From the top down, here are some guidelines: Roofs: again, cheaper is better: Whenever the design allows for a simple wood truss-roof with blown-in insulation and venting above the insulation, do this! And allow for a good, high-energy heel. Good design can work around aesthetic challenges. Embrace the energy heel and work with it. Every contractor knows and loves this system. Implementations challenges will be low. When the design calls for a low-slope, flat, or vaulted roof, you have more limited options, but there are still options. The most conventional strategies are to “flash-and-batt” or add rigid insulation over the sheathing. Minimize the amount of rigid or foam insulation you use, but it must be adequate enough to prevent condensation. As always, use mineral wool (Toprock) or EPS, or anything besides XPS. Maximize the amount of dense-packed insulation. You can get air-sealing with the flash and batt strategy, but only as long as the spray foam does not become brittle and crack, and only if installed correctly. In the high-performance building science community, spray foam is not considered a “durable air seal.” But, based on the availability of innovative, educated, or like-minded contractors, this may be the option with the least harm. The next option, when adequate slope allows excellent venting, a structural rafter, truss, or joist roof dense-packed with low carbon insulation can be covered with a cold roof such as this5. In areas where labor is extremely expensive and depending upon the pitch of the roof, contractors may argue that this is more expensive than spray foam. It is possibly true, but it is not necessarily true. It depends on the contractor more than the system. For a flat or low-slope roof, it is more imperative that the contractor be educated and willing6. A high-R-value, densepacked roof such as the one above can be left unvented if there is no chance of moisture entering the insulation cavity and if whatever moisture may sneak its way in can get out. This requires excellent air sealing with tapes and caulks at every penetration and a chase to eliminate most penetrations, such as an electrical chase framed below the roof framing. This assembly is risky, and building scientists argue about how risky. Idaho has, in general, a very dry climate, so the risk is lower here. In the two dense-packed options, the use of “smart membranes” and excellent air sealing is crucial. A smart membrane with smart installation and a plan for applying it at each junction keeps moisture vapor out of the assembly and allows it to escape to the interior. Not every contractor is aware of smart membranes, but there are some common options, and most contractors in our market are familiar with CertainTeed products and will be more open to this version, Membrain, rather than an unknown brand. Performance, installation, and permeability vary, but an article discusses the most common7. Walls: champion a thick wall. Thick walls are the best walls. Thick walls inherently feel better to humans than paper-thin ones – they are more sheltering. And, at each window, there is an opportunity for something interesting: angled window jambs allow light to reflect and fill a room more softly, and deep sills create places to sit or place objects. We give up some R-value per inch with low-carbon or carbon storing insulations, so we have to add inches back in. There are a couple of options for where those inches can go relative to the sheathing layer (similar to a roof): • Exterior insulation provides better performance by reducing thermal bridges, and we benefit from keeping the sheathing warm, thus avoiding condensation. It creates some challenges with fastening siding, but this is a reasonably Credit FineHomebuilding — continued from page 39 Credit 475 Building Supply 40 AIA IDAHO ARCHITECTURE | 2022-23 | aiaidaho.com
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