Extra Thick Walls Make Room for Lots of Insulation
UPDATED November 1, 2013
An easy way to build a thick wall is with alternating studs
Double 2x4 walls are built in the same way as conventional 2x4 walls. Instead of a single exterior wall, however, the house has two parallel exterior walls, set about 5 inches apart. Door and window openings must, of course, line up. Advanced framingHouse-framing techniques in which lumber use is optimized, saving material and improving the energy performance of the building envelope. techniques can minimize material use and maximize insulation.
No special skills or tools required
One advantage of double 2x4 walls is that the necessary materials and techniques are familiar to all framers. The learning curve for double 2x4 walls is much smaller than other innovative techniques, and no special equipment is required.
Double 2x4 walls are usually tied together with wide top and bottom plates (2x10s, 2x12s, or, if permitted by code requirements for fire-blocking, 8-foot rips of plywood or OSB). It's always important to include an air-sealing detail — either caulk or a gasket — between the bottom plate and the subfloor.
Thick walls don't have to mean dark rooms
The deeper the wall, the less light a standard window opening will let in. One alternative to more glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. is angling the window jambs. The beveled edges will also soften the visual transition between bright windows and dark walls.
Thick walls can create useful nooks. Don't let deeper window and door openings go to waste. Well-lit window seats, work surfaces, and built-in shelves or cabinets are just a few of the details that could put these serendipitous spaces to work.
Windows may be the tricky spot
The most common construction problem with double 2x4 walls involves layout errors that result in window or door rough openings that don't line up. It pays to double-check the rough openings before the walls are built.
However, making the inner frame an inch bigger on each side makes it easier to fasten through the backs of deep extension jambs into the framing with pocket screws for a solid joint.
For good-looking wood trimmed windows, use deep extension jambs. Don't bother trying to keep the extension jambs flush with the window's frame. Instead, offset biscuit slots 3/16 in. to make a decorative (and consistent) reveal. This reveal can be seen on the top jamb. The stool should be flush, so it's best to remove the factory stool and make a custom one.
2x10 plates can help with fire-blocking requirements
There are no specific code sections dealing with double-stud wallConstruction system in which two layers of studs are used to provide a thicker-than-normal wall system so that a lot of insulation can be installed; the two walls are often separated by several inches to reduce thermal bridging through the studs and to provide additional space for insulation. framing, although all the provisions in Section 602 ("Wood Wall Framing") still apply. Fire-blocking (Section 602.8) can be more difficult in double-stud walls than in conventional framing because the two stud layers can provide a chase that spreads or contributes oxygen to a fire. A simple, cost-effective way to deal with the vertical fire-blocking requirement is to use 2x10 or 2x12 plates that receive both 2x4 stud layers. Horizontal fire-blocking is also required and must occur at intervals not exceeding 10 feet. While glass-fiber or mineral-wool insulation batts that are securely retained in place are acceptable fire blocks, loose-fill insulation is not, unless specifically tested and approved to retard the spread of fire and hot gases.
ABOUT DOUBLE 2x4 WALLS
Getting to R-40
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 sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , 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-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. . A 5-inch gap between 2x4 walls provides room for 12 inches of cellulose, yielding an R-value of about 40.
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, drywallers 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.
Include a ventilated rainscreen gap and permeable sheathing
Because the exterior sheathing on a double-stud wallConstruction system in which two layers of studs are used to provide a thicker-than-normal wall system so that a lot of insulation can be installed; the two walls are often separated by several inches to reduce thermal bridging through the studs and to provide additional space for insulation. is colder in winter than the sheathing on a thinner wall, double-stud walls are at risk of moisture accumulation. Research has shown that the risk can be lowered by including a ventilated rainscreen gap between the siding and the sheathing.
Ventilated rainscreen gaps have multiple benefits. While rainscreen gaps may be optional for conventional walls, they are mandatory for double-stud walls.
Building scientists also advise builders to avoid OSB when sheathing double-stud walls, since OSB is moisture-sensitive. Instead, choose a more durable and vapor-permeable sheathing like plywood, diagonal boards, structural fiberboard, or DensGlass Gold.
For more information on these issues, see Monitoring Moisture Levels in Double-Stud Walls.
DISADVANTAGES AND ALTERNATIVE APPROACHES
The inside just got smaller
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.
Consider a different approach. If the size of a home's foundation is fixed, a home with 2x6 walls and thick foam sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. will take up less interior space than a home with double 2x4 walls.
If the work schedule is tight, SIP(SIP) Building panel usually made of oriented strand board (OSB) skins surrounding a core of expanded polystyrene (EPS) foam insulation. SIPs can be erected very quickly with a crane to create an energy-efficient, sturdy home. 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. (For more information on Larsen trusses, see All About 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 superinsulation 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 sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. . 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.
For more information on Larsen trusses, see "Retrofit Superinsulation" by John Hughes (Fine Homebuilding No. 20, April/May 1984), and "High Efficiency At Low Cost" by Jim Young (Fine Homebuilding No. 87, Spring 1994).
- Bob La Pointe/Fine Homebuilding SIP #15
- Martin Holladay
- Gary Williamson/Fine Homebuilding #150
- Dan Morrison/Fine Homebuilding #189
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