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Moisture levels inside homes in winter can sometimes be high enough to cause mildew and other problems. On the other hand, winter dryness inside some homes is also a problem. Therefore, controlling moisture levels inside a house in winter is desirable. 

Winter moisture problems can usually be divided into two groups: those caused by liquid water, and those caused by moisture in the air. Other than the usual plumbing and roof-leak type liquid water problems, the most significant winter liquid water problem is an ice dam. Ice dams occur on the roof when outside temperatures are below freezing, and some precipitation has or is falling. Heat from inside the house causes snow or ice on the roof to melt and run further down the roof. Once this liquid water hits an unheated section, like an overhang, the water freezes causing a dam. Liquid water builds up behind the dam and runs back under shingles, and into the house. Quite often, an incomplete coverage of insulation or air leakage from the house causes an ice dam situation.

Other winter-time moisture problems are situations created by moisture in the air. Relative humidity (RH) is based on temperature. Warming air (without adding moisture) reduces the relative humidity, while cooling air (without removing moisture) increases relative humidity. Quite often, the coldest place in a house in the winter is a window. Condensation on a window indicates that the window is cold enough to increase the RH to 100% at the window surface. Mold on the inside of exterior walls or on ceilings around the perimeter of the house are also indications that these surfaces are cold enough to cause an increase in the RH near those surfaces. Mold may also grow on clothes and shoes in closets, where the temperature is lower than in the adjoining room.

Controlling condensation and mold growth requires either warming the surface or drying the air. In some instances, adding insulation is an option. Closets can be warmed by installing louvered doors, or by adding a heat register in the closet. Window surfaces can be warmed by adding an exterior storm window, adding a heating vent located beneath the window or by replacing the window with more a energy efficient window. Contrary to popular opinion, turning a light on in a closet does not discourage mold because of the light. Rather, the light produces heat, which in turn lowers the relative humidity, and that discourages mold.

If moisture condenses between the permanent window and the storm window, leakage of air around the permanent window is allowing warm moist air from inside the house to seep into the air space between the glass panes. In this case, an effort should be made to seal the leaking spaces. Another option is to ventilate the air space between the two windows. Do this by drilling two 3/8″ diameter holes at the top and bottom of the storm window or loosen the storm window slightly.

Drying the air requires knowing the source of moisture. Crawl spaces can be a significant amount of moisture. If the crawl space soil is not covered with a ground cover, one should be added. Another major source of moisture in a house is an unvented combustion device, such as a gas fireplace or kerosene heater. Burning one gallon of kerosene produces about one gallon of water. Burning a 20,000 BTU gas fireplace for one hour produces about 1.5 pounds of water. Venting a clothes dryer inside can produce about five pounds of water per load. Other sources of moisture include people, pets, plants, cooking, cleaning, bathing and hobbies. In high moisture producing areas (kitchens, baths, and laundry), ventilation is required.

Only 4 to 6 pints of water are necessary to raise the relative humidity of a 1000 square foot house from 15% to 60%. A comfortable winter-time level would be between 30% – 50%.

At other times, or even in other locations in the house, excessive drying may occur in the winter. As outside air enters the house and is heated, the RH decreases. Wood and other materials exposed to this dry air may shrink, resulting in cracks, squeaks or gaps in floors, cabinets, doors or sheetrock. Hardwood floors may even warp as the underside dries more than the upper side.

In these overly-dry situations, the best solution is to decrease the rate of outside air entering the house. Look for leaks in ductwork, leaks around windows and doors, and through other penetration through the floors, walls and ceilings. Dampers should also be closed on fireplaces when not in use.

Contrary to popular opinion, heat pumps do not produce dryer or wetter air than furnaces or other heating systems. Often times, though, a house with ducts in the crawl space or attic may bring in more outside air that, when heated, tends to dry out the house.

The best solution to winter-time moisture problems is to create a tight house with a continuous, contiguous insulation barrier and air barrier, then control internal sources of moisture. This solution will also help alleviate or prevent summer-time moisture problems as well.

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Condensation on a window occurs when the surface of the window is cooler than the “dew point” temperature of air in contact with the window. Condensation is a result of a combination of surface temperature and moisture in the air.

Window condensation is usually a wintertime situation, when outside temperatures are very low. Condensation can also happen during the summer, when outside air is very humid and inside temperatures are kept relatively cool. Winter condensation occurs on the inside of windows, while summer condensation occurs on the outside of windows.

Summer condensation problems are mostly visual, whereas winter condensation problems can be destructive. Since outside window and building surfaces often get wet from rain, a little extra liquid water will not be detrimental. The condensation should disappear as outside air temperatures rise. Winter condensation though, can cause decay, mold and paint problems.

Several factors affect heat flow through a window and therefore the surface temperature of the glass, including inside and outside air speed across the glass, sun shine, sky temperature, inside and outside air temperature, and glazing types and treatments.

Energy efficient windows and other window treatments reduce the amount of heat moving through a window system. Double pane glass, low E coatings and inert gas fill help to reduce the flow of heat. This reduced heat flow results in cooler surfaces on the cold side of the window and warmer surfaces on the warm side of the window.

Winter Condensation

Winter condensation occurs when the inside surface of the window is cooler than the dew point temperature of the inside air. The dew point temperature is related to relative humidity. Condensation may occur under high relative humidity at only cool outside temperatures. As the outside temperature drops, the inside surface will also get cooler. Therefore condensation will form at lower relative humidity on cold days.

The chart below shows when condensation will occur for several window types at various outside temperatures and inside relative humidity. This chart is based on an inside temperature of 70F. The chart shows that condensation occurs at colder outside air temperatures with energy efficient windows, indicating that the inside glass surfaces are warmer than less efficient windows.

To prevent winter condensation, either warm the window surface or dry the inside air. Options to warm the window surface include 1) opening drapes, 2) blowing air across the window surface, 3) replacing the window with a more efficient window, 4) adding storm windows, or 5) raising the temperature inside the building. To reduce the relative humidity in the room, control or eliminate moisture sources. People are sensitive to low relative humidity, so in extremely cold weather, the only option to prevent condensation may be to warm the window surface.

Window Condensation Potential Chart

The following chart indicates the conditions where windows may start to show condensation. Based on an outside temperature (and assuming the inside temperature is 70 degrees F), condensation could occur at any indoor relative humidity higher that that shown for that type window.

For example: The bottom line shows a single-pane window. If the outside temperature is 30 degrees, condensation would for whenever the inside relative humidity is greater than about 32%. In contrast, if the window was double-glazed, condensation would not occur on the window until the RH was higher than about 58%. At 10 degrees outside, condensation will not occur on a single-glazed window until the indoor RH is above 17%, while for a triple-glazed window, the indoor RH would have to be above 62%.

Summer Condensation

Summer condensation occurs when the outside window surface is cooler than the dew point temperature of the outside air. In the southeast US, summer dew point temperatures range from about 65F to 75F. When temperatures inside the building are within this range, summer condensation problems can occur.

The outside glass surface in energy efficient windows will be closer to the outside air temperature, while the outside glass of an in-efficient window will be closer to the inside temperature. Low-E coatings help reduce the amount of radiant heat transfer through a window. As the summer sun warms the outside glass, a Low-E coating reduces the amount of this heat that moves inward. (The outer glass can warm significantly in the sunshine. During the winter, the inside glass is warmer because of the reduced radiant heat movement outward, and you don’t get that “cold” feeling sitting next to a window.) At night during the summer, heat is radiated from the outside glass to the cold sky and other objects. The Low-E coating reduces the heat transfer from inside, so the outside glass surface can cool significantly below outside air temperatures.

In cases where the inside temperature is below the outside temperature, a Low-E coating will allow the outside glass temperature to drop to about the same as that of an inefficient window. In cases where the outside air is colder than the inside temperature, a Low-E coating allows the outside glass to get even colder. Therefore under the right conditions, windows with Low-E coatings can develop more summer condensation than inefficient windows.

Since we cannot control the outside dew point temperature (or relative humidity), the options for preventing summer window condensation problems are to warm the inside surface of the window as a way to warm the outside surface. Raising the thermostat setting is about the only option. Exterior shutters, shades or even trees can help reduce summer condensation problems as well.

In summary, condensation occurs when a surface falls below the dew point temperature of the air. The outside glass in an energy efficient window will be closer to the outside temperature, and the inside glass will be closer to the inside temperature. The glass in an inefficient window will be more heavily influenced by both inside and outside temperatures. A Low-E coating (that reduces radiant heat transfer) will tend to warm the inside surface in the winter, and the outside surface in summer sunshine. A Low-E coating will also lower the outside surface temperature at night in the summer. Therefore, a low-E coating will reduce the potential for winter condensation, while creating more potential for summer condensation situations (especially if the inside thermostat is set near or below the outside dew point temperature.)


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Note: The information provided here and in the accompanying pdf document is provided for a better understanding of the science and physics of how buildings work so that we may make them work better, providing more stable, durable and efficient buildings. This information is NOT intended to support a particular product or company. 

Spray-in-place polyisocyanurate (polyurethane) foam is a high performance building material. Spray foam is primarily used as an insulation material. When installed, the foam expands in place and fills in around plumbing, wiring and other obstructions in the framing. For this reason alone, spray foam often outperforms batt insulation. Other characteristics of the foam, as described below, provide additional benefits.

Spray foam used in building construction typically comes in two forms: low-density or open-cell foam, and high density, closed cell foam. Because of the different physical properties and make up, these two foams cannot routinely be interchanged. Open-cell foam is better in some situations than closed-cell foam and vice versa.

Moisture permeability: Open-cell foam is described as being somewhat moisture permeable. In other words, some water vapor can move through the foam under the right conditions. By contrast, closed-cell foam is said to be moisture impermeable, or waterproof. Water will not readily pass through this foam. By comparison, fiberglass and cellulose insulation are both considered highly moisture permeable. The kraft facing on some batt insulation has approximately the same moisture permeability as open-cell foam, but when installed improperly, moisture will move around or even through this facing.

Air permeability: Both foams are essentially air-impermeable. (And so is plywood, OSB and drywall.) At much less thickness than typically installed in buildings, no appreciable air will move through either foam. By comparison, air will readily move through fiberglass batts and blown-in insulation. High density systems such as “blown in blanket” fiberglass and cellulose insulations are less air permeable than batts, but still much more air permeable than spray foams.

For a material to be called “air impermeable”, the maximum leakage rate at 75 Pascals (Pa) pressure difference is 0.02 liters per second per square meter. (0.02 L/s-m2) The air permeance of insulation material is measured using ASTM E 283 as listed in section R806.4.2 of the 2006 IRC (International Residential Code). ASTM E 283 is the Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen. For comparison, the air permeance of 3/8″ plywood sheathing is 0.0067 L/s*m2 @ 75 Pa. Some open-cell foam measures at 0.009 L/s*m2 @ 75 Pa at 3.5 inches thick. Closed-cell foam is less permeable.

But what are the implications? From a simple approach, a house with walls measuring 8 ft high by 24 ft wide by 60 ft long might have an insulated wall area of 1200 square ft (or maybe 114 sq meters.) Air will leak in only half that area (because it leaks out the other half). Over an hour, about 65 cubic feet will leak into this house at 0.009 L/s-m2. (And that is when the wind is blowing 25 miles per hour. So we are realistically leaking maybe a third of that under normal conditions.) We want leakage in a house to be about 1/3 air changes per hour, or in our example house, 3840 cubic feet per hour. Either open-cell or closed-cell foam will make air leakage through the foam insignificant.

Heat flow: One measure of the performance of insulation is its resistance to heat flow. This resistance is stated in a number called the “R” value. Building codes typically require R-13 insulation in walls. As such, fiberglass batts are rated at R-13 at a thickness of 3 ½”, which is the thickness of a typical wall. (Or is it the other way around? In reality, fiberglass at a thickness of 3 ½” can’t economically be manufactured much better than R-13, so the codes have really been written to deal with this limitation.) Open-cell foam has a similar R-value of near 3.6 per inch. For a 3 ½” installation, this would be R-12.6 or a nominal R-13. For closed-cell foam, its R-value is closer to 7 per inch. When installed in walls, only 1 ½ to two inches of closed cell are commonly used, providing an R-value near R-13.

R-value is a measurement of the resistance to heat flow through a substance, or what is scientifically called conduction heat transfer. In this case, the substance is the insulation. Two other methods of heat transfer are encountered in buildings. One deals with air movement, and is called convection heat transfer. Air containing heat can move through porous material and carry that heat with it. Since fiberglass and cellulose are somewhat porous to air movement (air permeable), some heat can move into or out of a building with air movement through the insulation.

Another type of air-flow heat transfer that occurs in porous insulation is called “convective looping” where air moves just within the insulation rather than through the insulation from one side to the other. This looping is caused by the phenomenon that warm air tends to rise, and cold air falls. Temperature differences between upper parts of walls and lower parts, or an inside surface versus an outside wall surface can cause this form of heat transfer. Air impermeable insulations such as the spray foams eliminate convective heat transfer. This characteristic allows R-13 of spray foam to outperform R-13 of fiberglass or cellulose insulation.

A third form of heat transfer is the flow of radiant energy. A hot surface can transfer energy to a colder surface across an open space. This mode of heat transfer can be felt when standing in front of a fire. No conduction is occurring because you are not touching the fire. Convective heat transfer is not causing your front to get warm while your back stays cool because the heated air typically goes up the chimney. Energy “radiating” from the fire moves through the space to warm you and other objects and surfaces around you. Foams as well as other insulations can affect radiant heat flow when placed in the proper location. But foams can be used in places and under circumstances where other insulations cannot be used, and can greatly reduce radiant heat transfer.

Heat, and and moisture flow: In building construction, controlling the flow of heat, air and moisture are important. Heat flow is typically controlled by insulation. Controlling heat flow is important to control indoor comfort and energy costs. A secondary, but important, consideration in controlling heat flow is to control temperatures of surfaces in the building envelope. This aspect will be discussed in more detail in following paragraphs.

The control of air flow is important because air contains pollutants, dust, dirt, heat (or cold) and moisture. Air flow control is typically accomplished with caulks, tapes and housewraps. Lots of publications show details and methods of air sealing buildings. Many show how to seal the outside of a building or the inside of a building (such as the air tight drywall system.) These methods are designed to prevent air from getting from one side of a wall to the other through air permeable insulation.

The control of moisture has, until very recently, been pretty well ignored. It happened, but we only dealt with it if we could find a leak. Now that we better understand the relationships between air and water, water and fungi, and concerns with fungi (mold) and health, much of the construction industry is working to address potential issues. In the last several years, the industry has come up with building materials of various moisture permeability such as synthetic roof underlayments and housewraps, “drainage planes,” dehumidifiers, “thermidistats,” and energy recovery ventilation systems. These materials and systems help keep water out, or help deal with it once it gets in.

Moist air: Two forms of moisture typically affect buildings in warm, humid climates: liquid and vapor. Common liquid water sources are roof and plumbing leaks, leaks around windows and doors, and condensation. Common water vapor sources are air, clothes dryers, bathing and other family activities. In these cases, liquid water is turned into a gas where it can then move freely through planned and unplanned openings in buildings.

Air, as we know it, contains some moisture. A phenomenon with “moist” air is that the amount of moisture the air can hold depends on the temperature of the air. As air is heated, it can hold more moisture. As air is cooled, it can hold less moisture. The amount of moisture air holds is commonly stated as “relative humidity” or the relative amount it is holding compared to the maximum amount it can hold, at that temperature. For example, air at 70 degrees and 50% relative humidity (RH) is holding 50% of the moisture air can hold at 70 degrees. Air at 100% is saturated, and cannot hold any more moisture.

As a hunk of air is cooled, its capacity to hold moisture decreases, so it’s RH goes up. If cooled enough, it reaches 100% RH and becomes saturated. If cooled further, the water vapor turns into liquid water; it becomes condensation. (An air conditioner helps dehumidify air because it cools the air below the air’s condensation or dew point temperature, and condenses some of the water out of the air.)

Decay fungi need liquid water. Molds and mildews typically need a humidity higher than 80% RH. If plumbing and roof leaks aren’t enough to worry about, condensation can also provide the liquid water necessary to cause problems. Even without liquid water, high relative humidity can lead to mold growth.

In buildings, cold surfaces exposed to warm, humid air can result in condensation and high RH. In the winter, inside warm air can leak outward and contact cold exterior materials and condense. In the summer, warm, humid outside air can leak in and condense on or raise the RH near cold air conditioned surfaces.

In South Carolina, the dew point or condensation temperature of outside summer air ranges from about 72F in the Greenville area to near 75F along the coast. If this air leaks into a building cooled by air conditioning below its dew point, condensation, mold and decay are possible. To deal with this possibility, air flow needs to be stopped as much as possible, surfaces need to be kept warm and objects that do get wet need to be able to dry.

Buildings and building materials will get wet. To prevent fungal problems, they must dry quickly. Fireplaces, lack of air conditioning, leaky walls and windows, and a lack of insulation actually helped historic buildings dry relatively quickly. With the advent of tighter buildings, indoor plumbing, air conditioning, and insulation, buildings were exposed to more moisture and to slower drying conditions. Controlling moisture is now more important than ever.

Spray foam’s use in building performance

Spray foam is a superior insulating product. It expands as it is installed and fills wall cavities better than batts. Spray foam is not compressed around obstacles or during installation, another way batts lose insulating value. Spray foam does not allow air movement, so air leakage and convective looping do not occur. Both open-cell and closed cell can perform these functions about equally. Both foams provide better insulation, and help keep warm surfaces warmer and cold surfaces colder.

When it comes to addressing moisture, the differences between open-cell and closed-cell foam become important. A simplified, initial difference is that open-cell foam is better suited for use against materials that can be damaged by water, and that closed-cell foam is better suited for use against materials not affected by water.

Even though open-cell foam is considered air impermeable, it is somewhat moisture permeable. Under conditions where warm humid air could contact a very low moisture permeable or very cold surface, sufficient moisture could move through the foam and condense against the surface. Examples of this situation are AC ducts in vented crawl spaces, or walls with vinyl wallpaper. In both situations, moisture cannot pass freely through the system at an acceptable rate and builds up to a detrimental level. The ducts could be coated with closed-cell foam to address the situation since the duct material typically won’t be harmed by water, but the walls likely cannot be fixed with closed cell foam. (Vinyl wall paper is bad news in the south, and very intricate details need to be in place to make it work OK.)

In wood frame structures in the south, much of the drying of a building occurs to the inside. For this reason, everything inside the exterior weather layer needs to be somewhat moisture permeable. Open-cell foam works well for this application. Closed cell foam does not. If closed-cell is used inside exterior sheathing, and the sheathing gets wet, it cannot dry fast enough to the outside to prevent problems. The sheathing could rot before any water issues become apparent. The same situation applies to attics: open-cell foam works well applied to the underside of roof sheathing, while closed cell does not. Closed-cell can prevent any water leaks from being apparent until the sheathing has been destroyed.

I have personally witnessed a leak occurring above open-cell foam. Water was sitting on a surface below the foam, and the foam was covered with drips. I actually thought a pipe below the foam had leaked and sprayed water up on the foam. But when I started tracking the leak, I realized the foam was soaked in about an 8-inch diameter area. Digging up through it, I found the leak. Had this been closed-cell foam, it would have taken significantly longer to find the leak.

Closed-cell foam can be used successfully on the outside of a wood frame structure. For instance, closed-cell foam can be applied to the exterior of roof sheathing to create a water-resistant, well insulated roof system. In this case, the foam acts as a water resistant barrier, while the wood sheathing is still able to dry to the inside as necessary. (Note, though, that even in this situation, the foam needs to be protected from the weather with some weather resistant material.)

Closed cell foam can also be used successfully against bricks, rock and concrete work. These items are typically not harmed by water. Closed-cell foam can also be applied to the inside of metal siding and roofing. (Open-cell foam can also be used in these situations in cooling climates.) Against water permeable materials like brick or block, closed-cell foam can be used to provide an interior waterproof coating. This can be beneficial in basements or above grade block foundations where exterior waterproofing is not possible. (In situations where exteriors are sufficiently waterproofed, open-cell foam can be used on the interior of these walls.)

Crawl spaces: Spray foam and crawl spaces can work, but several constraints and issues exist. Conditions in a vented crawl space are typically wetter than outside air. As such, dew points are higher. As a result of these high dew points, decay and fungal issues are prevalent in vented crawl spaces. Floors over crawl spaces need to be protected from crawl space air and moisture. Spray foam can be used to accomplish this, though the penalties can be severe. If open-cell foam (or other moisture permeable insulation) is used to insulate the floor, low inside temperatures and impermeable floor coverings can lead to floor problems. Condensation can occur under vinyl flooring, resulting in fungal growth and decay. Hardwood floors can cup or buckle.

If closed-cell foam is used to insulate the floor over a crawl space, any inside water leaks will necessitate removal of flooring. Water will not be able to drain through the floor into the crawl space, and the sheathing cannot dry to the crawl space. Also, since closed-cell foam is extremely difficult to remove, temporary removal to aid drying and repair is not a feasible option.

In addition to the above problems and issues with insulating floors over crawl spaces, additional effects are likely. Any beams and sections of joists exposed below the foam are not protected from the crawl space environment, and will likely experience mold and decay. Ducts and AC equipment in the crawl space can also experience condensation and other moisture issues.

For these and other reasons, crawl spaces in the southeast should be unvented and semi-conditioned. Here, semi-conditioned means that humidity levels are controlled. Closed cell foam can be used on the inside of foundation walls to insulate and waterproof the foundation (though exterior waterproofing is more effective.) Other types of insulation can be used on foundation walls when sufficient waterproofing and air sealing details are provided. With foundation wall insulation, floor insulation becomes unnecessary and even counter productive. Ducts should still be insulated and air tight. As mentioned earlier, closed-cell foam is best for insulating ducts, though under the right crawl space conditions, open-cell foam can perform well.

In summary, spray foam is a high-performance insulation material that also provides other benefits to the building and occupants. Due to its ability to completely fill voids and cavities, and its air and moisture permeability characteristics, spray foam is an efficient material for controlling heat, air and moisture flow in a building. Spray foam is one of the best components for providing the environmental separation critical to making buildings work properly.

Open-cell foam is used on the inside of materials that can be damaged by water. Open-cell is superior in walls and under roof sheathing. Open-cell foam can work well under floors over conditioned crawl spaces. Open-cell foam should not be used against moisture impermeable surfaces that are exposed to high-dew point air (such as ducts), low-permeable floors over humid crawl spaces, or against wet surfaces such as basement walls.

Closed-cell foam is used against metal, brick and masonry. Closed cell foam can also be used effectively on the outside of wood sheathing or other material that has the ability and need to dry to the inside. Closed-cell foam should not be used on the inside of wood materials, or under wood-framed floors.

For more information about the specific product you want to use, contact the manufacturer.