Tags: wall

17 Apr 2009, Comments Off

CFC Reviews CMHC Design Report: GUIDELINES FOR DELIVERING EFFECTIVE AIR BARRIER SYSTEMS – WHY PROBLEMS OCCUR

Author: admin

WHY PROBLEMS OCCUR

The air tightness, continuity, structural integrity and durability of the air barrier system are dependent upon three factors; materials, design and installation practice. Flaws in any of these elements can have negative ramifications on the ability of the completed system to perform to specification in the short and/or long run.

Materials

When specifying air barrier materials, the designer must confirm that the material or materials chosen have an air permeance rating equal to or less than 0.02 L/(s·m2) measured at an air pressure difference of 75 Pa. Many materials may meet this requirement, but care must be taken to ensure that the material will maintain its air permeance rating (and not have any adverse effect upon the system’s ability to meet the other three requirements of continuity, structural integrity and durability) once it has been installed in the wall. For instance, two-part materials that are fabricated on site, such as some spray-applied materials, may be rendered ineffective if not mixed correctly. All relevant information regarding the material, including air permeance, fabrication instructions and material characteristics, can be found in the technical literature as supplied by the manufacturer.
Most commonly specified air barrier membrane materials demonstrate similar air and vapour permeance characteristics (in reference to their scope of use on a building). However, other performance characteristics, such as adhesion, elongation, puncture resistance and tensile strength may vary considerably and must be taken into consideration when specifying materials, especially when used around roof/wall junctions, wall/window junctions and control joints where movement is expected. The variance may be enough to compromise the ability of the system to function correctly. As an example, the elongation of regularly specified self-adhered air barrier membranes can range from 4% to 200%. Where movement between system components is expected, materials with greater elongation properties should be selected.

The installed materials must not react adversely to either other materials that comprise the air barrier system, or adjoining components within the building envelope. While it is beyond the scope of this paper to document every potential incompatibility, the designer must be aware that incompatibilities can occur, and should carefully consider the physical and chemical properties of the materials being specified. Physical incompatibilities occur when the physical characteristics of different materials make them incompatible. A common example is where a hot-applied material is installed over heat-sensitive material. For instance, if torch-grade membrane is installed over self-adhered or spray-applied membrane, the excessive heat may cause the self-adhered or spray-applied membrane to melt (this may also occur if hot mopped asphalt is used around the roof/wall junction). However, specifications often allow for different trades to select between a range of acceptable materials, and a situation may occur where one trade has selected self-adhered membrane and a second trade chosen torch-grade.

The general contractor should monitor the work of the sub-trades and identify any concerns regarding material compatibility or sequencing to the designer, who should be aware of the materials being used on the project. Chemical incompatibilities occur when the chemical properties of different materials make them incompatible. Consider substrate preparation. If walls are not primed properly and in keeping with manufacturers’ recommendations, or the incorrect primer is used, not only may the membrane not bond adequately to the substrate, but the chemical composition of the primer may damage the membrane itself. In fact, the chemical compositions of certain membranes may make it impractical to use them concurrently on a wall section. The chemical composition of asphalt membranes is such that it will cause certain rubber membrane to decompose. Similar results may be attained when a membrane of a particular makeup comes in contact with high solvent-based sealants or uncured solvent-based primers.

The Canadian Construction Materials Center (CCMC) has published technical guides that detail specific structural, durability and air leakage test criteria for air barrier materials and systems. Air barrier materials can be tested both as stand-alone materials (tested for air permeance) and as part of a system (tested for air permeance, structural integrity and durability). For optimum results, all system materials should be evaluated under this protocol. However, while the results of evaluations like this can be used as a reference to provide assurance of the material’s ability to perform as part of a system, the evaluations do not pre-approve the system. It is the responsibility of the designer and installer to bring the individual materials together as an effective system.

Design

Meeting specifications does not necessarily guarantee that the air barrier system will perform well.  An incorrectly designed system will not function effectively regardless of how well it has been installed. It is not uncommon for an air barrier system failure to be attributed to a flaw in design. Common examples are improperly locating the air barrier within the wall; discontinuity within the system (for instance, gaps in the system at major joints, such as roof/wall, wall/foundation, and window and door frames to wall junctions); sequencing of structural, mechanical and electrical systems which may make air barrier continuity impossible to achieve, and; failure to differentiate between air barriers, vapor barriers and/or materials that act as both.
In cold or severely cold climates2, where a material is to act both as an air barrier and a vapour barrier, it should be placed on the warm side (or high-vapour pressure side) of the wall3. It should be placed at a sufficient depth within the building envelope so dew point temperature occurs to its exterior side. Where air barrier and vapour barrier functions are to be performed by different materials, the vapour barrier should be placed on the warm side of the wall. Again, it should be placed so dew point temperature occurs to its exterior side. In this instance, the air barrier may be placed anywhere within the wall provided it restricts the flow or movement of conditioned air, preventing this air from coming in contact with cool surfaces where temperature is below dew point.

If the air barrier is placed outside the insulation plane, the air barrier material must have a vapour permeance characteristic, or the system be designed, such that water vapour will diffuse to the exterior of the building envelope, or a vapour barrier of lesser permeance is used on the inside. In comparing warm and cold climates, the ‘science’ behind where the vapour barrier is placed within the wall does not change ?? it is always placed on the warm side of the wall. However, in warm climates, because the warm side of the wall will be closer to the exterior than in areas of cold climates, the vapour barrier will be placed closer to the exterior as well (and may even form part of the exterior wall).

In most instances, to best meet the requirement of durability, the air barrier should be placed within the exterior cladding and outward of the structural frame. This not only protects the air barrier from exterior environmental conditions, but by keeping the structural frame of the building within the air barrier, the system design is more straightforward in terms of maintaining continuity at penetrations associated with structural elements. Reviewed by Moishe Alexander the CEO of Canadian Funding Corporation

3 Apr 2009, Comments Off

CFC Reviews CMHC Design Report: Construction Tolerances – The Problem

Author: admin

Most designers think the builder should just cope when parts are built out of position, and builders are accustomed to doing so, often all too literally. Their methods are sometimes rough and ready.
When a part is built too far out of position to fit, it is more common to alter the details to ensure acceptable finished appearance, even at the expense of function, rather than demolish and rebuild.

It is not difficult to find:

• brickwork with too little bearing, because the structure was built too far back from the face of the wall;
• modified shelf angles, either burned off at the outer edge, or with structurally inadequate extensions;
• brick cut to make thinner veneer, and insulation omitted altogether, in order to keep a finished wall flat where the structure behind it was too close to the face of the wall;
• precast panels with joints varying in width from zero to more that twice the detailed joint size;
• fasteners for steel stud track that can be removed without tools, because they were placed too close to edge of slab, spalling the concrete.

When gaps are designed into assemblies to provide for structural live and dead load deflections, creep, and differential movement caused by thermal expansion and contraction or moisture, oversized or mispositioned parts may reduce the required gaps. Such problems are not confined to the brick veneer steel stud example. Tolerances need to be considered in the design of all building assemblies.
Builder and designer often share responsibility for these problems. Construction variations commonly exceed established norms. However, applicable codes and standards often explicitly allow variations that details do not accommodate. When there is no formal codification of allowed variation, the common practices of trades involved establish de facto standards. Designers who do not state an alternative implicitly adopt these standards, even if they are unaware of having done so.
The wall cavities measured during a study by CMHC a few years ago, were as much as 17 mm smaller, and as much as 20 mm larger than the dimension indicated in the details. The range from smallest to largest on the same building averaged 19 mm. The difference between the largest and smallest cavities on a building was never less than 10 mm, and in the worst case it was 37 mm. For most construction types, there is not much information available about what levels of accuracy are being achieved. Similar levels of variation do occur in other kinds of construction.
Designers should explicitly determine if the tolerances implied by the relationships between parts and provisions for adjustment in a detail are realistic. If a detail cannot accommodate variation that is acceptable under the applicable standards, by established trade practice, or by tolerances stated in the specifications, what will happen on site? The detail would have to be revised, perhaps at additional cost. Or, the builder may leave some insulation off to make room for the structure to avoid an ugly bulge in the wall. Some brickwork may be left insecure because of inadequate bearing. If the designer allows for adequate tolerances, and the builder meets them, there will be a great reduction of such problems. Reviewed by Martin Lapedus.

3 Apr 2009, Comments Off

CFC Reviews CMHC Design Report: Window Installations – General

Author: admin

It is important to ensure that sealants are applied at the appropriate point in the installation process and at the appropriate location to ensure that the air barrier is continuous. In the case of a metal window, the thermal break should be properly situated over the insulation in the wall, to maintain the continuity of the thermal barrier. If this is not done, a thermal bridge will result in high heat loss and condensation problems.
Improper installation can also impair window performance. For example, improper fastening or shimming of the frame may result in operable units not working properly. For example, casement units or awning windows will not open or close without considerable force. The CSA A440.4 Standard is a reasonably good guide to installation.
Installation details for a specific product should be reviewed as part of the design stage, to ensure that installation details do not conflict with the intended applications. For example, using a strap anchor to fasten the top of the window is not advised, as it is difficult to provide an air and water seal around this type of anchor.
For more information on this topic, refer to the CMHC report Design of Durable Joints between Windows and Walls. A mock-up installation during construction is strongly recommended.
This allows all parties (designer, installers, contractor, etc.) to review the installation details and sequencing. It is also appropriate to field-test the mock-up to fine-tune the installation details so the installed window passes the E1105 test (see Section 3). Sequential photographs of the installation process are often useful, especially if additional installers are expected to be added to the contractor’s forces. The new staff can review the photographs of the mock-up as a supplement to the installation instructions.
In general, the CSA A440.4 installation procedures should be referenced in the project specifications, but project-specific details may require special attention.
Wherever possible, designers and specifiers should maintain a design philosophy of continuity of air barrier, thermal barrier, water-vapour retarder and weather barrier. Checklist for window specification The following list is a general guideline for preparing specifications for window projects. These suggestions are in no particular order and individual projects may require additional items.
Site measurement of rough openings is critical to ensuring that the windows will fit. This measurement is usually the responsibility of the contractor or installer.
Windows should be delivered to site as shortly before they are to be installed as practical. They should be stored vertically in a clean, dry area, away from possible sources of breakage.
Shop drawings for windows are often useful. The design drawings may address a specific aspect of the design (waterproofing details, structural considerations, etc.) for a generic window. Shop drawings show how the specific window design is to be integrated into the wall assembly, including structural loads, anchorage, and drainage of hollow elements.
The window supplier should provide information on proper maintenance procedures, to ensure long service life of the installed products.
The designer should request full laboratory test reports for the ABC ratings (unless the designer already has that information).
Specifications should explain which field tests will be conducted (see 4. “Achieving Field Performance”) and at what level of performance and with what frequency. Specifications should state who will do the tests and the pass-fail criteria. State what happens in case the windows do not pass, and must be re-tested.
The window supplier should clearly state all warranties and guarantees, and should provide relevant information in writing. Installation and maintenance instructions should be reviewed to ensure that they do not contain any directions that will void the warranties.
Where “equivalent” and “approved substitution” products are permitted, the designer should clearly state the parameters that define the desired performance. If resistance to wind-driven rain is the primary concern, for example, then the desired B rating should be clearly stated. Then, if the contractor proposes an alternative, the contractor knows what information to provide the designer for evaluation of equivalence.
Note that it should be the contractor’s responsibility to provide such documentation for the designer to review, rather than making the designer obtain the appropriate data to determine whether the contractor’s choice of products is acceptable.
Installation specifications should ensure that weepholes and related drainage paths are not blocked
Specifications should ensure that hardware is properly adjusted for use and windows are cleaned before the project is completed.
For low-e coatings, the type of coating (product name and emissivity) should be defined, and which surfaces are to be coated. Reviewed by Jan Luistermans.

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AIR Alexander Building Canada Canadian Funding Corp. Canadian Funding Corporation CFC cmhc construction Design IDP integrated design Moishe Alexander performance report Reviews Window Window Installation
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