A.6.2.1 Model Energy Code (CABO)
3. Coating Membranes, which include paints
The proper location and installation of vapor retarders is very critical. Some important guidelines to follow to insure the performance of the vapor retarder are:
1. Install the vapor retarder at the warm side of the insulating surface.
Vapor retarders should not be used above any other form of retarder at ceiling level.
2. When installing either a membrane type of structural type, make sure that all seams, laps and joints are properly sealed. Sealing a vapor retarder may be achieved by gluing, taping or stapling. A method of stapling is described in Figure A6.4.2.1.
3. In the case of a membrane type retarder, make sure that any punctures or tears in the material are repaired.
Figure A6.4.2.1
Proper Sealing of Vapor Retarder
Because there is a growing trend to add insulation to existing metal buildings, a proper vapor retarder is of critical concern. One of the most common methods is to add an additional layer of insulation to the bottom flange of the purlin system.
This can create an air space where moisture-laden air can accumulate if the integrity of the new vapor retarder closest to the warm insulating surface of the
building has not been maintained. It is important that an intact vapor retarder is not left within the insulation mass.
A6.4.4 Signs of Condensation
1. Visible Surface Condensation – Condensation occurring at cold exposed surfaces.
a. Water, frost or ice on windows, doors, frames, ceilings, walls, floors, insulation vapor retarders, skylights, cold water pipes and/or cooling ducts.
2. Concealed Structural Cavity Condensation
a. Damp spots, stains, mold and/or mildew on walls or ceilings.
b. Delamination of laminated surfaces, bubbles or blisters in asphaltic surfaces and peeling paint.
c. Damp insulation.
A6.4.5 Controlling Condensation
Table A6.4.5.1 provides guidance on controlling condensation problems as listed in this section.
1. At the Source – Limit the amount of water vapor within the heated interior.
a. Provide a well drained base course such as crushed rock or washed gravel under grade level slabs to prevent moisture from permeating into the building through the slab.
b. Provide for adequate ground water drainage.
c. Divert rain and melt water accumulations away from the foundation.
d. Vent all mechanical heating apparatus with hydrogen-oxygen by-product exhausts to the outside.
e. Reduce supplemental interior humidification.
f. Be aware that the pouring of concrete within a newly erected structure presents special considerations. While the practice should be held to a minimum, an individual project assessment must be made to prevent both visible and concealed condensation.
2. By Insulating – A proper insulation scheme effectively raises surface temperatures within the building envelope above the dew point temperature. This is accomplished by controlling the heat loss at the exposed (exterior) sides of those surfaces. However, insulation added above an existing roof should be placed at the existing roof level and not against the retrofit roof if possible. Adequately ventilated space is required because any trapped warm air may condense at the cold metal roof.
a. Provide additional insulation in ceiling and/or wall cavities or replace existing damp or water soaked insulating material within these cavities.
b. Install double or triple glazed windows or insulated storm panels with thermal break frames.
c. Install insulated doors.
d. Install insulation wrap (with exterior vapor retarder) around cold pipes and/or cold air ductwork.
e. Insulate floor slab edges exposed to exterior temperatures with a rigid insulation-pressure treated plywood combination.
f. Paint condensation prone surfaces with moisture absorbing paints.
3. With Vapor Retarders – A vapor retarder is a vapor resistant membrane of polyethylene film, aluminum foil, paint, asphaltic laminate and/or glazed, asphalt saturated building paper that exhibits a permeance of less than one perm. Since the membrane is, in effect, a seal to prevent water vapor from combining with a given cavity air mass, it must be continuous and unbroken. All joints and seams must be lapped, sealed, and secured.
a. Employ (or repair) a vapor retarder at the warm side surface of all insulating material. In the case of the fiber blanket
insulations, the retarder may be laminated to the insulating fiber.
All joints at crawl spaces, under slab ductwork, attic openings, ceiling fixtures and/or other wall, ceiling and floor penetrations must be properly sealed.
b. Install a vapor resistant ground cover over interior, exposed ground surfaces.
c. Install a vapor retarder between sub-flooring and ground slab.
d. Install a clear vapor retarder over skylight openings and seal off to warm side insulation barrier.
e. Install a vapor retarder on both sides of the insulation in buildings with a cooler, controlled atmosphere and in cold storage buildings to prevent condensation inside the insulation.
4. Through Ventilation – The dilution of a moist interior air mass with drier outside air for the express purpose of lowering the relative humidity of the air mass. For retrofit applications, venting above existing roof may not be necessary if the source below is eliminated by adequate means of vapor retarders. The natural amounts of water vapor from outside air exchange will dissipate gradually without any effect.
a. Cold Side Venting – The venting of “exterior” cavities (cavities at the cold side of the insulation envelope but contained within the general building envelope) of the building’s structural elements. One square foot of “free” vent area must be provided for each 300 square feet of convective cavity area. Vents should
be uniformly distributed to provide the best overall airflow and also should be screened and louvered to prevent insects and rain from entering the cavity.
1. Provide ridge and eave vents in building “attic” areas.
2. Provide for base and eave line ventilation to wall cavities.
3. Install foundation vents to any crawl space areas.
4. Install exhaust fans.
b. Warm Side Venting – The venting of the interior building envelope.
1. Install convective type venting apparatus.
2. Install remote exterior air changers with heating and distributing systems as required.
3. Install exhaust fans. (Note: Borderline effectiveness – depends heavily upon infiltration for air change).
Table A6.4.4.1
Dew Point Temperature (oF)1 Relative
Humidity
Design Dry Bulb (Interior) Temperature (oF)
32oF 35oF 40oF 45oF 50oF 55oF 60oF 65oF 70oF 75oF 80oF 85oF 90oF 95oF 100oF 100% 32 35 40 45 50 55 60 65 70 75 80 85 90 95 100
90% 30 33 37 42 47 52 57 62 67 72 77 82 87 92 97 80% 27 30 34 39 44 49 54 58 64 68 73 78 83 88 93 70% 24 27 31 36 40 45 50 55 60 64 69 74 79 84 88 60% 20 24 28 32 36 41 46 51 55 60 65 69 74 79 83 50% 16 20 24 28 33 36 41 46 50 55 60 64 69 73 78 40% 12 15 18 23 27 31 35 40 45 49 53 58 62 67 71
30% 8 10 14 16 21 25 29 33 37 42 46 50 54 59 62
20% 6 7 8 9 13 16 20 24 28 31 35 40 43 48 52
10% 4 4 5 5 6 8 9 10 13 17 20 24 27 30 34
1. Chart adapted from ASHRAE Psychometric Chart, 1997 ASHRAE Fundamentals Handbook.
Table A6.4.5.1
Condensation Problem Methods of Control (See Controlling Condensation)
1. Moisture, frost or mold on underside of uninsulated metal roof.
1d, 1e, 2a, 2f, 3a, 3b, 4b1, 4b2, 4b3
2. Moisture or frost on skylights. 1d, 1e, 3b, 3d, 4b1, 4b2, 4b3
3. Moisture or frost formations on interior vapor retarder.
1d, 1e, 2a, 3b, 4b1, 4b2, 4b3
4. Moisture dripping from ceiling fixtures. 2a, 4a1, 4a4
5. Moisture, dampness and/or mildew on floor areas.
1b, 1c, 2e, 3c, 4a3
6. Moisture and/or frost on exterior windows,
doors and metal frames. 1d, 1e, 2a, 2b, 2c, 4b1, 4b2, 4b3
7. Dampness, stains, mildew or blistering and peeling paint on ceilings.
1d, 1e, 2, 2a, 2f, 3a, 3b, 4a1, 4a4, 4b1, 4b2, 4b3
8. Dampness, stain, mildew or blistering and peeling paint on walls.
1d, 1e, 2, 2a, 2f, 3a, 3b, 4a2, 4a4, 4b1, 4b2, 4b3
9. Moisture dripping from cold water pipes or cold air ducts.
1d, 1e, 2d, 2f, 3b, 4b1, 4b2, 4b3
10. Soggy or damp insulation in ceiling or walls. 1d, 3a, 4a1, 4a2, 4a4
A6.5 Ventilation
All metal buildings require some level of ventilation, and more often this ventilation is becoming the responsibility of the Metal Building Contractor. A lack of ventilation can create an uncomfortable working condition through elevated heat levels and stale air. It can also contribute to condensation problems.
Ventilation can best be represented by the number of times per hour the building air is replaced with outside air. This is referred to as air changes per hour. The number of air changes required per hour widely varies per application. A rule of thumb is 3-5 air changes per hour for warehouses, 5-10 air changes per hour for light manufacturing facilities and 10-20 air changes per hour for heavy manufacturing. The matter in which airflow requirements are determined is as follows:
Assume a 100 ft. wide x 250 ft. long x 30 ft. high building being used for light manufacturing, assembly and storage. The first task is to determine the total cubic content of the building.
Cubic content = 100 ft. wide x 250 ft. long x 30 ft. high Cubic content = 750,000 ft3
Next, the total airflow can be determined through air changes. CFM stands for cubic feet per minute of airflow.
CFM = Cubic content x air changes/60 minutes CFM = 750,000 ft3x 5 air changes/60 minutes CFM = 62,500 ft3/minute
Thus, 62,500 cubic feet per minute of air must be moved through this building, providing 5 air changes every hour, and proper air ventilation.
Allowances must be made for a place for air to enter the building and for air to exit the building, and the airflow must be evenly distributed throughout the building. Typical methods of moving air include exhaust and supply air fans, ridge ventilators and louvers.
A6.6 Cool Roofs
In general, lighter colors reflect more sunlight than darker colors because of a darker color’s ability to absorb energy from sunlight. This principle holds true for roofing products as well.
Higher temperatures on a roof surface can, through the process of conduction, raise the internal temperature of the building envelope. This results in significantly higher cooling costs. Since most of the energy in the United States is generated through the combustion of fossil fuels, a greater demand for energy can be directly associated with smog, greenhouse gas emissions, acid rain, and eventually global warming. This problem is compounded by a phenomenon know as “Urban heat islands”, in which the surrounding air temperature around a group of buildings is significantly higher due to the radiation absorbed by the surrounding surfaces and a lack of vegetation.
The U.S. Environmental Protection Agency (EPA) recently announced its Energy Star Roof program, under which manufacturers will be allowed to use the Energy Star Label on reflective roof products that meet the EPA’s strict specifications for solar reflectance.
Roofs that bear the Energy Star logo have proven that the initial reflectivity of the material is at least 65 percent or greater and that it will maintain a reflectivity of at least 55 percent for a three-year period.
The American Iron and Steel Institute (AISI), along with the Metal Building Manufacturers Association (MBMA), Metal Construction Association (MCA), National Coil Coaters Association (NCCA) and the Galvalume Sheet Producers of North America (NamZAC), are involved in a project at Oak Ridge National Labs (ORNL) which will compare the solar reflectivity of metal panels with that of other roofing materials.
A7.1 Introduction
Owners of metal building systems may want to consider providing lightning protection. Providing lightning protection for a typical metal building is a simple, inexpensive task and the rewards in personal and equipment protection far outweigh the installation time involved.
A7.2 Where To Ground
The nature of the fastening systems used in most metal buildings virtually grounds all the panels and accessories to the main structurals. The only task then, is that of connecting the bases of the main structurals to some effective grounding device. On small buildings, ground wires from main structurals to the grounding device need be connected only at two opposing corners. On longer buildings, connections should be made at the intermediate structurals no more than 100 feet apart, as well as at each corner.
The simplest means of tying the connections mentioned above to the grounding device is to bury a low resistance wire around the building and join at each appropriate structural line. This "main line" can then be led into the selected grounding device. Always use low resistance wire, preferably lower in resistance than the conductor used to ground the electrical equipment within the building.
A7.3 Grounding Devices A7.3.1 Water Mains
The most reliable and permanent low resistance grounding device is an underground leaded joint, cast iron water main. Attaching to these can provide effective grounding from as far as 75 feet away. When such a main is not available a ground connection to a threaded or welded steel water pipe three inches in diameter or larger is satisfactory. Connections should be made using copper or brass clamps attached to a space on the pipe that has been cleaned of all corrosion and paint.
A7.3.2 Driven Rods
Driven rods or pipes make an effective grounding device in some kinds of soil and have the advantage of low cost and easy installation. Rods should be copper clad steel or galvanized iron of at least 5/8 inch diameter. If pipe is used, 3/4 inch galvanized iron is recommended.
Length should be approximately 8 feet, and no less than 6 feet. Just one driven rod or pipe is seldom sufficient to accommodate a lightning strike, and the recommended practice is to connect two rods or pipes in parallel, about ten feet apart, in order to get the desired low ground resistance.
A7.3.3 Buried Plates
Buried plates can also be used as grounding devices. Plates should be 1/16 inch thick copper of at least 9 square feet, buried below permanent moisture levels, with 2 feet of charcoal or crushed coke above and below each plate.
A7.4 General Recommendations
In the driven rod and buried plate method, a test of ground resistance is recommended. A ground resistance in excess of 5 ohms calls for more grounding devices connected in parallel, in order to lower overall system resistance. Many electrical contractors can test the grounding resistance of a lightning system.
Consultation with a qualified local engineer is recommended to insure that a solution is adequate. Periodic tests by qualified technicians will insure continued protection of any structure.
A8.1 Introduction
One of the most detrimental climatological conditions to metal buildings is snow and ice buildup on the roof. Snow buildup to any significant depth greatly increases loads on the roof. While much of the snow will tend to slide off steeper roofs, (over 4:12 slope), much will remain that falls on a cold surface or previously covered surface. It is common to prevent snow slide by having devices placed on the roof in strategic locations. Snow will tend to slide more readily on a warm roof, caused either from sunshine or heat loss through the roof.
Relatively little snow will slide off low slope roofs.
A8.2 Drainage
Gutters, downspouts and interior roof drains allow for the controlled removal of water from a roof system. They must be kept open and free flowing to work.
During cold temperature conditions, gutters, downspouts and drains can freeze solid allowing for ice build-up on the roof. This ice build-up causes additional water back-up on the roof deck. These circumstances create extreme loading conditions on the roof system and building. Freezing conditions are particularly likely on the north side of a building and in shaded areas of a building.
One simple precaution is to have heat tape installed in gutters and downspouts.
This will help maintain open and flowing gutters and downspouts. However, in extremely low temperature conditions, heat tapes may not be 100% effective and should be checked regularly.
A8.3 When to Remove Snow
Defining a specific depth of snow that a building has been designed to support is not possible because the density of snow is variable and dependent upon weather conditions both during and after a snowfall, as well as affected by the total depth of snow at a location. With the variability of snow density, it is possible for conditions to exist that exceed the designs specified by the building codes. Snow density also changes as the snow melts. Not all water drains off the roof as the underlying snow absorbs some water from the melted snow above. This leads to ice build-up on the roof as the temperature varies from day to night.
Fresh snowfall may weigh as little as 10 to 12 pounds per cubic foot (pcf) but the density will greatly increase as it compacts and becomes heavier with water.
Typical densities on a roof will range from 16 pcf to 30 pcf depending on snow depth. When there is snow on the roof of a building and rainy conditions occur, excessive loads can develop rapidly. Snow acts as a sponge in these conditions and loads can approach the weight of water, 62.4 pcf or 5.2 pounds per square foot (psf) per inch of depth. Rarely will a cubic foot of snow and ice equal the weight of water due to the expansion that takes place as water freezes. However, these conditions must be monitored with extreme caution.
Snow will build up in areas around firewalls, parapet walls, valleys, dormers and on lower roof levels where a step in the roof occurs. All current building codes require design for snow build-up conditions so that the structural systems in these areas can support the additional loads. However, due to the variability of snow density, as noted above, it is possible for conditions to exist that exceed the designs specified by the building codes.
While it is not possible to accurately determine a specific depth of snow that is considered a safe maximum, an approximation can be made. The first step is for the building owner to obtain information as to the snow load that the building has been designed to carry. For example, a building designed for a 30 psf snow load can be at design load with just 18 inches of snow at a density of 20 pcf and could be overloaded with less than a foot of snow under wet conditions. Cleaning the roof is, of course, the only way to relieve this. It is recommended by Factory Mutual (Ref. B2.44) that roofs be cleared of snow when half of the safe maximum snow depth is reached. The maximum snow depth can be estimated based on the design snow load and the density of the snow and/or ice buildup.
A8.4 Snow/Ice Removal Procedure
Following are some suggestions that generally apply, however, it is recommended that the building manufacturer or a structural engineer be consulted before snow removal is initiated.
1) Remove all hanging icicles from eaves and gutters. These will be quite heavy and if snow hangs up on them during removal, it can only increase this load. Care must be exercised to not damage the building and to not endanger pedestrians.
2) Always provide proper safety precautions when working on the roof. If possible, remove snow by not getting on the roof. Using draglines through the snow, working from the endwalls, can sometimes do this.
3) Place ladders at the end of the building so sliding snow will not dislodge them.
4) Never send one person on a roof to remove snow.
5) Remove snow in a pattern that does not cause an unbalanced loading condition. Avoid large differences in snow depth between adjacent areas of the roof. Do not remove all of the snow from small areas and then move on to another area. Instead, remove the snow in layers from all over the roof. This gradually decreases the load.
6) Remove drifted areas first, down to a level with other snow. If an area has drifts four feet deep and the main roof is two feet deep, trim off the drifts to two feet before proceeding.
7) Remove snow from the eave towards the ridge, sliding the snow off the roof over the gutter.
8) Remove the snow from the middle one-third of each bay for the full width of the building, beginning with the most snow packed bay.
Complete snow removal on the remainder of the building.
9) On gable buildings, remove snow on both sides of the ridge at the same time.
10) Never use metal shovels on any type of roof. Do not use picks, axes or other sharp tools to break up ice on the roof. It is quite easy to damage the roofing with these tools.
11) Do not remove snow to less than a 3" depth over roof sheets. Care must be taken to eliminate hitting panel fasteners, snow guards, etc. If an ice layer is next to a panel, it should be left, if not extraordinarily thick.
12) Care must be taken in removal of ice and snow around ventilator bases, pipe flashings, and HVAC units, due to the ease of damaging neoprene boots, pipes, conduits, etc.
13) Be cautious of snow or ice breaking away and sliding down the roof, even on low slope roof buildings.
14) Use extreme care when working along the edge of the roof.
15) Watch for extreme deflections and listen for unusual noises when snow and ice build-up conditions exists.