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Larsen Truss / Super-Insulated Homes |
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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. 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".
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. 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. 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 |
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Robert Riversong 
