Last fall, Dartmouth College realized that it needed to build four new single-family homes, pronto. Beginning this summer, the homes will be occupied by the “house professors” assigned to new “house communities” — the term that Dartmouth uses to describe the college’s dormitory clusters.
To build the four needed houses, Dartmouth hired Unity Homes, a company in Walpole, New Hampshire, that assembles homes from panelized walls and modular components. Founded by Tedd Benson, Unity Homes specializes in superinsulated buildings with very low rates of air leakage. (For more information on Unity Homes, see the links in the “Related Articles” box below.)
My son Noah, who’s a junior at Dartmouth, noticed the new homes under construction and tipped me off. So I drove to Hanover, New Hampshire, to check out the new buildings. Tim McNamara, Dartmouth’s associate director of Campus Services, graciously met me at one of the job sites. I was given a tour of the sites by three representatives from Unity Homes: Ryan Lawler, Justin Pouliot, and Brad Moore.
Panelized walls and modular bathrooms
Although Unity Homes is expanding, it only sold 12 homes last year, so it’s still a relatively small company. At its indoor manufacturing plant in Walpole, Unity Homes assembles building components into wall panels and bathroom modules. (Bathroom modules are shipped to the job site with the tub, vanity, sink, and wall-hung toilet already installed. Once these modules arrive on site, they are moved from the truck to their final location by crane.)
Unity Homes is convinced that its German-made CNC (computer numerically controlled) cutting machine and its focus on indoor assembly helps the company achieve a higher standard of construction quality.
Unity has developed a panelized wall system using 9½-inch I-joists…
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Thanks for the pictures and the report.
The only thing I'd change...
The only thing I'd change is the use of XPS where a lower cost, lower global warming potential foam (EPS) could be used.
The attention to detail is very impressive. I hope that this is in fact the future of residential construction.
I'm not sure trying to move bulk water to the exterior at the window sill is best done by sandwiching membrane between two layers of siding.
One of the reasons to include a rain screen is to provide a drainage cavity, and typically all flashing is kept on top of it so that any water that does penetrate the siding can move unimpeded to the bottom.
Response to Charlie Sullivan
Thanks for your comments (which as you know are based on more knowledge that is reported here -- since you were at the job site the day I visited).
Ryan Lawler told me that Unity Homes is thinking of making modifications to their specifications. Among the modifications under consideration are the use of a deeper service chase (which I think is a good idea) and switching from XPS to EPS (also a good idea).
Response to Malcolm Taylor
I understand your point. I think this detail is debatable -- but in the long run, either way works.
Most people overestimate the volume of liquid water that a rainscreen needs to handle. In most cases we are talking drops, not tea cups. The main way this water disappears is evaporation, not drainage. So the skirt flashing on these homes, though unconventional, probably works fine.
I agree, and it's a very minor detail among what are very well built houses. It's just an unnecessary complication.
Given the amount of attention and money spent on energy efficiency, I am surprised by the choice of a resistance water heater as opposed to a heat pump water heater. Is there a reason that resistance is a good choice for these houses, or that heat pump is a bad choice?
Response to Reid Baldwin
I'm guessing that the electric resistance water heater was chosen for all the usual reasons: heat-pump water heaters are expensive to buy, noisy, and rob heat from the room in which they are installed.
No technology is perfect.
I agree that the flashing would be fine without the detail shown, but I am used to seeing flashing that either just barely works, doesn't quite work, or doesn't even come close. So it's a breath of fresh air to see someone going beyond the minimum.
To underscore that I will note that Martin's pictures are not cherry picking. I saw one of these in person and the details are carefully and consistently done throughout the house.
Martin, nice to hear that my one reservation (XPS) is being addressed.
They sure look well thought out. Back to the skirt flashing though: If it isn't really going to deflect bulk water intrusions, then I can't see why it's necessary. If it is, then the chances are it will cause the water to pool where it is sandwich between the pieces of siding. Cement board doesn't allow much drying through the material, so you now have an area which has been identified as potentially problematic that is closed off from the ventilation and drainage the rain screen cavity provides. I think no flashing would be preferable.
Thanks Malcolm. That makes sense.
Coroplast to form the rain screen
Does anyone else use the Coroplast material to form a ventilated rain screen? What's your experience? I see I can order this in either 6 mm or 10 mm thicknesses. Since I can cut the sheet material any way I want, I can make the corrugations run vertically and so it seems like an excellent and inexpensive way to ventilate behind vertical wood siding on a double stud wall.
My main worry is that this is not an officially tested building material. The intended use for Coroplast seems close enough (weather resistant & etc.) and I'm quite encouraged when I see Unity using it. Thoughts?
Response to Bill Dietze
I first heard the Coroplast trick from Rollie Peschon of Spirit Lake, Iowa, back in 2005. I wrote an article for Energy Design Update mentioning the use of Coroplast for this purpose. Rollie takes a lot of photos -- some of his Coroplast photos from 2005 are posted on the Web:
I've never heard of any problems associated with using Coroplast this way.
Can I-Joists be used as studs in a structural wall? I realize they are here but is that only possible because they are part of a structural panel with sheathing on both sides? Could one, for example, use them in a conventional stud wall to replace 2 X 6's?
Very interesting article with a lot of thoughtful detailing.
Using I-joists as studs
For more information on using I-joists as studs, see The Klingenberg Wall.
Katrin Klingenberg used I-joists as studs when she framed her house in Urbana, Illinois in 2003-2004. I wrote an article about that house for the May 2004 issue of Energy Design Update. In that article, I wrote, "Klingenberg framed the thick walls of her house with vertical 12-inch TJIs (I-joists from Trus Joist). Trus Joist has developed details allowing their TJI floor joists to be used as studs; however, since U.S. builders show little interest in superinsulation, the publication is available only in German. (The document, which has the unlikely title of “Balloon und Platform Framing Details,” is posted on the Web at http://web.archive.org/web/20051111012117/http://www.trusjoist.com/PDFFiles/GE-R05.pdf.)
"According to Klingenberg, when TJIs are used as studs, they require structural sheathing on both sides. For interior sheathing, Klingenberg specified OSB, which functions as a vapor retarder. On the exterior, Klingenberg wanted a more vapor-permeable sheathing; she settled on 1⁄2-inch Stedi-R structural fiberboard (R-1.28) from Georgia Pacific."
Martin: thanks for the response and the link.
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