This article was originally published in the March/April 1999 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online March/April 1999
|Insulation is often installed over the top of a dropped ceiling. This results in thermal by-pass because the thermal barrier is not in full contact with the sheetrock air barrier.|
|Furred walls should be blocked and sealed at both the top and bottom of the wall to reduce convection in these small cavities.|
|Bay windows often have their floor open to the exterior and joined to the second story floor system.|
|Typical kitchen soffit open to assemblies above and to an exterior wall. Each location should have had plywood or oriented strand board (OSB) in place before the soffit was framed.|
|Kneewalls that do not have top plates often contribute to moisture-related roof failures.|
|Figure 2. Covering a hole WRT the attic is an easier way to complete a 3C barrier than trying to follow the surface of a dropped ceiling.|
|Figure 3. Following the surface of a tray ceiling is the best way to install a 3C barrier around this kind of hole WRT the living space.|
|Bringing fireplaces inside the 3C barrier results in safer operation because cold chimneys don't have to be heated.|
To emphasize the importance of maintaining the integrity of this joint thermal/air barrier, we at Advanced Energy have developed what we hope is a memorable name for this concept: the Continuous Contiguous Complete--or 3C--barrier.
What exactly do we mean by the 3C barrier?
- By continuous, we mean that there are no breaks in the air and thermal barriers.
- By contiguous, we mean that all components of the two barriers must be in physical contact with each other.
- By complete, we mean that the barriers should be installed together in such a way that they completely contain the living environment of the house.
Although the terms thermal barrier and air barrier may be new to some builders, the concepts probably aren't. The thermal barrier is nothing more than the insulation that is normally installed in every house. What we are calling the air barrier some builders know as house wrap, sheathing, foam, caulk, and gaskets. The critical difference between insulation and sheathing put up by separate contractors and a correctly installed 3C barrier is that the 3C barrier is a team process. The designer must blueprint the 3C barrier, the builder must enforce it, and the framer must construct it.Designers, Builders, and Framers--All Have a Role It takes teamwork to make sure that the 3C barrier correctly follows the planes of the walls, ceilings, and floors--but it's worth the effort. An improperly designed or installed 3C barrier can dramatically affect the durability of the building, the comfort of the occupants, and the amount of energy required to heat and cool the home (see The R-Value Killers). Here's how each team member's roles should be played out.
The designer's job is to specifically and carefully blueprint the details of the 3C barrier, preferably on the elevation and floor plan pages of the house plan. Details should include the location of the 3C barrier, the materials to be used, and--most important--how they should be joined at breaks in the building planes (see illustrations).
The designer must be very specific in giving the framer details so that the barrier can be installed properly, even in tricky breaks in the plane, such as dropped ceilings. The more complex the planes are, the harder it is to produce a high performance house, especially if the 3C barrier is not drawn into the plans. Following an irregular plane is difficult at best. If the plans don't show where the barrier is located, the confusion about where to install the barrier doesn't stop at the framer. All the other trades that follow the rough framing stage will also be confused.
The builder's job is to ensure that the plan is followed. This means that the materials specified must be available, and that the framer must know the plan and know how to achieve it. To do a good job, the framer must resolutely follow the plan. All the other trades should look to the elevation and floor plan pages of the house plans and then to the framer-created planes. They should be trained that whenever they break a plane by wiring, plumbing, venting, piping, or installing ductwork, they are responsible for sealing and reestablishing that plane.Irregularities in the Building Planes The 3C barrier would be quite easy to install in a home in which all three building planes--floor, walls, and ceiling--were flat, even, and unbroken. Designing and installing the 3C barrier for a boxy, ranch-style home, for instance, would be fairly simple. But this is the real world. So we end up framing--and insulating--houses where the three planes are anything but flat, even, and unbroken. Technically, breaks in the building planes are called intrusions and protrusions. At Advanced Energy, we call them holes and bumps.
Whether a break is a protrusion or an intrusion depends primarily on one's point of view. An intrusion into the living space is a protrusion on the other side of the surface, and vice versa (see Figure 1). The term with reference to, or WRT, is helpful when discussing the question of perspective. A dropped soffit would be a protrusion WRT the attic, but an intrusion WRT the kitchen. A bay window would be a protrusion WRT the living space, but an intrusion WRT the outdoors. Most intrusions and protrusions occur in the ceiling and wall planes. Floor systems are not as likely to have bumps and holes because people don't like stumbling over or into them.3C Treatment of Bumps and Holes Why is WRT critical? Because it's important to recognize that what appears as a bump on the ceiling to anyone walking through the living space will appear as a hole to the insulation installer in the attic. And, of the two, holes are easier to cover than bumps.
For example, suppose that the living room has a dropped ceiling. WRT the attic, this appears as a hole in the attic. Rather than having the 3C barrier follow the surface of this hole, it is better to simply cover the hole with a rigid material specified by the designer. This material--plywood, for example--becomes the air barrier, and the attic insulation can now be installed as if the hole did not exist (see Figure 2).
It's a more difficult task to treat a bump. For example, suppose that the living room has a tray ceiling. Now the 3C barrier must follow the surface of the bump. Theoretically, the entire ceiling could be raised to create an even, unbroken ceiling plane. The problem with this approach is that in most cases the higher plane would intersect the roof of the building in such a way that many ft2 of kneewalls would need to be built--not a good approach from either a building science or an economic perspective. The better solution is simply to install the 3C barrier by following the surface of the plane (see Figure 3).The Devil Is in the Details Framing details that are often not recognized as breaks in the planes include strapped ceilings, furred walls, and kneewalls nailed to rafters without top plates. When a ceiling between conditioned and unconditioned space is strapped with 1 x 2s or 1 x 4s, care should be taken to ensure that the end of the strapping is also air sealed from unconditioned space. When masonry and wood walls are furred for the installation of gypsum, plumbing, wiring, or realignment of framing, a horizontal strip of wood should be nailed to the top and bottom of the wall if the furring is open to unconditioned space. Kneewalls should never be nailed to the rafters just to save on the lumber used for top plates. Top plates should be used in all cases. These details may seem small, but failure to pay attention to them can mean big losses in heat, moisture vapor flow--and comfort.
The detailing where the 3C barrier changes directions--the corners--pose another challenge. We know from years of experience that even in the wall plane, where corners are commonplace, framing is seldom done so that the 3C barrier can be properly installed. (See Beauty and the Beast Upstairs, HE Mar/Apr '95, p. 27 for some advanced framing techniques.)
We can not emphasize enough how critical it is to pay attention to this type of framing detail and to every step involved in the proper installation of the 3C barrier. Without a fully functioning 3C barrier, a house may be a home, but it won't be a high-performance home.
3C Barriers Part 2
|Figure 1. An intrusion into the living space is a protrusion with reference to (WRT) the attic, and vice versa.|
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