Post date: Oct 10, 2014 10:37:10 AM
Once I got started with details of a fence to block the view of the utilities, I was mired in detail.
Structure
So I made some sketches of how the sills and studs would be laid out at the corners of the double-wall construction. This method uses two separate stud walls with a space between. My idea was to support the roof on the inner wall, leaving the space between the studs for wiring and plumbing. The outer wall would extend above the roof to hide the fact that I want a shed roof - a no-no here in up scale houses.
I then proceeded to sketch how the roof trusses would be supported by the inner wall and connect to the outer wall. One of the basic premises of the double stud wall is that the cold studs of the outer wall are not bridged to the warm studs of the inner wall. But structurally, the roof must use the sheathing of the outer wall to bear the shear forces. So there is a ledger near the top of the outer wall. The edges of the roof sheathing connect to this ledger.
The roof trusses (I-joists) align directly with the studs of the inner wall. Some double stud wall designs have the studs of the outer and inner walls aligned, but the only place this is necessary is at the openings - the doors and windows. Elsewhere we can place the studs optimally either for sheathing layout (so the edges align with 4’ studs as nailers), or for roof truss support.
Insulation
Once the structure has been settled, we need to determine where the thermal barrier is located. This wall design allows for placing Fiberglass batts between the studs of the outer wall and (horizontally) in the cavity between the walls.
Fiberglass has gotten a bad reputation from poor installation. It is hard to fill the space around pipes and electrical boxes without overly compressing or leaving gaps - which reduces the insulation value. But in my situation these obstructions will be located in the inner wall, leaving the outer wall open for easy installation of the insulation. (An alternative is to use blown cellulose insulation, but this entirely fills the wall, making future changes more difficult because of the presence of insulation.
The outer studs still are cold, so we put a layer of foam board insulation outside the sheathing, with the joints of the insulation not aligned with joints of the sheathing. This helps prevent air movement through the wall. Since air movement is the largest source of heat loss (not conduction through the materials) we also tape all the joints of the sheathing and insulation board. This seal must carry around all the corners, so the tape is added to the detail diagrams.
Moisture
In addition to the roof details - which clearly keep out moisture - we need to keep out rain that strikes the outer wall and prevent moisture from condensing within our wall. While the climate in Taos is much drier than that in Seattle, making the problem easier, we still have to be careful that steam from the shower does not condense and rot the insulation and/or the structure.
The vapor barrier needs to be on the warm side of the wall. During summer in Taos the air is generally dry, so the location of the vapor barrier is not critical. During winter, the warm side of the wall is clearly inside. So by rights, the vapor barrier should be near the inner wall. However, this wall is full of holes - from pipes and electrical connections. These are hard to seal.
So we move the vapor barrier outward. Another possibility is the outer side of the inner wall studs. This would be behind the pipes and wires that run in the wall and inside the insulation. However, I cannot figure out how to install this barrier, which is usually polyethylene sheet. Why? Because once both walls are raised, I want to install insulation in the cavity between the two sets of studs. This is best done after the walls are erected and cannot be done if the vapor barrier is in place. Alternatively, once the insulation is in place it is not possible to install a plastic sheet behind the inner studs.
Can we move the vapor barrier farther outward? Yes, we can make the outer sheathing the vapor barrier. There is even a modified OSB (Oriented Strand Board) with an integrated vapor barrier. This requires sufficient rigid insulation outside of the sheathing so that the sheathing is always above the dew point.
Or maybe not always. If the outdoor temperature drops to zero for a week, it is possible that moisture will condense in the sheathing. But the temperature of the sheathing will be around freezing, and mold and rot do not thrive at this temperature. The average winter low temperature, which is closer to ten or fifteen degrees Fahrenheit, is more realistic. Once the sheathing and its condensed moisture warm to the point that the vapor pressure of water exceeds that inside the house the moisture will evaporate and move indoors.