WOULD YOU RATHER HAVE THE STANDARD R-20 WALL? Code-minimum walls are framed and insulated conventionally, and are prone to air leaks and thermal bridging. The 2009 IRC requires that a residential structure in a heating climate (zone 5) must have the following R-values: Windows: R-2.8 Slab: R-10 Basement walls: R-10 Wood-frame walls: R-20 Roof: R-38
YOU CAN SAVE ENERGY WITH R-40 Progressive builders and designers have redefined what it means to build an efficient envelope: Windows: R-5 Slab: R-10 Basement walls: R-20 Wood-frame walls: R-40 Roof: R-60
If we look even a mere 20 years into the future, we’ll see a very different world. For one thing, fossil-fuel-based energy will be much less abundant and much more expensive as we become independent of foreign oil. Eventually, we will be living in a world without oil. But the places we live in last a lot longer than 20 years, so we should start thinking now about how to construct houses that will function in a post-petroleum economy.
To that end, a good deal of money and brainpower is being put to work to come up with efficient solutions. The German Passive House Institute and its U.S. affiliate (Passive House Institute US; www.passivehouse.us) have designed building-enclosure standards based on energy use that’s 15% to 20% of today’s typical residential design load. Their model suggests that an envelope ratio of 5:10:20:40:60 should be considered a minimum in the northern tier of the United States. In other words, windows at R-5, subslab at R-10, basement wall or slab perimeter at R-20, walls at R-40, and ceiling or roof at R-60.
Fortunately, you can achieve this goal now with one of six high-performing wall systems, each capable of achieving or exceeding R-40 (not including the added values of the siding, the drywall, or the surface-bound air layers). Each can be interpreted or varied in many ways. Although there are still other approaches to consider, I’m focusing on systems currently in use (or in development) in the northern United States, where the climate demands more efficiency from a structure.