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.
What we're working on right now, "outrageous green" tree bark siding, antique pine floors and counter tops, sixty solar tube collectors on the roof, radiant floor with a weave of heart pine and stone tiles, a vanity made out of a tree trunk! just outrageous is all I can say. (and Beth threw in some articulated steel arch beams to support a natural cooling cupola sheathed in local pine with concealed rope lighting) Creative collaboration at its funnest!
The team on this project, Michael, Scott Terry, Guillermo Vilegas, Mat McDonald, Chris Kerscher, Paul Rockwell, Gabby Garcia, and Beth Williams.
It’s a sea of mud right now but we are collaborating on a landscape plan with Caroline Siverson of Kinetic gardening and by spring this will be magical.
The front door, awaiting log columns and a huge stone Budda to go in the rock garden behind the wall at left which will be flanked by delicate copper rain chains where the temporary gutter pipes are now.
The view back towards the front door, the column rising from the fireplace is a steel post wrapped in a salvaged heart pine 8x8 from Heartwood Pine floors in Pittsboro NC who also supplied all the "Character grade" heart pine floors .
The view back towards the front door, the column rising from the fireplace is a steel post wrapped in a salvaged heart pine 8x8 . The cooling monitor is bound with a boxed beam (at the top of the post) that has a concealed notch which hides ninety feet of rope lighting so the upper cupola glows at night.
The kitchen, a blend of cherry wood and heart pine. The hood surround is antique heart pine as is the island top and legs. the toe kick will be mirrored to reflect the heart pine floors and give the impression of a floating island.
The cabinets are designed around aging in place principles, with the microwave hidden under the island and full wheelchair access under the kitchen sink. There is a special display niche planned for over the fridge for showing off local pottery.
While we had a rustic theme going with the lighting we made an exception to incorporate this sculptural ceiling fan over the breakfast table.
Sixty Apricus tubes on the roof supply heat to the drain-back solar water heater which in turn helps heat the radiant floor, with back up from a Quietside condensing gas water heater.
The view from the screen porch out across the boat landing to the private lake.
The view from the deck across the valley to the swimming pool and pool house. The gutters look like copper but they are actually aluminum with an oil-rubbed bronze finish.
The stone chimney rises from the roof.
The breakfast nook.
The Radiant floor in the front hall and guest wing is covered by a heart pine grid with tumbled travertine marble tile inserts.
Mat's branch vanity project. It's developed a life of its own, great to watch it come together.
Huge heart pine timbers to frame the opening from the entry hall to the living room were crafted by Chris Kerscher who also created the frame and panel work around the front door to give it presence inside and out.
Here Chris and Mat raise the 3x14 salvaged heart pine timber and “persuade it” into the notch cut to receive it.
The articulated steel I-beam created by a collaboration between designer Beth Williams and engineer Rob Munach. in the back is a similar but lighter articulated flitch plate beam hidden in the hip rafters at the bend in the monitor.
