Why Review ] [ How to Achieve Thermal Envelope Goals ]

How to achieve thermal envelope performance goals

The problem

An architect and/or energy consultant sets a performance standard and provides details and specifications for the thermal envelope; the contractor builds it per the plans and specifications, and the building fails to meet the standard. Whose fault is it? How can this situation be avoided?

Setting a performance standard for a building is only half of the battle.  Most builders are not trained in building science and methods for achieving a high-performance building envelope.

Providing a standard in combination with a set of details and specifications that result in construction that is significantly below that standard will probably lead to disputes over which party is at fault, and who should pay to repair the causes of poor performance at the end of the construction process.  Remember, the client expects, and is paying for, a building that meets the standard.  Someone will have to be responsible for achieving the performance that the contract sets forth.

So, how do you set a standard that is achievable?

Proposal for achieving high-performance thermal envelopes:

First, inadequate thermal envelope performance is likely if there are no explicit standards, as the bidders will all assume that the other contractors will only bid on normal construction practices, i.e. not a high-performance building.  Therefore, set a standard to allow the GCs to all bid on a building with the same high level of performance, and include the appropriate details and specifications capable of achieving that goal.

Second, it is important to include statements of the intent of the various thermal envelope components.

Finally, because the general contractors and their sub-contractors are typically not experienced in building high-performance buildings, it is important to include an on-going, non-quantitative, quality-assurance process for each thermal envelope component that will result in a successful, overall quantitative performance test at the end of the construction work.

During the design phase

Plan review

  1. Someone with experience in building envelope theory, and ideally familiar with common causes of building failures, should review the plans and details. Certain building configurations are “red flags” for thermal envelope problems. It is possible to identify the critical areas that typically lead to failures and avoid them by working with the architect and/or engineer(s) to refine problem details during the design process.   Plan reviews are a win-win situation for everyone involved as they can save the costs of call-backs and disputes over the responsibility for building failures.

  2. Each component of the building envelope should be addressed:

  • Insulation - Conductive losses

  • Insulation designs should be based on system R-values, not nominal cavity insulation values. Metal stud or joists can degrade system R-values to less than half of the nominal insulation values.  Details for difficult building detail areas (such as at rafter tails, horizontal and vertical building transitions, and at major structural elements) should be included in the contract documents, not just typical “flat” or simple wall and roof sections. Increases in thermal envelope design R-values should be considered as a means of reducing HVAC system sizes and/or system types. Insulation systems installed in the outermost planes of the building will generally be more successful than complex insulation designs that follow the interior finished surfaces (such as interior wall planes that jog around structural members vs. the enclosed exterior walls, dropped ceilings vs. the overlaying roof slopes, the inner roof planes and transition walls vs. the overlaying “cricket” roofs, and attic floors vs. attic roof slopes).

  • Vapor control

  • Vapor retarder designs should require perm ratings related to dew point calculations for the building use. Long-range planning should include an examination of possible changes in building use.

  • Air barriers

  • Air barrier systems should be considered as one of the most important components of the thermal envelope design.  They are often overlooked in the details and specifications, but can have the most significant impact on building loads and indoor air quality.

  • Roof/attic ventilation system

  • When and how to ventilate a roof or attic should be an easy decision, but ventilation is generally used by default, even when it is not appropriate. Complex roof and air-barrier configurations, skylights, snow infiltration and vent blockage, and mechanical systems in attics are all situations where ventilation is difficult.  Maintaining adequate and balanced air flows, and uniform distribution can severely limit design flexibility and puts additional pressure on the need for a flawless air-barrier installation.

  • Keys to Success

  • Having the plans reviewed is only half the battle. Incorporating the adopted recommendations in the bid documents is necessary to ensure their implementation.  All too often the review comes late in the design process, and recommendations that could be low-cost or no-cost during the design and construction phases do not make it into the original contract documents.  Attempting to address problem details discovered late in the construction process is more expensive because the work is out of sequence and repairs create delays. As a result, the upgrades usually aren’t done, performance is compromised, and call-backs or disputes occur.

  • Specification review

    Reviews of architectural specifications used for projects that have had building envelope failures indicate that the major components of the building envelope are often not addressed adequately.  Typically, only the R-values of the insulation materials and a vapor retarder are specified with only the obligatory “installed in accordance with the Manufacturer’s recommendations” included to cover the intent of the building envelope’s performance. Air-sealing procedures, air-barrier materials, ventilation designs, and other air/vapor-control measures, as well as their intent, are usually not covered.  The inclusion and implementation of these important elements of the building envelope are left to the discretion of the general contractor and his sub-contractors.  Common building practices are generally inadequate for preventing building failures of some type and at some scale in almost every structure.  This is especially true in many types of special-use buildings which require more knowledge of building science to properly address the details related to severe indoor environments.  As a result, building performance rarely meets the potential that modern building materials could provide, and energy is wasted over the entire life of the building.

    Keys to Success

    All thermal envelope specifications should be in the original bid documents. When the specifications do not include the critical elements of a successful building envelope, it is very difficult for even the most responsible and knowledgeable contractors to put these measures in their bids.  They can only afford to include what is specified in their prices due to the extremely competitive nature of the construction industry.

    Having a good project specification is only half the battle. Confirming that buildings include all of the specified work assures that the Owner is receiving the intended value and performance of the design. If the contractor is held accountable for his work by regular inspections throughout the construction process, the proper installation of the materials that make the difference will be assured.  Having the inspections conducted by someone who understands the intent of the design can also be helpful in communicating the science behind the specifications to the contractor and his sub-contractors.

    Performance Specification Reviews

    Most reviews of architectural specifications show that they rarely include performance standards for the thermal envelope materials or the combined performance of component systems. Performance standards should be a required aspect of the building process. It is not unreasonable to ask contractors to install building components in a manner that will achieve the desired performance.  In-place performance standards for many other building systems are routine and expected.  Examples of performance specifications for energy-efficient buildings are well developed and readily available.  Testing equipment used in the evaluation of building performance is not new and these services are also readily available. Increased use of performance standards could only serve to promote the availability and standardization of industry-standard documentation and procedures.  Again, establishing performance standards is a low-cost element of the construction process and one of the few that has a short-term payback. Standardized specifications and industry familiarization with performance standards would keep up-front costs low.  Savings in energy usage, and all of the environmental implications, provide the return on investment that any increase in design and construction costs may create.  Additionally, performance testing is a scientific process resulting in objective data that eliminates the likelihood of disputes over responsibility related to problems identified during the construction phase, reducing project delays, and avoiding possible litigation related to failures that might otherwise occur after the building has been completed.

    Keys to Success

    Having a good quality assurance specification review process will ensure that the standards are appropriate for a given building type or use and result in an effective building envelope. This review should include a discussion of critical details and identify the likely areas where special attention should be paid to the installation work when normal construction practices will not be adequate to achieve successful building envelope performance.

    During the construction phases

    1. Convey the intent, methods, and performance goals of the thermal envelope in the initial project meeting after the contractor has been selected.

    2. Include construction inspections by someone knowledgeable in thermal envelope theory.

    3. Perform the required QA test procedures for each component or section of the thermal envelope as they are installed.  This can be done by building section or major element (roof, walls, etc.).

    • Insulation - Conductive losses

    • Exposed insulation installations are relatively easy to quality control by visual inspection.  Small defects or missing insulation materials are easy to repair or complete before the finishes are installed.  Regular inspections can be a reliable method of quality assurance for this envelope component.  Infrared thermography can be used to verify insulation completeness and performance, but this method is seasonal.  It is important that the inspector is knowledgeable in the proper execution and method for achieving maximum performance of the insulation materials used in the project (some insulation materials do not work effectively unless related building envelope materials are installed properly to complete the insulation system).

    • Vapor control

    • Vapor retarders are relatively easy to quality control by visual inspection. Small defects in vapor retarder materials are not problematic as vapor control materials are effective against vapor transport by diffusion in proportion to the area that they cover, i.e. if they cover 95% of the envelope surface, they are 95% effective. Most building failures related to vapor transport are not caused by diffusion. Regular inspections can be a reliable method of quality assurance for this envelope component. Air barriers (below) should be considered an integral part of vapor control as air movement is usually the most significant means of vapor transport.

    • Air barriers – convective losses

    • Air barriers are usually the most problematic thermal envelope component.  The majority of building failures are related to heat or moisture transported by air flow into and through the thermal envelope. This is why the primary standard for building envelope performance is the measure of air leakage.  Elements of most air barriers come early in the construction process and are concealed by building finishes.  This makes repairing them at the end of the project, if performance testing standards are not met, very costly as building finishes must be removed to access and improve the air-barrier components. Test methods used for quantifying building air-leakage performance (usually blower door testing) are not viable during the construction phase as the entire building envelope is usually not all in place at once and often includes some of the interior finishes. Alternative methods of testing air-barriers that do not require a completed envelope do exist.  By temporarily covering building sections enough to allow minimal pressurization, air-leakage sites can be located using visual indicators such as theatrical fog. For example, this approach would allow roof air-barrier components to be tested before sections of walls, windows, and doors are in place.  This will not quantify leakage, but it will allow the installation crews to quality assure each air-barrier component (walls, roofs, and their intersections) while they are still accessible.

    • Roof/attic ventilation system

    • If ventilation is needed, special attention is required for the roof or attic air-barrier installation.  In vented roof and attic designs, ventilation rates, inlet and outlet balance, and even distribution should be verified during the construction phase.  Inspecting inlet and outlet vent openings to verify that net free areas are not blocked and will function in deep snow coverage is critical.  Fog testing can be used to verify that ventilation pathways are open. Mechanical systems in vented attics should be insulated and air sealed to the same standards as the rest of the thermal envelope and should be inspected and tested as in the quality assurance sections above.