Saturday, March 5, 2016

Too Much of a Green Thing: Passive Solar, Triple Glaze, and SIPs Gang Up


What a difference a year makes. Last year this time.
(All photos by author.)
In our approach to designing our mountain-top home we planned on taking advantage of a southern exposure that also, fortuitously affords a gorgeous view of a large and majestic Cardigan Mountain facing our Tug Mountain. To minimize energy consumption we opted for triple-glazed Fibertech fiberglass windows (see blog Sunday, February 14, 2016, Window Shopping in Canada). We also purchased and planned for multiple wood stoves as major sources of heat. Just this week we made another important design decision that will make our house even more energy efficient. The entire construction will be out of structural insulated panels or SIPs.
A section of a SIP
Doug at right conferring with Pete
in  front of foam at Foard Panel
 
The main section of the house is a barn-like timber frame with the open floor plan of high-ceiling living room and dining area, a kitchen with 9-foot ceilings and a loft over it. This rectangle was always going to be sheathed in SIPs, a popular choice for timber frames. SIPs are ideal for timber frames because they can cover a large span without additional structural elements. Two “wings” off the timber frame accommodate entrances, sun room, laundry room other living (and sleeping) areas, and bathrooms. We weren’t sure if they would be “stick-built,” traditional two by six construction, or made of SIPs, which are super-insulated. After careful evaluation of price proposals and other considerations (such as how to integrate two stick structures with a timber frame) we’ve opted for all-SIPs.
What are SIPs? They are fabricated building panels, a sandwich of expanded polystyrene (EPS) or Styrofoam (think cheap picnic cooler) insulating foam core between two sheets of oriented strand board (OSB). OSB is considered an environmentally responsible building material made from fast-growing young trees and residual parts of larger trees that have been sized for lumber. The SIPs are manufactured (in our case by Foard Panel in West Chesterfield, NH) to fit exactly the walls and ceiling of the house. After the timber frame is up, the SIPs are trucked in and put in place with a crane, then fastened together and to the sill plane, and sealed with triple expanding spray foam.


Foard Panel's headquarters
According to the Structural Insulated Panel Association web site (sips.org): “Building with SIPs generally costs about the same as building with wood frame construction when you factor in the labor savings resulting from shorter construction time and less jobsite waste. Other savings are realized because smaller heating and cooling systems are required with SIP construction.” Fans of SIPs say that they have less impact on the environment than traditional construction methods over the life cycle.
Our roof SIPs will be 10 ¼ inches thick with an insulating value of R38. The walls will be 6 ½ inches thick, with an insulating value of R24. With such major insulation we will have what is considered a “tight” house, one that has very little air transfer. In New Hampshire, with traditionally harsh winters, that means the warmed air stays in, the cold air stay out. Mission accomplished. We’ve surpassed our green goal.
However, we have a created a new problem. The house also needs to breathe, to let out musty, humid air in exchange for fresh, oxygenated, new air. In an old-fashioned, drafty house such as the 1912-built Victorian we are renting while under construction on the mountain, there is no issue of breathing. Air is exchanged regularly with the outside through inferior construction methods, abysmally inefficient windows, and poor (or no) insulation. The combination of SIP construction and triple-glazed windows raises the concern that we could create a negative pressure condition in the new house. If we had a wood stove burning, drawing oxygen from the living room, and the oven baking, pulling in air from the kitchen, we could turn on the range hood and create a dangerous smoke situation in the house. So we will have to add another system, another layer, an air exchange system to breathe for the house.

Breathe; it’s what I tell myself when I am anxious. Too bad a mantra doesn’t work on a house.

2 comments:

  1. I can't wait to hear what THAT involves! But... makes sense. Everything needs to breathe!

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    1. As in every additional layer, money is involved. Lots of building technology involves letting spaces breathe. Keeping air moving around minimizes moisture and mold. The housebuilding adventure continues. . .

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