While ASHRAE 90.1 has been pushing continuous insulation (CI) for the past decade, the building codes are catching on. And now that the U.S. Department of Energy (DoE) has mandated all states to adopt a commercial building energy code that meets or exceeds ASHRAE Standard 90.1-2010, CI specifications are really being cast into the spotlight.

Defined as “insulation that is continuous across all structural members without thermal bridges other than fasteners and service openings,” CI is a proven strategy for boosting building enclosure thermal performance and durability, which is why it is morphing from a best practice to a full-fledged requirement in building codes like the 2013 International Energy Conservation Code.

“The code officials are attempting to keep up with the best of building science, which has demonstrated that installed insulation between studs frequently doesn’t perform at anything close to the R-value listed for the insulation,” explains David W. Altenhofen, AIA, The Façade Group, Philadelphia. “Eliminating thermal short-circuits with continuous insulation is necessary to help reduce the energy consumption of our buildings.”

In fact, continuous insulation has been documented to be as much as twice more effective than cavity insulation per R of the same material. Or stated in the inverse, thermal bridges can be responsible for significantly compromising the building envelope’s thermal performance.

For example, in referencing Table A9.2B Effective Insulation/Framing Layer R-Values for Wall Insulation Installed between Steel Framing, which is found in ASHRAE 90.1-2010, one will see that by filling the cavity between the metal studs spaced 16 inches on center with R-19 insulation, this will result in an effective insulation value of just R-7.1, thereby reducing the insulation value by 62 percent.

Not only does heat escape through these conductive thermal bridges, but cold spots can also form on the interior. “If these cold spots are at or below the dewpoint temperature of the interior environment, latent water vapor in the air may condense and cause water damage,” explains Michael Harrison, AIA, architect, technical advisory group, Shepley Bulfinch, Boston.

Of course, thermal bridging doesn’t only occur when metal studs penetrate through the insulation. Rather, any penetration such as mechanical fasteners, floor support beams, cladding fastening systems, balconies or overhangs will create thermal bridging as well.

Noting some additional highly conductive “frying pans” which compromise the insulation, Jeff Diqui, architectural engineer, building enclosure specialist, Sto Corp., Chicago, lists:

• Steel shelf angles in contact with the structural frame with no stand-offs
• Structurally mounted steel window/door lintels as opposed to loosely laid lintels
• Exposed concrete floor slabs
• Parapet wall details
• Fenestration thermal break not aligned with the thermal control layer in the opaque wall
• Major structural steel penetrations that may support canopies

“You can keep piling in more insulation to overcome a thermal bridge, such as a shelf angle that one did not want to design in stand-offs, to allow for true continuous insulation. But the reality is all the heat transfer is still funneling through the shelf angle,” he says.

In terms of effectively dealing with thermal bridges, Maria Spinu, Ph.D., LEED AP, building science and sustainability leader, DuPont Building Innovations, Wilmington, Del., points out there are only a few ready-made solutions such as thermally broken window frames. So, in many cases, thermal bridging requires customized solutions and highly skilled of detailing and workmanship.

To get started, Spinu’s colleague, Ben Meyer, RA, LEED AP, a building science architect with DuPont, defines the first step as recognizing exactly where the thermal bridges are occurring and then developing a strategy for minimizing loss in those areas. “When specifying these connections, the shim or ‘fill’ materials and responsibilities among contractors at the transitions can be very important.”

One approach, says John Broniek P.Eng, senior engineer, Icynene, Pittsburgh, is covering the thermal bridge with a sufficient amount of continuous insulation— i.e., R-5 to R-10—on the exterior side to prevent the bridge from transferring heat into or out of the building.

However, the insulation must be added at precisely the right place. “Simply adding additional insulation in the opaque wall to compensate for these ‘frying pans is not an effective method as the heat transfer is smart enough to find the path of least resistance; you guessed it the infamous thermal bridge,” says Diqui.

In addition, Harrison points out that installing additional insulation above the prescriptive code limits, which would mean exceeding 4 inches on the vertical surfaces, is not practical and will reach a point where the cost of the materials outweighs the potential energy savings, resulting in a diminishing return on investment.

Getting It Right

When it comes to attaching cladding materials, Harrison’s colleague Maria Mulligan, AIA, LEED AP, architect, technical advisory group, Shepley Bulfinch, recommends intermittent clips, as opposed to continuous girts, or clips made of non-conductive materials to preserve the insulation R-value.

However, when specifiers are dealing with something more complex, such as the relieving angles commonly used to support brick veneers, this can be tricky.

“One solution for is to move the back vertical leg out, so it is no longer up against the backup wall, leaving space for the insulation to continue behind it. The angles are then supported by knife plates or back-to-back clip angles or brackets approximately 4 feet on center,” suggests Richard Keleher, AIA, CSI, LEED AP, senior architect, The Thompson & Lichtner Company, Canton, Mass.

“Some structural engineers will balk at this,” he admits, “but my experience is that most eventually agree to this design. For even better performance, there are proprietary—and expensive—systems for lessening the thermal bridge of the plates/clip angles/brackets.”

For specifiers seeking other ideas for dealing with this particular challenge, as well as other steel frame thermal breaks, Altenhofen recommends “Thermal Bridging Solutions: Minimizing Structural Steel’s Impact on Building Envelope Energy Transfer,” published by the Structural Engineering Institute and American Institute of Steel Construction’s Thermal Steel Bridging Task Committee.

As for balconies, they are a popular amenity in mid-rise residential and hospitality projects. However, Altenhofen categories them as big “radiator fins” as they are typically built as a continuation of the interior structural slab. While the market offers proprietary devices that create a thermal break while maintaining structural integrity, these products do come with a price tag.

Another detail to look out for is building drawings which propose exposing the edge of floor slabs in mid-rise projects. “This practice is not only bad for thermal bridging, but it also has a tendency to leak water and air,” says Altenhofen. “It’s better to locate the cavity face of the back-up wall flush with the edge of slab and then continue insulation past.”

Offering some additional design tips, Altenhofen references a DOE specification document, “TO2 7.2.3 External Insulation of Masonry Walls & Wood Framed Walls,” with methods for securing furring over insulation using long screws to minimize thermal bridging. Note that this approach will not fully eliminate bridging, nor is it ideal for air and water sealing, he says.

In the case of metal girts extending through the continuous insulation, Altenhofen recommends installing the girts perpendicular to the stud framing. “Even better is to use clips spaced about 4 feet apart vertically to extend through the continuous insulation and then support the girts for the cladding.”

While Keleher also recommends this approach, he cautions that installers may protest due to the fact that they can’t install the girts snugly against the back-up wall for better stiffness. To address this, the clips need to be specified at a heavier gauge and will therefore be more expensive.

Technology Solutions

With CI in mind at a project’s outset, there are a variety of products and systems—each with its own set of pros and cons—which can help specifiers work toward a continuously insulated building enclosure.

For instance, panelized or pre-engineering enclosures offer a high level of quality control as they are fabricated in a factory controlled environment. However, they are not immune to thermal bridging and generally require a layer of CI on the exterior. In addition, the panel-to-panel joints must be properly sealed and it can be a challenge to install a proper redundant two-stage defense that is vented, drained and weeped to control air and water, says Altenhofen.

Altenhofen is also concerned about the long-term longevity of taping insulation layers together for air, water and thermal control. In addition, he points out that including pre-engineered enclosure assemblies in construction documents can be tricky due to the fact that they are proprietary systems, and therefore difficult to solicit comparative bids.

“We have done a few projects where the back-up wall, air barrier, girts and insulation gets panelized, but not the final cladding,” he explains. “That way once the panels are erected, the air barrier joint can be properly covered with membrane and the CI patched in.”

While generally a more expensive solution, due to the added protection, shipping and set-up costs, Bob Dazel, AIA, LEED GA, marketing manager, Dryvit Systems, Monroe, Mich., sees panelized walls as ideal for building sites with challenging site access, repetitive module designs or projects with a very short construction period.

Another key strategy on the road to CI is using spray polyurethane foam (SPF). Essentially, the product is spray applied into the wall cavity, expands and dries – without any joints or fasteners – thereby delivering a solid thermal barrier. SPF is also a great insulating solution for oddly-shaped and difficult to access spaces.

Particularly suited for cement-based or masonry materials, SPF will easily adhere to these rough surfaces while it is curing, and ultimately create a durable, continuous bond.

At the same time, the use of SPF does add a few complexities to the project. For starters, because it is a combustible material, building codes require that it be separated from the interior environment by a protective layer of non-combustible layer, like gypsum board, or coated with an intumescent-type paint, according to Altenhofen. In addition, the insulation layer’s thickness should be studied in a WUFI model in order to determine the most appropriate permeability of the foam to allow the masonry wall to breathe and not trap liquid water that could freeze within the masonry layers.

In fact, it is not uncommon for a building team to inadvertently install a vapor barrier, which would be problematic in certain climates, potentially leading to the growth of mold and mildew in the stud cavity and ultimately compromise indoor air quality and cause damage to the building.

Harrison also points out that spray foam formulations are constantly changing as a result of increasingly stringent code requirements restricting chlorofluorocarbons and hydrochlorofluorocarbons. As a result, some products have different properties which affect compatibility with other enclosure materials. “For example, great care must be taken when SPF foam is sprayed onto membrane air barriers or membrane flashings. Based on the formulation, curing the foam can generate a high temperature that can degrade surrounding materials,” he cautions.

Something which concerns Altenhofen about SPF products is their ability to adhere to metal wall studs or cavity girts. Because girts are typically made from galvanized steel, they are covered with residual oil from the roll-forming process. While SPF manufacturers specify that a solvent wipe must be applied to remove the oil prior to SPF application, Altenhofen is concerned that installers are not always following these instructions and therefore, the insulation’s ability to remain watertight will be compromised.

“While SPF is gaining popularity, we are less likely to want to use it as the air barrier and water-resistant barrier on institutional projects with a long design life or on projects with high wind loading, such as a high rise, and near the coast,” he says.

Furthermore, the application of SPF requires a highly skilled installer who is knowledgeable about how weather and field conditions could potentially affect the spraying and curing process, and ultimately SPF’s long-term performance.

“Applying SPF is essentially doing advanced chemistry out in a dirty, dusty, too hot or too cold, too wet or too dry job site. There are applicators able do this, but it is not everyone out there with a spray rig,” he cautions.

One alternative is rigid insulation. For example, extruded polystyrene offers a good thermal barrier, as long as the joints are tight or filled with sealant, and doesn’t come along with any cupping or curling problems. It also offers a good vapor barrier and compared to SPF, absorbs just a tenth of the water.

At the same time, more onsite labor required, particularly if the boards need to be cut and taped, says Broniek. And unlike SPF, foam boards will require a separate air barrier within the assembly. However, if the rigid insulation is installed outboard of the support frame, it can meet CI requirements as long as only fasteners penetrate the insulation thickness.

Because both SPF and foam boards do come along with the issue of fire resistance and meeting NFPA 285 requirements (see sidebar, “The Buzz Around NFPA 285,” for more information), non-flammable mineral wool is a common alternative although it only offers two-thirds of the insulating R-value per inch.

Inside vs. Outside

While circumstances will vary from project to project, installing the thermal insulation layer on the exterior, outboard of the structural support frame and air/vapor barrier, for new construction is the generally accepted best practice. In addition to protecting the structure from corrosion and leveraging the wall’s thermal mass to buffer interior conditions, detailing the continuous insulation is a less arduous task, as compared to an interior installation.

“This is because at the exterior location it is often, easier because of accessibility, to fully cover the entire enclosure surface particularly at floor edges,” explains Broniek.

In addition, an exterior installation gets around the issue of water vapor condensation which will happen under cold weather conditions where there is a significant temperature difference between the building enclosure exterior and interior, in cases where the insulation is placed on the interior.

Either way, building teams must carefully consider where to place penetrations for things like ties and supports to ensure that air and thermal continuity is maintained. Experts also advise selecting an insulation material based upon its ability to perform when coming into contact with rainwater in the masonry cavity walls or open joint rainscreens.

Another issue with the exterior installation is attaching the exterior cladding over the CI, which must connect through to the structural frame. “In most cases, the cladding will have to be limited to one inch of thickness because most fasteners cannot support the wind load requirements beyond one inch,” says Lucas Hamilton, manager, building science applications, CertainTeed, Philadelphia.

When dealing with heavier cladding, such as brick on cavity walls, it becomes even more challenging to place the cladding even further away from the slab as a significant amount of steel will be required to achieve this.

“Then there are logistic issues like incorporating the fenestrations or openings when the brick is 3 inches further away from the interior drywall,” he explains. “The issues just keep adding up when you try to add more insulation to the exterior. For this reason, we often stop at about inch of insulation on the exterior.”

With regards to placing the insulation on the interior, this is commonly done for retrofits, particularly historical restorations, as well as mass, concrete and CMU construction for buildings such as large warehouses where the exterior aesthetic is not as important. Generally speaking, in the latter case, an interior insulation installation is relatively straightforward.

However, if a foam plastic is selected, then the codes require the CI to be separated from the occupied interior space by a 15-minute thermal barrier. “Now, that is all fine, but traditionally, the 15 minute thermal barrier – i.e. drywall – is fastened to the mass wall with metal Z-furring or studs with the CI placed in between, but the metal stud acts as a thermal bridge,” explains Dazel. “This cannot be done in accordance with the energy code any longer, which now makes integrating CI into a mass wall quite challenging.”

Regardless of where the CI is placed, perhaps one of the biggest challenges is detailing the enclosure so that the insulation is, in fact, continuous with no breaks, openings or penetrations.

As a basic test, Sto Corp.’s Diqui recommends taking out a red pen and tracing the thermal control layer around the wall sections and details on the design documents. “If you have to lift up your pen, you have to go back to the drawing board.”

In terms of avoiding those pesky thermal breaks, Diqui reviews some of the previously mentioned suggestions such as utilizing stand-offs on shelf angles, loose laid lintels, new technologies to integrate thermal breaks at concrete balcony connections, avoiding exposed concrete floor slab conditions, new materials and technologies for cladding attachments, carefully designed parapet wall details, aligning the fenestration thermal control layer with the opaque wall thermal control and avoiding z-furring that interrupts the CI.

“We like to incorporate a mechanical device to hold down the insulation instead of relying solely on adhesive,” suggests Altenhofen. “Brick ties can be selected with an insulation retention washer and similar features can be incorporated into the clips that support cladding substructure.”

While thermal continuity is one thing, experts are quick to point out that ensuring the continuity of the air and water barriers is a completely different animal. Built in to thermal insulation materials is a certain tolerance for thermal expansion and contraction, and will not compromise thermal performance. On the other hand, this will impact air and water barrier continuity, which is why many advise not fully relying on the CI to provide thermal, air and water control. In addition, says DuPont’s Spinu, detailing the air and water barriers must be performed within the air/water barrier plane, and not on the insulation plane.

Offering some design and installation advice in this realm, Shepley Bulfinch’s Harrison recommends extensive modeling to determine ideal insulation thickness, permeability of the vapor retarder and locations within the assembly to allow maximum drying of the enclosure. In addition, flashings should be provided at all of the opening sill and head conditions, and at the bottom of the wall, at grade, so that penetration water will be drained out.

Prior to installation, Harrison is a big fan of building a complete mock-up of the exterior wall system including the roof edge, foundation and window opening conditions. The mock-up, as well as the final building construction, should then be tested for air and water infiltration.

Not understating the importance of building envelope commissioning and testing, Dazel anticipates that LEED v4 and the IECC will propel these issues to unprecedented prominence.

“It is one thing for the model building codes to say it, it is another for the architects to know it, and even another for the building inspection departments to understand and implement it,” he says.

But in this new era of energy efficiency and high performance buildings which are required, post-occupancy, to actually perform as designed, building enclosure designs and installations will be held to a much higher level of accountability.

“We have now reached a time where every square inch of that building envelope will need to be coordinated for material compatibility, proper installation and then tested for performance. This will require substantial coordination and likely increase the amount of required building envelope specialist or consultant,” says Dazel.

As such, architects will have to be more specific in their specifications with regards to material choice and compatibility, and proper detailing at all envelope transitions, he says. In addition, general contractors will have to implement increased supervision and coordination.

A Performance-Based Approach

While CI is one way to boost thermal performance and meet code requirements, it’s not the only way. Alternatively, specifiers can also choose the performance-based route which can include other creative design solutions to reduce thermal bridging, or trading off CI for other energy improvement strategies, notes Spinu.

To meet code, designers must determine the U-value of the total assembly. As opposed to simply reporting the continuous insulation’s R-value, with this performance-based approach, all of the wall components – such as sheathing, drywall and framing – are all figured into the U-value analysis.

One common application where this U-value compliance path is embraced is for healthcare and other institutional projects where high interior humidity is a factor. In an effort to keep the studs warm and dray, the U-factor approach allows for placing all insulation outside of the sheathing with no batts between the studs, relates Altenhofen.

Designers may also take this approach when the prescriptive configuration for a particular climate and application is undesirable. For example, in the DOE’s Climate Zone 5 – i.e., Boston – ASHRAE’s prescriptive requirement is for the thermal insulation in a metal framed wall is to provide a CI of R-7.5, in addition to insulation in the stud cavity of R-13.5.

However, Shepley Bulfinch’s Mulligan points out a number of disadvantages with splitting the insulation between the exterior and interior such as where to place the vapor retarder. “It is advisable to consider a vapor-permeable retarder between insulation layers, in this case, to promote drying to both sides of the wall.”

But even so, Mulligan cautions that within the interior insulation, the dew point could reach a point where water condensation corrodes the stud framing.

Consequently, to avoid these issues, Shepley Bulfinch would select the U-value alternative and install a thicker layer of insulation on the exterior.

One point to be aware of is that the energy code’s prescriptive R-vale and performance U-factor values are actually equivalent in many climate zones, while it’s the path to compliance that varies.

For example, the prescriptive requirement in a typical 4-inch metal framed wall in climate zone 4 is R-13 in the cavity + R-7.5 CI. “ASHRAE will take the perceived rated value of the cavity insulation of R-13 down to R-6, while there is no applied reduction for CI, so what you really have is an effective total R-value of 13.5.”

To convert the R-value to U-factor, 1 is divided by 13.5, which equals 0.074. But looking in the code, one will find a performance-based requirement of 0.064 for the U-factor.

“Because the performance U-factor approach also lets you add in the other building components, this typical wall will actually offer a total R-value of about 15.5. When converted to U-factor, it is now 0.064 – exactly what the code calls for and exactly the same prescriptive or performance,” states Dazel.

At the same time, Altenhofen points out that the U-factor compliance method assumes that an analysis of the entire wall – taking into account thermal bridges – has been performed, particular in cases where CI is not used.

“Unfortunately, we see that sometimes the U-factor method does not account for the loss through metal studs or other short-circuits and incorrectly adds up the U-factors of the layers of the wall assembly,” he notes. “The U-factor compliance path is much more challenging because it takes a good bit more effort to calculate the average U-factor, compared to just selecting the insulation with the proper R-value.”

It should also be noted that bypassing CI is usually a more expensive endeavor. For example, applying a thicker layer of insulation will make the walls thicker and may require additional structure to support the added weight and size.

The Role of Energy Modeling

As previously alluded to, energy modeling of the building enclosure is playing an increasingly important role in helping proposed designs verify predicted performance and code compliance.

Whether it’s free programs put out by the DOE – such as RESCheck and COMCheck – or more sophisticated software – such as eQuest, Trane Trace 700 or Carrier HAP – there is quite a variety of tools available to building teams. Some designers are also using programs like WUFI, IES Virtual Environment or THERM to help provide input parameters for the engineers running the energy models.

On the other end, architects usually request that the energy modeling data be provided in a timely manner so that the building design can be adjusted accordingly during the project’s early phases.  

“Using these parametric tools, usually in some combination, we can predict the performance of the building enclosure components closer to the eventual real world performance,” reports Altenhofen. “Of course, many of these tools are relatively new and we are constantly working to validate their output.”

In fact, a number of folks are rather critical of energy modeling programs and are quick to point out that many don’t currently include all available energy-efficient strategies for minimizing the building envelope’s thermal loads.

“For example, air leakage has not been imputed into energy simulations, even though it is now well accepted that air leakage could contribute up to 40% energy loss,” states Spinu. “Also, phase change materials and radiant barriers are not commonly simulated, even though both strategies could contribute to reduced envelope thermal loads.”

Keith D. Boyer, P. E., director, architectural wall technology, CENTRIA, Moon Township, Penn., also cautions users that energy models can be skewed to both predict better performance than is actually achievable, as well as demonstrating code compliance when the intent of the code is not truly met.

However, with more stringent energy codes, Boyer anticipates that this situation will change for the better as the codes are placing the onus on owners and designers to verify, through measurement, that actual performance matches modeled performance.

That being said, Boyer recommends Therm VI as a powerful thermal program for determining the thermal efficiency of the wall section and components. “Different components modeled with Therm VI can be tied together using the ASHRAE U-zero equation – the U-value weighted average based on wall area – to determine the whole wall insulation value.”

Another design strategy that modeling can help execute is “off-setting,” which enables designers to enhance one area of the design in order to offset another area.

“For example, something as simple as over insulating the roof in a building with specific wall-to-roof ratios may allow for less insulation to be used in the wall,” says Dazel. “However, an approach like this typically requires much more sophisticated whole building energy simulation software analysis done by a professional consultant.”

The New Normal

In the grand scheme of things, whether specifiers choose EIFS, SPF, mineral wool or some other insulating material, one thing is for certain: continuous insulation is becoming a required aspect of building design.

As more technologies and wall assembly combinations are tested for performance, longevity and indoor environmental quality, the industry is sure to benefit from the ever-growing body of research and data. But in the meantime, building product manufacturers and building science already has a lot to offer in this realm. So while it may take some more time and effort, on the part of the building team, to design and assemble a continuously insulated, high performing building enclosure, it has been done and can be done.