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This article was originally published in the November/December 1999 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.

 

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Home Energy Magazine Online November/December 1999


Wrap It Up: 
Building Houses
with the Skin on 
the Outside


by Chris Makepeace

Chris Makepeace is the manager of the Walls and Windows Section, Building Sciences Branch of Alberta Infrastructure, Supply, and Services, a government organization in Alberta, Canada.


Sealing the building envelope to eliminate air and moisture seepage can be one of the toughest elements to home construction--but it's among the most important. One simple way to do it is by putting the insulation and the air/vapor barrier on the outside of the house's frame, minimizing penetrations that cause holes and thermal bridging. 

 
 

Figure 1. The plane of air seal and insulation can be both single and continuous if PERSIST is used. The structure of the building is totally interior to these planes, with overhangs built as extensions that do not penetrate the air seal or the insulation. 
Architectural details are installed on the exterior of the thermal/air barrier.
To ensure that that the membrane has been installed continuously, patches of insulation can be cut away (then replaced) before the cladding is installed.

What PERSIST Means

PERSIST stands for Pressure Equalized Rain Screen Insulated Structure Technique. But what does this mean?

The term pressure equalized refers to the equalization of the pressure behind the rain screen cladding with that of the pressure being exerted by wind on the exterior. Because the wind constantly changes, the pressure will not be precisely equalized all the time, but the pressure difference across the rain screen cladding is modulated to minimize water entry. This pressure modulation minimizes the forces that can draw water through even small openings in the cladding.

This term also refers to overpressurizing (or underpressurizing) by mechanical equipment. In cold climates, such overpressurization can result in condensation in the building envelope as interior humidified air is cooled below its dew point. The pressure balancing that occurs behind the rain screen minimizes water entry, while the pressure balancing across the air barrier membrane minimize the potential for condensation in the construction if there is an imperfection in the membrane.

The term rain screen refers to the cladding elements that protect the interior construction from direct solar radiation, wind, and water. Unlike the face seal approach, such as exterior finish insulation systems (EFIs), which makes the cladding the only barrier to outside water penetration and therefore must be 100% perfect, a rain screen cladding will still work if water gets through imperfections. The water is drained from the cavity air space behind the cladding by flashings and designed openings in the cladding. The structure of the building envelope is not subjected to wetting, since the air barrier materials that are used on the exterior structure are actually a roofing membrane, are waterproof, and are fully adhered to the sheathing.

The term insulated structure refers to the practice of placing the insulation on the outside of the structural frame as opposed to insulating on the inside of, or within, the structural frame. An air seal membrane is installed on the exterior sheathing, where it can more easily be continuous. The sheathing provides the support to resist wind, stack effect, and mechanical pressurization loads; it transfers these loads to the structure of the house, while the membrane (which is fully adhered to it) provides the seal. The membrane, the sheathing, and the structure of the building together form the air barrier system. The increased airtightness this creates for the building envelope makes it necessary for mechanical system designers to adjust their building leakage estimates so that overpressurization does not occur.

Figures 2 and 3. At present in Alberta, there are only a few aluminium window extrusions available that allow membranes to be tied directly to the air seal plane of the frame. In lieu of that, the membrane can be returned into the rough opening. 
Most techniques for creating a tight building envelope in residential construction focus on how to create a continuous air barrier on the interior of the building's frame. In cold climates like Canada's, it often makes more sense to put the air barrier on the exterior of the building's frame, where there are fewer structural penetrations, so that thermal bridging of the structure can be minimized. An added advantage of this technique is that, because the building is thoroughly sealed on the exterior and its structure is kept warm throughout, it is not subjected to moisture problems caused by water from exterior sources or by condensation from interior environments. In other words, this technique deals effectively with water from both the outside and the inside.

This unique approach to construction was originally recommended by Canada's National Research Council in the early 1960s. Since then, it has been advocated by building science researchers, investigators of building envelope problems, design professionals, government departments, and materials suppliers. My organization (Alberta Infrastructure, formerly Alberta Public Works) has implemented this approach in most of our construction projects over the last 15 years. Barrie Dennis, our manager of roofing, coined the name Pressure Equalized Rain Screen Insulated Structure Technique (PERSIST) to describe this method for Alberta Government Standards documents. Simply put, with PERSIST, the building construction from interior to exterior consists of: the walls, a waterproof membrane, the insulation, and the exterior cladding (see Figure 1).

Thousands of commercial buildings have been built to date using PERSIST. Although only about a dozen houses have been built this way, the method is gaining acceptance for residential construction--especially where moisture has been a problem. The houses that have been built or retrofitted with PERSIST are mainly large luxury homes whose owners don't mind paying a little extra for added performance. Although there are many other approaches to air sealing the building envelope, I feel that those methods must be done with utmost care or moisture problems may result.

The Advantages of a Thick Skin An important function of buildings is to provide a separation between the interior and exterior environments. If air passes through the wall, it can contribute to moisture problems in the construction assembly, affect indoor air quality, and reduce the energy efficiency of the building. While most simple single-family houses provide adequate separation and control, many larger, architecturally explicit, and multifamily homes have experienced problems with their performance and durability. These problems often arise because of hidden discontinuities in the building envelope's air and thermal barriers.

Over 15 years of building investigations, we have found that the problems that occur with building envelopes are usually caused by one of the following:




  • Details for penetrations through, and connections with, walls and roofs that are often inadequate. Often these details are left to the contractors, who must develop on-site solutions or copy what was done before without considering how the present situation may differ. For example, window manufacturers' details often describe only how their product fastens to a typical wall; they don't consider different wall constructions.
  • Designs that often ignore the constructability of the air barrier system. What impact do structure and aesthetics have on the constructability of the building as a whole?
  • Construction assemblies that do not allow for drainage and drying. Single-family residential wood frame, batt-insulated homes usually have a great capacity to take on water, hold it for a short time, and then dry out under more favorable conditions. Degradation occurs when more water enters the walls and roofs than can be safely held. A concern that is taken more seriously now than in past years is the potential for mold and mildew. Air movement through the construction assembly can carry these contaminants into the interior building environment, potentially affecting human health.
  • Materials are used that do not withstand the loads imposed by stack effect, mechanical pressurization, and wind. Many products claim to be air barriers, and in small tests they may seem to be effective. Once installed in real construction, however, these materials may not be buildable, may not be continuous, or may not be able to withstand the rigors of repeated positive and negative loads over their service life.
We have found that the PERSIST method avoids these problems because of its very simplified approach. The Technique PERSIST can accommodate demanding, high-humidity interior environments (such as indoor swimming pools, greenhouses, and saunas) while at the same time allowing the flexibility that is necessary for a variety of aesthetic design approaches. The technique is based on a sequence of steps in the design and construction of the building envelope that differs only slightly from common practices.

When considering a PERSIST assembly, it is helpful to think of the envelope as a series of planes. The structural framework and infill (nonstructural) walls are constructed to create a single plane from foundation to wall and from wall to roof on the exterior. A wide variety of structural and infill systems can be used. The structure can be exposed to the interior, as in cathedral ceilings, without special airtightness detail changes. Services can also be installed in these structural walls without special details.

A membrane is fully adhered to the exterior plane of the structural framework. Penetrations of this plane that are required to support the exterior claddings are designed to minimize thermal bridging, endure occasional wetting, provide for construction tolerances, and allow the membrane to easily achieve an air seal. The membrane acts to seal the structure and infill walls to fulfill the air barrier function of the building envelope. It also fulfills the water barrier (drainage plane) function. Note that this system is designed to withstand the imposed loads of stack effect, mechanical pressurization, and wind.

The insulation is mechanically fastened tightly to the exterior of the air barrier system (the membrane). This prevents air from circulating behind the insulation, which can reduce its effectiveness, and ensures that the membrane is kept warm.

The exterior cladding is installed on the exterior of the insulation, creating an air space. Pressure equalization of the cavity is promoted through protected openings designed into the cladding. Openings at the bottom of the cladding must allow water that passes through the exterior cladding to drain to the exterior.

For windows, glazing systems should be selected and positioned for similar performance. At present, most residential windows available in Canada do not provide for continuity of the air barrier and water drainage systems. The membrane is therefore wrapped into the window rough opening to protect the structure from water entry. The window should be placed into the structure of the wall, where it can be sealed to the membrane on its interior face. The cavity created between the frame and the membrane should be protected from water entry and drained at the bottom of the exterior (see Figures 2 and 3).

The use of nailing flanges around window edges should be avoided, since they often act as a reverse flashing at the head, funneling water into the wall structure and interior. A low-expansion foam bead can be installed at the head and jambs, between the outer edge of the frame and the membrane, to divert water from entering between the frame and the membrane in the rough opening. An interior-seal bead of foam is installed on the interior on the head, jambs, and sill to act as the air seal. If water should enter the area between the frame and the rough opening, it can be drained to the cavity at the sill. Large interior sills should be avoided in colder climates, as these can restrict warm air movement at the lower junction of frame and glass, causing condensation or ice to form there.

Benefits of PERSIST The PERSIST system has significant construction and performance advantages--advantages that can provide long-term cost benefits:








  • Electrical and mechanical services for the interior can be installed in exterior walls without special construction techniques for air sealing.
  • No special detailing is necessary for future interior renovations to ensure airtightness of the drywall.
  • The structure of the building is interior of the plane of insulation. This minimizes thermal movement, as well as thermal bridging of the structure.
  • The sequential approach of material installation promotes good construction detailing, on-site construction, and inspection.
  • If small imperfections do occur within the plane of the air seal, any resulting condensation (in cold climates) will occur in the drained cavity between the cladding and the backup wall. (That is, moisture-tolerant materials are the only materials outside the air barrier.)
  • If warm, humid, exterior air contacts a membrane that is cooled by interior conditioned environments, the resulting condensation will be contained in the cavity exterior of the structure and drained. If a significant air leakage problem occurs, in either warm or cold climates, it will be isolated and can be identified either visually or through thermography or other test methods for repair. In other wall systems, moisture problems in walls are hard to spot because the air seal plane of the wall is hidden.
  • Although exterior caulk must still be maintained to minimize mass water entry in the wall cavity, maintenance is now mostly a visual concern. It is no longer key to providing separation between the inside and the outside environments, as it would be in a face seal design.
  • The air seal is protected from UV degradation and thermal cycling.
  • Some additional insulation may, under some conditions, be installed interior of the membrane within a stud wall infill system in cold climates. This should be done cautiously, and only in buildings with low interior humidity. In conditions of high interior humidity and cold exterior environment, condensation could occur within the wall structure.
  • When retrofitting existing buildings or building enclosures with PERSIST, the building can remain occupied during most of the implementation.
Membrane Details Since the early 1980s, when the use of membranes adhered to the backup wall was introduced in Alberta by J.C. Perreault, this method has gained acceptance in other parts of Canada and the northern United States. The membrane can be either self adhered (SA) or thermally fused (TF) to the backup wall. Both types of membrane come in rolls 3 feet (1-meter) wide, and of varying length.

The self-adhered membranes have a release paper on the sticky side. The SA membranes that are used most commonly in Alberta have polyethylene as the carrier and reinforcement for the membrane (styrene butadiene styrene, or SBS), which is usually about 50 mils (1.2 mm) thick. The release paper is pulled from the membrane, and the membrane is adhered to the primed sheathing. The polyethylene surface is then rolled with a laminate roller to ensure adhesion and to eliminate air bubbles. The polyethylene can more easily be damaged by subsequent subcontractors, and voids can be created, if it is not carefully applied. Once it is placed on the primed surface, it usually cannot be removed. Joints should be lapped in shingle fashion to allow water to flow over them. The amount of lap typically is specified by the manufacturers.

Some manufacturers have additional compatible sealants that they recommend for some joint applications. With adhesion on only one side of the membrane, it is important to simplify the planes to which the membrane must adhere. SA membranes are recommended for retrofit projects in which the building is occupied or where backup wall construction may be prone to ignition if exposed to a flame. Temperatures during installation below -5°C may restrict the use of SA membranes in some projects.

Thermally fused membranes have either fiberglass or polyester reinforcement with SBS on both surfaces. Various release materials are used to keep the surfaces from adhering while they are in the roll. TF membranes are applied by melting off or embedding these release materials so that the SBS can be melted to a point where a pool of liquid SBS is created at the surface as the roll is being applied. If an open-flame torch is used, the designer and contractors must carefully review the potential for fire. When a torch is used around wood and wood dust that could smolder, a fire watch should always be taken after every day's work.

Joints are lapped and then heated to fuse the SBS of both sheets. A few manufacturers have formulated an SBS that can be installed as a peel-and-stick; some heating can be done to fuse joints and/or promote adhesion without damaging the reinforcing of the membrane. Most PERSIST house construction uses the peel-and-stick method with no problems, but it should be noted that the torch method bonds the sheets together more thoroughly.

Design and Performance PERSIST is by no means a panacea for all the ills of home construction today, but it does have distinct advantages in terms of design flexibility, constructability, and building performance. It costs somewhat more, because it is not yet standard practice. For an above-normal home costing $125,000 in Edmonton, the premium would be in the neighborhood of $3,500.

It is important for builders who use this method to take a whole-house approach. For example, they must recognize the need to carefully size all ductwork according to the operations of the appliance, because they cannot rely on leakage to balance air pressures. Furthermore, humidity levels in PERSIST houses (in cold climates) can be quite high because they are so airtight. Condensation can occur on windows if the windows are of poorer quality. Forced-air gas fired furnaces in heating systems in PERSIST homes should not use humidifiers and should include a two-stage fan on the circulation motor so that more air flow is generated when the furnace comes on. Circulating air across the windows (this is where supply grilles are always placed in Canada) will prevent condensation.

If these guidelines are followed, builders and renovators who use the PERSIST method will obtain excellent results, not only in the performance of the buildings, but also during the construction phase. For example, contractors who have worked with the PERSIST method have commented on how helpful it is that, once the membrane is installed, the building is waterproof and interior trades can commence their work. Innovation in installation during wall fabrication could further speed up construction and reduce costs.

Cladding systems can be more flexible, since they function mostly to shed water; much like shingles on a sloped roof. New lightweight claddings could reduce costs. Architectural fenestration around the functional building envelope can be more varied, since it does not have to conform to the plane of air seal. Indeed, the design freedom afforded by this approach is just now being recognized.
 
 
 

 


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