Posted on

Choosing Energy Efficient Windows

When considering the windows for an energy efficient house, there are 2 things to look at. The first is the material the frames are constructed of, and the second is the type of glass in the window.

The most common type of window on the market currently are the PVC framed windows. These are constructed of an extruded PVC channel that is cut to length and then welded together to form the frames. If you haven’t seen the film Blue Vinyl you really should and then you will understand how toxic PVC really is and why it has no place in the home.  Unfortunately it is cheap and easily available so it is very commonly used.  PVC is also not very thermally stable and will expand when heated, causing problems with windows binding or frames warping in the heat.

Another type of window that you will see in some older homes is aluminum frames. These are the worst kind of window to have in a heating climate like Canada, as the aluminum is a conductor and will rapidly conduct heat to the outdoors. The only aluminum windows available in Canada currently are for commercial buildings, although they might show up in condominiums and apartments.

Another choice for window frames is wood.  Wood has the advantages that it is a renewable resource and has a very high aesthetic appeal.  The disadvantage is that it rots and will need regular maintenance in order to maximize its lifespan.  Also a lot of wood windows are treated with a chemical fungicide to reduce rot.  If you are chemical sensitive, this may cause a problem.

My personal choice for window frames is fibreglass.  The windows are formed much like the PVC windows, with the fibreglass being pulltruded into long channels that are cut to length and then connected with mechanical connectors to form the frames.  Fibreglass has the advantages of being very stable, and so will not rot.  Fibreglass is also thermally stable, so there are fewer problems with binding and warping, fibreglass is also stronger than PVC.  This all results in a stronger, longer lasting window.  Three companies in Canada that produce fibreglass windows are Inline Fibreglass, Thermotech, and Fibertec.

The next decision is the type of mechanism for the window. The choice of mechanism makes a difference to the amount of air infiltration that you will get through the window. Remember that air infiltration equals heat loss. The best type of window is a fixed window with no mechanism, but in most cases you will want a window that can be opened for ventilation. Of the windows with an opening mechanism, a well sealed hopper style window has the lowest air infiltration, followed closely by casement windows. Of the more classical window styles, single hung windows will have a better performance than double hung.

Next is the glass. There are a lot of choices regarding the glass for windows. In Canada windows are rated with an Energy Rating (ER). A positive ER window, in a south wall, will gain more heat than it loses. If the ER is not available, look at the Solar Heat Gain Coefficient (SHGC) which is a number between 0 and 1. The higher the number, the more heat a window will allow in as radiant heat from the sun. The other factor to look at us the U-value. This number is the inverse of the R-value commonly seen in insulation, and indicates the amount of heat a window will lose. The lower the number the better. A double pane window with an low-e coating will have as SHGC of around .45 an a U-value of .35. A triple pane window with a low-e coating will have an SHGC of .35 and a U-value of .25. Another factor is the spacer at the edge of a double or triple pane window. Most newer windows come with plastic spacer that has a lower conductivity than the aluminum spaces that were common in the previous generation of windows.

Typically a triple pane window will have lower U-values, lower SHGC and higher ER values and so will lose less energy, but the price can increase substantially over double pane. You will have to do a heat loss calculation in order to determine whether or not the extra cost is justified. When I built my house I went with double pane low-e windows on the south, west and east side of the house and triple pane on the north facing windows. I did the heat loss calculation for my house and having triple pane in all windows would save about $100/yr in heating costs, but would cost $8000 more, for a payback of 80 years. Even with higher energy prices, in my home the triple pane was not justified. If you are in a colder climate, like Winnipeg, then the triple pane becomes more justified. If you are concerned with having the lowest energy use possible and have no budget constraints, then triple pane, or even quad pane is the way to go.

Posted on

Insulated Concrete Forms

You may have seen the Insulated Concrete Forms (ICFs) being used to build a house, or heard about them in discussions about green building. ICFs are Styrofoam blocks that are stacked to make walls and then they are filled with re-bar and concrete. Unlike a normal concrete wall the forms are left in place. There are different variations on this theme, with most ICFs consisting of two flat 2-3″ pieces of foam separated by a 6-8″ space and attached together by wire or plastic braces. Other ICFs have a waffle like structure on the inside of the form which reduces the amount of concrete used and increases the insulation.

If you read the literature presented by the ICF manufacturers, they come up with statements about the “effective” R value up into the 50 range. This is misleading advertising. Concrete has essentially no insulating value (.08/inch) and the foam has an insulating value between 4 and 5 per inch. An ICF with 4 inches of foam (fairly typical) would then have an insulating value of between R16 and R20, way shy of the R50 advertised. The way that an ICF make houses more energy efficient is that the shell of the house is truly air tight. If the window and door penetrations are properly sealed (spray in foam and caulked), an ICF house would be essentially air tight. As noted in an earlier post, most heat is lost through air infiltration.

There are several disadvantages to ICFs. The biggest is that they use a lot of concrete, and the manufacture of concrete is one of the larger contributors to greenhouse gases in the world. Second is that they are made of plastic, which comes from fossil fuels. Another thing I don’t like about ICFs is that they have plastic foam on the inside of the structure. I don’t like this for 2 reasons. The first is that in the case of a fire, it could possibly produce very toxic smoke. Secondly having the insulation on the inside reduced the effect of the thermal mass of the concrete.

The advantages of ICFs is that they are much more flexible in the way that concrete walls can be formed. With conventional forms, it is much more expensive to have walls taller than 8 feet, as the forms have to be stacked which is much more labour intensive, whereas the ICFs don’t have that limitation. Also, ICFs can be installed by a Do It Yourselfers with the help of a few friends, but be careful to follow the instructions and make sure the walls a thoroughly braced, as a blowout can make quite the mess. Also if you use the waffle type of ICF, you can create a very solid wall that uses less concrete than a conventionally poured wall.

For my house I decided to go with the conventional poured concrete wall for the foundation. I did this because I designed the house as a walkout with passive solar input and needed as much thermal mass exposed as possible. If the foundation was not a walkout, I would have strongly considered using waffle type ICFs for the foundation, but I would be reluctant to use it for the above ground walls due to the high greenhouse gas emissions from the manufacture of the concrete.