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Double Stud-Wall Construction

One alternative to standard frame construction is a double stud-wall.  This is constructed by having a 2×4 stud-wall, then a gap, and then a second stud-wall, either 2×4 or 2×3.  The walls are then filled with insulation.  The two walls are built so that the studs are offset, so there is the maximum amount of insulation at all points.  In my house I used two 2×4 stud-walls separated by a 3 1/2″ gap in which I put one layer of Roxul vertically, the another layer in the gap horizontally, and the final layer vertically again.

The advantages of the double stud-wall, beyond the obvious extra insulation, is that there are few, if any, thermal bridges.  A thermal bridge is where a place within a wall with a lower insulating value, such as a stud, extends from the inside of the wall to the exterior.  In the typical 2×6 stud-wall, the 2×6’s create a thermal bridge, because the wood has an R value of R1/inch for R6 versus R20 where there is insulation.  This thermal bridging creates spots within the wall that will conduct heat much faster than the rest of the wall causing cold spots.  This is part of the reason to go to a 24″ spacing of the 2×6 studs if possible, as it will increase the overall R value of the wall.  Another advantage is that the amount of insulation is doubled with only using 25% more lumber.  The 2×4 studs can also come from smaller trees than the 2x6s.

There are a couple ways you can construct the double stud-wall as explained in the Canadian Home Builders Association’s Builders Manual.  One is to build the outside wall in line with the outside of the foundation, which has the advantage of being easier to build, but will reduce the floorspace in the building.  The other is to cantilever the outside wall from the inside wall, which is built on the edge of the foundation.  This will maintain the floorspace but will be more difficult to construct.  In either method of construction, the vapour barrier can be placed on the outside surface of the interior wall which allows you to run all the electrical and plumbing lines inside the vapour barrier and maintain a continuous vapour barrier.  If using this method, the vapor barrier should be protected by a thin board (1/4″ osb or masonite board would work) to avoid accidentally puncturing it.  The vapour barrier can be placed within the wall because it is still inside the part of the wall where the dew point will be reached, so no moisture will condense inside the vapour barrier.  In my house I just put the vapour barrier on the inside of the wall and taped and caulked all the holes in order to simplify the construction.  If Roxul is used, a solid top plate is not required, as the Roxul provides the fire barrier.

In my house I put up the first wall, then installed the trusses on top of those and then built and lifted the second wall inside the house, leaving a gap between the walls for insulation.  I did this as I did not have a lot of help for lifting the finished walls.  If you had a crew, you could build the walls complete on the floor and them lift them into place.  Using a double stud-wall is a good method of getting a lot of insulation into a wall if you are doing your own construction, as it is simple, the materials are relatively inexpensive, but it is labour intensive.  If you were having the house built, it would probably be more cost effective to consider other methods such as Structured Insulated Panels (SIPs).

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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.