can you open the windows? (Or even, can you have windows?)

Of course you can. But I like to follow up with, why do you open your windows? The usual reply is to let in the fresh air. I have heard things like to let in the white noise, to hear the birds chirp etc. but usually it is to let in fresh air.

What if the air in your house was always fresh? Remember we have a ventilation system that is running 24 hours a day bringing in the fresh air - and it is filtered, so there is no pollen, bugs, and dust from the outside getting in either. I'll get to the ventilation system in a later post, let just talk windows now.

Windows are a very important part of the passive house design - in some cases more important than the wall. A passive house window, properly orientated and shaded will let in more solar energy in a year than it will let out. In a heating dominated climate that is HUGE! 

So what is the big deal? Why are European Windows so much better? See for yourself:
The upper "baby" window is an Energy Star Anderson Window - there is even a sticker on the window that is touting its superior energy efficiency!
The lower "daddy" window is an Intus triple pane window. This window is NOT even passive house certified. America, we have our work cut out for us!

It is tough to read in the photo - but the sticker on the glass reads: "HIGH-PERFORMANCE INSULATING GLASS Microscopically coated to give 2 panes more insulating power than 3. "HP" near Anderson trademark assures High Performance"

Window Efficiency has 2 ratings "U-Value" and "SHGC" 
U-Value technically is the "Thermal Conductivity", so the lower is better.
SHGC is the percentage of solar heat transferred though the window, the higher the SHGC the more solar energy is lets through.

Now to dig a little deeper into what exactly the U-Value is: The U-Value is the inverse of the R-Value. The R-Value is typically how insulation is rated - ie: R-19 Fiberglass Batts or R-38 Attic Insulation. 

The Efficient Window Collaborative is a great resource for window lingo. 

If you look at the NFRC label for any window you will see
The U Factor of 0.35 = an R-Value of 2.85
The SHGC of 0.32 means 32% of the solar energy will get into the house
The additional performance ratings may or may not be filled out. They are optional. 
See the above link for more details on the other ratings.

American window performance ratings are based on a standard size window - so larger windows will perform better than smaller ones. The glass is a better insulator than the frame - so you want a large glass to frame ratio. The U-Factor accounts for the frame insulation.

There are basically 5 different components that make up a window
  • Glazing or Glass: This indicates the number of panes of glass and the thickness of the glass. 
  • Coating on the Glass: I'm sure you've heard the term Low E Coating (and much like the 4G network, you just nod your head). This has to do with the solar gains of the windows

So if a window is properly oriented and shaded you don't necessarily want low e coating. I know, it is all that is available - again, windows in this country tend to be designed for cooling climates. 
  • Gas between the panes (Argon): Argon and Krypton are inert gasses that don't create the natural convection currents as well as standard oxygen, so they provide the "insulation" The greater the distance between the panes of glass, the greater the insulating value of the window
  • Spacer: We've all seen the condensation around the edge of a window before - some of us may have even seen frost. This is due to the spacer of American made windows. It is typically made of metal, either stainless steel or aluminum - VERY CONDUCTIVE, NOT INSULATING. Passive House windows have an insulating material as the spacer - cork, resin, etc. This will eliminate that condensation / frost issue
  • Frame: Usually Vinyl or Wood - maybe an aluminum clad on the outside. Either way it is important to have the frame thermally broken - either with multiple air pockets, foam filled, etc. 
So lets compare the above picture and see how they are made:

So how to these differences effect the performance:
Energy Star American Window
U-0.30 (or R-3.3) SHGC-0.3 (30%)
Passive House Window
U-Glass 0.088 (or R-11) U-Frame 0.139 (or R-7) SHGC (0.3 to 0.6 based on orientation)

Now If you look really hard, and ask around and actually get to speak with someone at the factory you might be able to find a U-0.20 (R-5) window. 
That's right R-3.3 window or R-9 window. Chances are your window is more insulated that sections of your wall.


Next time I will introduce you to a Tilt and Turn Operation of a window - a very common window in Europe. 
Please enter your comments below - Thanks!