Tight Construction

Without a doubt, modern construction practices have resulted in houses that are much tighter than those built a hundred years ago. One of the reasons for this evolving tightness is the use of sheet materials—with drywall and plywood, there are fewer random holes for air to pass through. Plus, windows and doors continue to improve—today, virtually all manufacturers make products that seal very tightly.

 

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In the 1970s, builders began experimenting with houses that were built as tight as possible. These super-insulated houses had phenomenal energy efficiency—in the midst of an energy crisis, they had heating bills of perhaps only $100 for an entire year. Of course, there were some early mistakes, but these houses eventually led to a very good understanding as to how a house can be both healthful and energy efficient.

 

Today, builders specializing in tight, energy-efficient construction often have reputations for building high-quality, healthy homes. But some people still remember the early mistakes and they condemn tight construction, citing the poor indoor air quality and excess moisture found in a few of the first houses that were built as tight as possible. And medical professionals sometimes link tight construction to indoor pollution, without having an understanding of building science. That’s like having a contractor discuss brain surgery. Some people have gone so far as to suggest that we return to building loose houses. This isn’t very forward thinking.

 

If we built houses like they used to, we wouldn’t be able to afford the energy bills and, even with the high bills, we’d still be cold in the winter. A large turn-of-the-nineteenth-century midwestern farmhouse could have easily used a train-car load of coal in the winter—and still be drafty and uncomfortable. Instead of moving backwards, we should look at the tight houses that have no moisture problems—and good indoor air quality—and learn from them. Even with houses getting tighter, there is still room for improvement. In other words, with a little effort, many of the houses being built today could be built even tighter. In previous articles, repeated references were made to tight construction. This article will tie together what is known about tight construction, show why it is a good idea, and explain how it is integral to the systematic approach to house design and construction.

Misconceptions

Misconception #1: Tight houses have poor indoor air quality. Some do, but so do some loose houses. The problem isn’t related to tightness as much as it is to the use of unhealthy building materials—and a lack of mechanical ventilation.

 

Misconception #2: Tight houses have moisture and mold problems. Some do, but excessive moisture and mold are also common in loose houses. Moisture and mold problems are often due to foundation or roof leaks, leaky plumbing, poor construction, a lack of mechanical ventilation, choosing an improper ventilation strategy, or accidental ventilation.

 

Misconception #3: Chimneys don’t function well in tight houses. Again, this is true for some tight houses, but chimneys don’t always function well in loose houses either. One answer is to use combustion-fired heating devices that have sealed combustion chambers.

 

Problems of poor indoor air quality, moisture, and poor chimney function are typically not due to tightness itself, but rather a failure to view a house as a system. According to one informed architect, “You can build tight buildings as long as you don’t fill them full of poisons.” Fortunately there are ways of achieving energy efficiency without sacrificing health. According to an editorial in the Journal of the American Medical Association, “There is no doubt that unless a reasonable and logical plan is developed, the deleterious health impacts of excessive home tightening will be enormous.” 

 

The good news is that a reasonable and logical plan has been developed—the plan involves understanding a house as an integrated system consisting of various components and sub-systems that continually interact.

 

Loose vs. tight construction

 

With tight construction, healthy materials, controlled ventilation, and direct-vent, sealed-combustion heating devices, you can have a tight house and good indoor air quality. If you consider a house to be a system, tight construction has many advantages over loose construction. Most building scientists are of the opinion that tighter houses are an excellent idea, and the tighter the better. Here’s how tight and loose compare.
 

Comfort

Loose houses can be drafty and uncomfortable. This is especially true in the winter when wind and stack effect (natural ventilation) cause excessive cold air to enter the living space through the random holes in the structure. Too much of this natural ventilation during cold weather leads to excessively dry, uncomfortable indoor air. Tight houses are more comfortable because there are no holes for the cold air to enter through. Thus, tight houses aren’t drafty, and they’re generally not too dry in the winter—unless they are over-ventilated by a poorly designed mechanical ventilator. Though it takes some effort, existing houses can be tightened enough to reduce drafts significantly.
 

High energy bills

 
Loose houses can be expensive to heat. This is also most noticeable in the winter when stack effect is responsible for excessive infiltration and exfiltration. This form of natural ventilation often results in far more air exchange than necessary during the coldest part of the year—just when heating bills are already high. Tight houses are cheaper to heat because there are no holes for stack effect (or wind) to move air through. In fact, there are mortgages available for energy-efficient houses that take into consideration the fact that such houses have low energy bills. RESNET is a clearinghouse for information on energy-efficient mortgages.
 

Pollutant entry

 
If a loose house becomes depressurized (something that happens fairly regularly in all houses) pollutants originating outside the living space can be sucked indoors. For example, air passing from the outdoors, through a wall, into the living space, can bring with it mold and pollen from outdoor plants, exhaust gases from passing traffic, or particulates and gases from materials inside the wall cavity—most notably from insulation. This is where the second healthy-design principle of Separation was derived—separate problem materials from the living space by building an air-tight barrier.
 
It isn’t unusual for air to be sucked from the outdoors, through porous soil, through cracks in a foundation, and into a basement when the lower portion of a house is depressurized. As the air moves through the soil, it can pick up contaminates like radon, lawn chemicals, termite-killing chemicals, spills from underground storage tanks, and ground moisture, and bring them all into the living space. In a California study, trichloroethylene was pulled into the air of a school from contaminated ground water. In Ontario, methane from the soil of a former landfill site leaked into townhouses built there. With tight houses, pollutants and moisture can’t be sucked in because there are no random holes for them to pass through. Of course, pollutants could be brought indoors through a deliberate hole by a controlled ventilation system, but in such a situation you have the option of installing an air filter.

 

Tight construction not only keeps unwanted pollutants out, it also keeps insects out. Flies, spiders, even wasps, find their way indoors through the many gaps and cracks in a loose house.
 

Hidden moisture problems

 
When an air-pressure difference causes air to move through the random holes in a loose house, it can carry moisture with it. If that moisture finds a cool surface inside the wall cavity, it can condense there—leading to mold growth. This can happen in the winter when a house is pressurized, and warm humid indoor air passes into a wall cavity and condenses on the cold sheathing or siding. If an air-conditioned house in the summer is depressurized, hot, humid outdoor air can get into a wall cavity and condense on the back side of a cool interior vinyl wall covering. With tight houses, there are still going to be air pressure differences, but with no holes, there are no pathways for the moisture to travel through to get into building cavities.
 

The cost of ventilation

 
Tight houses require mechanical ventilation, and ventilation equipment costs money to install and operate. Yes it does, but when you analyze all the factors involved, the energy costs in a tight, mechanically ventilated house are typically lower than in a loose, accidentally and naturally ventilated house. Plus, in a tight house, you have the option of installing an air filter on the incoming air supply. The actual operating cost of mechanical ventilation is routinely less than most people pay for telephone or TV-cable service, and far less that we routinely pay for health insurance coverage. Ventilation is just not a significant expense. In a loose house, there is definitely a cost associated with excessive natural ventilation—often a high cost—that is hidden in the high heating bills. The bottom line: The cost of mechanical ventilation in a tight house is typically less than the hidden cost of infiltration in a loose house.
 

Lack of control

 
The big disadvantage to loose construction is lack of control—you simply have no control whatsoever over the indoor air supply. Mother Nature moves air in and out whenever she pleases, and she doesn’t care if the natural ventilation rate is too high, too low, too uncomfortable, or too expensive. Chimneys and leaky ducts provide accidental ventilation which, in some cases doesn’t cause problems, but it’s common for combustion by-products and other pollutants to be sucked indoors. With no control, you sometimes have too much ventilation, but at other times you have none. On a calm day, when the outdoor temperature is mild, Mother Nature won’t move any air into or out of a house, even if the windows are open.
 
For sensitive people, the control afforded by tight construction is an especially important advantage. This is because it allows them to operate their ventilation system intermittently, whenever the outdoor air is clean. Consider a house next to an orchard where trees are sprayed periodically with pesticides. In a loose house, pesticides will enter the living space due to natural and accidental ventilation whenever the orchard is sprayed. But with a tight house, you can turn off the mechanical ventilation system during spraying, and be perfectly safe. Then, once the outdoor air clears, you can turn the system back on. Most areas have some form of periodic outdoor pollution that can be avoided in a tight house with a ventilation system that is occasionally shut off. Smoke from a neighbor’s wood stove or fireplace often hangs near the ground at night, polluting the neighborhood. In a tight house with the ventilation system turned off at night, the indoor air will be smoke-free.
 
When you have a tight house built with healthy materials and a controlled ventilation system, you have the best of all worlds. You’ll have none of the disadvantages of loose construction and all the advantages of tight construction. If a house is over-ventilated naturally, energy is wasted, and the indoor air will be overly dry in the winter. When a house is under-ventilated, it can be unhealthy. But with controlled, mechanical ventilation you can have a rate that’s “just right” all the time.
 

Tight construction

 
In order to prevent an air-pressure difference from pushing or pulling air (which contains moisture and pollutants) through the random holes in the walls (and floors, and ceilings) of a house, you have two choices: 1) you can eliminate the air-pressure differences or 2) you can eliminate the holes.
 

Eliminating air-pressure differences

 
You can certainly eliminate some of the accidental pressures—and that’s often a good idea. For example, leaky ducts should be sealed whenever possible. On the other hand, most people wouldn’t want to give up using a clothes dryer, which is also responsible for accidental ventilation. You might choose a controlled ventilation strategy that has balanced airflows, because it will not contribute to air-pressure differences in the house. But you can’t possibly stop Mother Nature from applying pressures due to wind and stack effect. So, if you decide to eliminate air-pressure differences, you soon realize that you can eliminate some, but not all.
 

Sealing holes

 
As it turns out, it’s fairly easy to seal some of the holes in a house. In an existing house, you can plug up the holes that are easy to visualize and easy to get to, but there are other holes hidden inside building cavities that were cut by plumbers, electricians, and carpenters when the house was originally built, that you can’t seal. If, for example, all the holes in an existing house added together equaled a hole about 2 sq. ft. in size (this is called the effective leakage area or ELA), after conscientious sealing, you might be able reduce the ELA to about 1 sq. ft. In existing houses, some leaks just can’t be sealed—they must be lived with.
 
Some of the biggest holes in houses transport air currents inside building cavities. For example, older houses often have huge gaps around chimneys, inside walls, above dropped ceilings, within kitchen soffits, etc. Weatherization contractors call these by-passes and, when air moves through them, there can be a significant amount of heat loss in the winter.   However, the air moving through bypasses may not affect the indoor air quality—after all, the air is moving inside building cavities, not within the living space itself. Sealing by-passes will often significantly lower heating bills.
 
In new construction, with a little thought, you can eliminate almost all of the random holes. While it is not possible to seal 100% of the holes, many builders are able to build houses with ELAs of just a few square inches. Although not hermetically sealed, such a house will have many advantages over typical loose construction. The process of tightening a house is often called air sealing.
 

Materials for sealing

 
To seal up the holes and prevent an air-pressure difference from moving air through the structure of a house, it’s more important how materials are put together, than what materials are used. For example, if you built a house entirely of paper and securely taped every seam, there would be no holes, thus no way for air to move into or out of it. Granted, a paper house wouldn’t be very sturdy, but the point is this—even something as flimsy as paper can be used to make an air-tight house. It’s just a matter of sealing all the seams.
 
When you look at how a typical house is built, you quickly see that there are many different materials involved—wood, drywall, masonry, insulation, etc. The secret is to assemble all the pieces so, when two different components come together, there is some way to prevent air from moving between them. In the paper-house example above, we used tape where the different pieces of paper came together.
But you could also use library paste, or silicone caulking, or anything else that would make a tight seal between the individual pieces of paper.
 
Library paste isn’t a good sealing material in house construction because it isn’t durable enough. If you want a house to remain sealed for several decades after it’s built, you must use a long-lasting product. Some popular, and durable, materials include gaskets, caulking, aerosol-foam insulation, and drywall joint compound over paper tape. Yes, in some situations, paper tape works just fine. The durability of the sealing materials is actually quite important. After all, you don’t want to create an airtight house using materials that will degrade over time, yielding a house that isn’t airtight a year into the future. Following are some materials that should stand the test of time.
 
A long-lasting caulking can be used in a variety of places to seal a house. A product called acoustical caulking has often been recommended because it remains non-hardening and flexible, so it has less chance of cracking over the life of a house. The Acoustical Sealant manufactured by Tremco is widely used by builders of energy-efficient houses. It has a fairly strong odor when wet but tends to lose its odor over time. Because, when used, it’s sandwiched between building components, and not directly exposed to the living space, it should not affect indoor air quality. As an alternative, 100%-silicone caulking also works well, and aerosol-foam insulation is widely used to seal around doors and windows.
 
Seams between pieces of polyethylene sheeting can be sealed with 3-M #8086 contractors tape (3M), which can be mail-ordered from Shelter Supply. Two companies, Denarco, Inc. (Sure-Seal) and Shelter Supply, sell gaskets that are often used by builders who specialize in tight construction. Airtight electrical boxes are sold by Thomas & Betts (Nu-Tek brand) and Ryeco Products (R & S Enviro brand).
 
If the airtight electrical boxes mentioned above aren’t sealed properly, or if they develop a leak after installation (perhaps resulting from a loose gasket) they can’t be easily resealed—unless you tear out some of the drywall. There is a Lessco plastic box (Low Energy Systems Supply Co.) that can be used to make a conventional plastic or metal electrical box airtight. If a Lessco box develops a leak, it can be made airtight by drilling a small hole next to the electrical box, and injecting some urethane foam into the space between the Lessco box and the electrical box. A variety of these types of energy-related construction products can be mail-ordered from EFI.
 
Ryeco Products (R & S Enviro brand) offers an insert device to make existing electrical boxes airtight. And K-Products has a gasketed cover plate, with small easy-to-use sliding doors over the receptacles, to make an electrical box both air tight and child safe. However, one of the easiest ways to tighten an existing electrical box is to install a foam gasket behind the cover plate. Although they won’t be as tightly sealed as in new construction using airtight boxes, these gaskets do a reasonably good job. They are routinely available in hardware stores and building-supply centers.
 
Most of the recessed ceiling lights on the market are quite leaky. They can allow excess heat from the living space to enter the attic, which results in energy losses—and moisture in the attic, which can result in damage. It’s been estimated that a single leaky recessed ceiling light is responsible for $5-30 worth of energy per year being lost. But airtight recessed fixtures—though not commonly used—are readily available. Manufacturers include Hubbell Lighting (ICX7-ES, ICX&-ES2), Juno Lighting (Air-Loc), and Cooper Lighting (Halo AIR-TITE).
 
Scientific Component Systems has a series of recessed airtight light fixtures that accept compact-fluorescent bulbs. Some are designed for new construction, and some can be retrofitted to existing recessed fixtures to make them airtight. Retrofitting existing fixtures can be much easier than removing old fixtures and installing new ones.
 
Ado Products has a Quick-Seal Insulated Attic Hatch Cover that seals to the ceiling with a foam gasket. It can easily be removed for access by turning four special turnbuckle fasteners.
 
Air-pressure barriers (retarders)
 
People often talk about an air barrier (more correctly called an air-pressure barrier, or an air-flow barrier) as if it were a single material. But an air-pressure barrier is really more of a concept. In our paper-house example, two materials—paper and tape—were combined to form an air-pressure barrier. An air-pressure barrier typically consists of a number of different materials combined to form a continuous surface with no holes in it.
 
Actually, because it is impossible to seal a house 100%, it isn’t technically correct to use the word barrier—the word retarder is more accurate. That’s because the airflow can’t be blocked completely, but it can be retarded a great deal. So, we should say air retarder, or air-pressure retarder, or air-flow retarder instead of using the word barrier. Still, the term air barrier continues to be widely used.

 

There are a number of ways an air-pressure retarder can be incorporated into new house construction, but two are popular: sealed plastic sheeting and the airtight drywall approach (ADA).
 

Sealed plastic sheeting

 
After a house has been insulated, many builders cover the inside surface of the insulation with large sheets of polyethylene plastic. This is often referred to as a moisture barrier or moisture retarder. (We’ll talk about this later. For now, we’ll only consider the concept of an air-pressure retarder.) The plastic sheeting is typically applied directly to the studs prior to attaching drywall, plaster, or wood paneling.

In most houses where plastic sheeting is used, it isn’t sealed very well. Two sheets may overlap, but there is no tape or caulking between them. A sheet may be stapled loosely along its edges, without being secured to the floor. Openings around windows are rarely sealed, nor are the openings in the plastic sheeting around electrical outlets.
 
In order for plastic sheeting to block the flow of air due to air-pressure differences, it must be combined with other materials to form a single unbroken surface completely surrounding the living space. This takes a bit of thought, but it can be, and is being, done by builders throughout North America. For example, to prevent air from moving around the edges, plastic sheathing can be caulked to the subfloor, caulked to window and door frames, and caulked to other pieces of sheeting. Special airtight electrical boxes are manufactured to prevent additional air movement, and details have been worked out to seal the sheeting around plumbing lines and heating/cooling duct penetrations.
 
For more information about the techniques for sealing an air-pressure retarder consisting primarily of plastic sheeting, see the Builder’s Field Guide which is available from the Bonneville Power Administration. A few key details are also available in the Shelter Supply catalog. And climate-specific details can be found in a series of excellent Builder’s Guides produced by Building Science Corp.
 
One disadvantage to polyethylene is that it can deteriorate inside a wall cavity and become damaged and ineffective without the occupants being aware of it. This generally occurs behind electric baseboard heaters where the elevated temperature causes the plastic to break down, become brittle, and fall apart. Ozone, generated by electrostatic precipitators or other appliances, will also contribute to deterioration. Sto-Cote Products, Inc. (Tu-Tuf) and Yunker Plastics, Inc. (Dura Tuff) produce heavy-duty polyethylene products that are quite durable.
 
Some sensitive people are concerned about outgassing from plastic sheeting or caulking migrating into the living space. While this is possible, it is unlikely that enough outgassing would get through the drywall or plaster to be a problem. After all, if you seal up the holes, there will be no air pressure differences to push pollutants through a wall assembly.
 
Airtight drywall approach (ADA)
 
Because of difficulties some builders have with sealing flexible polyethylene sheeting, a different method of construction has been developed. It avoids the pitfall of having the polyethylene deteriorate inside the wall cavity. It’s called the Airtight Drywall Approach (ADA). The actual construction techniques are not difficult and they can be applied to virtually any style house.   
 
With ADA, drywall is the principle material used to form the air-pressure retarder. During installation, care is taken to insure that the joints between the drywall and various other materials are sealed thoroughly. Special care is taken around windows, doors, electrical outlets, etc.
 
A common leakage point in houses is the small gap formed where a wall meets the floor. With ADA, a bead of caulking, or a special gasket, is placed on the subfloor before the wall frame is even erected. Once this is done by the framing crew, the wiring, plumbing, insulation, etc., are installed in a conventional manner. Then, another bead of caulking, or a gasket, is applied to the inside face of the lower 2x4 (or 2x6) wall plate. When the drywall is installed, it will compress this second gasket, and will be sealed against the plate. So, by using just a few different materials, the floor is sealed to the lower wall plate, and the wall plate is sealed to the drywall, forming a continuous well-sealed surface.
 
Simply using paper tape and drywall joint compound in the standard manner will prevent air from moving between the sheets of drywall. With special provisions around electrical outlets, windows, doors, etc., ADA can provide a continuous and tight air-pressure retarder.
 
Some of the materials used in the ADA technique can be bothersome to very sensitive people. For example, rubber gaskets, caulking, and aerosol-foam insulation can be problematic if they are exposed directly to the living space. But with ADA, these materials are not exposed to the living space, so they are rarely a problem. In fact, the advantages of tight construction far out weigh the small chance that these materials would be bothersome. Besides, these materials are installed relatively early in the construction process, so they will have some time to outgas before the interior of a house is finished and occupied by a sensitive person.
 
Good sources of information about ADA include the Builder’s Field Guide, published by Advanced Energy, The Airtight House, published by the Iowa State University Research Foundation, and the Builder’s Guides published by Building Science Corp. Healthy House Building for the New Millennium by John Bower shows the technique in a step-by-step manner in both photos and text.
 

Determining tightness

 
You can tell a builder to build a tight house, but how do you know if he really did a good job? And how can you tell if a weatherization contractor or insulation installer really tightened up your existing house? In other words, how can you tell just how tight a house is?
 
The blower door test is being used more and more by general contractors, insulation installers, weatherization agencies, and utilities to determine house tightness. It involves using a special fan (a blower door) to pressurize (or depressurize) a house under controlled conditions.   Most blower-door operators exhaust enough air to cause a house to be depressurized to 50 Pascals (A Pascal [Pa.] is a tiny unit of pressure measurement. There are about 7,000 Pa. in one pound per square inch.), then they precisely measure the airflow through the fan. If they need to exhaust a great deal of air to reach 50 Pa., it means the house is loosely constructed. But if they get to 50 Pa. by only exhausting a little air, the house is tight.
 
So, if you want to know just how tight a house is, you should have it tested with a blower door. In leaky houses, you may need to exhaust 4,000 cfm or more to reach 50 Pa. (Technicians refer to this as 4,000 cfm50.) Some of the tightest houses can be depressurized to 50 Pa. by blowing as little as 100-200 cfm out of them (100-200 cfm50). Although there are a number of factors to consider, it’s generally stated that if a house is tighter than about 1,500 cfm50, it should have mechanical ventilation. There are several blower-door manufacturers, including The Energy Conservatory (Minneapolis Blower Doors) and Retrotec, Inc.
 
House tightness can also be measured by using a tracer gas. This involves injecting a certain quantity of an inert gas into a house, measuring its concentration, then waiting a specified time and measuring the concentration again. The second measurement will be lower because some of the gas will have leaked out through the random gaps and holes in the house. The leakier the house, the less gas will be left when the second measurement is taken. The problem with this approach is that it will only give you information for the time the house was tested. Because the amount of air flowing into and out of a house is a function of many factors (e.g. wind, stack effect, duct leakage, etc.), a blower door is more commonly used because it can overcome all those factors and give you a standardized evaluation that can be compared to other houses. Still, tracer-gas testing has a place and is used by some researchers. Both ASTM and ASHRAE have standard protocols available.
 

What about diffusion?

 
When an air-pressure difference moves air through a hole, the air typically contains gaseous pollutants, particulate pollutants, and water vapor. But there is another way pollutants and moisture can move through a wall—by diffusion. With diffusion, some components of air can actually move through a solid material—holes and air-pressure differences are not a factor. But diffusion can only account for the movement of gases and vapors, it cannot account for the movement of particulates. So if you build an airtight house with no holes, particles of insulation, mold spores, and pollen won’t be able to pass through. But diffusion can be a factor.
 

How does diffusion work?

 
Diffusion only moves a gas or vapor through a solid surface if there is a difference in concentration of that gas or vapor from one side of the surface to the other. For example, if there is a low concentration of formaldehyde both inside and outside a house, but for some reason there is a high concentration within a wall cavity, then the formaldehyde will diffuse through the materials making up the wall, both towards the indoors and the outdoors, to try to make the concentrations the same indoors, outdoors, and within the wall cavity.
 
If there is more water vapor on one side of a solid material than on another side, it may diffuse through the surface until the concentrations are the same on both sides. But if a gas or vapor is present in the same concentration on both sides of a solid material, the concentrations will remain equal and unchanged.
 

Is diffusion important?

 
Something that surprises many people is the fact that diffusion is extremely slow—so slow that the quantity of pollutants and moisture that travel through walls (and floors, and ceilings), just because of diffusion, is generally insignificant.
 
In fact, if you carefully analyze the laws of physics that apply, it can be determined that, in a typical house, 99% of the moisture and pollution travels through holes in the structure because of air-pressure differences—and only 1% of the moisture and pollution travels through a typical structure because of diffusion. So, if you build an airtight house by sealing up all the holes, you will solve 99% of the problem. Unfortunately, there is widespread misunderstanding in the construction industry about how insignificant diffusion is, and how important it is to seal holes.
 

Why the confusion?

 
The confusion surrounding diffusion goes back to the energy crisis of the 1970s when serious house tightening first got under way. Part of the problem is related to terminology. When early super-insulated houses started experiencing moisture condensation inside insulated building cavities in the winter, it was quickly understood that the moisture was getting into the cavities from the living space. For example, in research studies, water was found condensing inside walls in the vicinity of holes in polyethylene sheeting near electrical outlets. It didn’t take long before there were heated discussions about moisture barriers. Builders started carefully sealing the seams in the plastic sheeting they were using, and the moisture problems went away. When scientists started analyzing the way moisture was getting into the building cavities, they realized there were two mechanisms at work—moisture was primarily moving through holes because of air-pressure differences, and secondarily because of diffusion.
 
As builders started sealing the plastic sheeting very well, they began calling the plastic an air/moisture barrier or an air/vapor barrier, because it was actually performing two functions—it was blocking the transport of water vapor by diffusion, and blocking the air-pressures that moved air through holes. Then, the experts realized that most building materials weren’t true barriers to diffusion. For example, plastic sheeting did an excellent job of blocking diffusion, but it wasn’t perfect. So, they started referring to air/moisture retarders and air/vapor retarders. About the same time, a few people realized you could use two different materials to perform the two different functions. So, they started calling the materials that blocked the air movement through holes air barriers (and later air retarders), and the materials that blocked water vapor by diffusion vapor barriers (and later vapor retarders).
 
Because water vapor could be transported through a building because of both air pressure and diffusion, vapor retarder evolved into vapor-diffusion retarder. Then, people realized that we were really talking about more than water vapor—we were talking about gases like carbon monoxide and formaldehyde too. So, today, vapor-diffusion retarder has been shortened to simply diffusion retarder.
 
As was mentioned earlier, because it’s impossible to seal all the holes in a house, the terms air barrier and air-pressure barrier and air-flow barrier aren’t technically correct. So, as with diffusion terminology, when blocking air movement the word barrier is being replaced with the word retarder. Thus, the more correct terminology to use is air retarder (or air-flow retarder or air-pressure retarder) and diffusion retarder.
 
The thing that has gotten lost in all the jargon is the fact that diffusion is not very significant. In fact, some experts believe that, if you do a very good job of sealing up all the holes in a house, diffusion can be ignored! Well, that may be so, but a diffusion retarder isn’t very expensive, and it’s easy to install, so most tight-house builders use one anyway—as cheap insurance. Plus, because building codes often require them. (Because building codes tend to lag somewhat behind state-of-the-art technological knowledge, they don’t yet address the importance of sealing holes, but they do address the less important diffusion retarder.)

Diffusion-retarder materials

Diffusion describes how gases and vapors pass through a solid material. Gases and vapors will move through some materials faster than others. If a material only allows for a very slow rate of diffusion, it is said to be a diffusion retarder.

Most common building materials have been tested to determine their perm (permeance) rating. This is the rate at which water vapor travels through them. A perm rating of zero means no moisture will pass through. Aluminum foil and glass have perm ratings of zero. Polyethylene sheeting (4 mil thickness) has a very low perm rating of 0.08, and enamel paint has a perm rating of about 1.0.
Although diffusion ratings are different for different gases, a low perm rating for water vapor generally also means a low gas-diffusion rate.
 
When used in an existing house, a paint with a low perm rating is usually sufficient to minimize diffusion. Many manufacturers have primers that function as diffusion retarders. They typically contain an ingredient called styrene butadiene which is also used in carpet backings, and has been suspected of causing health problems related to new-carpet installations. Actually, if you have a good air-pressure retarder, most ordinary paints have enough of a perm rating to prevent most diffusion.
 
In new construction, several common materials function well as diffusion retarders. The asphalt-impregnated paper on fiberglass insulation, and polyethylene sheeting, are commonly used. Builder’s foil, which is a sandwich of aluminum foil and Kraft paper, also works well. It’s usually available in 3'-4' wide rolls. Most manufacturers offer it as a solid material, which functions well as a diffusion retarder, and perforated with hundreds of tiny pin pricks, which can block airflow but not diffusion. Denny Sales Corp. sells four different foil/Kraft paper sandwich products (Denny foil). They have a solid material with foil on either one or both sides of the Kraft paper, and a perforated material with foil on either one or both sides.
 
Foil-backed drywall also works well as a diffusion retarder, but it must usually be special ordered. Foil-backed drywall is readily available from all drywall manufacturers, but it’s rarely stocked by local suppliers, though they can usually get it if given a few days lead time.
 

Diffusion-retarder location

 
The most important component of air that diffusion must address is water vapor. This is because water vapor is often found at a significant difference in concentration from one side of a wall to another. To minimize the amount of water vapor that will diffuse through a wall, and to minimize the possibility of condensation within wall cavities, a diffusion retarder needs to be in a certain location inside a wall—and that location is different in different climates. Most of the books dealing with diffusion retarders say they should be located on the warm side of the wall.   The rule of thumb is this: in cold climates the diffusion retarder should be installed close to the indoors, and in hot climates it should be located close to the outdoors. So, in Minnesota, a diffusion retarder might be installed just behind the drywall, but in Florida it should be close to the exterior siding.
 
If you extend the above rule of thumb to apply to the central U.S., where it’s cold part of the year and hot part of the year, you might think that the diffusion retarder should go in the center of a wall. And that’s exactly where it should go. In practice, that can be difficult to accomplish, but the same effect can be achieved by using a layer of insulating foam sheathing on a house. This works because the foam itself acts as a diffusion retarder and its thickness (typically 1-3"), means the inner surface is thermally in the center of the wall. While a detailed discussion of diffusion and moisture transport is beyond the scope of this article, one of the most complete sources of information is the Moisture Control Handbook. Although not nearly as complete, the 51-page Controlling Moisture in Homes discusses basic moisture-control concepts.
 
In a cold climate, a diffusion retarder near the indoors will help prevent water vapor from the living space from diffusing into insulated building cavities, and it will also help prevent gases from the insulation from diffusing in the other direction—into the living space. In hot climates, a diffusion retarder near the outdoors will help prevent water vapor from outdoors from diffusing into the insulated building cavities, but it won’t prevent the diffusion of gases from the insulation from diffusing into the living space. However, this isn’t a significant concern, because the quantity of gases transferred by diffusion is so insignificant when compared to quantity of gases transferred by air-pressure differences through holes.
 
Diffusion retarders should always be installed where they will best minimize moisture problems. It’s been estimated that half of the houses in hot, humid climates have the potential for hidden moisture problems—often because the diffusion retarder is in the wrong location.
 

Sealing a diffusion retarder

 
An air-pressure retarder should be as tight as possible because it is the primary line of defense at reducing moisture and pollutant transfer through walls. On the other hand, because diffusion is such an insignificant factor, a diffusion retarder can be imperfect. So, if you plan to use polyethylene sheeting as a diffusion retarder, and regular drywall as an air-pressure retarder, you should seal the drywall as tightly as you can, but the diffusion retarder doesn’t need to be sealed very well at all. Of course, if you plan to use polyethylene sheeting as both a diffusion retarder, and an air-pressure retarder, it should be securely taped and very well sealed wherever there could be air movement through or around it.
 

Summary

 
Airtight construction, heating/cooling, ventilation, and filtration are the most interrelated parts of a house and they should all be planned for together. In general, airtight construction has many advantages, not the least of which is improved indoor air quality, but it must always be combined with the use of a controlled ventilation system. An airtight house without ventilation is like a scuba diver without an air supply.
 
When designing a tight house, pressure-tight is far more important than diffusion-tight, but in most cases a diffusion retarder is a good idea because it’s easy to install.

 

(Note: This article is part of the original HHI Archives, and was believed to be accurate at the time of writing. The views expressed in this article are those of the author, and do not necessarily represent those of The Healthy House Institute, LLC.) 

 

 

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