Duct and HVAC insulation · Stone wool
ROCKWOOL ROCKDUCT
- Max. temp.
- 350–540 °C
- Density
- 40–80 kg/m³
- Thickness
- 25–50 mm
How should kitchen exhaust and hot exhaust ducts be insulated?
In short
Kitchen exhaust and hot exhaust ducts are insulated to reduce the heat given off to the space around the duct and to stop people touching a hot duct surface, and it helps limit water vapour and grease condensing inside the duct. Stone wool is commonly used for this work because it is non-combustible and withstands high temperatures; it is fixed with pins and covered with metal jacketing. But insulating a duct does not automatically make it a "fire-rated duct". A duct system that must have a fire-resistance rating is a complete tested system and must be designed by specialists.
How is a kitchen duct different from an AC duct?
For AC ducts the main issues are lost cooling and dripping. A kitchen exhaust duct carries hot air laden with grease vapour and smoke, so there is more to consider.
- Hot duct surface. Heat radiates into ceiling voids and neighbouring rooms, making the air conditioning work harder, and it is a hazard if someone touches it.
- Grease builds up inside the duct and can act as fuel. A National Insulation Association article states that grease duct fires can reach roughly 1,600–2,000 °F (about 870–1,090 °C) within minutes, hot enough to ignite combustible materials near the duct.
- Clearance to combustibles. The US standard NFPA 96, which many projects reference, sets a clearance between kitchen exhaust ducts and combustible materials and allows it to be reduced only with approved protection. The requirement that actually applies depends on each project's laws and standards; the design engineer must confirm it.
What insulation does and does not do
| Issue | How duct insulation helps | Limitation |
|---|---|---|
| --- | --- | --- |
| Heat given off around the duct | Reduces heat transfer from the duct surface | Thickness must be designed for the air temperature in the duct |
| Contact with a hot surface | Lowers the temperature of the outer surface people can touch | The acceptable surface temperature is a project requirement |
| Water vapour and grease condensing in the duct | The duct wall cools more slowly where it passes through cooler spaces | Does not replace cleaning the duct at regular intervals |
| Fire spreading from the duct into the building | Stone wool is non-combustible, so it adds no fuel | A fire-resistance rating belongs to the complete tested system, not to the insulation alone |
Why stone wool?
Insulation for ordinary AC ducts is designed for air-conditioning temperatures. Kitchen and hot exhaust ducts need a material with a high service temperature that does not burn, which is the strength of stone wool. Examples from ROCKWOOL Thailand data sheets:
| Product | Grade | Max. service temperature per data sheet | Reaction to fire |
|---|---|---|---|
| --- | --- | --- | --- |
| ROCKWOOL ROCKDUCT | ProRox BL 938(40) ALU SA (aluminium-foil faced) | 540 °C | Euroclass A1 |
| ROCKWOOL ProRox BL | BL 938 SA / BL 958 SA / BL 960 SA | 350 / 450 / 650 °C | Euroclass A1 |
These figures are material limits. The grade and thickness must be chosen for the real air temperature in the duct and the project requirements. The ROCKWOOL Thailand handout lists its exhaust-duct insulation set as 50 mm stone wool, which is the manufacturer's example and not a thickness that suits every job.
How is it fixed and finished?
Blanket insulation on rectangular ducts needs fasteners to carry its weight, especially on the underside; otherwise it sags and gaps open.
- Insulation pins and washers. The detail drawing in the ROCKWOOL Thailand handout shows an exhaust duct insulated with 50 mm ProRox BL, held by pins welded to the duct with washers at 300 mm intervals. That is the detail in that document; the actual number and spacing follow the project drawings.
- Metal jacket. It protects the insulation from impact and dirt. The same document states that exhaust ducts exposed outside the building are to be covered with a galvanised steel sheet jacket along their length.
- Insulation joints. Butt them tightly with no gaps, and do not crush the insulation. See Gaps and compression in insulation.
Adhesive-bonded pins such as ASAWA PIN, with an aluminium shank and galvanised-steel base, suit fixing insulation to walls, roofs and surfaces that are not hot. A ROCKWOOL installation guide sets out the principle that the duct surface temperature must not exceed the service temperature of adhesive stick pins. For a hot duct, check the limits of the adhesive and the pin first, or use welded pins as drawn. Ask us with the duct's operating temperature.
"Fire-rated duct" is a system matter, not an insulation matter
Duct insulation ≠ fire-rated exhaust duct
A kitchen exhaust duct enclosure that claims a fire-resistance rating or zero clearance to combustibles must be tested as a complete system to a dedicated standard such as ASTM E2336, which covers the non-combustibility of the materials, resistance to external fire and a fire inside the duct. The result belongs only to the system installed as tested, including material type, number of layers, fixing method and cleaning access openings. Wrapping a duct with general stone wool insulation does not give that rating. Work with fire requirements must be designed and certified by qualified engineers and installers.
Read what the fire terms mean in Fire performance terms for insulation and the role of stone wool in fire protection systems in Stone wool in passive fire protection.
Information to prepare before asking
- Air temperature in the duct in service, and the type of work (kitchen, oven, flue gas)
- Duct size and shape, length and number of bends
- Whether the duct runs inside or outside the building
- Distance to combustible materials such as ceilings, timber or lightweight walls
- Whether the project specifies a fire-resistance rating for the duct. If it does, a system backed by test evidence is required
For ordinary duct insulation, see How to choose duct insulation.
Sources
- ROCKWOOL (Thailand) — Handout: ROCKWOOL insulation for HVAC ducts, 200 602-9/25 V8 (TH)
- ROCKWOOL (Thailand) — data sheets ProRox BL 938(40) ALU SA / ROCKDUCT Hi-Temp (200 351TH-03/21 V1) and ProRox BL 960 SA (200 352TH-08/25 V4); the BL 938 SA and BL 958 SA values come from our product data, already checked against the manufacturer's data sheets
- ROCKWOOL Limited (UK) — Installation guide: Ductwrap and Ductslab (22/2/2021), note on adhesive stick pins and duct surface temperature
- National Insulation Association, Insulation Outlook — Clay Booth, "Will Current Grease Duct Enclosure Systems Pass the Test in 2009?" (September 2008)
- Consulting-Specifying Engineer — "Specifying grease ducts using new codes" (summary of NFPA 96 and IMC clearance and enclosure requirements)
- ASAWA PIN catalogue
Related products
Categories and example products for this topic
- ROCKWOOL ROCKDUCT — Stone wool for ducts, kitchen and exhaust ducts; foil-faced grade to 540 °C.
- ROCKWOOL ProRox BL — Stone wool blanket in 3 grades to 350 / 450 / 650 °C; Euroclass A1.
- ASAWA PIN — Adhesive-bonded aluminium pins, 32–125 mm, for surfaces that are not hot.
Industrial and high-temperature insulation · Stone wool
ROCKWOOL ProRox BL
- Max. temp.
- 350–650 °C
- Density
- 60–100 kg/m³
- Thickness
- 50–100 mm
