2.8.1 Internal insulation of rectangular air ducts
Internal insulation can only be applied to rectangular ducts. In principle, circular ducts can only be insulated externally. However, in order to prevent contamination of the installation and other problems, it is strongly recommended that air ducts are only insulated externally. If the choice is made to insulate rectangular ducts internally, the following materials can be used:
- glass or rock wool blankets with long fibres and finished with protective film against the detachment of these fibres. Most commonthicknesses 0.5" or 1" (13 or 26 mm);
- vulcanised synthetic elastomers, often with self-adhesive foil with a thickness of between 10 and 25 mm, depending on the applicationand manufacture.
Insulation blankets shall meet the following requirements:
- Non-combustible, according to DIN 4102, class 2 and fire propagation,according to NEN-EN 13501, class 1;
- Smoke number: ≤1 (smoke density negligible).
22.214.171.124 Processing of insulation blankets without self-adhesive foil
The insulation blankets are glued into the duct using contact adhesive. The entire duct surface is glued by means of a roller or spray adhesive. The contact adhesive can be applied at a temperature specified by the manufacturer/supplier. The duct wall must be dry, clean and free of grease to achieve good adhesion. For proper attachment, at least 2 adhesive pins, welding pins or split pins with cover sheet should be applied to the duct walls per m² of wall surface.
The insulation should be finished at the ends of the duct using galvanised sheet metal profiles or plastic profiles. This is to prevent the insulation from coming off when duct sections are connected.
2.8.2 External insulation of rectangular and circular air ducts
External insulation can be divided into three sections:
- thermal insulation by means of glass wool or rock wool blankets (see 126.96.36.199);
- thermal insulation using synthetic rubber (elastomeric) foam (see 188.8.131.52);
- thermal insulation using glass wool or rock wool blankets finished withaluminium sheeting (see 184.108.40.206).
220.127.116.11 Thermal insulation by means of glass wool or rock wool blankets
This type of insulation is only used indoors. Rectangular and circular metal air ducts are, depending on the application, usually thermally insulated externally by means of solid glass wool or rock wool blankets with a thickness of 25 mm. These blankets are made up of upright fibres, the so-called lamella mat, covered on one side with a foil of reinforced pure aluminium (thickness 0.02 mm). lamella mats are available without and with a self-adhesive foil. In case of rectangular ducts the insulation is placed against the insulation filling and finished on the suspension construction. The brace, which is used for circular ducts, is incorporated within the insulation. In the case of rectangular ducts, the suspension structure is outside the insulation. To this end, a hard pressed insulation filling of equal thickness is applied between the suspension structure and the duct wall and the seams are finished vapour-tight. It is also possible to lay the air ducts on MDF blocks with the same thickness as the insulation. The insulation must then be continued between the blocks (see 18.104.22.168). It is also possible to insulate the air ducts before installation. The insulation is then slightly pressed into the suspension bracket. The protective film around the insulation may not be torn.
Lamella mats shall meet the following requirements:
- Non-combustible according to DIN 4102, class A2 and fire propagationaccording to NEN EN 13501, class 1;
- Smoke number: ≤ 1 (smoke density negligible).
22.214.171.124.2 Processing of insulation blankets without self-adhesive foil
The insulation blanket should be cut to the following length: For rectangular duct:
- L = 2x (duct width + 2x insulation thickness) + 2x (duct height + 2x insulation thickness) + overlap.
For a circular duct:
- L = (duct diameter + 2x insulation thickness) x 3.14 + overlap.
126.96.36.199.3 Processing of insulation blankets with self-adhesive foil
This insulation is used in poorly ventilated areas and installed as visible work because of the absence of adhesive pins in the case of rectangular ducts up to a width of 1200 mm. The insulation should be cut to the following length:
For rectangular duct:
- L= 2x (duct width + 2x insulation thickness) + 2x (duct height +2x insulation thickness) + overlap.
For a circular duct:
- L= (diameter duct + 2x insulation thickness) x 3.14 + overlap.
188.8.131.52.4 Adhering and finishing for rectangular ducts (without foil)
The lamella mats are glued to the duct using contact adhesive. The entire duct surface is glued by means of a roller or spray adhesive. The contact adhesive should be applied at temperatures as specified by the manufacturer/supplier, preferably above approx. 0°C. The duct wall must be clean, dry and free of grease to ensure a good adhesion between the insulation and the duct wall.
In order to prevent the insulation from coming off the duct wall during the drying process of the adhesive, at least 2 adhesive pins, welding pins or split pins with cover sheet must be fitted on the underside and side walls of the duct per m² of wall surface for horizontally installed air ducts wider than 600 mm. With vertically installed ducts, pins must be fitted on all walls wider than 600 mm.
It is recommended that the insulation be placed exactly between the flange connections. If there is a risk of condensation on the flange, it is necessary to insulate the flanges. The seams between the lamella mats are glued together using an aluminium all-weather tape, with a width of 75 mm and a minimum adhesive strength on steel of at least 9N/25 mm², and then smoothed.
184.108.40.206.5 Adhering and finishing of rectangular ducts (with foil)
After cutting the insulation to the correct size remove the protective sheet approx. 10 cm lengthways, fit the insulation in the correct position and press firmly over the entire surface (note that once the insulation is pressed it cannot be moved).
Then remove the remaining protective sheet in stages and press the insulation firmly against the duct wall over its entire surface. It is recommended to apply the insulation exactly between the flange connections. If there is a risk of condensation on the flanges and it is necessary to insulate the flanges, a separate strip should be laid over the flange connection. Ducts up to a width of 1200 mm do not need to be provided with adhesive pins, welding pins or splice pins. The ambient temperature during processing should be between +10° C and +50° C. The seams between the flanges should be covered withan aluminium "all weather" tape, with a width of 75 mm and a minimum adhesive strength on steel of at least 9N/25 mm², glued together and then smoothed.
220.127.116.11.6 Adhering and finishing for circular ducts
Spray adhesive and/or contact adhesive are not used for circular air ducts. The insulation is cut to the correct length and the overlap is prepared by cutting the insulation loose from the aluminium foil. The seams are then taped and smoothed using the tape mentioned in 18.104.22.168.5. In addition, a tension band of at least 10 mm wide should be tightened around the insulation every 600 mm.
22.214.171.124 Thermal insulation using synthetic rubber (elastomeric) foam
This type of insulation is only used inside buildings. Rectangular and circular metal ducts are insulated externally using the abovementioned insulating material if there are large temperature differences (e.g. external air ducts) or if the ducts are visible. This material is available in various thicknesses, depending on the manufacturer and the temperature differences that occur. This insulation material is available without and with a self-adhesive foil. In order to obtain the correct insulation, the suspension structures are kept outside the insulation for both rectangular and circular ducts. Furthermore, a separate strip of the same insulation thickness is always glued over the duct flanges.
The insulation shall meet the following minimum requirements:
- Non-combustible in accordance with DIN 4102, class A2 and firepropagation in accordance with NEN-EN 13501, class 1;
- Smoke number: ≤ 1 (smoke density negligible);
- practical fire behaviour: self-extinguishing, non-dropping and non-flammable.
The insulation should be cut to the following length: For rectangular duct:
- All sides are cut to size of the side + 1x insulation thickness.
For a circular duct:
- L= (duct diameter + 2x insulation thickness) x 3.14.
126.96.36.199.3 Adhering and finishing
For both rectangular and circular ducts the insulation boards are glued over the entire surface with an adhesive provided by the insulation supplier. All bumpers should be pressed properly and therefore also glued properly. This also applies to the use of boards with a self-adhesive layer. The surfaces to be glued should be free of dust and grease. This form of insulation does not require any further finishing with tapes or adhesive pins.
188.8.131.52 Thermal insulation using glass wool or rock wool blankets, finished with aluminium sheeting
184.108.40.206.2 Installation of aluminium sheeting
Externally insulated ducts in the open air can be finished with (stuco) aluminium sheeting. In coastal areas the seawater-resistant quality (AlMg 3/ ENAW 5754) is recommended. After the metal air ducts have been installed on the roof or along a façade in the open air, the external insulation is applied with the required material and thickness. Then the aluminium sheet is applied. The thickness of the aluminium sheet is at least 0.8 mm. The sheet is applied by overlapping the neighbouring sheets. These sheets are provided with so-called grooves which are grooved in the sheet approx. 20 to 25 mm from the sheet end. These grooves are placed on top of each other and attached to each other with at least 13 x 3 mm stainless steel sheet screws (8 pcs/m) with screw thread up to the head. It is also possible to use rivets (aluminium/aluminium version) instead of sheet screws. However, installation and dismantling is more labour-intensive when using blind rivets. The sheets are installed tightly around the insulation, overlapping and draining in such a way that raining in and leakage are not possible. For this purpose, all seam connections in the sheet metal are sealed with transparent silicone sealant during or after the installation of the aluminium sheeting. In order to obtain a tight fit (especially in the case of larger ducts), it is possible to attach aluminium reinforcing strips to the duct during the insulation process. The aluminium sheeting can then be fixed to these strips. Care should be taken not to fasten the aluminium strips directly onto the steel. This is due to the risk of electrolytic voltage corrosion. A solution for this is to apply sealant, paint or a tape between the duct and the reinforcement strip.