Vapor Barriers and House Wraps: where and why?

House wraps must stop bulk water from entering on the cold side and also be permeable enough to allow water vapor to pass through from the warm side.

House wraps must stop bulk water from entering on the cold side and also be permeable enough to allow water vapor to pass through from the warm side.

The building envelope is defined as those parts of a house that keep the indoor and outdoor environments separate. The building envelope includes the exterior walls, roof, windows, doors and the foundation and/or ground floor.

As elements of the building envelope, vapor barriers and house wraps are a critical part of controlling moisture and air flow through your home.

If selected and installed properly, these products can help you conserve energy, prevent mold growth and maintain the structural integrity of your home. On the flip side, not using these products or using one incorrectly can have the opposite effect.

Vapor barriers on the warm side

A vapor barrier, also known as a vapor retarder, is a layer of material designed to slow or nearly block the movement of water vapor by diffusion. How much a vapor retarder impedes the movement of water vapor is referred to as its permeability rating, or “perm” rating.  Six-mil-thick (0.006 inch) plastic sheeting is a typical vapor retarder material prescribed by residential building codes in cold climates, as its perm rating is extremely low.

In standard cold climate frame construction, the plastic vapor retarder is located on the warm-in-winter side of the wall — typically it is applied over the studs directly behind the drywall.

All homes contain moisture inside — cooking, bathing, breathing all create water vapor. In winter time the challenge then becomes keeping this water vapor from reaching places in the building envelope where it can condense.

Ventilation, which is essential to exchange moisture-laden air with clean, dry air, helps reduce the quantity of moisture in a tight home, but not enough to eliminate the need for a vapor retarder.

Where it gets interesting is that 98 percent of water vapor in a home travels by air leakage, while only the remainder moves by diffusion — through solid materials such as the drywall and sheathing in your walls. So, with proper sealing around penetrations and by sealing overlapping layers, we can also rely on the plastic vapor retarder to serve as an air barrier.

House wraps on the cold side

House wraps, on the other hand, are primarily designed to cope with the elements on the outside. They must be permeable enough to allow water vapor to pass through them from the warm side, but still stop bulk water like rain from entering on the cold side — similar to a Gore-Tex jacket.

By nature, house wraps must be vapor permeable enough to allow for drying if moisture finds its way into the wall cavity from either the inside or the outside. In addition, house wraps can help minimize the movement of air in and out of the exterior walls. Air movement through the building envelope in an uncontrolled manner, means you’re losing heat, which can become a burden on your budget.

To effectively repel water and reduce airflow, house wraps must be detailed correctly and applied using the manufacturer’s recommended methods and adhesives. All the penetrations into your walls from the exterior, such as vents, electrical connections, and architectural features, must be carefully accounted for.

The right types of house wraps can perform an important job in windy places by stemming significant heat loss and keeping the framing protected from precipitation that gets past the siding.

Final thoughts

The placement and permeability of vapor barriers and house wraps are addressed by building codes, but vary by region. Vapor barriers are required on the warm-in-winter side of the exterior walls in Fairbanks.

This article only touches on the details required to choose and install vapor barriers and house wraps. Placement and water vapor permeability can be a fairly complicated issue because of the wide variety of products on the market today.

You can find resources at CCHRC, the University of Alaska Fairbanks Cooperative Extension Service, and your local building department to help you make the right decisions. Doing your research up front will help maximize home performance and prevent problems later.

Full Scribe Log Home Building Workshop in Nenana

The log home made of freshly peeled white spruce sitting on the side of the road about two miles north of Nenana could be the first of many. At least, that’s what the Toghotthele Corporation is hoping.

“We’ve already had four people pull into the driveway and say, ‘Is this for sale?’ We have interest just from passers-by seeing the activity going on. If these guys are interested in it, we could sell them a kit,” said Jim Sackett, CEO of the Nenana-based corporation.

Earlier, a group of students placed the tie log for a roof truss. This included scribing the logs (or tracing the shape of one log onto the other), cutting the notches, and using an excavator to lift and position the tie log on top of the side walls.

The 16-by-20-foot full scribe log home is the product of a three-week construction workshop that wrapped up earlier this month offered by Toghotthele and the University of Alaska Fairbanks Cooperative Extensive Service. It was taught by Robert Chambers, who teaches log building around the world, and local log builder Rich Musick.

The students are Toghotthele shareholders, plus a contractor from California who flew up specifically to take the class. They’re learning the craft not only to build log homes of their own, but also to possibly start a new business in Nenana that uses local resources and provides new housing.

Sackett is anticipating a potential housing boom if oil is found during exploratory drilling in the Nenana Basin this summer. Toghotthele owns roughly 140,000 acres of white spruce and is developing two subdivisions for construction.

“If people start moving into the area to take oil-related jobs, we don’t have much of a housing surplus in Nenana. Log homes are a natural fit to Alaska in general and specifically to Nenana. About half the homes in Nenana are log already.”

Full scribe construction means the logs retain their natural shapes and irregular surfaces. They are precisely hand-fitted to each other with no gaps, nails or other hardware. The method Chambers is teaching allows for logs to shrink as they dry (which typically takes up to six years) to ultimately form an airtight joint.

During the course, students practiced techniques familiar to log construction including scribing, notching, using chain saws and hand tools for sculpting and fitting, and other tools like spuds (for removing bark) and plumb bobs and levels (to orient layout lines and logs in horizontal and vertical directions relative to one another).

In today’s trend toward super-insulated homes, there is debate about how much insulation is enough. When it comes to logs, the R-value of wood is lower than other insulating materials. Wood is about R-1 per inch, compared to fiberglass (R-3.2 per inch) or rigid foam (R-4-5 per inch). So in theory, it would take a 16-inch log to achieve the same R-value as a standard 2×6 wall filled with fiberglass (if you don’t count extra heat loss through the studs).

“By the numbers, you’d be really hard-pressed to find Interior Alaska logs big enough to perform above R-21. But it’s a matter of perspective. You can build a decent wall system, and by making improvements to the rest of the shell — putting in an efficient heating system, good windows, good foundation and roof insulation — you can do really well,” said Ilya Benesch, building educator at the Cold Climate Housing Research Center.

As Chambers puts it, the embodied energy of building with local logs (the total energy used to produce all the building materials) beats most other construction methods.
“Homes that have a lot of concrete, aluminum and glued manufactured wooden products have a very high embodied energy,” he said.

In addition, well-built log homes can last 1,000 years, making them even more sustainable, he said.

You also can build very tight homes using full-scribe construction. Chambers seals the space between logs with a double gasket made from open-cell foam that is compressible but acts as an air and vapor barrier.

He noted a log home in Soldotna that tested at 0.5 air changes per hour, which rivals some of the tightest homes out there.

Chinking with elastic caulking is another way to air-seal joints between logs, though alters the appearance of the otherwise natural fit.

As Sackett points out, some Alaskans just prefer log.

“Log is just natural to Alaska. A lot of the early homes were log, and people just have this attraction to log homes,” he said.

“It’s a resource Toghotthele already owns, so figure out what you can do with what you already have.”


Check out Chambers’ DVD and book series here.

Placing the cross tie log onto the end wall

Placing the cross tie log onto the end wall

Green logs take 4-6 years to settle and form an air-tight seal.

Green logs take 4-6 years to settle and form an air-tight seal.

Placing the cap log on the cabin.

Placing the cap log on the cabin.

Scribing, which means tracing the shape of one log onto the other for a perfect fit.

Scribing, which means tracing the shape of one log onto the other for a perfect fit.

The class

The class

Shallow Frost Protected Foundations: a good option for the right site

Photo Courtesy Wisdom & Associates. The frost protected shallow foundation uses insulation to create a heat bubble under the structure. The heat bubble keeps the ground underneath and around the structure from freezing, effectively raising the frost depth. The shallower frost depth allows for a shallower footer.

Photo Courtesy Wisdom & Associates. The frost protected shallow foundation uses insulation to create a heat bubble under the structure. The heat bubble keeps the ground underneath and around the structure from freezing, effectively raising the frost depth. The shallower frost depth allows for a shallower footer.

When building homes in cold climates, traditionally the foundation is placed on undisturbed (or compacted) soils and below the frost line to better resist the potentially destructive effects of ground freezing and frost heaving. In Alaska, every region has building codes and/or generally accepted design standards that specify the depth of the local frostline. In Fairbanks, the design depth for footings is a minimum of 42 inches below grade. Installing footings and a foundation wall at this depth can be expensive, and in some cases a shallow frost protected foundation (SFPF) might present a more economical option. As a general rule, a SFPF system is feasible only on ground free of permafrost.

Unlike a standard foundation, a shallow frost protected foundation can be placed well above the frost line — often at depths of about 16 inches below grade, and in some cases less. Since the foundation now rests on soils that normally would freeze seasonally, the key issue is to keep the ground underneath and on the sides of the foundation from freezing. SFPF designs usually depend on foam board insulation laid out far enough horizontally around the perimeter of the footing to ensure that the ground underneath remains thawed year round, no matter how cold it gets.  In essence, the horizontal insulation creates a “heat bubble” in the ground under the building.  A frost protected foundation can accommodate a variety of designs including thickened edge/monolithic slabs and shallow footings.

By code, the horizontal foam board insulation must be protected from sunlight and physical damage. Typically, this means the insulation will get covered with a layer of backfill thick enough to protect it for the life of the structure — although concrete or pavement coverings also might be options (in a high traffic area, for example). Typically, foundations including SFPF systems should extend a minimum of 6 inches above grade to keep wood framing away from ground moisture. Any vertical area above the horizontal insulation also must be well insulated.

In Interior Alaska, SFPF systems are fairly new and a professionally engineered design will buy a lot of peace of mind. Because of site-specific variations in soils conditions and foundation designs, a professional engineer will best be able to calculate the insulation values and installation methods to ensure the foundation will perform properly.

This model is included just to show how heat leaks from the foundation into the ground. “Warm” colors indicate temperatures above freezing. “Cool” colors indicate soil temperatures below freezing. The dashed blue line is the freezing front, which you do not want to contact the foundation.

How can I use thermal storage in my home?

 A 5,000 gallon tank acts as thermal storage in a home heated by a solar thermal system. Photo Courtesy Reina LLC.

A 5,000 gallon tank acts as thermal storage in a home heated by a solar thermal system. Photo Courtesy Reina LLC.

CCHRC recently completed a study on how you can use thermal storage as part of your home heating system.

Thermal storage has recently gained interest in Alaska as it has the potential to increase the efficiency of heating appliances, enhance the use of renewable energy in cold climates, and reduce emissions of certain appliances like wood boilers. It is most suited for renewable energy systems such as solar thermal, geothermal and biomass, but can be adapted to a wide variety of heat sources. The report looks at different design considerations and describes several examples in homes around Alaska.

Thermal storage is a common concept. Many households use water storage tanks to provide domestic hot water, which can range from just a couple gallons to more than 100 gallons. Thermal storage also can be used in space heating systems to store heat for a certain period of time. For example, storing the heat from solar collectors in a buffer tank to use at night; storing heat from a wood boiler in a water tank to allow for a hotter, more efficient burn; or storing heat in the ground to harvest later with a ground source heat pump. In each case, thermal storage can be thought of as a “heat battery” because it holds energy to be used later. In this way, it can enable a heat source with intermittent delivery (like the sun or wind) to still meet demand.

Every thermal storage system needs three basic components: a heat source, a storage medium to store the heat (such as a tank of water, rocks or soil), and a discharge method (heat exchanger) to distribute the heat. Technically, any heat source can be used to charge a thermal storage material, however you should select the fuel and storage material based on availability, cost and compatibility with your home’s needs.

Also, many factors will drive the design of a thermal storage system for your home — such as your heating appliance, your distribution system, your heating demand, your lifestyle and many others. The design of the system also will depend on whether the system is being installed in a new home or being retrofitted into an existing one, as retrofits must accommodate the existing distribution system and available space in the home.

There are various applications of thermal storage throughout Alaska. A net-zero heating home built in Fairbanks several years ago uses solar thermal collectors and a masonry heater to charge a 5,000-gallon insulated water tank that provides heat to a radiant floor system.

The tank also heats domestic hot water in the house.

A different system, located at CCHRC, uses a wood-fired boiler to charge an insulated 1,500-gallon tank of water in the lab. The goal was to fire the boiler hot and fast, which produces more Btu and fewer emissions, and save the heat to use when it’s needed, rather than damping down the boiler so the fire lasts longer.

The water tank heats 1,900 square feet of lab space in the building. The tank was sized to hold as many Btu as the boiler could produce in one firing per day and to provide enough heat for the entire lab over a full winter day.

If you’re considering a thermal storage system, the first step is to consider what your goal is. Do you want to use renewable energy instead of fossil fuels? Are you looking for short-term (a few hours or overnight) or seasonal storage? Systems that are recharged daily are smaller and less expensive than seasonal systems.

Check out the report for an overview of various types of systems used in cold climates, case studies in Alaska, and tips for designing your own system.


A breath of fresh air

Schematic of the BrHEAThe system being used in the Buckland prototype house.

A new prototype system developed by CCHRC locks together heating and ventilation—two crucial elements of life in Alaska.

CCHRC researchers developed the integrated heating and ventilation system, called BrHEAThe, to ensure that new energy efficient homes are getting ample fresh air.

As homes are being built tighter in Alaska to save energy, less air is able to leak into or out of the building, so things like water vapor and chemicals generated from cooking or off-gassing furniture can be trapped inside. Without ventilation, they can build up to harmful levels for both humans and buildings.

Some Alaska homeowners are wary of mechanical ventilation, such as fans or heat recovery ventilators (HRVs), because they use electricity and replace heated air with cooler air. As a result, some people turn off or disable their ventilation systems.

The BrHEAThe system marries heating and ventilation so that incoming air is always hot and fresh.

Here’s how it works.

The HRV brings in fresh air and recovers most of the heat from outgoing stale air. The fresh air enters a filter box and then passes through a heat exchanger, absorbing heat from a coil connected to a boiler.

“If we pump hot water through this heat exchanger, it’s going to warm the air that moves across the heat exchanger from 40 degrees to 140 degrees,” said CCHRC research engineer Bruno Grunau.

The heated air is then distributed via ductwork throughout the home.

The high efficiency boiler also heats a domestic hot water tank.

During a test run in the lab, the system worked smoothly: 25-degree air from outside  was raised to 58 degrees by the HRV, then further boosted to 139 degrees by the heat exchanger.

The BrHEAThe system will be installed at a CCHRC research prototype home in Buckland that breaks ground this week.

“As building envelopes are getting warmer, heat loads are being reduced. As a result, systems like this can now be used to meet heating needs,” Grunau said.


Video of a test of the heating system.

What should I be aware of when building on permafrost?

If pilings are used on permafrost, they must be installed to a depth that will both support the structure and resist frost jacking due to seasonal ground movement.

Permafrost is loosely defined as soil and/or rock that remains frozen for more than two years. In the Fairbanks area, permafrost tends to be discontinuous and is concentrated primarily on north-sloping hills and in lower elevations with heavy ground cover. Big trees do not guarantee the absence of permafrost; it might just mean that permanently frozen ground or ice is down far enough that the soils in that spot can support a larger root system. The only way to be certain of what the ground contains is to have a soils test drilling done.

With permafrost, the safest bet is to it avoid it altogether and move to another piece of land. This is easier said than done, particularly because of the scarcity of buildable land near Fairbanks that is affordable. If you decide to build on permafrost, be as strategic as possible. Smaller and simpler structures will tend to fare better than larger, more complicated ones.

Minimal site disturbance is the accepted practice. The trees and the ground cover are your best friend. They protect and insulate the ground from the heat of the summer. A great example is the green moss you find on many of the shaded low-level areas in Fairbanks. Moss has a high insulating value, and in many cases if you dig down a couple of feet, the ground might still be frozen in the middle of summer.

Strategies for construction on permafrost include:

• As a general rule, the organic layer of ground cover provides insulation and should not be removed, as this will increase the risk of thawing any frozen ground underneath.

• Elevate and properly insulate the bottom of your house to prevent heat losses through the floor system from reaching the ground underneath, which can lead to thawing.

• In post and pad construction, use a thick gravel pad that is significantly wider than the house itself (also insulated if possible) to stabilize the ground and spread building loads.

• If wood or steel piles or helical piers are used, they must be installed to a depth that will both support the structure and resist frost jacking from seasonal ground movement.

• Cut trees sparingly to maximize site shading (while permitting for a fire break).

• Build a wrap-around porch, which will help shade the ground around and underneath the house.

• Incorporate large roof overhangs to shed water away from the house and provide shade.

• Install gutters and manage site drainage well away from the house.

• Retain an engineer familiar with local soils conditions to assist in designing a foundation system that will adequately and safely support your home on the soils specific to your site.

• Septic systems also must be engineered to function on permafrost, and remember that conventional systems might risk thawing the ground.


Other Resources 







Permafrost Technology Foundation case studies:

U.S. Permafrost Association website:

UAF Cooperative Extension Service online publications at


What’s going on in my crawlspace?

Crawl spaces are an area of the house that tends to get neglected. The old adage “Out of sight, out of mind” might apply here. Unfortunately, this also means crawl space problems can go unnoticed until they have an effect on the living space above. At this point, a problem that could have been easily remedied might have progressed into an expensive structural or health-related issue. The crawl space also can present a significant hidden energy drain on a home if not insulated properly.

Good moisture control is of primary concern in a crawl space. This starts outside the building envelope, and many problems can be stopped here in their infancy.

Gutters are a relatively inexpensive addition to a house that can provide huge preventative paybacks. In a climate with lots of rain, a house without gutters can direct lots of water against its foundation. Soils, wood and especially concrete are good conductors of water through capillary action. Picture a paper towel soaking up water — concrete works this way and can carry water great distances. If gutters are not an option, then at minimum the soils around the house should be sloped to direct water away from the building.

Once water reaches the foundation, things get a lot tougher. The structure must be prepared to resist infiltration. Ideally, both concrete and wood foundations should have some form of waterproofing on the outside. If this has deteriorated or was never installed, this might need to be remedied.

Assuming all external sources of moisture penetration have been addressed, the next step is to inspect the interior. With few exceptions, exposed dirt floors should be covered and well sealed with a continuous vapor retarder such as polyethylene with a minimum 6 mil thickness. If the floor will receive traffic, then it might be necessary to use either thicker and/or reinforced polyethylene sheeting or an even more durable membrane such as EPDM rubber. Even a dirt floor that looks and feels “dry” can release significant amounts of moisture, especially after heavy rains.

Another important consideration is radon, a cancer-causing radioactive gas that occurs naturally in the earth. The University of Alaska Fairbanks Cooperative Extension Service advises that if you have never tested your crawl space or basement, cold seasons are the best times to do so. The negative pressures created by combustion appliances, and stack effect in winter time, can bring radon into the home at a higher rate. Although high radon concentrations are considered hazardous, it’s possible that remediation after detection can be relatively simple. Testing crawl spaces is strongly recommended in areas known to have soils with radon concentrations. Test kits and information are available through the CES at 474-1530.

How well a crawl space is insulated and sealed can affect the entire building envelope. In Fairbanks, building codes require foundations to be a minimum of 42 inches below grade to protect the footings from freezing and frost jacking. Anything above that point could be at risk for freezing during the winter. This can mean serious heat losses if the crawl space is under-insulated.

Inspect the foundation walls and floor system closely. If fiberglass insulation was set directly against the inside walls with no moisture protection, or the dirt floor was left exposed, it might be wet and need to be replaced. If the floor joists were insulated, the floor system should be looked at closely. Any exposed ducting should be inspected to make sure all seams are sealed and connected. Be sure that exhaust fan piping (such as dryer ducting) doesn’t just terminate under the floor, but vents directly outside.

If you need to add or replace insulation, rigid foam and spray foam are good options. These types have high R-values and also qualify as vapor retarders. If you use foam, especially below-grade, make sure it’s approved by the manufacturer for your specific application. Spray foam and foam board may have certain restrictions or limitations in crawl spaces because of local fire codes. Some brands of foam insulation might meet fire code at a given thickness, while others might not.

In addition, it might be possible to use either a coat of fire retardant paint, drywall or fiberglass insulation to protect the foam board if required. The best source of information regarding current fire code considerations for foam insulations can be found at the local building department. Keep in mind that typically the local fire codes will need to be met if the home is put up for resale and is subject to inspection.

Tomorrow would be a good time to peek under the floor. The crawl space is integral to the foundation of the house and, in some cases, the largest source of unregulated airflow into the home. It is not a good place to let moisture, poor air quality or bad insulation go unchecked.

Is a pellet stove right for me?

First firing of the pellet stove at the UAF Sustainable Village, which serves as a backup heater in the northwest house.


Pellet stoves are a relatively new wood heating appliance, similar to wood stoves in concept but they have automated operation and burn processed biomass.

Pellets are manufactured from compacted sawdust, wood chips, agricultural crop waste, waste paper and other materials. They can also be made from biomass fuels such as nutshells, corn kernels, sunflowers and soybeans. Pellets are about 1 inch long and look like rabbit food. The pressure and heat created during production binds them together without the need for glue. Pellets are manufactured in Alaska, including at Superior Pellet Fuels in North Pole, and are available at local hardware stores and by delivery from manufacturers.

How it works

Stoves are designed to heat a space directly. The stove consists of a combustion chamber, ashtray and flue to vent exhaust gases. In a pellet stove, the flue can be direct-vented through a wall, meaning that no chimney is required. Pellets are stored in a hopper near the stove. The hoppers come in various sizes, but generally can hold enough pellets for the stove to run for more than a day.




Pellet stoves use electricity to run three motorized systems:

  • A screw auger feeds pellets into the fire at a controlled rate
  • An exhaust fan vents exhaust gases and draws in combustion air
  • A circulating fan forces air through the heat exchanger and into the room

The motorized systems are controlled by a control system and allow pellet stoves to operate automatically.

Pellet stoves do not have a distribution system. The fire inside the combustion chamber causes the stove to warm up and radiate heat throughout a room. Pellet boilers are available that use a hydronic distribution system.


As with other wood-burning devices, pellet stoves require frequent maintenance, yet less than a wood stove. The stove should be inspected regularly. Also, the hopper must be filled and the ashtray should be emptied on a weekly basis (though this depends on the size of the hopper and ash tray and the frequency of use).

Additionally, the stove should have a yearly check-up. Heating professionals can check that the doors, gaskets, electric connections and seals on the stove are in good condition. They can also check the chimney for creosote, rust, and corrosion.

Efficiency Range

Pellet stove efficiency ratings are published by manufacturers. The efficiency ratings combine electrical efficiency, combustion efficiency (a measure of the heat produced from burning fuel), and heat transfer efficiency. Efficiencies can range from 78–80%. More efficient stoves lose less heat up the chimney and deliver more heat into the home.

For more information on home heating devices check out these resources:

–Consumer Guide to Home Heating:

–Your Northern Home:

I’ve heard that “ECM” (electronically commutated motors) can help home appliances save energy. What are they, and are they worth the extra expense?

There are many ways that manufacturers are increasing the energy efficiency of their products. You’ve probably seen the Energy Star rating on new appliances. Since 1992, the federal government has been giving tax incentives and rebates to manufacturers and/or consumers for making improvements like reducing the amount of water needed to wash a load of towels or the electric load of your refrigerator.

One way to reduce energy use is by using electronically commutated motors (ECM). This is a high-efficiency motor that can work in home systems like air handling and heat distribution (or cooling). ECMs allow the motor to run at different speeds, depending on the demand from the appliance, rather than maintaining one speed constantly. This type of motor has been in use in the U.S. since 1985 and uses as much as 67 percent less power than that used by standard motors (PSC). That’s because sensors in the motor determine the system’s need and provide just the amount of energy needed. ECM motors are also quieter and cooler than standard motors.

Radiant floors are one example. The ECM runs the pump that distributes hot water to heat your floors. A sensor in the system measures the temperature of the fluid in your system and tells the pump to run only as fast as it needs to to heat your rooms. When running most efficiently, a system using an ECM could use less power than a standard light bulb.

HRVs (heat recovery ventilation systems) are also now made with ECMs. Just as with the hot water circulator pump, the HRV’s motor will vary its speed (and therefore energy use) based on the demands from the building.

When you push your “booster” button in the kitchen, the motor will run the fan at a faster rate and exchange more air for a set period of time. When the HRV is operating at its normal (lower) level, it will use less power and run less forcefully.

While it is possible to have a professional retrofit your existing furnace, HRV or other appliance with an ECM motor, it is generally more cost-effective in the long run to purchase a new appliance. Some appliances are not configured to allow the conversion at all — the older it is, the more this is likely.