Increasing attention is being drawn to double wall construction as a cost-effective, high R-value alternative to competing high R-value systems such as ICFs, SIPs, AAC, ThermaSteel, etcetera.
Advantages of double wall construction are:
Simplified wiring and plumbing due to wide open access prior to insulation, especially important when smart home wiring requires holding low voltage wiring well away from line voltage. The blown-in fiber insulation used in them is also easier to fish wires through than foam after the sheetrock is up and to modify for future additions.
The embodied energy and global warming impact of the double wall system also compares favorably with that of the concrete and foam in ICFs, Rastra and SIPs and the concrete and aluminum in AAC walls. These walls, including exterior OSB, are framed off-site and all scrap diverted to an I-joist plant and the walls for a typical house go up in a day. We’re generally dried in with tar paper on the roof in two weeks.
Whole wall “Steady state R-value” of a 10” AAC is R-12, 10” Rastra is R-16.5, 9” ICF is R-20, 6.5” SIP is R-23. We can expect a 12” double wall assembly with R-46 cellulose or JM Spider formaldehyde-free micro-filament fiberglass insulation in a thermally broken assembly to be well above any of these.
But most significantly the cost of double wall per R-value delivered is very favorable. Our wall panel quote on a recent 2,474 heated sf home was $12,810 total, $5 / square foot of heated floor area for double 2x4 construction (and $6.15 w/ 2x6 exterior walls needed with 2” of exterior foam on the foundation). The total cost for the JM Spider R-46 formaldehyde-free micro-filament fiberglass insulation was $1.30 per SF and this was done in a day and a half. So the wall cost for our 2,500 sf house was $16,000 with the 2x4 exterior walls (and $18,500 w/ 2x6 ext. walls). By comparison the panel quote for a similar house with energy framed 2x6 walls was $3.40 / sf and the 5 ½” R-23 spider insulation quote was $0.70 so the up-charge to go from 2x6 w/ R-23 to 12” dbl 2x4 walls w/ R-46 would be from $10,250 to $16,000. At $5,750 this is less than the up-charge for a solar water heater.
At $4,000 this is less than the up-charge for a solar water heater!
The wall panels arrive on the site, in two days we'll be setting trusses.
Dbl walls w/ offset studs, day two, braced and floor trusses going on.
Window openings are aligned but are not wrapped with plywood except at the bottom
Exterior panel is caulked to the slab. Blocks hold the walls 2.5" apart.
A mesh scrim is stapled to the wall and insulation is blown in to completely fill the cavity.
Some tight spaces are stuffed with conventional insulation, rough openings are shimmed after windows are installed.
Computerized panel plan is generated at the plant from our pencil drawings.