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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.

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Passive Solar Design

Passive solar design is about the orientation of a building and the placement of windows and mass in order to capture the heat from the sun and then to store it in the mass of the building.

One of the first things to consider when designing a house is the orientation to the sun.  The long axis of the house should be oriented directly east-west, with one of the long sides of the house pointing south.  The south wall should then be designed so that the majority of the windows in the house are on that side and that there is a minimum of windows on the north side.  One of the errors that many people make when doing a passive solar design is to have too many south facing windows.  In the average house you want no more than 15% of the floor space as windows on the south side of the building.  If you have more windows then you are prone to overheating, particularly in the fall and spring. When specifying materials you need to look for a window that has as high a Solar Heat Gain Coefficient (SHGC) as possible if you are in a northerly climate. The SHGC is a number between 0 and 1 that is the portion of the solar heat that is allowed through the window. A window that allowed all the heat through would have a value of 1. Most windows have a value of between 0.3 and 0.6. In addition you want to design overhangs that will shade the windows in the summer and let in as much light as possible during the winter (I will cover this more in depth in a later post)

In addition to the windows you want to have thermal mass in the building.  Thermal mass acts like a flywheel for heat.  If the sun shines on it, or the air around it is warm, it will store some of the heat, and once the air temperature drops, it will release some of that stored heat to the air. Once again, however, you can have too much of a good thing.  If you have too much thermal mass, it can never get above room temperature and will not contribute to heating the house, as in order for a thermal mass to be effective, it has to reach a temperature that is greater than the desired air temperature in the house.  This is because heat will always travel from a hot object to a cool object.  If the thermal mass is at the desired air temperature, it will only transfer heat to the air when the air temperature is below the desired temperature.  Also, with concrete, the most common thermal mass, the active zone is only about 4 inches deep, so there is little to no advantage having a 10 inch thermal mass.  A thermal mass will be most effective if it is directly exposed to the sun and is dark in color, as dark colors absorb more heat and light colors reflect the heat.

If the thermal mass is in an exterior wall, it is best if you can insulate on the exterior of the wall and leave the interior exposed to the air.  If you insulate the thermal mass on the inside of the house, such as in an Insulated Concrete Form (ICF) the heat will have to travel through the insulation first before it can heat the concrete, resulting in a less effective transfer of heat.  In my house I used 4 inches of Roxul Drainboard for the insulation. It has an insulating factor of R4.3 per inch, which is comparable to foam, and it has the advantages of being cheaper, being a more environmentally friendly product and providing a drainage plane for the walls. The only drawback I could see is that it has to be protected from the weather (I used concrete board).