The High-
Performance House--

The R-Value Killers Without proper installation of the 3C barrier, misalignment, gaps, voids, and thermal bypasses will plague a home. Advanced Energy Building Science engineers call these barrier breaks the R-value killers. Here's how the R-value killers eat away at a home's comfort. Misalignment Misalignment occurs when insulation and the other components of the 3C barrier do not touch. For instance, if batt insulation is rolled over a typical dropped soffit, a space is created between the air barrier (the surface of the soffit) and the insulation. As the air in this space is heated, it will rise through the insulation into the attic. Due to this misalignment of the thermal and air barriers, the insulation is no longer insulation; it is now a high-efficiency filter. This phenomenon will occur even if the space between the thermal and air barriers is 1 inch or less. Other typical areas of misalignment are plumbing chases (wet walls), chimney framing, and floor systems that lack blocking and air sealing. Gaps and Voids Gaps and voids are areas where insulation should be installed but isn't. This causes conductive and convective heat loss. A gap is a place where the insulation doesn't come all the way to the edge of the space to be insulated. A void is a hole in the envelope of the building. Examples include plumbing chases, wiring penetrations, fireplace cavities, dropped ceilings, soffits, and venting. Thermal bypass Thermal bypass is a catchall name for any situation where air or heat can flow through or around the insulation. Misalignment, gaps, and voids are examples of thermal bypasses.

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

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

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

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

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.

Figure 1. An intrusion into the living space is a protrusion with reference to (WRT) the attic, and vice versa.

Front elevation. Front elevation with 3C barrier drawn in. Rear elevation. Rear elevation with 3C barrier drawn in. Left side elevation. Left side elevation with 3C barrier drawn in. Right side elevation. Right side elevation with 3C barrier drawn in. House plans should clarify where the air, thermal, and moisture barriers (3C barrier) are to be installed by each building trade. The location of the proper installation of the 3C barrier must be clearly defined on the floor plan and elevation pages of the house plans so that complex details such as chaseways, stairwells, and dropped ceilings can be easily identified as being inside or outside the boundaries of the conditioned space. For each elevation view, a duplicate maps the 3C barrier location. The framer can follow the wall, ceiling, and floor plane as drawn when encountering dropped ceilings, shafts, chaseways, and other features. For example, when the ceiling plane is drawn unbroken and the framer is constructing a dropped ceiling, he would cap the top with sheathing to frame the ceiling as drawn "unbroken." The dropped ceiling would then be inside the conditioned space, as planned. First story floor plan. First story floor plan with 3C barrier drawn in. Second story floor plan. Second story floor plan with 3C barrier drawn in. Bold lines drawn on a small duplicated image of the original floor plan should be used to define the separation of conditioned space from unconditioned space.

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