Ice Dams Reveal Heat Loss Issues

Building Science Tells Us About Ice Dams

DSC_0660Last winter was an unusually cold and snowy here in my hometown of Bend, Oregon. As I walked through the snowy streets, I observed ice dams with their associated icicles cascading from roofs all over town. It paints a pretty picture, but the reality is that it’s very damaging to buildings. Ice dams growing along the eaves prevent water from draining off the roof as snow melts. It doesn’t take long for water to back up under roofing shingles and run into walls and ceilings. This damages insulation, gypsum board and, if uncorrected, can lead to structural decay. It’s sad that many of the homes I see with this problem were built in recent years, even though the solution is well known.

But first, let’s revisit the process that leads to this problem. An ice dam is caused largely by heat loss from a building. Warm air in the attic melts snow on the surface of the roof. As snow turns to water, it flows down to the eaves. The eaves are cold because they stick out into the surrounding air away from the building, so the water freezes. It doesn’t take long for a small glacier to form, and when the glacier gets high enough, water begins to back up. I’m seeing ice accumulations as thick as 10 inches here. Fluffy snow has an insulating value of about R-1 per inch, so a thick layer of snow can actually make the problem worse by isolating the roof surface from the cold, outdoor air.

When I teach building science to builders, designers and others in the construction industry. I always mention ice dams. It’s one way to emphasize the importance of two key elements of high-performance home design: air sealing and insulation. Improving these elements can greatly reduce, and even eliminate, the chances of ice dams forming in the first place.


Air Sealing

Much of the heat in an attic comes from warm air that leaks out from the living space. Small streams of indoor air rise through openings around pipes and wires. Recessed lights can be a major source of air leakage into attics. That’s one reason air sealing the attic is a key component of zero energy construction that we cover extensively here at the Zero Energy Project.


Forced-Air Ductwork

If ducts run through an unconditioned attic, they will increase the temperature and drive ice dam formation. When the heating system is running, air in ducts will be anywhere from 105 to 140 degrees F. Standard duct insulation is far lower than building insulation. Ducts have R-8, even in certified energy-efficient homes. Compare that with attic insulation that should be at least R-50. To make matters worse, ducts leak warm air directly into the attic. In new construction, it’s best to create a design that allows ducts to run through conditioned spaces rather than the attic. For existing homes, sealing the ducts can reduce leakage.


Insulate The Attic Perimeter

In most homes, the roof structure is built in a way that prevents adequate insulation depth near the eaves. The typical roof truss leaves little, if any, room for insulation above the top plate of exterior walls. This leads to snow melting in the perfect location to form an ice dam. Building scientists have been suggesting ways to increase the insulation value at the roof perimeter for decades. Ideally, the R-value at the perimeter should be same as in the middle of the building. trusses2Raised heel roof trusses can be ordered with the required depth at the point where the truss sits on the exterior wall. Vaulted ceilings offer a slight advantage in that rafters don’t pinch the space at the eaves the way that ordinary trusses do. Rafters framed with 2x12s provide enough depth at the eaves for sufficient insulation. While there are many solutions, we like the idea of using i-joists for rafters, because they can be 12 to 16 inches deep.


Cold-climate Roofs

In extremely cold climates, and especially those that receive an abundance of snow, the best approach is to place rigid insulation on top of roof sheathing in addition to the cavity insulation between rafters. Insulation boards are held in place with furring strips that also create a ventilation space. Placing ventilation above the insulation and below the roof sheathing is the gold standard for preventing ice dams.


Roof Ventilation

The key to preventing ice dams is to keep the roofing material as close to the outside temperature as possible. That’s one function of ventilation. Outside air is allowed to enter and exit through a series of vents. About half of these vents are located near the bottom of the slope while the other half are located near the top. Ventilation is required by code for attics and vaulted structures. Unfortunately, deep snow can cover vents located on the upper surface of the roof, making them ineffective for preventing ice dams. Vents located in soffits actually displace the much-needed insulation in it’s most critical location. One solution for soffit vents replaces a typical cardboard or plastic vent baffle with a sheet of rigid insulation. The sheet is positioned to direct air into the attic space above the fiberglass insulation. Depending on its thickness, the rigid insulation could provide R-5 to R-10, which, while a touch low, is certainly better than nothing.


Bottom Line

It’s true that all these ideas cost more than typical construction. But they are far less expensive than repairing damage from ice dams. All the solutions outlined above have the added benefit of improving energy performance and saving money, proving that the investment in good design and smart construction practices comes back in performance and durability.
We seem to view ice dams as a result of the weather, and therefore, beyond our control. Instead, we suggest assigning blame for ice dams to construction practices. That gives builders and designers the opportunity to take a risk and turn it into a benefit.


On-Site Storage, the Great Equalizer

We are now in a world where decentralized electricity production, such as rooftop solar, is more viable than ever for the public and more threatening than ever to utilities. The public is more willing to adopt in-home renewables thanks to reliable technology, solid performance, declining costs, and the growing availability of loans. Utilities are threatened by uncertainty about how to integrate thousands of new decentralized energy sources into the existing network. And ratepayers without rooftop solar are fearful that their rates will rise. On-site energy storage is one new technology that promises to address all sides of this dilemma.

The Need for On-site Storage Technology

As companies such as Tesla/SolarCity and Mercedes produce more affordable batteries for electric vehicles, energy storage technology is spilling into the housing sector. Grid connection is essential for zero energy buildings which need to bank excess production from sunny times to use at night and on cloudy days. The disconnect between when energy is produced by solar panels and when it is needed to meet peak demand remains a problem. On-site battery storage offers a solution.

While utility-scale, grid-based storage is coming online in California and Hawaii, battery products can now be installed behind home electric meters and intelligently integrated with the grid. Tesla’s new Powerwall 2 includes a built-in inverter that converts the direct current from panels into alternating current for household use and exports power to the grid when energy is most in demand. Thinking big, every one of California’s  “million solar roofs” could easily contribute to the overall energy mix making the energy grid more efficient.

On-site storage allows solar owners to store excess energy for later use, even during a power outage. Residential scale batteries hold between 5 and 20 kWh of electricity. This is in the same range as the 15 to 20 kWh used per day by a typical newly-constructed American home. Batteries currently available will help a typical zero energy home even out the peaks and valleys of daily energy fluctuation.

The Challenge of Net Energy Metering

Most utilities have reluctantly agreed to “net energy metering” arrangements when negotiating rate cases with state regulators. Net metering means that utilities accept energy from decentralized sources, although contract terms vary widely around the U.S. The phenomenal growth of these systems shows huge promise for addressing climate change, pollution and national energy security. When there were few decentralized producers, mostly rooftop photovoltaic arrays, there wasn’t much problem for the grid. Unfortunately, electric utilities are largely dealing with an electricity grid that doesn’t look much different from the days when George Westinghouse and Nicola Tesla installed the first large-scale powerhouse at Niagara Falls. The grid is in desperate need of a 21st Century upgrade in order to better meet peak demand and integrate decentralized energy production.

Benefits for Owners and Utilities

Utilities benefit from on-site storage in two ways. First, they don’t have to supply power to the home during peak hours. Second, with smart integration and a fair net-metering agreement, utilities could pull stored energy into the grid to meet a short-term need. Later, when demand subsides, batteries could be recharged with grid power or the customer could be paid a fair price for the amount of energy he or she supplied.

On-site storage is good for solar panel owners, because it allows power shifting from daytime production peaks to evening consumption peaks. Few zero energy homes use as much energy at midday as they produce. But when evening arrives, families begin to use lights, appliances, and energy-sucking devices like game consoles. On-site storage allows them to use banked energy to offset evening use when demand and utility production costs are higher.

Whether specific customers benefit from on-site storage depends on how they are credited for power returned to the utility under their net metering agreements. One factor is the timing of credit expiration. Some agreements generously set an annual expiration, usually each Spring. This allows credits to accumulate over the sunny months to be used during the cloudy winter months. These customers don’t have much to gain from on-site storage other than as an emergency back-up, because they gain full value for the excess energy they return to the grid. In other agreements, credits may expire each month. During sunny months that could mean leaving hundreds of kWhs on the table. For these customers, batteries allow the excess energy from each day to be used that same night. This reduces the amount that would be lost each month, and saves the customer money.

Under some net metering contracts there is a difference in the customes’ buy price and their sell price. These customers receive a lower price when selling excess power than it costs them to buy power from their utility. The sell rate is often based on the utility’s wholesale power costs, which can be anywhere from 20% to 50% of the retail price. Solar owners may generate excess power during the day and sell it at the wholesale rate, but buy power later the same day at the full retail price. Batteries store excess power on-site, so customers use it themselves. This replaces power that they would have purchased from the utility at the higher retail price. In essence, on-site storage allows owners under these types of net metering contracts to capture the full retail value of the energy they produce.

Time of Use Pricing

There is a third possible scenario that would benefit a solar/storage setup most of all and it involves time-of-use pricing. Although uncommon, this rate structure changes prices to reflect the actual cost of utility power. The most expensive energy coincides with the highest demand for electricity, which usually occurs in late afternoon or early evening. Retail customers would save money if they used batteries to carry them through the high-priced peak. This would be true even if the batteries were charged with grid power rather than solar.

A Win-Win-Win Solution

The grid will still be needed to address the bigger seasonal fluctuations from sunny seasons to cloudy ones. However, on-site storage can be valuable to consumers and utilities as it balances daily supply and demand variations. In the future, successful grid integration will require each zero energy building to capture excess production on site for later use and make that stored energy available to the grid as needed. Properly done, it will make the grid more efficient and keep energy prices lower for everyone. On-site energy storage will be a benefit for homeowners with solar collectors, for electric utilities, and for ratepayers without solar. It’s the kind of win-win-win solution the world needs to make the shift to mainstream zero energy living.

Solar Prices Keep Dropping

Prices for installed solar electric systems dropped again in 2015. Residential systems are 5% cheaper and commercial systems are 8% cheaper than the previous year. Tracking the Sun, an annual report published by Lawrence Berkeley National Laboratory, follows upfront cost before incentives. Prices for residential scale installations in the U.S. have dropped consistently from $12/Watt in 1998 to about $4/Watt in 2015 with some leveling off in the last few years. From 2008 to 2012, the main driver was a steep decline in photovoltaic (PV) module prices. Since 2012, module prices have remained relatively flat, while other components (inverters, mounting) and soft costs (labor, administration, marketing) have dropped faster. As system prices have declined, governments and utilities have deliberately reduced incentives, which now stand at less than $1/Watt in most areas.

Of particular interest is the relationship of price in the U.S. compared to other countries. While hardware costs are similar across countries, the authors assign the higher costs in the U.S. and Japan to soft costs, which might include business models, interconnection standards, labor rates, and incentive levels, among others.


Not surprisingly, larger systems cost less per Watt. A typical 4,000 Watt system comes in around $4.50/Watt, while a 20,000 Watt system drops to $3.60/Watt. The report notes significant differences between states with Minnesota and New Mexico at the high end. Nevada and Texas cost the least. Most states are below the median price, but large numbers of installations in higher-cost states, such as California, New York and Massachusetts, push the national median higher.

While high-volume installers showed lower prices in Arizona, that wasn’t the case in other states where there was no apparent relationship between sales volume and price. Of particular interest to zero energy builders, installation prices were substantially lower in new construction than in retrofits. This may be due in part to the economy of scale in housing developments. Tax-exempt customers, such as schools, government, nonprofit and religious organizations, paid more than residential buyers, although the price has declined for all customers over the years.

The steady decline in the price of PVs, has supported the growth of zero energy buildings. Although module prices have probably reached their low, other hardware components still have room to drop. More importantly, soft costs still offer substantial opportunities. Incentives are likely to target these areas in the near future. Continued growth and evolution in the solar industry is good news for the zero energy movement.

Induction Cooks Better than Gas

Induction cooktops have been dropping rapidly in price. I recently acquired two, single burner induction units plus a set of stainless steel cookware for $100. At those prices, you might want to consider retiring the old electric or gas range. There is no doubt that induction uses less energy than an electric resistance burner, but comparing it to a gas burner is more complicated. Thanks to Paul Scheckel, author of Home Energy Diet, we now know that induction cooking is 74% efficient while gas is only 32% efficient. Other sources show the efficiency of induction even higher, making 74% a conservative figure. Induction also heats faster. In Paul’s test, water boiled in 5.8 seconds using induction, while it took 8.3 seconds to boil the same amount of water with gas.

If you live in a zero energy house and power your electric cooktop and oven with solar it is, in Scheckel’s words, “fossil-free cooking.” If you’re living in a conventional home and want to chip away at your carbon footprint, you’ll want to consider the carbon emissions of your fuel choices. Burning natural gas directly to get 1 million btus releases 117 pounds of CO2. Burning coal directly for the same amount of energy would release just over 210 pounds. There aren’t too many people burning coal in the cookstove these days, so it makes the most sense to compare the total system efficiency of electricity with that of gas.

In Scheckel’s experiment the electric induction cooker released 0.29 pounds of CO2 to heat the water, while burning natural gas released 1.16 pounds. (CO2 emissions from propane are even higher than natural gas). So, even taking into account the gross inefficiency of coal-fired electricity, induction cooking dramatically reduces carbon when compared to gas (or to less efficient standard electric cooktops).

In addition to reducing energy use and increasing cooking speed, induction cooking has several more advantages. The electronics allow a wide range of programming. You can start, stop or change temperatures at pre-programmed times. You can precisely set the temperature by degrees. The durable glass top doesn’t get hot, so fingers don’t burn and neither does spilled food, making cleanup easy.

Induction cooking works by creating an electromagnetic field (EMF) that excites the molecules in cookware. So, you need pots and pans made with ferrous metal, i.e., stainless steel or cast iron. If you don’t have those, then you may need to buy new cookware. Those issues aside, induction cooking is safe, efficient, and a perfect addition to a zero energy lifestyle.


Keep Your Cool as Summer Heats Up

Keeping your cool is more important than ever – since this year is officially the hottest on record. Indeed, each of the last six months have set global temperature records.

deck shadeAlmost everyone in developed countries relies on standard active mechanical air conditioning to stay comfortable during the warmest months. It’s one of the fastest growing end uses in the energy sector and a major contributor to high home energy bills. What on earth did everyone do before the vapor-compression cycle was invented to cool our homes? The ancients dressed appropriately, designed their homes wisely, stayed in the shade, gathered around water and sought a breeze. Keeping cool in the modern world doesn’t have to be so different. Using these tried and true passive methods works today. They use very little energy and some are free. And they can be supplemented with active energy efficient mechanical cooling strategies if needed.

Cool Structures

If you live in a high performance home, you have a big advantage. The extra insulation, air sealing and high-performance windows keep the heat outside in summer, just as they keep the heat indoors in winter. Your heat recovery ventilator circulates fresh air through the building while keeping the heat outside. Facing your building toward the south soaks up sun during winter, but blocks it during summer. Properly designed fixed window overhangs help, too. In these homes the structure itself works to keep you cool. If your home lacks any of these cooling features, you may want to consider remodeling your home on the path to zero. No matter what kind of structure you live in, you can take very inexpensive – or free – cooling measures today.

Cool Behaviors

Here’s an approach that requires little or no equipment, takes little effort, and can be completely free: sitting in the shade. Your shady spots will change as the sun travels across the sky each day, so select the best spot for the time of day.

Cool drinks can take the edge off a hot day. Keep a jug of water in the refrigerator for a refreshing break. Wear light clothing to reduce overheating. Spray your face with a cooling mist from a water-filled spray bottle, apply an ice pack or a wet, cold wash cloth, or take a cool shower during the hottest part of the day.

Cool Spaces

If you don’t have shade, you can create it with an awning or umbrella. Try to block windows from direct sun to keep heat out of the house. Sun screens mounted to the windows are a good idea for east and west windows that suffer from direct solar penetration. Thinking long term, you can plant trees or shrubs to shade the yard or house. If you’re less patient, consider fast-growing climbing vines, such as wisteria, grapes or hops and fast growing trees suited to your climate zone.

Ideally, cool outdoor spaces will naturally catch a breeze. If they don’t, you can hang a ceiling fan on a covered patio or place a box fan under your favorite tree. You can also use a misting fan system for additional outdoor cooling.

Cool Nights

You may live in a place where night air is cooler and drier than during the day. If so, you can open windows at night to flush out the house, removing excess heat and preparing it for the next day. This method requires some experimentation, but a temperature difference of 15 – 20 degrees from day to night should be enough to significantly cool your home. You’ll need to open as many windows as possible to get cross ventilation and lots of air flow. A ceiling fan or portable fans to stir the air up will be very helpful. Then, close the windows in the morning to hold in the cooler temperatures during the day. Lowering the shades on any east or west facing windows will help prevent overheating during the day.

Cool Mechanicals

If these passive cooling measures do not provide sufficient cooling, you can move on to low energy use, less expensive, mechanical upgrades to more actively cool your home this summer – without using inefficient standard air conditioning.

You know that a breeze blowing across your skin feels good on a hot day. Moving air pulls heat away from your body faster than still air. Fans create an artificial breeze that helps your body’s own cooling system – your skin – work better. While a simple fan can make a big difference in your comfort, it uses very little energy and costs much less than an air conditioner.  Fans do not reduce the air temperature in the room, so they only work when you are there to enjoy the benefit. And since a fan motor generates a small amount of heat that warms the room, you should run a fan only when people are present. You can use either a permanent-mounted ceiling fan or one or more portable fans.

Another energy efficient mechanical option is evaporative cooling. Sometimes called “swamp coolers,” these devices cool air by evaporating water. While the image of a swamp cooler may not seem glamorous, in the right climate zones they are very effective. They are less expensive to purchase and operate than a standard air conditioner. Evaporative cooling works best in areas with low relative humidity during the cooling season. Deserts and “mediterranean” climates, such as much of the Pacific coast, are very suitable for evaporative cooling. The downside of using water for cooling is… using water for cooling. In many dry areas, where the climate is well suited for this approach, there are also severe water shortages. Newer technology called two-stage evaporative cooling can be more efficient for very hot climates.

Finally, a mini-split heat pump can be installed in any climate zone. While more expensive than other methods a mini-split, which both heats and cools can easily be installed in a single room of existing homes.They are much quieter and use much less energy than standard air conditioners.

Using any or all of these energy saving suggestions, starting with the passive cooling measures, will keep you comfortable during hot summers and at the same time will help fight climate change by reducing or eliminating the high energy use of standard air conditioners.



Banish Payback

green_house-300x300Every conversation about zero energy homes (ZEHs) eventually comes around to the question of “cost.” The negative connotation of added cost and, even worse, “payback” always puts ZEH advocates at a disadvantage. For years, I’ve encouraged advocates to call energy expenditures investments rather than “costs that must be recovered”.  So, let’s banish the entire idea of “payback” and “payback period.”

Would anyone judge a stock investment or an interest bearing bank account by calculating how long it would take the earnings to equal the principal? No, that would be absurd. Likewise, it’s counter-productive to consider funds used for energy improvements to be costs. They are investments with a financial return that is both significant and predictable – both immediate and long-term.


Which of these two houses cost more overall?

When you spend money to reduce energy use, you receive a tangible financial benefit that begins the first month and continues for as long as you own your home. Let’s say that you’re building a new zero energy home. You can calculate how much it will cost to increase insulation, reduce air leakage, improve equipment efficiency and add photovoltaic panels. In most cases the investment will be in the tens of thousands. This investment will return immediate benefits whether you finance the purchase or pay with cash.

To illustrate the idea, let’s use an example of an investment of $40,000 in energy efficiency measures needed to bring a new house to zero energy. In my area, financial incentives from the electric utility, state and federal government cover just under half and reduce the amount to $21,000.  If you finance this home with a conventional 30-year mortgage,  with current mortgage interest rates at 4%, you’ll pay $50 per month for each $10,000 you add to your principal amount. Let’s assume that you financed the additional construction cost of $21,000 (after incentives), then your monthly added payment for energy improvements  would be $100.  Based on energy modeling, let’s, assume that the home will save $200 per month for energy.


That $200 return starts the first month you live in your house,  and in this example, it exceeds the added monthly mortgage payment whether incentives were used or not.

Financing Summary Additional Investment Monthly Payment (30 yrs at 4% ) Monthly Energy Savings Net Return
With Incentives $21,000 $100 $200 $100
Without Incentives $40,000 $192 $200 $8

You can also turn this calculation around by first looking at savings and then calculating how much money you could afford to invest. By building a home that saves $200 per month, you could afford to invest $40,000 in energy improvements.

In return for your investment, you pay nothing or very little for energy from the day you walk in the door. The monthly savings almost always offset the additional mortgage payment. Many cost-effectively built zero energy homes will realize a profit on their investment during the very first month, as in this example. It’s a very simple idea. If the monthly energy savings exceed the monthly financing cost, you win!

— Bruce Sullivan, BASE zero, LLC,

– See more at: Zero Energy Project

Simple, Effective, Affordable Indoor Ventilation

IMG_17931-241x300We recently received this question from Keith in San Diego. I’m a regular volunteer for Habitat for Humanity, and I’ve got a question for you. How do we get variable amounts of tempered, filtered make-up air back into our houses? As you know, we’re building houses tighter than ever, and our affiliate is using 1 inch EPS foam on the outside of the house as well. ASHRAE 62.2 is great at calculating what to exhaust to meet their standards, but in a really tight house, you need something to exhaust.  An HRV would be a bit much. My question is – how do we meet the requirements in a logical and cost-efficient manner?

Many authorities require that automatic ventilation be provided in homes where the tested air leakage rate is below 7 air changes per hour at 50 Pascals (ACH50). It’s likely that the homes you’re building with Habitat for Humanity are at or below that threshold. Providing ventilation that is effective and also inexpensive can be quite a trick. Such a system must expel the odors, moisture and pollutants from the living space and replace that same amount of air from the outside. Your question concerns the replacement or make-up air.

Automatic ventilation is an essential element of high-performance homes. In leaky houses, this make-up air comes through openings in the building envelope — we call these drafts. By sealing the drafts, we can make the home more comfortable and energy efficient. But how does the make up air enter? Vents built into window frames or installed through walls are often used. They are inexpensive, but they are little better than the leaks they replace.

A better approach is to direct the make up air to specific locations in a building to complement locations where air has been expelled. Let’s call these locations supply points and exhaust points. You could use one fan to exhaust air and another fan to supply air. This is a “balanced system” and it has the advantage of delivering outside air to specific locations. Generally, you would want to exhaust air from bathrooms, laundry rooms and other wet locations. Supply air can be delivered to bedrooms. The combination of exhaust and supply flows balances the air pressure and mixes outside air throughout the home.

Balanced flow can be accomplished by a heat recovery ventilator or energy recovery ventilator. (For convenience, let’s call them both HRVs.) Inside an HRV cabinet there are two fans, one to exhaust stale air and another to supply outside air. With an HRV you get one package that serves both purposes. (Plus heat recovery, but let’s put that aside for now.) One HRV can provide whole house ventilation that meets the ASHRAE 62.2 requirement. Most equipment can operate at several speeds. In addition to whole house ventilation, an HRV can also provide spot ventilation for bathrooms. A simple 30-minute timer in each bathroom can boost the fan speed  to high when more ventilation is needed.

HRVs are not cheap. Equipment and materials alone can cost $1500 – $2000. But then high-quality bath fans can cost $200-300 when installed in new construction by a licensed electrician. If the HRV replaces two bath fans, that can offset some of the cost. While installing HRVs requires a certain level of knowledge and skill, I think Habitat volunteers could do a good job with proper training.

Nevertheless, it’s probably difficult for most Habitat organizations to justify the cost of HRVs. Here are two alternate ideas. First, you can install a balanced ventilation system without the heat recovery function. It’s cheaper and easier to install. Two in-line fans attached to a branched duct system can provide excellent ventilation for about the same price as a two, high-quality bath fans, and without the aggravation of passive vents in walls or window frames. Two manufacturers (Broan and Fantech) make a single unit that contains two fans for a balanced system.

Second, you could install a  Panasonic WhisperComfort Spot ERV for about $350 each.  A single unit installed in a central location would provide general ventilation for the whole house, while bath fans would cover spot ventilation. Another option would be to install one WhisperComfort unit in each bathroom, replacing the bath fans.

Both these solutions offer balanced ventilation to maintain good indoor air quality, but would be far less expensive than a HRV.

— Bruce Sullivan, BASE zero, LLC,

– See more at: Zero Energy Project

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