Continuous and contiguous air, thermal and moisture barriers are essential to healthy, high-performance homes, but it takes meticulous detail work to achieve this. A combination of rain screens, caulks and sealants, flashings and weatherstripping is required. Every penetration, transition and margin of the building must be addressed. Below are some general guidelines for exterior wall construction not only for long- term durability but also to achieve the performance that you expect from the product.
Thermal Barriers:
“Thermal barrier” is a fancy term for insulation. In a high-performance home, insulation should be installed on all exterior surfaces in an unbroken sequence. Any gaps, voids or breaks in the insulation coverage of the entire building assembly can result in heat loss or gain. This is shown in the image below with the use of a thermal imaging camera. In a color photo, heat loss shows up as warm yellow or orange and cool well-insulated areas are blue or black. In this black-and-white rendition, light and bright areas indicate heat loss.
Thermal bridging is the rapid transfer of heat through a building component when that component has less thermal resistance (R-value) than materials surrounding it. Framing materials offer a good example of thermal bridging through the building envelope. Wood has an R- value of a little less than one per inch, so a typical 2 x 4 stud has an R-value of around 3.5. Compared to the surrounding insulated wall cavities, if perfectly installed to manufacturers’ specifications to achieve R-13 or R-19, that’s quite a difference. So, if you look closely at the image above, the thermal movement through the wood-framing members allows you to see all of the studs and even the roof rafters glowing with the heat they are losing. Thermal bridging can greatly reduce the effective insulation value of a wall, floor or ceiling.
Another place where poorly insulated wood is typically used is for structural headers to displace the vertical loads over windows and doors, as was mentioned previously. This volume of uninsulated wood creates large areas of thermal bridging, significantly reducing the overall thermal performance of the entire wall assembly. This usually happens due to a lack of framing detail provided to them by the structural engineer or truss designer. Best practices are to install headers only where they are required structurally, to size them only for the actual load they are to carry and to insulate them. Many insulated structural header products are available on the market, or you can make your own by sandwiching a rigid foam board panel between two layers of wood (or structural wood product) to create a thermal break.
By adding a layer of rigid board insulation on the exterior of our entire wall assembly, we can reduce or eliminate thermal bridging, as this material provides an insulated break between the wood framing and the exterior heat source. By sealing the attic and insulating over the exposed roof rafters, we can reduce or eliminate thermal bridging there using the same approach. We could also choose to use an alternative building system, like SIPS, ICFS, AAC block or a natural material, which could significantly reduce thermal bridging in the building assemblies. In this article, we recommended raised heel or energy truss design as a remedy for insulation gaps between the top of the wall assembly and the edge of the roof assembly. It is evident from Figure 4.3 that this home suffers from poor insulation in the soffit area, a significant source of heat loss in the winter. These are like holes in the thermal envelope, and so the walls perform as if someone has left a window or door open, putting additional strain on the air conditioner or heater as it attempts to provide comfort under these conditions.
Finally, note the heat loss through the foundation or basement perimeter. This has become a more important issue as we built tighter thermal envelopes, which should enable us to reduce the size of air conditioning and heating systems required to keep them comfortable. However, this heat loss through the foundation assembly can result in raising the heating loads, negating any savings achieved in the main building assembly. In fact, we have seen instances in the last couple of years where heat pump system sizing is being determined by these heat losses, driving up heating loads even in cooling-dominated climates with very mild winters. This means that although we did a good job reducing the cooling loads through building science and envelope improvements, we were forced to install a larger HVAC heat pump system to handle the heat loss through the foundation in the few very cold days of winter that occur. This is the best argument for insulated slabs in any location that has any chilly winter days. An insulated slab or basement can reduce the heating load on the home by as much as 25 percent or more 23 depending on your climate and house plan.
Air Barriers:
Also, since insulation is a material full of air pockets, it is important to stop airflow through that material. For insulation to be effective, it must be encased by an air barrier. Air barriers function to keep air from freely flowing through insulation, allowing it to achieve the thermal performance (R-value) at which it was rated. To be effective, insulation and air barriers should be both continuous and contiguous, meaning that every exterior building assembly is insulated and encased by an air barrier. Research has proven that installing insulation without an effective air barrier results in a huge reduction in the effectiveness of the insulation and high bills with poor comfort.
Wherever the insulation is installed, there must be an air barrier in contact with the insulation on all six sides, leaving no insulation exposed; this prevents convection currents. The wall studs, along with the top and bottom plates, close up four sides. The exterior sheathing encloses the outside of the cavity, and drywall normally encloses the inside, but not without exceptions.
These exceptions are because there are areas of the thermal envelope that may be insulated but often do not have drywall installed on the inside of the cavity. This includes fireplace and HVAC chases and behind bathtubs when these features are located on an exterior wall. This can also include a stairwell on an exterior wall, even if part of the area under the stairs is a closet. Usually the under-stair closet ceiling slopes down to a point such that the bottom few steps of the stair would create a ceiling height too low to be usable. These bottom few steps, if on an exterior wall, will usually not have that area of the wall enclosed with drywall. In these areas, it is necessary to install some other type of air barrier to encase the insulation on the inside of the wall assembly.
Note that an air barrier is shown installed on the inside and outside of the wall common with the attic space, often called a kneewall or pony wall. These are vertical walls that separate a room from an attic space. They just stuff some batts into the cavities and call it good enough. They also don’t place air blocking in the big holes under the knee walls where the ceiling framing runs. This leaves dozens of big holes (16 inches by 8 inches) open so that outside attic air easily blows between the uninsulated floors and ceilings. In cold climates, the result is often frozen pipes between the floors of the home where you would think that cold air shouldn’t be able to go. Very often rooms over garages are uncomfortable because they suffer from both of these problems. These areas, even if insulated, are large holes in your thermal envelope when not sealed by some type of air barrier.
For blown-in insulation in the attic floor, significantly higher R-values are typically required by building codes to achieve the desired resistance to heat needed here. Since the insulation is not installed vertically, it is not as susceptible to convection loops and for this reason doesn’t need to be encased on the sixth side. The depth markers that are commonly seen in this type of installation ensure the depth of the insulation achieves its stated R-value. A common hole in attic insulation occurs at the location of an attic scuttle hole or attic stair.
Air barriers should be sealed at all penetrations. On the exterior side of the insulation, the house wrap or rigid foam board must have all seams taped. To complete the air barrier, it is necessary to caulk and seal all penetrations in the building envelope. Some of the more common penetrations in wall and roof assemblies include plumbing and mechanical vents, condensation drain pipes, fireplace chimneys and electrical conduits, fixtures, and outlets. Air infiltration into the building assembly occurs wherever these penetrations are not properly flashed or sealed.
The way to know if you have an effective air barrier is to test the house under pressure and then measure the air infiltration rate. Since 2009 this test, often called a blower door test, is required by code, but if you do not live in an area that mandates code inspections, you should make sure your builder is aware of the blower door test and that you see the results. The house must be tested and proven to be a very tightly sealed structure by achieving no more than 5 air changes per hour at 50 pascals of pressure (ACH50). This standard will be restricted even further to no more than 3 ACH50 in climate zones 3-8 by the 2015 International Energy Conservation Code (IECC) (advance information June 2014).
Ice Damming:
Ice dams are a source of tens of millions of dollars in home damages each year. Homeowners have tried heat tapes at the edge of the roof, rakes, more ventilation in the attic and a million other ideas to stop this threat, but with limited success. The building science community has been able to determine how and why ice dams occur, and this has led to a successful strategy to stop them. They found that the problem isn’t at the eave — that’s just where it becomes evident and does its damage. The ice dam itself is just a symptom of the real problem. Preventing ice dams is all about creating an effective air and insulation barrier at the ceiling of the house. An ice dam starts when heat is allowed to rise from the ceiling of the house at what building science calls thermal bypasses (explained below) and melts the underside of the snow pack on the roof. This meltwater runs down the warm roof as liquid at just above freezing. It stays liquid until it reaches the exposed eaves of the house, and there the temperature of the roof deck suddenly drops because the deck is fully exposed on the bottom side to the cold outside air. The meltwater flash freezes at the eave, and the ice dam begins to form. As more water melts and runs down the roof, it builds up a small lake behind the ice dam and backs up under the shingles to run down the walls of the home. If the snow pack stays frozen, there can be no ice dam. The solution is to correct the cause, not to deal with the ice dam itself. In other words, stop the heat from rising at the ceiling of the house and the snow pack will stay frozen and no ice dam will be able to form in the first place.
This requires us to seal all thermal bypasses and insulation defects in the ceiling of the home. A thermal bypass is a place where the insulation and ceiling air barrier have holes in one or both system (remember: “continuous and contiguous”). These are places like open utility chases for plumbing, wiring and ducts, dropped ceilings and other irregularities that provide opportunities for breaks in the drywall air barrier and insulation at the ceiling. These openings will act as a chimney for warm air to escape into the attic where they rise and warm the roof deck.
All thermal bypasses must be sealed airtight with rigid materials like plywood or foam board, then air sealed with caulk or foam sealant and then covered thoroughly with insulation. This is a job best done by the framing crew during the initial framing of the home. Subcontractors who later penetrate these air barriers must be held responsible for resealing them once they have installed the plumbing, wires or ductwork that runs through them. Any areas of missing insulation must be fully insulated.
Recessed can lights are notorious thermal bypasses. They are not only ventilated to allow warm house air to pass through, but they generate their own heat when the lights are on. The ENERGY STAR Certified Homes Program has an excellent list called the Thermal Bypass Checklist showing everything that must be sealed for a home to be certified.
The best solution in new construction is to seal everything on the Thermal Bypass Checklist and to use only recessed light fixtures that are airtight rated and also rated for insulation coverage. These lights are often called AT/IC-rated recessed fixtures. They should meet ASTM E-28325 and be labeled as such.
If you can’t replace the old leaky can lights in your existing home, consider a code-approved option. Build a sealed box out of drywall that leaves a clearance around the fixture as specified by the manufacturer. Cover the cans with the drywall boxes and seal the boxes down to the ceiling drywall. This will stop the air from rising through the cans and heating the roof deck and starting the process that leads to ice dams.
Radiant Barrier:
A radiant barrier is the foil-faced roof decking (with the foil facing the attic side) that stops radiant heat gain through a vented roof assembly in hot climates. This is usually one of the first upgrades for existing construction if your roof design allows good access for installation, along with improving the insulation in the attic if needed. If you live in a cooling-dominated climate, you should also strongly consider using aluminum foil-faced radiant barrier roof decking material with an emissivity of 0.05 or less26 to keep your attic cooler in the summer. This will at least prevent much of the radiant heat gain through the roof assembly. This results in helping to take some of the heat load off your air conditioner and ducts during hot weather.
Moisture Management:
Water is the enemy of all building materials, and can easily destroy everything we do to make our home healthy, safe, comfortable and durable. That may sound a bit over the top, but it’s the plain truth. Water can come from outside in the form of rain, or it can come from inside in the form of humidity that condenses on a cold surface. Either way, the materials are now unhealthy waste products. The best way to prevent moisture issues is to ensure that your home has an effective raincoat, or drainage plane as building science often calls it. This is achieved by the proper layering of waterproof materials in the wall assembly. Starting with the top of the house, the roof, you should be able to follow the flow of water down and off or out of the building assembly and verify that is a clear, unobstructed direct path. To do this, we’ll begin at the roof in order to understand how shingles work to keep water out of your home. They are always laid from the bottom up so that the upper shingles overlap the bottom shingles. Remember, water runs downhill. As the water runs down the roof, it would have to run uphill to get under the next shingle. Since this is very unlikely, the overlapping layers form a water-resistant system.
Moving from the roof to the walls, all the other waterproofing materials, like house wrap and metal flashing, must be installed from the bottom up, overlapped the same way roof shingles are laid. This way water flows across the top of the layer and has no way to get under and inside the flashing. This sounds simple, but it is very common to see these materials installed backwards! Too many crews today are not well trained. They only know that one layer must go under another, but reverse the order. When this is done, the water has a clear path inside the waterproofing. In fact, the lower layers act as an opening directly into the house, catching the water like a gutter.
Moisture management details mean the difference between living with years of worry-free enjoyment of your home and major, expensive repair costs. To make sure it’s done correctly, you need to understand the function of each material used. Cladding is the material that forms the exterior façade of your structure and provides ultraviolet (UV) light protection for the materials inside the wall assembly that would otherwise be degraded by direct exposure. Some green cladding choices include natural materials, like stone, and man-made materials that are durable, like cement-based siding. It is important to understand that exterior cladding materials are intended only to shed bulk water during heavy rains, not prevent water penetration. Stone, brick and mortar are porous and water will soak through them in a matter of only a few (15-30) seconds! Wood, vinyl and fiber-cement siding all readily leak. Since all claddings and windows leak, it is the responsibility of those building homes to take the steps necessary to manage the water that penetrates them.
The most aggressive water management system requires a clear one-half to one-inch air space between the cladding and the drainage plane, with weep holes at the bottom of the wall to allow rain-driven moisture to escape. This type of drainage system is more commonly referred to as a rainscreen. To assure a consistent, unobstructed air space, 1 × 4 furring strips are installed, which allow for gravity drainage. The drainage plane material is typically a house wrap (with a 25-year warranty) and/or asphalt-impregnated builder’s felt meant to keep liquid water out of the wall assembly.
One of the important components of the rainscreen is house wrap, which is a very interesting material. These wraps are engineered to be highly resistant to penetration by liquid water, but highly permeable to water vapor/humidity. This is so that they can do both of the jobs we need them for. First, they keep the walls dry by stopping liquid water like rain from getting to the wood. Second, they allow the walls to dry out once water has penetrated past them by allowing water vapor to pass through.
Remember, every wall cladding system and every window eventually leaks so we must plan for this. While most house wraps can hold out liquid water for a long time, they have water vapor permeability ratings from 7 perms up to 58 perms. The perm rating tells us how quickly water vapor will pass through a material. The perm ratings of house wraps are all high enough to allow for wet materials to dry out by releasing the water vapor from the wall system. The secret to how house wraps do this is that they control the size of the holes in the material. At a microscopic level, they have holes big enough to allow individual water vapor molecules to pass through, so any moisture inside will eventually dry out. But when water vapor molecules bunch up to make a molecule of liquid water, they are then too big to fit through, preventing liquid water from entering the building assembly through them.
If you are using stucco as your cladding, leaving a half-inch air gap behind that material is usually not possible, so the requirement for the water drainage plane is for a double layer of water-resistant underlayment. This double layer can be comprised of two layers of house wrap or asphalt-impregnated builder’s felt, a layer of either of these and a layer of paper-backed lath or paper-backed lath over a layer of rigid foam sheathing with the seams sealed with approved tape. The first layer serves as a bond break for the second layer where the real drainage takes place. Two layers of drainage plane behind stucco walls is now a code mandate and the position of the Portland Cement Association.
If you visit the website of the manufacturer of your house wrap or drainage plane product, you will find step-by-step detailed drawings and instructions showing how the builder must flash the windows, doors, porches and other areas of your home vulnerable to water penetration. Be sure that this is done right and don’t be afraid to speak up if the crews mess up. If it’s not done right, your manufacturer warranty for your house will be voided.
Building science technologies are beginning to develop components that integrate several layers of assemblies into one product. These products are available for both roof and wall assemblies. Some combine continuous foam insulation to stop thermal bridging and an engineered panel that replaces structural wood sheathing in a single nail-applied product. These products incorporate a built-in water- resistive barrier, eliminating house wrap or felt, and are installed and taped at all joints. This provides a continuous structural, insulative air barrier and water drainage plane. These types of engineered, prefabricated and performance-tested multipurpose systems are available now, and represent the future of high-performance building materials.
Flashings:
There’s an old saying that fits this topic very well: the devil is in the details. That’s because it’s how we flash windows, doors, porches, chimneys and other key areas of the exterior walls and roof that will keep the water out of your home. This is not the place to cut money from your budget! Flashings of all sorts are manufactured to fit every joint and intersection of your exterior building envelope, to cover all the different angles, seams, gaps and penetrations. It’s not just the materials; even more importantly, how they are installed keeps the water running downhill.
With windows, the process begins with a sill pan or a layer of waterproof material covering the bottom of the rough opening that the window will be installed in. This material should be turned up at the corners at least six inches on the inside of the studs and then extend out on top of (not under) the house wrap below the opening. The house wrap has a flap cut above the window. The window is then installed, and the sides are flashed with an adhesive tape, followed by a layer of flashing tape across the top of the window flange. The flap of house wrap is then brought down over the top flange, and it is taped in place with flashing tape. The bottom flange of the window is not flashed. This will allow any water that does get into the opening to drain out and flow down the waterproof drainage plane of the house and out the weep holes or screed at the bottom of the wall.
The process for porches and decks is much the same. The drainage plane material is fully installed before the ledger board is put in place. After attaching the ledger board, the drainage plane is cut above the ledger board to which the joists will be attached. A flashing material like metal or flexible waterproofing is attached to the sheathing, and wrapped around the front of the ledger board. The drainage plane is then turned down over this flashing and taped in place.
The addition of an insulated sheathing to the wall assembly reduces thermal bridging. Or you can replace the rainscreen assembly that we just described with an insulated sheathing panel system that is taped at all seams (with a 50-year warranty). These types of water management systems are always employed behind masonry or stucco walls (because water penetrates those materials so rapidly) and behind wood walls, too, in rainy or marine climates.
Building a Dry, Healthy Basement or Crawlspace:
If your plans include a basement or crawlspace, building it to stay dry is a must to ensure a green, durable and healthy home. From a building science and physics perspective, a crawlspace is just a basement with short walls. The physics of moisture, temperature and condensation remain the same. If you build your basement or crawlspace incorrectly, it is very difficult, disruptive and costly to correct the error in later years. Just as we did in building our exterior envelope, building a basement right means that we deal first with liquid water (water in the liquid state can enter through either the floor as rising groundwater or through the walls), next with condensation as a source of water, and finally with capillary water movement.
There has been a great deal of research into the question of how to best build a basement. The Building America Program, the Canadian Mortgage and Housing Corporation (CMHC) and others have completed a multi-year investigation into this question and have concluded that the basement wall that performs the best is one that is damp proofed on the exterior to control capillary action, with rigid foam board installed as a drainage plane on the exterior. By placing the rigid foam board in this location, two benefits are achieved. The first is that the wall is now inside a layer of continuous insulation and thus fully isolated from the cold soil and air. The second is that this means that the wall stays warm and is very unlikely to experience condensation on the inside in either winter or summer months. This makes building a finished basement much easier because it eliminates the worry about warm inside air finding a cold surface and causing problems by condensing. The second-best option is to install the foam board either in the middle of the wall using a manufactured system like an insulated concrete form (ICF) or on the inside of the wall. Many homebuilders prefer to install the rigid foam board or closed-cell spray foam on the inside of the basement wall to protect it from damage during construction. This layer of foam can be XPS, EPS or spray-in-place foam. If you take this approach, be sure that all seams, gaps and edges are fully caulked and sealed so that no inside air can get to the concrete wall, because with this approach, it will be cold and condensation will occur. The seams between sheets, and the top and bottom of each sheet, must be effectively air sealed to keep warm moist inside air from coming into contact with the cold wall. The old-fashioned use of a framed wall with plastic vapor barrier and blanket insulation is no longer considered a viable approach.
Concrete is full of millions of tiny pores that inherently lead to very strong capillary action, wicking water into the structure unless that action is controlled. Dampproofing is essential to protect the structure from this. This is often accomplished by using a paint-on or trowel- on material formulated for this purpose.
In much of the country, dealing with groundwater often entails installing a French drain (underground drain) and a sump. The French drain should be located at the bottom of the concrete grade beam supporting the wall and below the level of the basement or crawlspace floor. It should be surrounded with a generous layer of free-draining river rock and the whole assembly wrapped in a geo-textile filter fabric to keep the drain pipe from becoming clogged with soil and to break the hydrostatic pressure of the water pressing against the wall, allowing it to drain down to the French drain. The next step is to dampproof the below-grade portion of the wall and ensure that soil-borne water can drain freely down to the French drain.
Due to the strong capillary action of concrete, you should also be sure that there is a capillary break like a sill seal or 30-pound asphalt- impregnated builder’s felt between the poured footing and the wood wall it supports. This also aids in creating an effective air barrier at this uneven location. The control of soil moisture can be aided by ensuring that the outside soil grade is sloped away from the house in all directions with a drop of six inches for every ten feet of horizontal run. Installing gutters and downspouts that take the roof runoff at least three feet out away from the foundation will contribute a great deal to keeping a basement or crawlspace dry.
If you have a concern with radon , this is the time to place a layer of coarse rock beneath the basement slab and cover that with a taped and sealed layer of ten mil polyethylene. Install a PVC pipe and stub it out so that it is above the slab. Later you will connect a pipe to a riser and run that up through the house and out the roof. An in-line fan rated for continuous duty is installed in this pipe to capture the radon from below the layer of plastic and exhaust it safely above the roof.
Since 2000, the International Building Codes (ICC) have recognized and allowed two methods of crawlspace construction. One is the old ventilated crawlspace. The other is a sealed, unventilated crawlspace. If your local building official needs a code reference, this can be found in Sec. R-408 of the International Residential Code and Section 402.2.10 of the International Energy Conservation Code.
In the past, it has always been common practice to build crawlspaces with ventilation openings at a rate of one square foot of open area for each one hundred and fifty square feet of crawlspace floor area. As most people who have ventured into a crawlspace will report, they are almost always damp and musty with some evidence of mold or decay on the wood. Research has also shown that even dry soil in a crawlspace will evaporate between 10.2 and 19.1 gallons of water per day. This, along with humidity in outside ventilation air, are the two primary sources for the moisture that makes so many crawlspaces damp and musty smelling.
Some people have thought that adding a powered ventilation fan would make the crawlspace drier. Sadly doing this in most climates actually makes the crawlspace wetter and speeds mold and decay of the wood. Powered ventilation of crawlspaces in areas with humid summers will make them wetter, not drier, contributing to rot and mold growth. The same is true for powered attic ventilation, which doesn’t result in the best home performance in any climate. Powered attic ventilation can actually pull conditioned air up from the house below, resulting in higher indoor humidity and negative pressure that can cause backdrafting of carbon monoxide.
With an unventilated crawlspace, the insulation is placed on the walls, not under the floor of the house. This is more energy efficient.
All crawlspaces would benefit from having the soil covered with a six-mil or thicker plastic vapor barrier as shown in the diagram above. This will stop the soil from contributing so much water vapor to the crawlspace. To quote from the research findings, “Building America research has found that closed, conditioned crawlspaces perform better than vented crawlspaces in most parts of the United States.”
There has been extensive research aimed at solving the excess water vapor problem by the building science community over the last 20 years. The Oak Ridge National Laboratory and the Building America Program of the US Department of Energy have published recommendations for crawlspace construction that have been incorporated into our building codes. The problem comes back to climate differences. If you are building in a mild climate where, for the majority of the year, the outdoor air is dry, with a low dew point and relatively low humidity levels, then a ventilated crawlspace can be a good option. On an annualized basis, there will be more days when the wood is dried by the introduction of outside air than when it is made wetter. But in areas where the outside air is humid with a relative humidity over fifty percent much of the year, the net annual impact of introducing that air into the crawlspace will make the wood damp, not dry. The farther south or nearer a coast you live, the more closely you should look at a sealed, unventilated crawlspace. If you live in a cold and dry, or very arid climate, then ventilation can work for you, but your pipes will still be in danger of freezing.
Vapor Barriers:
What about moisture in the form of water vapor? Water vapor moisture is produced both outside (humidity) and inside (steam from cooking and cleaning). The second law of thermodynamics says that stuff moves from areas of greater concentration and higher energy to areas of less concentration and lower energy. With regard to vapor, air carrying the moisture (vapor) always moves from high pressure toward low pressure, and water moves from wet toward dry and from warm toward cool. When water vapor hits a cool surface, it condenses and changes from vapor to a liquid. If this is within the wall assembly, that becomes a problem, leading to mold growth and issues with rotting assemblies and poor indoor air quality.
So you may or may not need or want a vapor barrier. The goal is to control or stop condensation. There are two ways to do this. One is to stop the warm moist air from coming into contact with the cold surfaces. The other is to warm the surfaces so that they are too warm for condensation to occur.
In the past, we primarily used the idea of stopping the moisture using vapor barriers in our wall assemblies. This is the concept behind the plastic vapor barrier covering the studs. With the advent of rigid foam insulation board, we can now warm the wall surfaces to prevent condensation. And this provides the additional benefits of increasing the total insulation of the wall assembly and reducing thermal bridging.
When we do use a vapor barrier, where we place it is determined by which direction the wall will dry towards. Remember, our goal is to stop the water vapor from finding a cold surface and condensing while still allowing the wall to dry in the other direction. Strange as it might seem, northern homes dry out while southern homes dry in.
For example, if your home is in a very humid, cooling-dominated climate like Dallas, Texas, (or in a mixed-humid climate like in the Midwest), the direction of water vapor drive is from the warm, humid outside air toward the dry, cool air inside of the air conditioned home. As moist air comes in contact with the backside of cool conditioned wall surfaces, condensation and related problems can occur. This is especially true if the owners have kept the house at a temperature below the outdoor dew point temperature. Many a builder has removed vinyl wallpaper and found the drywall under it covered with mold and not understood the source of the excess moisture. So the right place for the well-sealed vapor barrier/vapor retarder is to the outside of the wall assembly, allowing the moisture to dry to the inside.
Now, let’s consider a home in a heating-dominated climate like Minneapolis or Toronto. The warm/humid side of the house walls most of the year is the inside, and the cool/dry air is on the outside of the house. In this climate, the water vapor is driven from the inside toward the outside through the building assemblies in long winter. As this warm, humid air reaches the backside of the cold exterior sheathing, it again causes a condensation problem. The place to put the vapor barrier would be on the inside of the wall assembly.
Except for extremely cold climates, we can skip the vapor barrier entirely and opt for installing the exterior foam sheathing, which keeps the wall assemblies warm enough to prevent condensation. We call this putting a coozie on your house. The thickness of the foam sheathing required depends on your climate zone. In the mild winter areas, one-half inch to one inch will do the trick. In mixed climate zones (fairly equal heating and cooling seasons), you should use an inch to an inch and one-half of rigid foam on the outside of the wall. In areas with very cold winters, you will need to install one and one-half-inch or two inches of rigid foam board to ensure that you keep the wall cavity warm enough to be trouble free. If you check with your local building code official, they can look up what is recommended in their code books.
You can paint the inside wall with two coats of latex paint and that acts as an interior wall vapor retarder, slowing the rate of vapor diffusion. When combined with the exterior foam, this is an excellent system that works very well in any climate zone. This has been called “the perfect wall” by the building science community. It also works as the perfect floor when rotated ninety degrees and the perfect roof when sloped properly. It controls water vapor condensation, temperature and thermal bridging and allows drying to the inside.
In the following drawings you can see the direction of water vapor flow and therefore the direction of drying that occurs in heating- versus cooling-dominated climates. The fact that the way a wall dries is not the same in all parts of our country has led to many poor decisions and confusion about where to place the vapor barrier in a new home. The diagrams also illustrate how the vapor barrier reduces the moisture load on the wall assembly, thus protecting it. The rule of thumb is to place the vapor barrier on the side of your wall that is more humid and warmer for the majority of the year. In hot, dry climates, walls that have no vapor barriers at all, often called breathable walls, are a good option. Also, avoid using moisture-stopping drywall products, as are commonly used around bathtubs and showers, in areas where direct water contact is not an issue.
The Real Cost of Housing
Remember, it’s not just the initial cost of the material that represents its value. Think about how long each material will last and what it will cost to replace it over time. Also, you might talk with your insurance agent about some of the things that you could do in building your home that might lower your homeowner’s insurance. Some of those things might surprise you; it would be wise to take advantage of them. Building materials that are durable, that are going to last a long time and possibly survive natural disasters like strong winds, hail and flooding reduce the insurance company’s costs to repair your home. Metal roofs and stone facades are an excellent investment for this.
