Development of a Cost-Effective, Resource-Conserving, Super-Insulated Envelope
A 30-Year Design Odyssey
I’ve been building super-insulated homes since 1982 and using and modifying this truss-wall system since 1987, and can build a 12″ thick (R-40+) wall with no more lumber than a conventional 2×6 house, in part because I eliminate exterior wall sheathing and use T-braces and full 3/4″ drop siding over housewrap. By using a tinted solar thermal mass and radiant floor slab, I also eliminate the first floor framing and finish.
For foundations, I use either a shallow, frost-protected grade beam (with almost no excavation) or an insulated rubble-trench foundation on wet sites that allow downgrade daylighting of the trench drain. Both of these are major cost-savings over a full foundation and avoid the challenging problem of moisture-proofing a basement. Both systems also drastically reduce the volume of concrete, which is the most used material on earth (after water) and a significant contributor to global warming. Additionally, the grade beams are easy enough to form with conventional materials and labor and don’t require commercial forms or a foundation contractor.
And, with the air-tight drywall system instead of vapor barrier, and dense-pack cellulose, there’s almost no thermal bridging and a modest 3 bedroom house can be heated with less than a cord of wood per year in northern New England.
I also often use native, rough-sawn green full-dimension lumber, rough-sawn subfloor and roof deck, and rough-sawn exterior trim. The load-bearing wall is 2×4 24″ oc and the exterior chord of the parallel chord wall truss is 2×3, extending from mud sill to rafter tail and attached to studs with rough-sawn 1×4 gussets 24″ oc.
This creates a balloon-framed outer wall (which is erected after the roof is on) which bypasses the second floor assembly and eliminates the problem of insulating and sealing the band joist area (a common heat loss channel in many homes).
The second floor ceiling joists and roof rafters are supported by let-in rough sawn 1×4 ledgers, with the beveled rafter ledger 18″ above the ceiling ledger to maintain full insulation depth of 18″ (R-60+). The ceiling joists are cantilevered outward to form the soffit and meet the rafter tails (ceiling joists and rafters are lapped to opposite sides of each stud) with a 2″ block in between to create a thrust-resisting triangle.
Origins & Design Development
In the 1970s, when a few building pioneers were inventing superinsulated house envelope systems, John Larsen of Canada came up with a simple parallel-chord wooden truss as a method for superinsulating existing houses or having truss shops prefabricate elements for new construction.
The idea was to strip off old siding and cut off existing roof overhangs, nail trusses onto the walls and over the roof, creating a new overhang. Then new sheathing, siding and roofing could be applied, and perhaps new windows. Alternatively, the existing windows could be left in place and extended exterior window boxes and sills created. This would turn an old, drafty house into a superinsulated home without any disturbance to the living space (unless windows were replaced).
The truss cavity could then be filled with blown insulation. Additionally, an air/vapor barrier could be applied over the existing wall sheathing before installation of trusses without fear of condensation as long as 2/3 of the R-value of the envelope is outside of the barrier.
I learned about this wall system when I studied energy-efficient building at Cornerstones School in Brunswick Maine in 1982. The school was spun off from the nation’s first owner-builder school, Shelter Institute, and founded by Bowdoin College physics professor Charlie Wing (author of our textbook From the Ground Up, From the Walls In, and the more recent Visual Handbook of Building and Remodeling). The lead instructor was Dale McCormick (author of Against the Grain: A Carpentry Manual for Women – our practical textbook, and one of the first women to become a union journeyman carpenter). I also read about it in Don Booth’s 1983 book Sun/Earth Buffering and Superinsulation.
But, before I used the Larsen Truss approach, I experimented with several double stud wall systems, each of them problematic for a variety of reasons – some having to do with ambiguous load paths or excessive use of materials. When I was asked to design and build a low-cost home and pottery studio for (and with) a wonderful couple who had been saving for 14 years, I decided to try the Larsen Truss.
My First Superinsulated Truss House & Pottery Studio – Western MA, 1987
(All of my house designs are passive solar.)
This house was built on a foam-insulated reinforced concrete grade beam on an insulated rubble-trench foundation, with an insulated flagstone-on-sand-bed first floor. All the framing was green rough-sawn lumber custom milled to full dimension. The load bearing walls were 2×6 for the first floor and 2×4 for the second – all on 24″ centers – set on PT plywood subsills on recycled tin printing plates for an insect barrier and capillary break.
The exterior walls were sheathed with rough-sawn diagonal boards and then wrapped with 8 mil stabilized Swedish Tenoarm vapor barrier, installed like housewrap with one taped seam. The trusses were fabricated by ripping KD 2x4s, dadoing 3/8” slots 24” oc, and glue-nailing plywood gussets into the slots. They were a standard 16’ in length with dadoes at each end for splicing to another truss section.
The prefabbed trusses were then nailed to rafter tails, through the sheathing to the structural studs, and extended to sit on a bottom plate on the plywood subsill. Because the owners wanted vertical siding, we nailed horizontal 2x2s 24” oc and then wrapped Typar over the cross-hatched exoskeleton. Pre-stained shiplap siding was installed over all.
The system proved to be more than a bit labor- and material-intensive, but there was only one other paid carpenter and the homeowners comprised the rest of the crew.
The primary drawback of the system was that we were left with two insulation cavities. I wanted to use blown cellulose, but to get it into the truss cavity now that it was sided we had to use a holesaw to cut through the inner sheathing and the air/vapor barrier. I used the Tenoseal adhesive which came with the Tenoarm to seal the holes with plastic patches after the insulation was installed. But, since I didn’t feel like renting the cellulose blower again and making holes in the drywall, I decided to insulate the inner wall cavities with unfaced fiberglass. Because this was my first attempt at blowing cellulose and I had access only to a rental blower of questionable quality, I was concerned about potential settling under the deep plywood window boxes. So, since I had fiberglass on site, I stuffed an R-19 unfaced batt under each window box so that it would be compressed by the dense-pack cellulose and re-expand if there were to be any settling.
The house was certainly a success, operating as designed to require only 4/5 of a chord of wood per year to heat. The small Yotul woodstove did double-duty as the whole-house exhaust system (in addition to the bath exhaust fan and kitchen range hood) and relied on passive make-up air inlets, some of them site-built to take in air under the solar-heated south garrison overhang and duct it both to the downstairs ceiling and through the upstairs floor. The woodstove also had a subfloor combustion air inlet that terminated at a floor register near the hearth. And a radon vent went from under the slab to above the roof to help maintain good indoor air quality.
The 1400 square foot studio/house was completed in 1987 for $48,000 including site work, well and septic. It won a Citation of Excellence from a national Energy & Resource-Efficient Design Competition sponsored by the Northeast Sustainable Energy Association.
Building Our Swords Into Plowshares – Western MA, 1993-94
My next opportunity to use a truss-wall system was in 1993, when I volunteered to chair the design committee for a community-based non-profit building project called Building Our Swords Into Plowshares, which was the Gandhian “constructive project” of the 18-month long War Tax protest in Colrain Massachusetts following the IRS seizure and auction of the community land trust farmhouse of a life-long peace activist who had served prison time for refusing the Vietnam War draft. (See Constructive Program – Building on Our Ideals.)
With the help of architect Bruce Coldham, I eliminated the problems of my first experience by creating a “modified” Larsen Truss (which later came to be known as the Riversong Truss), using the load-bearing KD 2×4 studs as the inner chord of the truss, with ripped 2x2s for the outer skeleton, attached with plywood gussets. By installing metal T-bracing in the inner frame for rack bracing, we eliminated the need for sheathing, used less lumber (the equivalent of a 2×6) and created a single 12″ insulation cavity to be filled with dense-pack cellulose. We also used horizontal novelty drop siding so that additional nailers were unnecessary.
This proved to be a simpler, more resource-efficient, and easier-to-insulate wall system than the Larsen Truss exoskeleton. Instead of vapor barrier, we employed the Air-Tight Drywall Approach (ADA), which involved caulking each framing layer to the next at the perimeter with Tremco acoustical sealant, and applying the same sealant to top and bottom plates and around door and window openings as drywall went up. Lessco Polypans were also used around electric boxes, whose wide flanges offered a caulking surface for the drywall membrane. This was actually less work than installing a continuous vapor barrier. To make the inspector happy, we used Glidden vapor retarder primer (no longer available, but Sherwin Williams makes one) on all drywall.
The Plowshares duplex was completed with the help of 300 community volunteers, working mostly on weekends over two years, and eventually was transferred to Pioneer Valley Habitat for Humanity along with an extra lot on which they built another single-family home.
A Super-Insulated Farm House with Compost Toilet – Western MA, 1998
The next time I had an opportunity to design and build a superinsulated house was for some dear friends of mine I had known at the Noonday Farm Catholic Worker community in north central Massachusetts.
This would be a 1440 square foot, 3-bedroom home overlooking an organic farm field. Initially, they had considered an earth-bermed house with concrete walls tucked into a south-facing slope. But when I explained that they could have a highly energy-efficient passive solar home with views of their farm field and good daylighting and ventilation, they were happy to shift gears and scrap the plans drawn up for them by an engineer friend who had designed America’s first Peace Pagoda for the Nipponzan Myohoji Buddhist order.
So we spent the winter designing and planning a Riversong Truss home which was to include a solar-heated indoor planting bed, wood heat and cookstove, provision for heating hot water with a coil in the cookstove, and Massachusett’s first approved site-built composting toilet. It was also built to the Energy Crafted Home program which offered incentives for energy-efficiency.
All rough-sawn hemlock lumber was ordered in advance and stacked, stickered and covered on site to allow time to dry (this, of course, also allowed for some warpage – a lesson I did not ignore on my next project).
Because the homeowners wanted to store solar heat for an indoor planting bed in six 50-gallon black plastic barrels behind south-facing glazing, I suggested that we install radiant tubing in the slab with a circulator and aquastat so that, if there were enough temperature rise, excess solar heat could be stored in the floor. As I suspected, however, there was too little surface area to volume ratio in the barrels to get the water hot enough for efficient heat exchange, but years later they secured a grant to install solar hot water collectors on their roof and tied them into the existing radiant floor system.
The slab was poured inside a shallow, frost-protected grade beam and on top of radon mitigation piping. The frame was a hybrid stick and timber frame, with exposed interior load-bearing timber bents using a simplified pegged joinery system that I designed.
The house was built entirely of local rough-sawn hemlock, including subfloors and roof sheathing, and the siding and trim was planed local hemlock prestained in a dipping bath.
The interior load-bearing 2×4 frame was braced with let-in steel T-bracing, and the outer truss members were 2×3 with 1×4 gussets to tie them to the structural studs. The second storey deck frame was inset onto the inner load-bearing frame, thereby eliminating the thermal bridge to the outer wall.
The air-tight drywall system was used to make the envelope impervious to infiltration and to avoid the need for a vapor barrier.
The compost toilet chambers were built as a structural element of the superinsulated envelope, thoroughly waterproofed, wrapped with 4″ of XPS foam, and sized for at least one year output per chamber for this family of four.
The compost toilet has been supplying clean loam for years, with no energy input beyond an efficient in-line exhaust fan in the vent stacks.
[For more, see Composting Toilet Design-Build.]
The house cost a little more than $100,000 (about $75/sf) to build in 1998, and was designed to require 4/5 of a chord of firewood per year for heat, in addition to passive solar gain. Indoor ventilation is provided by passive fresh-air inlets, with bath and kitchen fans as well as woodstove and wood cookstove serving as exhaust fans. The woodstove has a nearby combustion air inlet register in the base of the chimney, supplied by a sub-slab duct.
The Final Evolution – Passive-Solar Super-Insulation in Northern VT, 2008
The house is built on a shallow, frost-protected foundation, which required merely scraping off the top soil and leveling the footprint. As the site soil was naturally well-drained, no foundation drains were required (though seamless aluminum roof gutters lead to buried drainpipes which are daylighted far from the house).
The 12″ thick reinforced concrete grade beam has 2″ (R-10) XPS foam preset inside the outer forms, with an additional 2″x6″ slab-edge thermal break inset into the inside perimeter. The exterior foam board is later parged with surface-bonding cement over hardware cloth stapled to the 12″ wide rough-sawn mud sills, which are placed over foam sill seal and thin copper flashing, as both insect barrier and capillary break.
All 2×10 form boards for the grade beam were kept clean with 6 mil plastic and re-used as floor joists or window headers in the house, thereby eliminating waste.
The pre-tinted slab floor is poured over Tu-Tuff sub-slab vapor barrier, 2″ (R-10) XPS, and welded wire mesh with attached radiant tubing. This provides a finished floor, solar thermal mass storage, radiant floor heat, and the base of a curbless shower stall.
The house was designed so that the owner’s living space was on the first floor (for aging in place), with master bedroom, full bath (tiled shower and soaking tub), living room, dining room, small office nook, kitchen, and mudroom/entry/utility room in a one-storey ell. The second floor has two additional bedrooms, another full bath with one-piece tub/shower, and a great room.
All exterior and interior doors on the first floor are wheelchair accessible, and the tiled stall shower is curbless. The soaking tub has grab bars and the shower includes a tiled bench.
An open timber-framed breezeway with PT deck provides the primary entry point, outdoor storage and connection to a 2-car garage/workshop with upstairs “in-law” apartment.
The interior load-bearing frame is green, rough-sawn, full-dimension 2×4, 24″ on center, with let-in steel T-bracing. The outer, non-bearing frame is rough-sawn 2×3 attached to the studs with rough 1×4 gussets pre-attached to the outer elements on a jig. Outer wall window and door framing requires no headers and uses single 2x3s.
The deep-set windows are finished with drywall returns (with bull-nose corner bead) on three sides and either a finished yellow pine sill or a tiled sill in bathrooms and kitchen. Cats and houseplants love the deep sills, and the perpendicular window wells offer reflected diffused daylight throughout the interior space.
The truss walls are wrapped with Typar weather-resistant barrier (housewrap) and finished with rough-sawn, pre-stained (all sides) corner boards and pre-stained spruce novelty drop siding (pattern #105), with no window trim beyond the factory frame.
Eliminating exterior casing not only saves a good deal of material and labor, but also eliminates the additional cladding joint that normally requires caulking (a temporary weather barrier, at best). I use almost no exterior caulking, but allow the outer “skin” to drain and dry. With no sheathing and solid-color latex stain on the exterior, the outer “skin” is highly breatheable and can easily dry if any moisture should find its way into the thermal/structural envelope.
The roof is framed with rough-sawn 2×10 on beveled let-in ledgers (thus eliminating the need to cut birdsmouths), which extend 18″ beyond the outer walls to meet the ceiling joists (also on let-in ledgers) to form both a soffit and the required rafter tie that forms a thrust-resistant structural triangle, while allowing a full 18″+ of insulation depth to the eaves with room for a site-built ventilation baffle.
Insulation and Air Sealing
For the air-tight drywall system, I use Tremco acoustical sealant between each framing unit (bottom plate to sill, band joist to top plate, subfloor to band joist, bottom plate to subfloor, etc), and then apply Tremco on the inside edge of the top and bottom plates as well as the perimeter of door and window openings as the drywall is applied, also using Lessco polypans behind each electric box to allow sealing the drywall around those penetrations.
The plumbing vent is sealed at the ceiling with a roof flashing boot, and the block chimney is flashed and firestopped at each floor and ceiling with cement.
The hybrid double-framing system (platform frame first-storey walls and balloon-framed second-storey walls) creates a continuous insulation cavity that bypasses the floor assembly (no troublesome band joist issues), continuous over the ceiling and back down the other side, for an uninterrupted insulation “blanket”.
Once interior drywall is installed, I dry-blow dense-pack cellulose into the cavities, using a “leap-frog” system of multiple hoses pre-positioned in wall truss bays, because it takes careful technique to get proper density in a cavity that is open both laterally and vertically – and I do my own insulating to make sure it gets done correctly.
The attic, which is accessible only through a “hayloft” door at the gable, has a “gangplank” to allow an above-insulation walkway, and pre-placed “story poles” create an indicator for proper insulation depth during the blow.
The downstairs walls are drywalled first, followed by insulating them from upstairs (first photo above). Then the upstairs ceilings and walls are drywalled and those walls are dense-packed from the attic. Finally, loose-fill is installed in the attic and the job is complete. I calculate the number of cellulose bales required for each cavity space in order to achieve a minimum density of 3 pcf and then blow until I reach that volume.
The nearly 2,000 square foot, 2-storey home required 311 bales of cellulose, and during construction in December we heated it to a comfortable level with a few quartz worklights in zero degree weather.
The open wall cavities make the installation of mechanicals (wiring, plumbing, venting) simple, since there is little drilling necessary. The three air barriers (drywall, dense pack, housewrap) make the walls virtually impermeable to infiltration. The dense pack cellulose makes the walls highly fire resistant and extremely quiet. Insects and rodents don’t like the boric acid used as fire retardant in the cellulose, so these two universal problems are minimized or eliminated. The cellulose is more hygroscopic than wood, so it not only can absorb and release any diffused moisture that might get into the wall cavities but also draws any potential moisture away from the wooden frame, thus protecting it (foam insulations will do the opposite and fiberglass is an invitation to rodents and air movement).
Heating, Hot Water & Ventilation
In addition to a 74% efficient woodstove with under-slab direct coupled combustion air, the house has a 94% efficient direct vent condensing propane boiler which heats two zones of radiant tubing (in-slab 1st floor, and suspended 2nd floor), and an indirect domestic hot water tank.
Fresh air supply is by exhaust-only ventilation, using the Panasonic bath fans (one each floor) on 24-hour programmable timers (as well as a kitchen range hood), and passive make-up air inlets in each bedroom and in living rooms (Airlet 100 by Aldes). The propane clothes dryer in the mudroom/utility room has its own make-up air supply through a louvered 6″ duct, and the door to the kitchen is weather-stripped to keep the utility area pneumatically isolated from the living space. This system guarantees a minimum of 1/4 air change per hour, plus additional venting when cooking or bathing to eliminate moisture at the source.
Because the house has a minimal heat loss rate (16,300 BTU/hr on the coldest design day of -10°F with no solar gain), a moderate amount of passive solar glazing can contribute at least a third of demand, reducing the supplemental heat load to negligible amounts.
This house was given 5+ stars (highest rating) by the Vermont Energy Star home program with a HERS rating of 46. The HERS (home energy rating system) Index is a total energy efficiency range from 100 for a reference home built to the International Energy Conservation Code to 0 for a net zero energy home.
The Vermont Energy Code requires an index of 85 (15% more efficient than an IECC house) and US DOE Energy Star requires an index of 80 for northern climate zones 6-8.
So a HERS index of 46 means this Riversong Truss home is expected to consume 64% less total energy (heat, hot water and electricity) than an IECC standard house, and 34% less than a threshold Energy Star home.
According to the VT Energy Star administrator, this house tested in the top 5% of Vermont’s Energy Star homes, with the blower-door test showing just more than 2 air changes per hour at 50 pascals (2.13 ACH50).
The only plywood in the house is for door and window boxes, as this makes a better air-tight seal than boards, and for a couple of interior shear walls. Let-in metal t-bracing in exterior and center load-bearing walls and wooden under-rafter diagonal bracing sufficiently stiffens the structure, particularly once the air-sealed drywall is installed.
I was able to build this house for $105/sf with a crew of 3-6 carpenters, in part because of the designed-in savings of no 1st finished floor (tinted concrete), no sheathing, no exterior door and window trim, and using band-sawn rough-sawn boards for corner boards, rake, fascia, and soffit. All lumber, including trim, was local hemlock. The interior has a couple of rough-sawn exposed timber bents, with pegged braces, from a mill just up the road; and the pine board subfloor and roof sheathing was locally-milled from trees that I felled the year before in the same town.
Going from conventional construction to superinsulation adds approximately 5% to the cost of a house and the payback is enormous, both in energy savings and comfort. Some banks are offering higher debt-to-income ratios to mortgagees who buy or build highly efficient homes, since they need so much less income to operate it. The payback begins the first month of occupancy, since the slightly higher mortgage cost is more than offset by the significantly lower utility bills.
For drawings of my KD 2×4, CDX plywood double-wall variations, see Pictures & Details.
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