On really cold days, you may notice condensation forming on the inside of your windows. This can be caused by one or a combination of factors: excess humidity, inadequate ventilation, or poor windows. To understand and correct a particular issue in your home, you need to know some basic properties of moisture.
Condensation occurs when water vapor (a gas) turns into water droplets as it comes into contact with a cold surface. The point at which this happens (called the “dew point”) depends on the temperature and humidity of the inside air. The warmer the indoor air, the more water vapor it can “hold,” and moisture can better remain in the vapor state. When air moves next to a cold window, the temperature drops and it can’t “hold” as much vapor. That’s when you start to see condensation forming.
For example, if the indoor temperature is 70 degrees and the outdoor temperature is 0, then moisture will begin to condense on a single-pane window when there is roughly 15 percent relative humidity in the house. A double-pane window will cause condensation at around 25-40 percent relative humidity, and a triple-pane window at between 30-50 percent. These are rough numbers are based on average window insulation values.
The recommended indoor humidity levels for occupant health and comfort range from 30-50 percent. The general rule in a cold climate, however, is to target the lower end of this spectrum due to the risk of condensation within walls and ceilings. If your house has adequate mechanical ventilation, humidity is less of a concern. In Fairbanks, it’s tough to maintain anything close to 50 percent humidity in a properly ventilated house, because the winter air is so cold and dry. Because of its low moisture content, the inherent dryness of Fairbanks winter air is good for homes but not always the occupants, since the dryness can cause discomfort.
What can I do about it?
Three things: make sure your home is properly ventilated, aim for less than 40 percent relative humidity to keep both you and your home healthy, and consider replacing your windows or adding moveable window insulation during cold months.
If you already use mechanical ventilation and have low interior humidity, but are still having problems, you may need to examine your ventilation setting. If you have a heat recovery ventilator (HRV), it may be recirculating too often, which can contribute to increased moisture build up in the air. Recirculation mode closes the connection to the outside and brings exhaust air back into the rooms. Recirculation mode keeps the HRV core defrosted and saves energy, but sometimes it can run too long. Some experimentation with the HRV settings may be necessary. For example, in 20/40 mode the HRV brings in fresh air for 20 minutes and then recirculates for 40 minutes, and likewise for 30/30. If you’re getting condensation in your current mode, try decreasing the amount of time the unit recirculates.
Also make sure air is allowed to circulate—either passively or mechanically—throughout the entire house. If you close the door to the bedroom, the air can become cold and moist enough to condense on windows.
Older, poorer performing windows can create problems no matter what you do to your interior air. Bad seals around operable windows, metal spacers between the panes, and inadequate insulating value can cause the window surface to get cold enough for condensation to occur. If you’re not ready to invest in new windows, consider some type of moveable window insulation like foam board (on the outside) or well-sealed plastic film (on the inside). A CCHRC guide to different types of window insulation can be found at
The Sustainable Village homes at the University of Alaska Fairbanks used less than half as much energy as an average new house in Fairbanks and substantially less than an average energy efficient house during their first year of occupancy.
The four 1,600-square-foot homes were built at the university in Summer 2012 as an example of affordable, low-energy housing. They provide a testing ground for experimental building techniques and energy systems and a chance for students to help design, live in, and help monitor sustainable housing. Each house has a unique mix of wall assembly, foundation type, and heating and ventilation system. CCHRC is monitoring energy use, indoor air quality, and other factors to see which designs are the most affordable and practical for homes in the Interior.
The best performing house was the Willow house (SE unit), using the equivalent of 366 gallons of heating oil No. 1, or 48.3 million Btu, for both heating and domestic hot water from September 2012 to September 2013. The average same-size house in Fairbanks uses about 920 gallons, according to the Alaska Housing Finance Corporation’s database on home retrofits. The average new house meeting AHFC’s energy efficiency standards (called BEES) uses about 660 gallons per year. Even the biggest energy user at the Village, the Spruce House (SW), used only 463 gallons of oil equivalent.
How do the homes save energy?
They are super-insulated with features like heat recovery ventilation, triple pane windows, and low-flow showerheads. The Willow House has a REMOTE wall with 8 inches of exterior foam insulation and 3.5 inches of fiberglass batts inside the wall cavity (for a total of R-51). By comparison, a conventional 2×6 wall with 5.5 inches of fiberglass insulation has an R-value of 21. A propane boiler and three solar thermal collectors provide both space heating and domestic hot water.
The Spruce House, on the other hand, has a double wall filled with 18 inches of cellulose insulation (R-64), and a forced air heating system with a small diesel heater that heats fresh ventilation air.
Because each house has roughly the same heating load, the difference in energy use can be largely explained by the differing mechanical systems and those living in the homes. The west houses, for example, use an integrated heating and ventilation system. Because the two were tied together, extra heat was delivered to rooms in order to meet ventilation requirements, which resulted in periodic overheating. So occupants opened windows to moderate the temperature, which in turn consumed more fuel.
Occupant behavior drives most of the difference in energy use. What’s the set point of the thermostat? How long are the showers in use? Two of the homes were unoccupied in the summer, so they were not using any fuel for domestic hot water.
A cost analysis showed the Sustainable Village homes were competitive with other energy efficient building in the Interior — averaging about $185 per square foot, including water and wastewater, electrical, and roads (does not include land).
CCHRC also monitored ground temperature and indoor air quality at the homes to study the effects of different foundations and mechanical systems. The insulated foam raft foundations on the two west homes rest directly on a gravel pad, with permafrost 2-3 feet deep in the summer. A thick mat of polyurethane foam isolated from the ground with wood beams was designed to protect permafrost from any heat leaking out of the house. Sensors underneath the house show that the temperature at 4 feet deep has risen slightly (less than 5 degrees) and at 24 feet has remained the same. An experimental cooling system embedded in the foundation (hollow PVC pipe with an in-line fan) is designed to circulate cold air beneath the home and lower the temperature. Though it didn’t noticeably affect ground temperature last year, this year researchers will try the fan for a longer period of time (it was only turned on in February last year).
Lastly, indoor air quality monitoring shows slightly low humidity levels in a couple of the homes during the winter (30-50 percent is recommended for human health). Indoor humidity is a balancing act in a cold, dry climate like Fairbanks — you want occupants to be comfortable while also keeping the walls dry. Monitoring this year will focus on whether Energy Recovery Ventilators (ERVs), which recover both heat and humidity from outgoing air, are effective in a cold climate.
To see the full reports on first year performance of the homes, visit www.cchrc.org/uaf-sustainable-village.
We often stress proper ventilation as the key to maintaining a healthy indoor environment in a home, and promote heat recovery ventilators (or HRVs) as the best option for energy efficient ventilation in a cold climate.
HRVs exchange stale indoor air with fresh outdoor air, capturing heat from the outgoing air to pre-heat incoming air. They exhaust excess humidity, carbon dioxide, and indoor pollutants from pet dander, cleaning supplies, offgassing furniture, and other sources. The role of the HRV becomes increasingly important as homes are built tighter to save energy, which cuts down on passive air exchange.
To maximize the benefits of having an HRV, it helps to understand the different operation modes. One of the often-debated modes included in most HRVs in the United States is the recirculation mode. This mode is not often used in Europe because it is believed that the health risks outweigh the energy benefits. This article provides a description of the recirculation mode and gives pros and cons for the house and its occupants.
Under normal operation, the HRV replaces moist indoor air with fresh outdoor air. While HRVs recover much of the energy from the heated air during winter months, a considerable amount of heat is still lost due to the frigid temperatures in Interior Alaska. In addition, extremely cold outdoor air contains virtually no moisture, which can result in very low humidity levels indoors—a negative for some homeowners.
In recirculation mode, the unit closes the connection to the outside and brings the exhaust air back into the rooms. This saves a lot of energy, since there is no cold air coming in from outside. On the other hand, moisture and indoor pollutants are no longer being flushed out of the home, and their concentration will continue to rise and can eventually reach harmful levels. Recirculation can also spread unwanted smells from more to less polluted areas, such as from the bathroom to the living room.
In order to maintain sufficient air exchange, HRVs offer modes where these two strategies can be combined. For example, 20/40, 30/30, or Smart Mode. In 20/40, the HRV will bring in fresh air for 20 minutes and then recirculate for 40 minutes (likewise for 30/30). Smart modes usually require some kind of sensor (humidity or carbon dioxide) to dictate when to ventilate and when to recirculate, based on which measurements the HRV controller decides is more relevant at any given time.
The major advantage of recirculation mode is that it saves energy and redistributes heat throughout the house, particularly helpful if you have a localized heat source like a woodstove. On the flip side, it can potentially transfer pollution from one room to another rather than expelling it altogether. While Smart Mode seeks a happy medium between the two, there are still times when recirculation mode should not be used at all—if someone is cooking, smoking, or during times of high occupancy. One way to override the Smart Mode during these situations is with a push-button timer, a common feature of HRV installations that temporarily ventilates the HRV during such events.
If you do use recirculation mode, here are some best practices to maintain good air quality:
- High quality filters (High Efficiency Particulate Filters, HEPA, in combination with activated carbon filters) should be added to the supply duct to mitigate odor or pollution from spreading
- Constant recirculation should only be used when the building is unoccupied
- If recirculation is used during occupied periods, make sure the HRV is exchanging indoor and outdoor air for at least part of every hour
While recirculation offers the perk of saving energy, relying on it too much can undermine the benefit of having an HRV—to maintain indoor air quality that is healthy for both humans and buildings.
How does stack effect work? See for yourself.
Stack effect (also called chimney effect) involves airflow into and out of a building caused by indoor and outdoor air temperature differences Everything starts with the fact that warm air rises and cold air sinks. In winter, your house acts much like a bubble of warm, buoyant air sitting on the bottom of a sea of cold, dense air. This creates a pressure difference, one of the key factors you need in order to have air flow. The actual distribution of pressures inside the house can vary, but generally the pressure is positive toward the top floors and ceiling (meaning air wants to escape outside) and negative towards the bottom floor (meaning air wants to come in). To complicate matters, a taller structure such as a multi-story house will contain a taller column of air that will produce greater pressure differences.
The other key factor allowing for airflow is a pathway for the air to move between the regions of differing pressures, which in your house means leaks in your building envelope. Things are fine if you have no air leaks, but even the tightest homes have some air leaks. As warm indoor air leaks through the walls or roof, it cools and deposits moisture along the way. The problems don’t necessarily stop there, however. New air to replace the air lost must come from somewhere. Replacement air will tend to take the path of least resistance. Typically air is drawn in through the lowest regions (the negative pressure zone) of the house, which is why problems with soils gases, such as radon, tend to increase in winter. Replacement air isn’t always just drawn in through the lower parts of the structure. Air can also infiltrate through poorly sealed or malfunctioning combustion appliances such as wood stoves and boilers, or plumbing traps that have dried out and are therefore no longer able to provide an air seal to the septic system.
The key to reducing potential problems with stack effect is good air sealing around penetrations in the building. If you are considering sealing air leaks in your house, it’s very important that you start at the top. If you start at the bottom, then you might be increasing the chances that air leaking out of the top will pull air from other sources such as combustion appliances. Some common air leakage points in the positive pressure zone of the house (if not properly air sealed) can include: can lights, chimneys, plumbing vents, wiring penetrations, bath fans, and range vents. Always be sure that you have a functioning carbon monoxide detector in your home and that your boiler and wood stove have a dedicated source of combustion air.
CCHRC recently tested a wall construction technique in the Interior that provides very high levels of insulation to maximize energy efficiency. The Arctic Wall is an airtight double-wall system using cellulose insulation and is designed to allow water vapor to diffuse through the wall.
The system was designed by Fairbanks builder Thorsten Chlupp and uses some of the principles of the REMOTE wall—another super-insulated building technique that places the majority of the insulation outside the load-bearing wall.
Conventional cold climate construction calls for a vapor retarder on the warm side of the exterior wall. This vapor retarder typically consists of a layer of tightly air-sealed 6 mil polyethylene plastic sheeting, which keeps water vapor generated in the living space in winter time from getting into the exterior wall cavities. Installing a traditional plastic vapor retarder properly requires a high level of detail around all penetrations to prevent air and moisture movement through the wall assembly. This is a known weak spot for conventional cold climate construction.
The Arctic Wall, on the other hand, has no plastic vapor retarder. Instead of stopping moisture movement with a barrier membrane, it works by remaining permeable so water vapor can move through the wall with the seasons, creating a super-insulated wall that can also “breathe”.
The key components of the Arctic Wall include:
- an extremely tight building envelope to prevent air leakage and moisture transport via air leakage through the wall
- the majority of the insulation outside the structural framing and air barrier
- a wall that is open to water vapor diffusion that has enough capacity within the insulation to absorb and release a heating season’s worth of water vapor without succumbing to moisture damage
Chlupp’s system under study by CCHRC contains a 2×6 interior structural wall filled with blown-in cellulose with taped sheathing and a vapor-permeable air barrier (Tyvek HomeWrap) wrapped on the outside of that sheathing. Spaced a given distance depending on desired insulation thickness from the 2×6 inner structural wall, a 2×4 exterior wall is installed and wrapped around the outside with another air barrier membrane. The space between the two walls is then filled with 12 more inches of blown-in cellulose. See diagram for details. Depending on thickness, a superinsulated wall of this type can attain R-values of 70 or more, more than three times a traditional 2×6 wall system,
CCHRC monitored the Arctic Wall’s performance over 13 months by placing temperature, moisture and relative humidity sensors in the walls. The goal was to determine whether the conditions would support mold growth, and how moisture would move through the walls.
Test results indicated that both temperature and relative humidity levels in the walls were not sufficient to support mold growth. Neither side of the air barrier covering the exterior of the 2×6 structural wall ever approached the dew point (the point at which vapor condenses to water), indicating the structural framing is well protected from moisture.
The relative humidity of the bathroom wall (the one likely to see the most moisture) never exceeded 65%, staying well below the risk level for mold growth.
CCHRC also used moisture modeling software to predict how the walls would perform over a 9-year period, which showed that humidity levels and moisture content within the walls should not reach a level where mold growth would be a concern.
Also noteworthy was the direction of moisture transport in the Arctic Wall—walls dried to the inside in the summer and to the outside in the winter. This is not possible with conventional cold climate construction.
The Arctic Wall is a specific system whose components must be carefully engineered and built to ensure proper performance and moisture management. Based on CCHRC testing, the Arctic wall has done very well in Interior Alaska and provides a new option for a super-insulated house design.
Read the snapshot and full report on the CCHRC website at http://cchrc.org/arctic-wall
How to mitigate radon in new construction
The hilly areas containing fractured schist and rock around Fairbanks are known for having high concentrations of radon. A good radon mitigation system will ensure healthy indoor air quality. Your single best chance at dealing with radon issues is during new construction.
In this video, Ilya Benesch, building educator at the Cold Climate Housing Research Center, demonstrates the essential steps of installing a radon mitigation system for a slab-on-grade foundation.
The video follows EPA guidelines for installing radon mitigation systems found here:
Examining a radon mitigation system
In this video, Ilya Benesch visits a construction site and explores how the contractor has installed a radon mitigation system.
A house should manage indoor air quality by regularly exchanging stale “used” indoor air with fresh outdoor air. You also can improve indoor air quality by avoiding unnecessary sources of contamination, such as restricting smoking to outdoors, storing fuels outside, and selecting low-VOC paints and furnishings. During the year, the air in the Interior can contain particulates from wildfires, wood smoke, dust, pollen, car exhaust and other sources that cause you to shut the windows. That’s where filtration systems can help.
Air filtration options
When it comes to indoor air filtration, the best choice for you depends on many factors, including the size and tightness of your house, your existing ventilation system, your sensitivity, and the amount of particulates and other contaminants in the air. Be aware that irritating and harmful particulates don’t just come from outside but also inside — sources like tobacco smoke, animal dander and mold spores. Other contaminants include gases in paints, carpets, cleaners and other household products. The most common filtration systems are mechanical and target particulate matter. Prices range from about $200-$300 for a one-room portable filter to $6,000-$8,000 for a heat recovery ventilator installation with filtration.
The simplest system is a standalone air purifier, which contains a fan and filter elements all in one unit and can be plugged into the wall. These systems are designed to be portable and recirculate air in a single space, and will reduce pollutants like allergens, pet dander and dust from that space. These work well in homes where air quality problems are isolated to one or two areas.
Multiple room air cleaners
Air filtration systems that can serve multiple rooms or even the whole house typically cost more and will require an in-line fan and ductwork, but tend to be more effective.
Keep in mind that whether large or small, filtration systems by themselves don’t introduce fresh outdoor air, but they can provide air cleaning and heat distribution. Whole house systems may be a good option for those with bad allergies or respiratory problems.
Many homeowners who heat primarily with wood install small circulation systems, with an in-line fan and ductwork in just a few rooms to move heat around the house, said Richard Musick, of Ventilation Solutions LLC. The size of the fan is based on how much air you want to circulate.
“If it’s only a couple of rooms, you can get away with a 200 cfm (cubic feet per minute) fan. Big houses can require up to 900-1,500 cfm,” Musick said.
Heat recovery ventilator filtration
While new HRV systems often have high levels of built-in filtration, older models are generally only equipped with coarse debris filters whose primary purpose is to keep the core and motors clean. To help ensure good air quality, a simple filtration system can be attached separately in line with the warm-side supply port on the HRV. All the HRVs at CCHRC have a prefilter to catch the big particles, a main particle filter to catch small particles, and a carbon filter to remove odors, aerosols and VOCs. These filters can be found at HVAC and hardware stores, and are inexpensive and easy to replace. Note that the carbon filters typically need to be replaced more frequently than other air filters.
Filtration systems are measured by a MERV rating — or minimum efficiency reporting value — which goes from 1 (traps bigger particles) to 20 (traps the smallest particles). You pick a MERV rating based on what you’re trying to filter. For example, MERV 1-4 will take care of pollen, dust mites, and most animal dander, while you’ll need at least MERV 13-16 to filter out smoke particles. HEPA (high efficiency particulate arresting) is in the 17-20 range, removing more than 99 percent of tiny particulates such as carbon dust from the air.
Typically MERV 15 represents the upper limit for residential HRV systems as anything finer may restrict too much airflow. The EPA Office of Radiation and Indoor Air notes that filters with MERV ratings between 7 and 13 are capable of reducing unhealthy particulate matter almost as well as HEPA filters. Additionally, activated carbon filters can be used to neutralize smoke and VOCs.
Homes built today are more energy efficient with better insulation and higher levels of air tightness than many of the homes built in previous decades. Building codes now require mechanical ventilation systems for all new residential construction in most if not all northern states. This is simply because uncontrolled air leakage can no longer be counted on to provide the fresh air needed to keep a home healthy. Generally speaking, the highest performing ventilation systems available today will include balanced and regulated fresh air exchanges, in combination with air filtration.
No matter what system you get, check to see what type of replacement filters are required. Some models may use proprietary filters that are more expensive to replace or have more limited filtration capacity.
The “rule of threes” highlights the basic necessities of life: The typical human can survive 3 minutes with no air, 3 hours in a harsh environment with no shelter, 3 days with no water, and 3 weeks without food. Not pleasant to think about, but it does make you consider how you fulfill these needs on a daily basis. And while Alaskans are fortunate to have access to an abundance of water – the Alaska Department of Fish and Game reports that Alaska contains more than 40% of the nation’s surface water resources in its thousands of rivers and millions of lakes – getting access to clean water every day can be no small task. In Fairbanks, many people pay for water delivery, or haul water themselves, no easy chore in below freezing temperatures. Additionally, many people heat water for laundry, showers and dishes, which adds to household energy costs.
In order to save both water and energy, many people turn to low-flow showerheads and faucets in their homes. Low-flow devices reduce the water coming from a faucet but add pressure to the remaining flow, so people don’t notice the overall loss in water volume. These devices save money in two ways. First, they reduce water usage. If you pay for city water, water delivery, or for gas to haul your own water, using less water means saving money. Secondly, the majority of homes have a water heater to provide hot water for showers, dishes, and laundry. A low-flow device saves you money because you heat less water overall, which translates into lower energy bills.
If you aren’t sure whether you already have a low-flow device, you can always measure the gallons per minute (gpm) that a faucet or showerhead delivers. A lower gpm rating means the faucet uses less water. The easiest way to do this is with a stopwatch and a gallon-sized jug (for a faucet) or bucket (for a showerhead). Turn the faucet on all the way, then use the stopwatch to determine how many seconds it takes to fill up the gallon jug or bucket. Then divide 60 seconds by that time to get the gallons per minute the faucet produces. For example, if your showerhead filled up a gallon bucket in 18 seconds then it has a flow rate of 3.33 gpm (60 ÷18 gpm).
What’s the difference between regular and low-flow devices?
With a flow rate of 3.33 gpm, a 10-minute shower will use 33 gallons of water. If you pay 9 cents a gallon for delivered water, the shower cost $2.97. Now let’s say you have a low-flow showerhead, which is 2 gpm or lower. A 10-minute shower would use 20 gallons of water, and cost $1.80. While $1 in savings doesn’t seem like much, if you take 5 showers a week it adds up to $20/month or $240/year. And that’s not counting other occupants in the house.
What qualifies as low-flow?
Bathroom faucets must use a maximum of 1.5 gpm and showerheads 2 gpm to receive EPA’s label for low-flow appliances. The certification program, WaterSense, aims to help people use less water in order to preserve America’s water supply. Products must use at least 20% less water with no drop in performance compared to standard options. Look for products with WaterSense labels in stores. As many of these products cost less than $100 and don’t take long to install, it can be an easy way to save energy in a single afternoon.