The Science of High-R Wall Construction, Part 2

DSC_0249-300x199Part 2: Zero-energy homes usually require a wall system with an insulating value of R-30 to R-40, at least in cold, heating-dominated climates. In Part 1 of this series, I suggested that you can’t go wrong with rigid insulation sheathing or with double-stud walls filled with fibrous insulation. While the double-stud approach does have advantages, it also has a potential drawback – uncertainty about moisture condensation in thick walls.

My experience over the last ten years has shown that wood-framed walls are moisture tolerant. Even though condensation does occur, wood fiber is capable of storing a considerable amount of winter wetting until it can dry out the following summer. As long as the moisture content of framing and sheathing returns to normal, decay won’t occur. When you purchase kiln-dried lumber, it arrives on the site with about 15 percent moisture content and dries further after construction. When condensation occurs on framing, plywood or OSB, the wood fibers can absorb this liquid water until the wood becomes saturated at about 21 percent moisture content. Only then does the surface become wet enough to  support decay. The amount of time above fiber saturation is a key factor in whether decay takes hold.

These ideas were supported by a 2015 study conducted by Building Science Corp. The report includes lots of hemming and hawing and conservative conclusions, but the main take away is that while double walls do indeed get a little moist, they will not support decay because in cold, heating dominated climates, the wall assembly will dry to the inside. If there is condensation on the inside of the wall sheathing during cold weather, it will be too cold to support decay organisms

Bill Hull of W.H. Hull Company in Bend, Oregon, who specializes in double-stud walls, opened up a 12-inch-thick wall cavity on the north side of a one-year-old home. There was absolutely no sign of wetting, let alone decay, even though it was a bathroom wall.

In fact, I  recently completed my own home with 10-inch double-stud walls. During construction, I embedded a temperature and humidity sensor in the north wall to track moisture. I expect to see some of the conditions that might cause condensation, however, I would be very surprised to see decay. I’ll update this post with the results in March.

We have two great wall systems to help us reach the zero energy goal in residential buildings. Walls with exterior foam sheathing provide great energy performance and provide a strong defense against condensation. Double-stud walls also give superior performance, including resistance to moisture damage.

For more detailed information, check out the full Building Science Corporation study or if you’re short on time, read the study synopsis at

— Bruce Sullivan, BASE zero, LLC,

– See more at: Zero Energy Project

The Science of High R-Wall Construction, Part 1

double-wall_1-300x261Part 1: There has been a lot of talk lately about which wall assembly is better for achieving a truly high-performance wall, i.e., R-30 or better.  The top contenders are exterior foam sheathing or off-set double-stud walls. They are both great, so you can’t go wrong. But for the sake of argument, let’s take a look at each.

Many years ago Joe Lstiburek, building science rock star and founder of Building Science Corporation, introduced his “perfect wall” concept, although this perfection extends only to energy and moisture performance. There are other elements that I consider worthy of discussion: climate impact, cost and buildability.

Climate Impact

Foam sheathing, when mounted to the exterior of the wall, blocks heat loss through the wall framing and keeps the structural sheathing warm, essentially preventing condensation. One type of foam sheathing, expanded polystyrene (EPS), has lower greenhouse gas potential than other foam materials making it our favorite choice, despite being made of petroleum. Extruded polystyrene (XPS) and closed cell spray foam currently generate far more greenhouse gases than EPS. Wood framing sequesters greenhouse gases rather than generating them.


Foam material is considerably more expensive than fiberglass or cellulose insulation commonly used in double wall construction. Few contractors have experience with foam, and some have been deterred by a variety of little details, so labor costs can be high too. Double wall is generally less expensive for both materials and labor. Overall, costs for double wall tend to be lower than with exterior foam sheathing or other foam insulation products.


Installing exterior EPS requires attention to a variety of details. For instance, furring must be installed over the foam, around windows and openings, at corners and other locations to prevent dampness and support siding. If you choose foam, make sure to get a product that contains a borate insect deterrence to keep little critters away.

With double wall just about any framer can achieve good results with proper guidance. The major complication is capping the double stud walls with plywood gusset. The only potential drawback to double wall construction is uncertainty about moisture condensation in thick walls. However my judgment over the last ten years is that wood-framed walls are moisture tolerant. To find out why, read part two of this series about moisture content.

I believe that you will get a great thermal envelope with either EPS or off-set double stud walls. Double-stud will probably cost less and flow more smoothly, and it has lower greenhouse gas impact. In the end, you’ll generally get a better house if your builder gets to do things his/her way. So, the builder’s opinion should carry a lot of weight in your decision about which wall system is best for you.

— Bruce Sullivan, BASE zero, LLC,

– See more at: Zero Energy Project


Design Strategies for Zero Energy Homes That Builders Will Thank You For

architect1High performance homes, especially those striving to achieve zero energy     performance, employ a number of materials and construction practices not found in conventional construction. Customarily designers and architects may not specify certain energy related details or may neglect to make certain decisions relating to energy performance strategies during the design phase. This lack of attention to energy related details and strategies could create obstacles and add costs for builders. Your zero energy home design project will be smoother, less expensive and more successful, and builders will thank you, if you use  these twelve strategies in the design process and detail them on your plans.

1)    Clearly define the thermal boundary. That means deciding what is inside and what is outside the conditioned space. (Example: vented attics and crawlspaces are outside.)

2)    During conceptual design consider using fewer larger shapes, rather than many smaller shapes with lots of architectural complexity. Simpler building masses will be easier to build, air seal and insulate in the field.

3)    Specify that wall insulation is fully enclosed with rigid sheets of OSB, Thermoply or similar materials, and never design walls where it is difficult to properly cover insulation. Pay particular attention to soffits, attics, bathtub surrounds, and fireplace enclosures. If you’re drawing double-stud walls, be sure to include details for enclosing the framing cavity, including a plywood cap across the parallel top plates and plywood bucks inside window and door openings.

5)    Identify the type of air barrier system to be used. Will it be air-tight drywall approachZIP SystemSIGA membrane and tape or something else? List air sealing materials and techniques on the plans.

6)    Specify that blower door directed air sealing be conducted before insulation is installed in order to locate unexpected air leaks and to effectively seal them.

7)     Locate all heating and cooling equipment, along with their pipes, ducts and refrigerant lines. Draw these on the plans and specify the need for sealing any penetrations.

8)     Draw mechanical ventilation equipment and ductwork on the plans, locating equipment and ducts within the conditioned envelope of the building where feasible. Remember that heat recovery ventilators need a condensate drain.

9)    Decide on the type of water heater to be used and the best location. Electric resistance water heaters should be centrally located inside the conditioned space in heating-dominated climates and outside the conditioned space in cooling-dominated climates. In heating-dominated climates, heat pump water heaters should be located outside the conditioned space in areas with about 1,000 cubic feet of volume and a supply of waste heat. If gas-fired water heaters are used in an air-tight home, they must be sealed combustion models.

10)  Consider using one type of ceiling throughout the house: either flat or cathedral. Whenever ceiling heights change, there will be a wall separating the room with the high ceiling from an unheated space, usually an attic. This “vault wall” can be very tricky to air seal and insulate. The insulation level of that wall should equal other exterior walls, and it will need to be covered with a rigid material to enclose the insulation. (See 3 above.) If more than one ceiling height is present, develop clear details for air sealing, insulation and rigid backing.

11)  Based on an accurate energy model, determine the optimal size of the photovoltaic system. Check that there is adequate roof area with the proper tilt and orientation to supply the energy needed to reach the zero-energy threshold. Make sure there will be nothing on the roof surface to interfere with solar panels, such as a chimney, plumbing vents, etc.

12)  Early in the design process, engage the relevant building trades, including framers, insulators, plumbers, electricians and solar contractors, regarding the energy efficiency measures in the design and the most cost effective sequence for implementing these measures. Ask them to review the design and incorporate their feedback.

Following these 12 steps will make construction of zero energy homes considerably more efficient and more cost-effective for builders.

— Bruce Sullivan, BASE zero, LLC,

– See more at: Zero Energy Project