In my last post on Pretty Good House (PGH) building details, I shared a concept house illustrating how to incorporate low-carbon building strategies. Here, I’ll highlight another approach to a PGH—one that is performance-based, with a focus on operational energy—as well as additional low-carbon building practices and a few software tools I find valuable.

Start with energy and carbon modeling

While you can build a decent house, or even a PGH, without energy modeling, using it has several advantages, and it’s not that hard to do once you learn how. What I find most useful is being able to optimize construction details by comparing the effect different assemblies will have on a building’s energy usage. Energy modeling also helps fine-tune window specifications for the lowest energy use, and determines heating and cooling loads for the house, among other options.

For anyone trying energy modeling software for the first time, BEopt, a free software from the National Renewable Energy Laboratory (NREL) using the U.S. Department of Energy’s Energy Plus modeling program, is good and relatively simple. It has an optional feature to automatically determine the most cost-effective path to reaching net-zero operation. You can also use it to balance expenditures and gains and to help calculate return on investment. In many jurisdictions you will need Manual J calculations for heating and cooling, and they are important for right-sizing HVAC equipment, but BEopt does not provide Manual J outputs.

Ekotrope, Wrightsoft, and REM/Design are a few professional-grade programs. Some BIM (building information modeling) programs also have energy plug-ins. Passive House practitioners use the PHPP or WUFI Passiv energy-modeling programs, which are labor-intensive to use but preferred for extremely low energy–use homes.

It’s also a good idea to see how your embodied carbon emissions stack up with different assemblies. My favorite tool for this is the BEAM Estimator, which Chris Magwood and Builders for Climate Action designed. It doesn’t provide a full accounting of all embodied carbon emissions because comprehensive, accurate data simply isn’t available for some things like electrical and plumbing systems. But you can compare the relative impact of anything building-enclosure related, from foundation systems to roofing types and interior finishes. (BEAM is available on a sliding scale to make it accessible to all; donations are well worth the value the tool provides.)

Determine targets

One way to design for PGH energy efficiency benchmarks is to divide the location’s heating degree days (HDDs) by 180 to get the R-value for a wall, and by 120 for a roof. For example, in a cold climate, such as climate zones 5 and 6, use:

R-10 SLAB (EPS or GPS expanded polystyrene, mineral wool, or recycled XPS or NGX sub-slab insulation)

R-20 FOUNDATION WALL (crawlspace wall and slab-perimeter too)

R-60 ROOF/CEILING

R-40 WALLS

R-5 TO R-8 WINDOWS (U-0.20 to U-0.13). Even the best windows make lousy walls, so don’t over-glaze.

1.0 ACH50 AIRTIGHTNESS (This is the maximum air leakage target many of us are using. Others say 1.5 or 2.0 ACH50 is tight enough. Tighter than 1.0 ACH50 may not add significantly to the home’s performance but high-performance builders are routinely getting as low as 0.1 ACH50.)

Meet PGH benchmarks with proven assemblies

R-40 pier foundation

A pier-and-beam system using helical metal piers or concrete piers have much lower upfront carbon emissions than a full concrete foundation. Piers are located to carry wood beams to support the floor framing. When the top of helical piers are more than a few inches above grade they need bracing, so I’m showing a bolster system to create the height needed to allow code-minimum 18-in. clearance from grade to the bottom of the floor system. (Pressure treated beams don’t need to meet that clearance requirement.) The beams are inset enough to allow for ventilated skirting—the more airflow, the better.

The floor system can be dimensional or engineered lumber; I prefer I-joists because the narrow webs nearly eliminate thermal bridging, allowing for about R-40 for the whole floor system when insulated with dense-packed cellulose, wood fiber, or fiberglass.

To keep out air and critters, 1⁄2-in. sheathing is installed below the joists and sealed to the beams. (If you’re in an earthquake-prone zone, this system may not work for you, but it pairs well with a partial basement that can provide additional lateral support and a place for utilities.)

R-40 double-stud walls

Builders have strong feelings for or against double-stud walls, but they have a long track record in New England, and the builders I work with tend to like them. In a cold climate, they need to have ventilated rainscreens so they can dry easily to the exterior. Dense-packed cellulose or wood fiber are ideal insulation materials for this type of wall.

In cold climates, I am most comfortable when a variable-permeance membrane is used at the interior, though many builders just use painted drywall. (Often called “smart” membranes, these products are made from plastics whose vapor permeability changes with humidity levels.) With a 12-in. insulation cavity, the wall will perform at about R-40. The taped sheathing doubles as the air control layer.

R-60 roofs with thick cellulose

The least expensive, lowest-carbon way to build and insulate a roof or ceiling is usually a raised-heel truss with loose-blown, bio-based insulation. I typically spec R-60 cellulose at 16 in. deep. Raised heels can make walls look tall, so I sometimes reduce the insulation depth at the eaves to as little as 10 in. The air control layer is at the ceiling, in the form of a variable-permeance membrane, but taped 1⁄2-in. sheathing works too, as long as any penetrations are carefully air-sealed. Inside of that, horizontal 2x furring creates spaces that can be used to install lights and run wires without puncturing a membrane air control layer.

Optimize the window package

European-style triple-glazed tilt-turn windows are significantly different from standard North American-style windows, but once you’ve experienced their quality and comfort, it’s hard to go back to comparatively flimsy American windows. They perform best installed at the center of the wall, but with a minor performance penalty they can be flush with the sheathing. They are usually installed with clips instead of flanges; specialty tapes are available to keep water out and provide airtightness.

Why shoot for PGH benchmarks?

Building energy-efficient homes makes for more comfortable, less-expensive living and better prepares the occupants for climate change. Doing so in a reasonably cost-effective manner, perhaps trading fancy finishes and complicated rooflines to better afford the upcharge, makes it accessible to more homeowners and homebuyers. Reducing your carbon footprint at the same time is better for the planet, and often better for occupant health and building durability, than building a house out of plastic or concrete. As Bruce King, author of The New Carbon Architecture and co-author of Build Beyond Zero  once wrote (borrowing from Michael Pollan): “Build, but not too big, and mostly with plants. Nuff said.”

____________________________________________________________________

Michael Maines is a Maine-based residential designer, co-author of  The Pretty Good House, and longtime contributor to Green Building Advisor and Fine Homebuilding magazine.