25. A Riversong Truss System Home

Affordable For the Owner, Affordable for the Earth

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1st Floor Plan - 900 SF Interior Space

1st Floor Plan – 900 SF Interior Space

 

2nd Floor Plan

2nd Floor Plan – 768 SF Interior Space

Building Really Green in Vermont

This 1922 SF home was built with native, green, rough-sawn lumber, crafted into a super-insulated envelope with a Riversong Truss wall system, 12″ of dense-pack cellulose insulation, and the air-tight drywall approach to air sealing.

trussed-frame

Full-Dimension Rough-Sawn Framing

General Specs

Location: north-central Vermont (8500 heating degree-day climate)
Bedrooms: 3
Bathrooms: 2
Living Space: 1668 interior sq.ft. (1922 sq.ft. exterior footprint)
Builder: Riversong HouseWright
Home performance consultant: Efficiency Vermont
Insulation contractors: Riversong HouseWright, with help from Bill Hulstrunk of National Fiber

Construction

Foundation: frost-protected shallow grade beam, insulated with R-10 XPS vertical and 12″ of R-10 wing insulation, tinted radiant slab on R-10 XPS and Tu-Tuf vapor barrier, with R-10 slab edge thermal break.

tinted-radiant-slab

Tinted Radiant Solar Slab

Walls: rough-sawn 2×4 inner load-bearing wall, platform-framed first storey, balloon-framed second storey with let-in wooden ledgers for ceiling joists and rafters – all framing 24″ on center. Outer parallel-truss chords of rough-sawn 2×3 extending from sill to rafter tails and gusseted to studs with rough-sawn 1×4. Let-in metal T-bracing (Simpson TWB), ½” CDX window & door boxes, all assembled with Tremco acoustical sealant or EPDM gaskets for air-sealing, creating an uninterrupted thermal envelope on five sides. Walls enclosed with Typar housewrap and pre-finished spruce novelty drop siding (pattern 105) and rough, band-sawn 1x exterior trim boards, filled with R45 dense-pack cellulose insulation (R-68 in the ceiling).

window-box

Air-Tight Window Boxes

Roof: rough-sawn 2×10 rafters, rough 1″ pine board sheathing, 15# felt and 35-year architectural shingles

vent-baffles

Site-Built Insulation/Ventilation Baffles

Windows: double-glazed, low-E², aluminum-clad wood, double-hung in south half and casements in north half (U-factor: 0.32, SHGC: 0.31, Pella Proline)

Garage: detached (future) 2-car, with upstairs accessory apartment

west-elevation

West Elevation with Garage & Accessory Apartment

Energy & Resource Efficiency

Heating/hot water: Triangle Tube Prestige 94% efficient 30,000-110,000 Btu/hr modulating, condensing direct vent boiler with outside reset, Triangle Tube Smart 40 indirect tank, two zones radiant heat (in slab 1st floor, suspended 2nd floor), outside-air-coupled woodstove with thermal mass surround venting into masonry chimney.

The chimney was built with dry-stacked concrete chimney blocks, finished with hand-troweled surface-bonding cement (fiberglass-impregnated acrylic-modified stucco), lined with round clay ship-lap flue tiles with refractory-cement-sealed joints, and capped with a stainless-steel “shanty cap” with mesh bird screening. Above the roof, the stucco finish was sealed with two coats of UGL DryLok latex masonry sealer.

The first block course of chimney was capped to create a plenum for the termination of the outside combustion air pipes which were brought through the slab with an inverted “U-trap” in the west wall cavity to prevent cold-air back-siphoning, and the plenum is direct-coupled to the woodstove for effective “sealed combustion”.

Annual energy use:
Energy Star estimated: 42.8 MMBtu heat, 15.6 MMBtu hot water, 19.4 MMBtu lights & appliances (total annual cost: $2418). HERS rating: 46

Actual (Nov 2009 – Mar 2010): 1 cord firewood, 145 gallons propane, 200 kWh/month electricity. Extrapolated from winter use to annual at same rate: $1563/year or $130/month total energy cost.

• ENERGY STAR appliances and hard-wired CFLs throughout
• Operable windows designed for effective cross-ventilation
• Open floor plan with 9.5% passive solar glazing (28% solar contribution)
• 3 ACH/50 with air inlets open, 2.13 ACH/50 with air inlets closed
• Supplemental heat load (after internal and solar gains): 266 therms/year

Water Efficiency
• Water efficient front-loading washing machine
• Low-flow shower heads & 1.6 gallon toilets
• No dishwasher

Indoor Air Quality
• Low-VOC paints and finishes
• Solid pine cabinetry
• Softwood, tile & concrete floors
• Borate-treated cellulose insulation
• Exhaust-only ventilation with timed Panasonic bath fans and Airlet 100 make-up inlets

Solid Pine Kitchen

Solid Pine Kitchen

Green Materials and Resource Efficiency
• Minimal site disruption – excavation only 12″ deep
• Minimal use of concrete – 10″ x 20″ grade beam and slab
• All 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 subfloor and roof decks
• Exposed interior load-bearing timbers felled and milled within 5 miles
• Negligible engineered lumber (plywood door & window boxes)
• Less board feet of wood than a standard 2×6 house, but with 12″ walls
• Tinted slab as finished floor

Design & Construction Philosophy

Design/build at hourly rate (time & materials with no markup), mixed skilled and trainee crew & sweat equity make this rural home affordable, comfortable, practical, and ecologically responsible.

A collaborative effort: Owner/client desired a home that would be affordable to build, affordable to live in, as green as possible, accessible as she aged, and able to generate rental income.

Designer/builder and client collaborated on design and planning, permitting and site work. We created a floor plan that would place all necessary living spaces on first floor, with a great room, two additional bedrooms and a second full bath upstairs so that she could rent to boarders. Entry doors have low-profile sills, interior doors are 2′-10″, curbless tiled shower and soaking tub and downstairs toilet have grab bars.

Pre-existing site was perfect for shallow foundation: Client’s land was on both sides of the street and she severed 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, in-ground septic, and the footprint of a house, 2-car detached garage/accessory apartment and 16′ connecting breezeway with PT deck. The well-drained ground was gravel to a depth of 6′ underlain by sand. It would allow a shallow foundation with no need for sub-soil drainage. Sub-slab radon vent mitigated any potential for soil gas entry, and underground gutter drains diverted rain-water to nearby woods.

thompson-sfp-house-detail

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. A simple rectangular footprint (with entry/mudroom wing), orientated east-west and with south façade facing an open, unobstructed field (and turf labyrinth), an open south-side floor plan, thermal mass floor, tight very well insulated thermal envelope, sufficient but not excessive south glazing, and engineered south overhangs on both levels – were all integrated into a home that could be heated with less than 1½ cord of firewood.

Building “green”: 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 such that, by the time the frame is wrapped and sided, the moisture content is less than kiln-dried lumber.

A moisture-tolerant structure: With the caulked and gasketed frame, the 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.

East Gable "Hayloft" Door

East Gable “Hayloft” Door – Combustion Air Inlet & Fresh Air Inlets

Back-priming all exterior wood, careful attention to flashing, healthy roof overhangs, seamless aluminum gutters with underground drains, and good site grading all keep environmental moisture where it belongs. With no sheathing and latex solid-color stain on the softwood siding, the exterior skin is highly vapor permeable and the inside skin is sealed only with 1 perm vapor retarder primer. Cellulose insulation and wood framing are both highly hygroscopic and 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 indoor relative humidity just as thermal mass helps to buffer indoor temperature variations.

Optimizing the thermal envelope: The Riversong Truss wall system, one that I developed during 30 years of building super-insulated 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 any depth, and I determined that 12″ was appropriate twenty years ago when 2×4 walls were still standard. This depth offers deep, inset windows with usable sills, a continuous thermal blanket interrupted only by doors & windows, and a structure that remains cool all summer, warm in winter and uncommonly quiet.

Downstairs Living Room

Downstairs Living Room

Upstairs Great Room

Upstairs Great Room

Borate-treated cellulose is not only almost entirely recycled material, but also highly resistant to fire, insects, rodents and mold while being completely non-toxic to humans. By designing flat ceilings, the amount of attic insulation is limited only by budget and distance between ceiling joists and rafters at the eaves. By using independent let-in ledgers for each, I can maintain full 20″ insulation depth to the outer perimeter and a 2″ 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 and 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.

Lessons Learned

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.

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If you need project consultation or design services, contact me directly at

HouseWright (at) Ponds-Edge (dot) net.

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by Robert Riversong: may be reproduced only with author attribution for non-commercial purposes and a link to this page