Vapor Barriers - Exterior Wall Assembly

Saturday, October 28, 2017

What about moisture in the form of water vapor? Water vapor moisture is produced both outside (humidity) and in- side (steam from cooking and cleaning).  e second law of thermo- dynamics 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. Vapor barriers (like vinyl, polyethylene or the asphalt-coated Kraft paper face of our insulation batts) can either help to prevent this from happening, or can actually contribute to the problem, depending on the climate.

So you may or may not need or want a vapor barrier. The goal is to control or stop condensation.  ere are two ways to do this. One is to stop the warm moist air from coming into contact with the cold surfaces.  e 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.  is is the concept behind the plastic vapor barrier covering the studs. Getting a perfect plastic vapor barrier installation is very hard and detailed work, and there have been far too many instances where small overlooked holes have caused major problems, so now there is another option. 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.  is is especially true if the owners have kept the house at a temperature below the outdoor dew point temperature. If we had placed the vapor barrier on the inside of an exterior wall, it would have exacerbated the problem by halting the ability of the water vapor to dry to the inside. 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.  e 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.  e 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.36  is has been called “the perfect wall” by the building science community.37 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  ow and therefore the direction of drying that occurs in heating- versus cooling-dominated climates.  e 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.  e diagrams also illustrate how the vapor barrier reduces the moisture load on the wall assembly, thus protecting it.  e 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, o en 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.

Vapor Barriers - Exterior Wall Assembly

Flashings - Exterior Wall Assembly

There’s an old saying that fits this topic very well: the devil is in the details.  at’s because it’s how we  ash windows, doors, porches, chimneys and other key areas of the exterior walls and roof that will keep the water out of your home. Flashings are one of the most important and overlooked components of the building envelope that make or break long-term resistance to damage. Make certain that your framers and roofers are using the right materials for the job and installing them in the proper fashion.  is is not the place to cut money from your budget! Flashings of all sorts are manufactured to  t 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.  is 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 open- ing. The house wrap has a  wrap 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.  is will allow any water that does get into the opening to drain out and  ow down the waterproof drainage plane of the house and out the weep holes or screed at the bottom of the wall. By the way, the order that these flashings are installed, from the bottom up, is critical. For curved windows and doors there are flexible flashing tapes on the market now that work very well.


The process for porches and decks is much the same.  e drain- age plane material is fully installed before the ledger board is put in place. A er attaching the ledger board, the drainage plane is cut above the ledger board to which the joists will be attached. A  ash- ing material like metal or  flexible waterproofing is attached to the sheathing, and wrapped around the front of the ledger board.  e drainage plane is then turned down over this flashing and taped in place. You can obtain detailed drawings showing exactly how these openings need to be flashed on the websites of the house wrap or window manufacturer for your project.

The addition of an insulated sheathing to the wall assembly re- duces 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.

Radiant Barrier - Exterior Wall Assembly

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.  is 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. Note that radiant barriers require an open, ventilated air space on the side facing the house, so they should not be installed in a sealed attic, as they simply don’t work when foam insulation is in contact with them.

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.  is will at least prevent much of the radiant heat gain through the roof assembly.  is results in helping to take some of the heat load o  your air conditioner and ducts during hot weather.

Ice Dammin - Exterior Wall Assembly

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.  e build- ing 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.  e 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.  is 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.  e meltwater  ash 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.  e 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.  is 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.  e energy
star Certified Homes Program has an excellent list called theermal Bypass Checklist24 showing everything that must be sealed for a home to be certified.

The best solution in new construction is to seal everything on thermal Bypass Checklist and to use only recessed light fixtures that are airtight rated and also rated for insulation coverage. These lights are o en 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.  is will stop the air from rising through the cans and heating the roof deck and starting the process that leads to ice dams.

Air Barriers - Exterior Wall Assembly

Also, since insulation is a material full of air pockets, it is important to stop air ow through that material. For insulation to be effective, it must be encased by an air barrier. Air barriers function to keep air from freely  owing 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.  e wall studs, along with the top and bottom plates, close up four sides.  e 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 o en do not have drywall installed on the inside of the cavity.  is includes  replace and HVAC chases and behind bathtubs when these features are located on an exterior wall.  is 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 out- side of the wall common with the attic space, o en called a knee- wall or pony wall.  These are vertical walls that separate a room from an attic space. Typically, builders do not install an air barrier on the attic side of these walls. 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.  is leaves dozens of big holes (16 inches by 8 inches) open so that out- side attic air easily blows between the uninsulated  floors and ceilings. In cold climates, the result is o en frozen pipes between the floors of the home where you would think that cold air shouldn’t be able to go. Very o en rooms over garages are uncomfortable be- cause they su er 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.

The exception to the air barrier installation requirement is if you are installing blown insulation on the attic  floor. 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.  e 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. Any attic access that penetrates the thermal envelope should be well sealed with weather stripping, with multiple layers of rigid board insulation applied to the attic side of the board cover, or by installing an insulated stair unit.

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.  ere can be no exception to this rule. Some of the more common penetrations in wall and roof assemblies include plumbing and mechanical vents, condensation drain pipes,  replace 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, o en 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.  e 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).  is 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).

Thermal Barriers - Exterior Wall Assembly

“ 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.  is 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 build- ing component when that component has less thermal resistance (R-value) than materials surrounding it. Framing materials o er 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 × 4 stud has an R-value of around 3.5. Compared to the surround- ing 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.  is volume of uninsulated wood creates large areas of thermal bridging, significantly reducing the overall thermal performance of the entire wall assembly. As was mentioned earlier, it’s unfortunate that many framing crews are taught to install headers over every window and door, even when they are located in non-load-bearing walls.  is usually hap- pens 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.

We recommended raised heel or energy truss de- sign 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  that this home suffers from poor insulation in the so t 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.  is 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 in- stances 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.  is 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.  is 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 depending on your climate and house plan.

Advanced Framing

Optimal Value Engineered (OVE) or Advanced Framing techniques are framing techniques that use less lumber than conventional framing, yet are just as structurally sound. Using 5 to 10 percent less lumber is cheaper and faster because it uses 30 percent fewer framing pieces,21 which equates to a direct reduction in lumber package costs. But, as important, these methods also present one of our best opportunities to improve the thermal performance of the structure.

Remember, thermal performance is one of our primary goals. Because Advanced Framing uses less lumber, this leaves more space in the cavities to allow for higher levels of insulation, resulting in better thermal performance. In fact, Advanced Framing results in a 75 percent improvement in thermal performance22 over standard 2 × 4, 16-inch-on-center framing.  e lumber cost savings can add up to enough to cover the additional costs associated with improv- ing the thermal performance, including the cost of adding insulated sheathing to reduce thermal bridging through the wall assembly.

This is definitely a case of “less is more,” in that using less lumber saves trees and uses less money in your budget, with the added bonus of leaving more room for insulation, giving more thermal performance.  ink about that because it represents significant improvement in your building performance for minimal additional construction costs and lowers your long-term utility bills. So if you are planning a framed structure, you should make certain that your framing contractor has been trained in and practices the methods discussed here.

Advanced Framing

Many computer-aided design (CAD) programs can be set to a grid of either 16 inches or 24 inches on center (the distance from the center of one framing member like a wall stud or ceiling joist to the center of the next) to allow for ease of designing to these two basic spacing criteria. Providing these Advanced Framing details in your architectural plan sets can assure that you bene t from those cost savings and performance benefits. Better efficiency, improved comfort, and reduced costs are all achieved in each of the following methods:

Framing 24 Inches on Center: Exterior wall studs, floor joists and roof rafters can be spaced at 24 inches on center (as opposed to the conventional 16 inches on center). Depending on the load bearing on walls, framing lumber may require 2 × 6 studs rather than standard 2 × 4 framing. Note that the total cost (material and labor) for framing with 2×6 studs spaced 24 inches on center is about the same (since 30 percent fewer studs and only a single top plate are
required) and o en less than what it would have cost for 2 × 4 studs spaced 16 inches on center. Because there are fewer studs to cut, there is less waste.  is also saves labor for both your electrical and plumbing contractors, who now have to drill fewer penetrations for mechanical runs.

However, with most types of cavity- ll insulation (depending on climate R-value requirements), it may cost more to  ll a 2×6 cavity than to  ll the same structure framed with 2 × 4s.  is is not only due to the increased depth of the studs but also to using less framing materials overall, so it will take more insulation material than it would have for the same depth, regardless. In addition, the added two inches of wall thickness will require extension jambs at all of the windows unless drywall returns are used.

In-line Framing: Aligning the floor, wall and roof framing members directly above one another so the loads are transferred directly downward, requiring no additional structural support, can save considerably on structural engineering and framing costs. With in-line framing for improved load stability, double top plates can be eliminated because the load is distributed evenly through the remaining single top plate. Note that studs that are 24 inches on center are placed in direct alignment with floor joists spaced 24 inches on center and directly below roof trusses spaced 24 inches on center.  e structural concept is to align all point loads to carry the weight directly down to the ground.

Headers Sized for Actual Loads: Structural headers are o en over- sized or installed over all window and door openings, regardless of whether or not they are structurally necessary. When the size of the window used is specified in conjunction with in-line framing, headers are not necessary because no studs need to be cut. If walls are not load bearing, no headers are required over window or door openings. Having your structural engineer specify which areas will require headers, as well as the size of each header required, will save both materials and money.

In most cases right-sized headers can be pushed to the outside of the framed wall assembly, allowing for insulation on the inside of each header cavity, which not only improves the overall thermal performance of the wall assembly but also eliminates thermal bridging at the headers. Note that it is possible and now required by code to insulate headers by using foam sheathing as a spacer in place of plywood or oriented strand board (OSB), either between or on one side (preferably the exterior side) of doubled headers.  is technique uses scrap foam sheathing to reduce thermal bridging through the wood header.

Two-stud Corners (California Corner) with Drywall Clips:  is method of corner framing uses only two studs, saving material and providing space for additional insulation in the corner. To attach drywall in a two-stud corner, drywall clips are fitted onto the edges of the drywall before being attached to wood or steel studs.  is eliminates the need for an additional stud in the corner to attach the drywall.

Window and Door Placement: By aligning at least one side of each window and door to an existing wall stud, use of an additional jack stud is not necessary. If the window or door width does not completely  ll the cavity and align with the next stud, you can attach the other side to the next stud with a metal hanger.  is eliminates the need to frame additional studs to support the load transfer around these penetrations in the wall assemblies.

Interior Partition Walls Intersecting with Exterior Walls (T-walls): Traditional framing addresses T-wall intersections by adding studs at each side of the partition solely for the purpose of providing a surface for attaching drywall. Ladder blocking between the exterior studs behind the partition wall uses two-foot scraps of lumber to provide the same supporting structure and allows for much better wall insulation and reducing thermal bridging. You can use scrap wood for ladder blocking, reducing the additional lumber you need to purchase.

 
 
 

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