A Thick Cocoon of Cellulose Protects This Superinsulated House
General Specs and Team
Location: Warren, VT
Living Space : 1922 sqf
Cost (USD/sq. ft.): $105/sqf
Builder: Riversong HouseWright
Home performance consultant: Efficiency Vermont
Insulation contractors: Riversong HouseWright, with help from Bill Hulstrunk of National Fiber
Foundation: Slab placed inside of a frost-protected shallow grade beam
Underslab insulation: Horizontal R-10 XPS under entire slab and vertical R-10 at slab edge
Foundation perimeter insulation: Vertical R-10 XPS at exterior of foundation and 12" of R-10 horizontal wing insulation
Walls: Inner load-bearing wall is rough-sawn 2x4s, 24" o.c.; platform-framed first story, balloon-framed second story with let-in wooden ledgers for ceiling joists and rafters. Outer wall: parallel-truss chords of rough-sawn 2x3 extending from sill to rafter tails, gusseted to studs with rough-sawn 1x4s.
Wall bracing: Let-in metal T-bracing (Simpson TWB)
Rough openings: ½" CDX window & door boxes; Tremco acoustical sealant or EPDM gaskets used for air sealing
Wall insulation: R-45 dense-packed cellulose
Siding: Pre-finished spruce novelty drop siding over Typar
Exterior trim: rough, band-sawn 1x boards
Ceiling insulation: R-68 cellulose
Roof framing: rough-sawn 2x10 rafters
Roof sheathing: rough 1" pine board sheathing
Roofing: Laminated asphalt shingles
Windows: Pella Proline aluminum-clad wood windows (double-hungs and casements) with double-pane low-E² glazing (U-factor: 0.32, SHGC: 0.31)
HERS rating: 46
Passive solar design features: Oriented for passive solar heat gain; solar gain satisfies an estimated 28% of the home's heat load.
Blower-door test results: 3 ACH @ 50 Pa with air inlets open; 2.13 ACH @ 50 Pa with air inlets closed.
HERS estimated energy use: 42.8 MMBtu heat, 15.6 MMBtu hot water, 19.4 MMBtu lights & appliances.
HERS estimated annual energy cost: $2,418
Actual energy use (Nov. 2009 – Mar. 2010): 1 cord firewood, 145 gallons propane, 200 kWh/month electricity. (Estimated annual energy cost, extrapolated from winter use: $1,563.)
Space heating: Triangle Tube Prestige 94% AFUE 30,000-110,000 Btu/hr modulating condensing direct-vent boiler with outside reset; wood stove with ducted outdoor combustion air, vented to a masonry chimney.
Heat distribution: in-floor hydronic (1st floor: tubing embedded in slab; 2nd floor: PEX tubing suspended 2" under subfloor with bubble-foil radiant barrier).
Domestic hot water:: Triangle Tube Smart 40 indirect tank
Appliances and lighting: Energy Star appliances and hard-wired CFLs throughout
● Efficient front-loading washing machine
● Low-flow shower heads
● 1.6 gpf toilets
● No dishwasher
Indoor Air Quality
● Low-VOC paints and finishes
● Solid pine cabinetry
● Softwood, tile, and concrete floors
● Borate-treated cellulose insulation
● Operable windows designed for effective cross-ventilation
Mechanical ventilation system: Exhaust-only ventilation with timed Panasonic bath fans and Airlet 100 make-up inlets
Green Materials and Resource Efficiency
● Minimal site disruption (excavation only 12" deep)
● Minimal use of concrete (10" x 20" grade beam and slab)
● All concrete form boards re-used in house framing
● Local, rough-sawn, green hemlock lumber
● Local, rough-sawn exterior hemlock trim
● Builder-felled pine trees milled into boards for 2nd floor and roof decks
● Exposed interior load-bearing timbers felled and milled within 5 miles
● Negligible engineered lumber (plywood window boxes)
● Fewer board feet of lumber than a standard 2x6 house, but with 12" walls
● Tinted slab as finished floor
This comfortable, practical, and ecologically responsible house was also affordable, thanks to a crew combining skilled workers and trainees, a sweat equity contribution from the owner, and “time & materials” billing by the builder
By Robert Riversong
I was hired to design and build a home for a Vermont land owner who wanted a house that would be affordable to build, affordable to live in, as green as possible, accessible as she aged, and able to generate rental income.
We collaborated on the design, planning, permitting, and site work. We created a floor plan that would place all necessary living spaces on the first floor. The second floor has a great room, two additional bedrooms, and a second full bath so that she could rent space to boarders.
To improve the home’s accessibility, the entry doors have low-profile sills. The interior doors are 2'-10" wide. The curbless tiled shower, soaking tub, and downstairs toilet all have grab bars.
The site was perfect for a shallow foundation
The client’s land was on both sides of the street. She decided to subdivide the section opposite her existing log house to create a second building lot. The level, grassy site had just enough room for a deep well, an in-ground septic system, the footprint of a house, a (future) two-car detached garage/apartment, and a 16-ft. connecting breezeway with a pressure-treated deck.
The well-drained ground is gravel to a depth of 6 ft., underlain by sand. The soil would allow a shallow foundation with no need for sub-soil drainage. A sub-slab radon vent mitigates any potential for soil gas entry, and underground drains divert rainwater to nearby woods.
Passive solar requires an integrated design approach
An initial set of plans and elevations, created by another designer, were shelved for a holistic approach to energy efficiency which optimized the free available heat of the sun. The home has a simple rectangular footprint (with an entry/mudroom wing). It is orientated east-west, with the south façade facing an open, unobstructed field. An open south-side floor plan, a thermal mass floor, a tight very well insulated thermal envelope, sufficient but not excessive south glazing, and engineered south overhangs on both levels are all integrated into a home that can be heated with 1½ cord of firewood.
Except for the weight, building with fresh-sawn green wood is a pleasure in many ways. It cuts like butter. Nails almost fly into it – even the 20d galvanized monsters that were required for the full-dimension lumber – and it's straight as an arrow. Once it's secured in the frame, it dries in the sun and air and stays straight.
Because I don't sheath my walls, they are fully exposed to the drying summer conditions. By the time the frame is wrapped and sided, the moisture content of the framing is less than kiln-dried lumber.
A moisture-tolerant structure
With a caulked and gasketed frame, Lessco polypan electrical box surrounds, careful sealing of all mechanical chases, and no penetrations in the upstairs ceiling except the plumbing stack (sealed with roof flashing) and the chimney (fire-stopped), the house is more than tight enough to prevent air-borne moisture from exfiltrating. Gasketed “hay-loft” doors allow service access to the major and minor attics above the thermal envelope.
To keep moisture where it belongs, we back-primed all exterior wood and paid careful attention to flashing. The site was carefully graded to keep water away from the foundation, and the design includes generous roof overhangs and seamless aluminum gutters connected to underground drains.
The walls have no sheathing, and the softwood siding is finished with a latex solid-color stain — details that make the exterior skin highly vapor-permeable. The only vapor retarder on the inside skin is 1-perm vapor retarder primer. The cellulose insulation and wood framing are both highly hygroscopic; in other words, they can safely store and release the minor quantities of moisture that any house — no matter how tight — can expect to experience over its lifetime.
Keeping water out is only half the equation for a durable structure — allowing the envelope to store moisture and dry in both directions as the seasons change is the other, and often neglected, half. The moisture storage ability of natural materials also helps to buffer the indoor relative humidity, just as thermal mass helps to buffer indoor temperature variations.
Optimizing the thermal envelope
The modified Larsen Truss wall system, one that I developed during 30 years of building superinsulated homes and additions, allows high levels of thermal insulation with the least amount of thermal bridging of any system except SIPS or structural strawbale.
The wall can be made to any thickness. Twenty years ago, when 2x4 walls were still standard, I determined that 12 in. was appropriate. This depth offers deep, inset windows with usable sills, a continuous thermal blanket interrupted only by doors and windows, and a structure that remains cool all summer, warm in winter, and uncommonly quiet.
Borate-treated cellulose is not only composed of almost entirely recycled material, but is also highly resistant to fire, insects, rodents and mold. It's also completely non-toxic to humans. By designing flat ceilings, the amount of attic insulation is limited only by the budget and the distance between ceiling joists and the rafters at the eaves.
By using independent let-in ledgers for each, I can maintain full 20 in. insulation depth to the outer perimeter and a 2 in. ventilation channel (site-built of overstock hardboard). Having reviewed nearly all the research on venting of roof assemblies, I remain adamant about a well-ventilated roof to avoid winter ice dams, keep the attic cool in summer, extend shingle life, and evacuate any moisture that does find its way into that space. I use the only ventilation system that's been independently proven to be efficient and reliable: continuous soffit vents and continuous wind-baffled ridge vents with no obstruction in any rafter bay.
Put everything in writing
I've made it a practice to draft a memorandum of understanding (MOU) at the start of any major building project, to make sure that initial agreements are not later misconstrued, and that each party understands their respective roles and responsibilities. I include a mediation clause, in the event of “irreconcilable differences.”
I had agreed to design and build this home because the client stated her intention to grow old in it and had somewhat limited resources. Because I work for a very fair hourly wage, pass on my material costs without markup, and don't charge for overhead or profit, I can build a house like this for as little as $100/sq. ft. in a market in which ordinary custom homes start at $150/sq. ft. I'm willing to put in a personal subsidy for someone with an authentic need for housing and in order to create another example of a truly “green” home.
So I was surprised and disappointed to learn that the house was put on the market less than one year after it was built. If I were to engage in such a project again, I would require an equity-sharing agreement to recover my subsidy if the house were to change ownership within ten years. The silver lining, perhaps, is that dozens of sustainable building students were able to tour the house during and after construction, it's received some attention on the Web and in the building science community, and the young couple who now own it are thrilled with its performance.
- Robert Riversong
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