Larsen Truss/ Super-Insulated Homes
Questions and Answers

Robert Riversong has created his own form of the Larsen Truss building system, and specializes in super-insulated houses. He teaches these techniques, and many other subjects, at the The Yestermorrow School in Vermont. From geodesic domes in the 1970s and a community land trust homestead program in rural Maine in 1981, Robert has worked as a project manager, trainer, and consultant for non-profit building projects from inner-city Boston to the hollers of Tennessee.  He has focused on passive solar super-insulated buildings, with one of his design/build projects receiving a Citation for Excellence in a national energy/resource-efficient design competition. Robert has also worked as an outdoor experiential educator and wilderness guide, a ritual leader and rites-of-passage facilitator, and a spiritual midwife for people in transition.


Q: Do you do anything to segregate the wall bays from each other? I find it hard to get a good dense cellulose pack when the cavities have too much cross talk.

A: No, I've considered stapling fiber mesh to the trusses, but that's a lot of work and it would interfere with utilities.

Instead, I've developed techniques to make it work.  For one-story walls, I might leave a 6" horizontal gap (first covered with mesh) between upper and lower sheets of drywall.  Through that, I can reach the hose and my arm if necessary and direct the cellulose downward, laterally, and then upward.  I might also reblow with a smaller and stiffer hose (1-1/2") that I can jamb down into the first blow to increase the density.  Then I fill the gap with 3/8 drywall for a flat seam.

For the 2-storey sections, I have hung the first floor drywall, then blown the first floor walls from the second floor using the leap-frog method of pre-positioning several 3" or 2" hoses into consecutive truss bays, blowing into the first, then moving that hose to the next bay in line and blowing into the second hose, the third, etc.  This will increase the density of the initial blow, and I've been able to achieve 3+ lbs/cf density.

Then I will hang the second floor ceiling and walls, and blow down from the attic (which is accessed through a hayloft door in the gable above the insulated envelope - no penetrations in my ceilings , not even ceiling lights).  And finally I'll do an open blow in the attic, filling the space at least twice as deep as the joists up to story poles which indicate proper and uniform depth.

Q: How do you prevent settling of the cells in those 20'+ walls?

A: The insulated wall cavity is actually 18' in total height, including the attic insulation above it.

While it's tricky getting good density in an open wall system like mine, I know I got approximately 3.1 lbs/cf (which is what my cellulose manufacturer recommends) because I compared the calculated bales per wall with the actual number used as I blew each wall.  If I didn't use as many as I had calculated, then I would insert a smaller diameter stiffer hose down into the already-blown cellulose and compact it further.

The initial settled density (by gravity) of blown cellulose in an attic is 1.4 lbs/cf.  While it's difficult to get full dense pack density at the top of a wall section (though I assist this by stuffing empty cellulose bags into the wall cavity to create some back pressure), it will still be considerably more than the gravity density of 1.4 lbs/cf and will always be attempting to expand.  Even if there were a small amount of settling in the wall cavity, I have 20" of cellulose in the attic on top of the wall.

Dense pack is more than double the settled density of cellulose, so it cannot settle unless it becomes totally saturated with water.

Q: Have you looked into a wall after few months or a year to insure that no settling has occurred?

A: I haven't had that opportunity, but I'm completely confident in the ability of dense pack cellulose to resist settling and this has been confirmed to me by the technical manager of Cell Tech, who is a friend and fellow teacher of super insulation.

I was impressed with cellulose nearly thirty years ago when I saw a demonstration wall (about 4'x8' with Plexiglas walls) that had been moved around the northeast on a utility trailer from one home show to another and showed zero settling.

Soon after, I did a gut renovation on a duplex in Boston and blew cellulose into the walls.  One piece of drywall popped a few screws, so I had to remove it and the cellulose had to be clawed out of the wall - it was dense enough to stand up with the drywall removed.

The only thing that can change the physical properties of cellulose is significant wetting (greater than 30% of its weight in water).  Placing it in a wall at twice its settled density is like compressing a sponge to half its size and placing it in a confined space.

It's one of the prevailing myths about cellulose that it may settle over time.  That can happen only with improper installation, most commonly in retrofits where it's not possible to know where all the wall cavities are because of embedded bracing, or irregularly-spaced framing, etc.  But a good installer will check the job with an infra-red camera to make sure there are no voids.

Q: So, no sheathing? That is one thing that would take some getting used to (for me). Seems like you would need to be careful installing the housewrap. Can other siding types be used with your system? How do your windows go on with no sheathing? No casing?

A: Yeah, it takes a little care and a ladder with a reversed stand-off, but you can also reach the housewrap from inside the house as it's being applied.

Since I frame on 2' centers, I wouldn't use anything less than full 3/4 horizontal siding.  If a customer insisted on clapboards, then I would sheath with diagonal rough-sawn boards and skip the metal t-bracing.  I did one house with vertical shiplap siding and had to nail horizontal 2x2 girts over the trusses.

All siding is back-primed and stained, not painted, with cut ends getting stained as the boards are installed and the second coat of stain applied as each lift of siding goes on so scaffolding has to be set up only once.

Aluminum-clad windows with nailing flanges are installed after housewrap and, if the customer wants window trim, then wider nailers would have been installed in the truss system to frame the window openings.

Q: Do you use a vapor barrier on the warm-in-winter side of the wall when you insulate with cellulose?

A: I haven't used vapor barriers for 20 years.  On new construction, there's really no need and they can even be counterproductive by concentrating moisture problems at weaknesses in the VB.

Full-scale season-long tests at the Univ. of Ill. Building Science Department have demonstrated that of the total winter-season moisture accumulation in a typical wall section, 99% comes from exfiltration of moist air and 1% is due to diffusion through building materials.

If a house can breath (absorb and release moisture daily or seasonally), then vapor diffusion is not the problem as long as indoor moisture remains within normal limits.

To create an effective vapor barrier with 6 mil poly or equivalent is, as many of you know, nearly impossible and a lot of trouble.  I've done it, with caulked or French-folded seams, sealed at top and bottom plates and around electric outlets and at each door and window opening.

In addition, there is some evidence that wrapping the walls with plastic creates a static charge which draws negative ions (the ones that make us feel good at beaches and waterfalls and contribute to health) out of the indoor environment.

I have no interest in building a hermetically-sealed house, which means no plastic vapor barrier and no plastic foam.  A house envelope is a third skin - after our biological skin and our clothing - and like the other two, it must breath to maintain a healthy and livable indoor environment.  All building materials used in the last 10,000 years met this criterion, until the last few decades.

What I DO use, to meet codes or Energy Star standards, is the Air-tight Drywall System, and a latex VB primer.

Q: No fire stops?

A: As far as fire stops, I've used this system for three homes in Massachusetts where codes are strict and strictly enforced, but three different building inspectors had no problem with a "balloon-framed" wall system that was full of dense-pack cellulose, which is a better fire stop than solid wood blocking.

Q: What are you blowing for attic depth?

A: I place 18"- 20" of cellulose in the attic, for an actual R-value of 61.2 to 68 (assuming R-3.4 per inch at 1.4 lbs/cf settled density).

Q: You don't use an HRV (Heat Recovery Ventilator)?

A: I don't use them, because I try to keep my houses as low-tech as possible to reduce initial and operating costs and long-term maintenance.

Since code and Energy Star requires bath and kitchen exhaust fans, why not use those as the central exhaust system with no additional ducting?

The heat load of my homes is so low that the small amount of recoverable heat from an HRV system seems hardly worth the initial and long-term cost.

In addition, because my homes include a wood stove, which is an exhaust fan, they will ventilate even during a power outage - in other words, they don't require "artificial respiration".

Q: How do you deal with green lumber (which I'm assuming is the rough sawn stuff ) shrinking? Ring shank nails?

A: I use 20d galvanized common nails for framing - great holding power, but it means hand nailing.

Dripping green, it is.  I keep the lumber closely stacked on site - no stickering - to keep it as green as possible.  It spits at you when you nail it!

But green lumber cuts like butter, and it stays straight and true.  Since I don't sheath the walls as they go up, they are exposed to sun and wind for a few months as I'm building and are mostly dry by the time I close in the walls. 

Careful design prevents problems with uneven shrinkage.  For instance, using flush beams instead of below-joist girders, allows the entire floor system to shrink downward as a unit. And, once the framing is locked in place, there's almost no twisting or bowing.  Many people who visited the site remarked how straight the framing lumber in the structure was.

Q: Also, how's the consistency of the rough sawn lumbers dimensions?

A: In this case, since I got my lumber from a computerized band saw mill, it was remarkably accurate and consistent.  However, with a double wall system, only one plane needs to be lined up.  The same is true for the roof and ceiling.  For inside partitions, other than the center load-bearing wall,  I use KD lumber.

Q: What is your approach to windows and doors on these super insulated houses?

A: I go with steel or fiberglass insulated entry doors with lowE/argon lights.  Steel doors have the advantage of an available magnetic weather strip (like a refrigerator door) which seals better than bulb weather strip, and adjustable oak/aluminum thresholds with thermal breaks.

For windows, I've found that double-glazed lowE/argon units offer the best balance of solar heat gain and insulative quality, on the S, E & W facades in a 7000-8500 DD climate with 50% solar availability. The only place more insulating windows can be cost-effective is on the north, but it's simpler (and less costly) to order the same windows all around.

It's getting more difficult, however, to find high solar heat gain lowE windows (Canada offers a better selection).  For passive solar construction, a window should have a SHGC in the .60 range.  Many commonly-available windows are now in the .40 range, and they significantly reduce the possible solar gain.  While they might be appropriate for E & W sides were low sun angle makes shading impossible, they make no sense on the south.

I've always used aluminum clad wooden windows as the best balance between embodied energy and environmental cost on the one hand and longevity with minimum maintenance on the other.  A building material or unit with a higher initial environmental cost can be "greener" if it outlasts a material or unit with a lower initial impact.

A material can be said to be "sustainable" if its life-span is longer than the time it takes for the Earth to recover from its impacts.

I also like to use casement windows, rather than double-hung, where the design permits.  A casement offers full (rather than half) opening for ventilation, can scoop prevailing winds into the house in summer, can be opened against high noise areas, offer more opening for emergency egress, and tend to seal more tightly.  However they are not appropriate where they will open into a traffic path, such as onto a deck.  In my last house, I used double hungs for the aesthetic on the front half of the house (living room, dining room, great room) and casements on the back half (kitchen, baths, bedrooms).

Q: Can you recommend some good reading on this stuff?

A: I wish I could, but I don't think there's been a book on the subject published since the 80s (following is a list).  And there isn't even a website that I can find that covers the spectrum of super insulation options. You might just have to come to Vermont and take one of my classes! :)

Booth, Don, Sun/Earth Buffering and Super insulation, 1983, ISBN 0-9604422-4-3

Nisson, J. D. Ned; and Gautam Dutt, The Superinsulated Home Book, John Wiley & Sons, 1985 ISBN 0-471-88734-X, ISBN 0-471-81343-5

Marshall, Brian; and Robert Argue, The Super-Insulated Retrofit Book, Renewable Energy in Canada, 1981 ISBN 0-920456-45-6, ISBN 0-920456-43-X

Shurcliff, William A., Superinsulated houses: A survey of principles and practice, Brick House Pub. Co, 1981, 1982 ISBN 0-931790-25-5

Shurcliff, William A., Superinsulated Houses and Air-To-Air Heat Exchangers, Brick House Pub Co, 1988, ISBN 0-931790-73-5

McGrath, Ed, The Superinsulated House: A Working Guide for Owner-Builders, Architects, Carpenters and Contractors, 1981, ISBN 0-918270-12-X

Q: I am interested to know if a larsen-truss can be insulated with fiberglass batts or is blow in cellulose a requirement to the system?

A: Fiberglass batts are the worst insulation every created. Why would you consider using them?

I appreciate your assertion, yet, my ignorance on the subject is why I thought to ask you. I considered them as an option because a framed house works well when it has insulation and I know there are different kinds, cellulose, fiberglass, spray polyurethane. The poly is $$$$, so I rejected that on first note. I intended to use the blow in as that is what I see used on the house on your website, yet a friend was concerned with all the chemicals in the blow in. I had no idea that fiberglass insulation was so bad. I have read quite a bit online about building, yet no one (as best I can remember) explained why it was the worst insulation ever created.

I don't know what your friend means by "all the chemicals in the blow in". First, blown-in insulation can be cellulose or fiberglass. Second the only chemicals in cellulose are borate fire retardants (avoid ammonium sulfate, which can offgas and corrode metal), which are non-toxic to humans and pets but toxic to all common household insects and an irritant to rodents. So cellulose is the only insulation that won't get riddled with insect and mice tunnels (which are always seen in fiberglass).

Fiberglass has high embodied energy, is a suspected carcinogen, and often contains formaldehyde binders which are chemical sensitizers that promote generalized chemical allergies. Fiberglass is very open to air and vapor movement, loses most of its R-value when air passes through it, and decreases in R-value when it gets either hot or cold (in other words, when it's most needed). Batts are impossible to install without voids and compressions that can diminish the R-value by as much as 40%.

Cellulose benefits: 100% recycled, very low embodied energy, completely non-toxic to humans, resists fungal growth, toxic to insects, irritates rodents, very fire resistant (can use as firestop), high sound attenuation (very quiet), prevents infiltration, highly hygroscopic (can absorb 30% of its weight in water reversibly), breathes water vapor, draws moisture away from framing, if installed properly will not settle, highest R/inch of fibrous insulations, R-value increases as it gets colder or hotter.

Q: I am building a 1 story (ranch) house in Southeastern MA which is approximately 2 miles from the ocean. First off I am building this myself (with a little help) and we intend on spending the rest of our lives in this house. I also would like to make it as energy efficient as possible but keep a very (VERY) tight budget. The house will be slab on grade with PEX for radiant heating. I am considering a Mooney wall design with 2x6 studs, cellulose and 2" XPS on exterior. What I would LOVE is for you to be part of the build, but I am a realist and know that will never happen. But, if you could give me some advise on how to make my house the best I can make it, it would be almost as good. With that being said, could you offer the best design for the slab construction as well as the walls, ceilings and roof? I really don't have a lot of money, but want to build the best I can afford.

A: Investing in an extremely efficient envelope will pay itself back many times in energy savings as well as comfort and security. The easiest way to improve a basic 2x6 wall is by cross-hatching on the inside (incorrectly referred to as the "Mooney Wall" to add depth and reduce thermal bridging) or adding rigid foam board on the outside (to add R-value and reduce thermal bridging). It's usually one or the other, not both.

But foam is expensive, creates challenges in integrating the weather-resistant barrier (WRB) and flashings and in mounting doors, windows and siding - typically requiring a rain screen approach which adds additional expense and complications.

If you want to go well beyond code-minimum wall R-values (a wise move), the easiest and least expensive method is the double stud wall. My modified truss wall also works exceptionally well for this and requires less framing material but is a bit less conventional to build than two standard stud walls. There are a variety of ways to design a double stud wall to meet code bracing requirements and minimize thermal bridging while maximizing R-value at less cost than using foam. And, depending on how it's sheathed and sided, may not require a rain screen gap.

As for a radiant slab-on grade, that can be an inexpensive, durable and very comfortable combination foundation and floor which can double as solar thermal mass storage. But it has to be designed correctly, both to meet code and for thermal integrity.

A monolithic slab (one piece with thickened edges for footings) can be the simplest approach, but it makes insulating a heated slab more problematic. I prefer a shallow, frost-protected concrete grade beam perimeter foundation (only has to be 12" below grade) and a separate tinted slab floating inside with slab-edge insulation separating the two. The combination of sub-slab insulation, slab-edge insulation and the exterior insulation for the frost-protected grade beam makes for a very efficient floor system. Having the perimeter beam already in place also makes it easier to properly locate sub-slab plumbing, radon/vapor/moisture barrier, radon vent pipe, and radiant tubing.

If you'd like more specific assistance for your project, I offer email design and consultation for a modest fee.

Q: I live in a double walled envelope home in Franklin, Wisconsin that was built in 1982 by Ken Miller. We are preparing to install new siding on our home and I would like to expand and replace the windows on the North wall of the home to bring in more sunlight. The window on the north wall have a sliding window on the outer wall and a sliding window on the inner wall. My question is whether I can remove the inner sliding window when expanding the windows. I also want to know if the remodeler would need to do anything special while expanding the windows and removing the inner window.

A: I'm not familiar with Ken Miller's work but am familiar with the double-envelope thermo-siphon solar design which quickly became obsolete because of its failure to perform as expected and its significant liabilities - including inconsistent air flow, heat variations and stratification in the living space, insufficient night-time air flow reversal, mold formation, and catastrophic fire hazard.

The inner and outer envelopes must be hermetically sealed, so an interior window cannot be removed without boxing the new outer window into the inner wall in an air-tight and fire-proof manner. But that will, particularly with a very large outer window and window box, interrupt the thermo-siphon air flow within the buffering envelope cavity. And adding more north glazing with a window box such that the glazing is contiguous with the inner living space would add considerable heat loss with no buffering air space. What you're describing would be a significant mistake, unless you installed new matching windows in both outer and inner walls (which would still be a net heat loss, but less so).

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