Eieio
August 27th, 2009, 05:11 PM
Builders of energy-efficient or near-zero-energy homes often aim for R-40 walls. Although a variety of methods have been proposed to meet this goal — for example, SIPs, Larsen truss walls, thick exterior foam sheathing, or the use of I-joists as studs — most builders settle on the most affordable option, which is to use double 2x4 walls with a total wall thickness of 9 to 14 inches. The space between the double walls is usually insulated with cellulose.
The spacing between the double walls depends upon the desired wall R-value. A 5-inch gap between 2x4 walls provides room for 12 inches of cellulose, yielding an R-value of about 40.
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If the walls are being insulated with cellulose, you need to choose between several possible installation methods. Usually the damp-spray method is not used because very thick walls dry slowly.
The cellulose can be blown behind air-permeable netting, or the cellulose can be installed after the drywall is hung. If the latter method is chosen, dry wallers are usually instructed to leave a continuous horizontal 4-inch gap between the lower drywall and the upper drywall so that the gap between the sheets is 4 feet off the subfloor. The gap is used by the cellulose installer and is later patched.
There are two potential disadvantages to using double 2x4-wall construction. The first is that the thick walls can rob a floorplan of interior space. Secondly, building double walls is time-consuming.
Make the foundation wider. If nothing but wall thickness is changed, a smaller interior will be the result. Designers need to keep this in mind and make the foundation larger to compensate.
Add insulation to the outside.
If the size of a home's foundation is fixed, a home with thick foam sheathing will take up less interior space than a home with double 2x4 walls.
If the work schedule is tight, SIP walls go up much faster than double 2x4 walls. But the total cost for SIP walls will be much higher than for double 2x4 walls, even when insulation costs are included in the calculations.
THICK WALL RETROFIT: LARSEN TRUSSES
A double row of 2x4 studs creates an affordable thick wall, but there are other ways to build thick walls. In some cases, especially retrofit situations, it makes sense to consider Larsen trusses.
A Larsen truss is a lightweight vertical truss attached to the exterior of a building's wall framing. Larsen trusses are ladder-like elements built from pairs of vertical 2x2s connected by intermittent gussets made from 3/8-in. plywood. The depth of a Larsen truss is usually 8 to 12 inches. Positioned like auxiliary studs, Larsen trusses have only one purpose: to make a wall thick enough to install additional insulation.
The Larsen truss system was developed in 1981 as a super insulation retrofit technique by a Canadian builder, John Larsen of Edmonton, Alberta.
When installed on an existing house, Larsen trusses can be attached to the existing siding. In new construction, they are usually fastened to the exterior face of the wall sheathing. The bottoms of the trusses are connected with a wide plywood bottom plate that cantilevers from the foundation. Larsen trusses are usually insulated with cellulose, although fiberglass batts can also be used.
Link
http://www.greenbuildingadvisor.com/green-basics/double-stud-walls
The spacing between the double walls depends upon the desired wall R-value. A 5-inch gap between 2x4 walls provides room for 12 inches of cellulose, yielding an R-value of about 40.
458
If the walls are being insulated with cellulose, you need to choose between several possible installation methods. Usually the damp-spray method is not used because very thick walls dry slowly.
The cellulose can be blown behind air-permeable netting, or the cellulose can be installed after the drywall is hung. If the latter method is chosen, dry wallers are usually instructed to leave a continuous horizontal 4-inch gap between the lower drywall and the upper drywall so that the gap between the sheets is 4 feet off the subfloor. The gap is used by the cellulose installer and is later patched.
There are two potential disadvantages to using double 2x4-wall construction. The first is that the thick walls can rob a floorplan of interior space. Secondly, building double walls is time-consuming.
Make the foundation wider. If nothing but wall thickness is changed, a smaller interior will be the result. Designers need to keep this in mind and make the foundation larger to compensate.
Add insulation to the outside.
If the size of a home's foundation is fixed, a home with thick foam sheathing will take up less interior space than a home with double 2x4 walls.
If the work schedule is tight, SIP walls go up much faster than double 2x4 walls. But the total cost for SIP walls will be much higher than for double 2x4 walls, even when insulation costs are included in the calculations.
THICK WALL RETROFIT: LARSEN TRUSSES
A double row of 2x4 studs creates an affordable thick wall, but there are other ways to build thick walls. In some cases, especially retrofit situations, it makes sense to consider Larsen trusses.
A Larsen truss is a lightweight vertical truss attached to the exterior of a building's wall framing. Larsen trusses are ladder-like elements built from pairs of vertical 2x2s connected by intermittent gussets made from 3/8-in. plywood. The depth of a Larsen truss is usually 8 to 12 inches. Positioned like auxiliary studs, Larsen trusses have only one purpose: to make a wall thick enough to install additional insulation.
The Larsen truss system was developed in 1981 as a super insulation retrofit technique by a Canadian builder, John Larsen of Edmonton, Alberta.
When installed on an existing house, Larsen trusses can be attached to the existing siding. In new construction, they are usually fastened to the exterior face of the wall sheathing. The bottoms of the trusses are connected with a wide plywood bottom plate that cantilevers from the foundation. Larsen trusses are usually insulated with cellulose, although fiberglass batts can also be used.
Link
http://www.greenbuildingadvisor.com/green-basics/double-stud-walls