Now days many textile players prefer Pre Engineered Building (PEB) Systems a complete package of low rise steel buildings manufactured and delivered to the site by a single supplier. The package comprises of rigid structural steel, cladding system and building accessories. Though the concept is simple, PEBs are extremely diversified in their usage and uniqueness in each design. The PEB concept has gained widespread popularity in the last few years as a result of the speed and simplicity with which such buildings can be erected. In addition, many traditionally designed industrial buildings are also now opting for metal roofing. A critical and necessary ingredient in the PEB System is the thermal and acoustic insulation. This is necessary to minimize the heat gain (or energy loss, for an air conditioned building) as well as to provide acoustic insulation from heavy rain and other outside noises. In a typical PEB structure, the roof accounts for approx. 40 to 50% of total heat gain, while walls account for approx. 15 to 20% of heat gain.
Almost 100% of PEBs world-wide are insulated for the following reasons, which are addressed in further detail below:
- Minimize heat gain.
- Maximize thermal comfort.
- Minimize energy loss, cooling load and operating cost for Air conditioned building.
- Provide acoustic insulation.
- Prevent unwanted moisture condensation.
Minimize heat gain and maximize thermal comfort
As MNCs have put up new facilities in India, they have insisted that the workplace meets internationally accepted norms. This is because of the increased awareness of harmful effects of thermal stress, the physiological effect of high temperature and humidity, and the harmful effect of noise on human health. As a result, architects and planners are paying more attention to this aspect in the last few years.
As per ISO 9241, a typical work place environment should have a temperature less than 23oC and noise level less than 60 dB(A). Depending on process/product, human activities and the number of occupants, the work place temperature varies. It is impossible to achieve thermal or acoustic comfort inside a pre- engineered metal building (which does not have centralized air conditioned system) without adequate insulation. Minimization of heat gain is also very important from medical point of view as heat stress not only reduces productivity but also aggravates heart problems.
By using fiberglass wool as under-deck insulation, heat transmittance through the roof and wall is reduced substantially. Sufficient temperature drop occurs across the roof and wall system, resulting in lower temperature and a comfortable workplace environment.
Minimize Energy Loss
If the PEB is centrally air conditioned, a project owner can reduce the capital as well as operating cost of chiller, motor and AHU by providing adequate thermal insulation. Thermal insulation having sufficiently high thermal resistance (R value) should be installed on roof and wall. Air conditioning load can be reduced substantially and hence one can save scarce fuel energy/electric energy. This is the reason why it is called the product for sustainable development.
A noisy workplace reduces working efficiency, concentration, and negatively impacts overall performance. Consider the noise inside a PEB shed without insulation which is bombarded with heavy drops of rain or hail which occurs during Monsoon every year. Mumbai recently recorded highest rain fall of 39 mm/hr for 24 hrs. With that sort of rain the sound level of a 10 m x 5 m metal roof can reach 78 dB(A) which is roughly equivalent to the noise in major traffic on a road at 10 m distance on a continuous basis. To maintain acoustic privacy or keep sound level under the specified level i.e. 55 dB(A), fibrous type insulation like Glass-wool is the best choice.
Control on Condensation
Condensation should be strictly controlled in PEBs as structural metal in contact with condensed water droplets can corrode. To prevent moisture, proper insulation thickness and thermal resistance of the insulation is of utmost importance so that the surface temperature of the same does not go below the dew point temperature. Please note, using aluminum foil on its own as a vapor-retarder does NOT control condensation.
When selecting insulation for a Pre Engineered Building, the following criteria should be considered:
- High thermal resistance or R value, for effective thermal insulation.
- Ease of application
- Lightweight material to avoid need for support from expensive metal skin or welded mesh
- Variety of aesthetic options / finishes
- Cost effective (total installed cost)
- High NRC for effective acoustic insulation
- Fire safe properties.
- Non-corrosive to steel.
Now we discuss how Fiberglass Wool meets the above requirements.
High Thermal Resistance (R value)
- Recommended thermal resistance is R > 1.25 sq.m. OC/W to get effective temperature drop across the wall and the roof.
- The final roof or wall composite should have thermal conductance value, U as low as 0.35 to 0.7 W/m2.oC.
- 50mm Fiberglass wool technically
and economically satisfies these requirements as evident from the tested values
of fiberglass wool given below.
Note: Thermal Resistance = Insulation thickness/Thermal conductivity.
wool insulation with a minimum density 16 kg/m3 and 50 mm thickness is
recommended for PEB insulation. The most widely specified density for metal
roof insulation is 24kg/m3 in 50mm thickness.
Ease of Application
- Fiber glass wool being soft, resilient and non sticky, can be unrolled easily over the purlins.
- Flanges are provided for easy stapling to make an uninterrupted layer of glass wool.
- High variety of roll lengths can be provided (from 7.5 m to 30 m) to match building design and to minimize number of joins.
- Can be easily cut and fold to accommodate openings through insulation.
- Dont have any shot content which can be awkward during installation.
- Fiberglass wool is available with various factory laminated and fire rated facing options, like Reinforced Aluminum Foil, High Strength Aluminum Foil or White Metalized Polypropylene (WMP-VR).
- All the facings used by Twig have undergone extensive testing across many different criteria (fire safety, bursting strength, tensile & transverse strength, ageing analysis etc) prior to being approved for use.
- High Strength Aluminum Foil and
WMP-VR can be installed without the need for wire mesh or weld mesh support,
thereby saving money and improving the finished look. Please note that this is
only recommended when distance between purling is less than 1.5M.
- Since Fiberglass wool is lightweight, it does not require a costly second metal roofing skin for support. It can simply be installed under the metal roof or inside the side cladding, with or without weld mesh support (depending on the type of facing used).
- Fiber glass wool being compressed cost of inventory and transport are less.
- Require less packaging less scrap.
- Cost of wire mesh can be eliminated by selecting right facing.
Glass wool being fibrous and having an open air cell structure is always preferred for its good noise reduction property and is almost exclusively used for acoustic insulation of multiplexes and sound studios. Its average range of NRC is 0.75 to 1.24. Glass wool can be used in internal partitions in addition to roof or wall.
Fire Safe Properties
Fiberglass Wool has the following fire ratings which are critical for PEB application.
- Not easily ignitable: conforms to Class P as per BS 476 part 5.
- Non-combustible: as per BS 476 part 4.
- Fire retardant Class 1 as per BS 476 part 7/Class O as per BS 476 part 6 and 7.
Non Corrosive to Metal:
Fiber glass wool being chemically very stable does not react with metal or any other matter under normal condition. Its pH value is very near to seven, so it is neither acidic nor basic in nature. Moreover it is free from shots, sulphur and chlorides. It is non hygroscopic, too. So the structural members are completely safe from corrosion.
This article was originally published in the September issue of magazine, "New Cloth Market - The complete textile magazine from textile technologists."