House Parts: Windows

Windows just keep getting better. Basically what you need to know as a homeowner is that the better you build and insulate your house walls, roof and foundation, the better windows you need to install. It all kind of goes hand in hand. If you put poor performance windows in a super insulated house you are going to have a lot of condensation-on-the-glass problems. Its physics. Moisture condenses out the air at the dew point. Dew point varies depending on conditions inside and outside.  If warm moist air hits a cold window, water droplets will form and rip down the inside of your window. Not so good. 

So we always put really great windows in our really well insulated houses. But doesn't that cost a lot? Not really. Not relatively... and not if you look at the big picture.

Here's what we look at when we consider windows: 

1. U-factor

U-factor is the inverse of the R-Value (i.e. the term we use to show a material's resistance to heat flow), and it basically tells us the same thing. R-Value tells us how well insulation performs from an energy-efficiency perspective. U-Factor tells us the same about widows.  With R-Value, the higher the better. With U-Factor, because it's the inverse of R-value, the lower the better.

Right now in Zone 5 (All of Ct and MA and pats of NY) to meet 2012 IECC Code your windows need to be at U.32,  to meet Energy Star you need U.28 or .27 depending on Solar Heat Gain Coefficient. Lets do the easy math. remember to U-Factor is the inverse of R-Value. R-Values are easier to understand so lets convert.

U-Factor of 0.32 = and R-Value of  R3.125

U-Factor of 0.27 = and R-Value of  R3.70

U-Factor of 0.17 = and R-Value of  R5.88

U-Factor of 0.13 = and R-Value of  R7.69

U-Factor of 0.10 = and R-Value of  R10

The normal windows that we use are in the U.13 to U.17 range. They are generally triple pane windows from Europe or Canada.  You can see that code seems sort of wimpy compared to the windows we normally use. Even Energy Star seems a bit tame. This is because the walls we are constructing also exceed the code required R Values by quite a bit. 

(Code requires a framed wall to be R20. Ours are usually min R30. Code requires Roofs to be R49 We like to get to at least R60.)

Most American windows get you to Energy Star and that is about it. We do use then for clients who want a 'lighter green' home. You can get a decent Energy Star window for the right house here in America. But we like to get the European whenever we can. The price is not that different when you compare apples to apples. They are not crazy expensive. 

What else is important when choosing a window?

2. Solar Heat Gain Coefficient

The Solar Heat Gain Coefficient (SHGC) basically refers to how much heat is going to come through the glass via the sun. Most Americans windows offer one SHGC and it is designed for the middle of the country. Basically north Carolina or Kansas. It blocks  a lot of the sun becasue the whole souther half of the US has high cooling loads and need s to block sun form coming in. This is great for them and makes it easy to sell windows- one kind, less complicated- but its not good for the North East. We WANT solar heat gain for at least half the year. as conscious designers, we design to let the sun in to warm the house...we can't have our windows working against us! Other windows companies will make windows with different SHGC. We often choose different types of glazing for different sides of the house (N,S,E +W). This way we cna let the sun in where we want it and block it where we don't. 

Below is a typical window sticker.  Its from 2009. You can see Energy Star's rating levels have gotten more stringent. 


3. Visible Transmittance

The third box shown here is Visible Transmittance.  Films that are applied to make the window more efficient will lead to less visibility and less solar heat gain.  So sometimes if we have designed a house to work in a passive solar way, yet still want it to be very energy efficient, we have to play a trade off game.  So, maybe we decide to make the south-facing windows less energy efficient than all of the others so that we can get that winter sun in to heat up the house.

Below are some triple-pane tilt-and-turn European windows that have a U-Factor of .15.