Ducts are an essential part of HVAC and our household. They experience both the highest temperatures (heating season) and the coolest temperatures (cooling season). To make them as efficient as possible, it makes sense to insulate air ducts.
When is insulation required on air ducts?
Most ducts require insulation with a specified R-value; this is true both for supply and return ducts. R-values for duct insulation range from R-1.9 to up to R-12. Duct insulation is not required only in very limited situations (for heating in very hot climates – Climate Zone 1 or for return ducts in indirectly conditioned spaces as specified by Table 6.8.2.).
What is insulation required on air ducts?
This is quite easy to address: Just check ‘Table 6.8.2 Minimum Duct Insulation R-Value’. This is an ASHRAE (American Society of Heating, Refrigeration and Air-Conditioning Engineers) issued table that specified exactly duct insulation R-values. This table tends to be a bit complicated to both access and read, however.
To help everybody out, we are going to look at Table 6.8.2 broken down into 3 charts (Heating, Cooling, Heating+Cooling) that tell you exactly what insulation R-value is required on different ducts (in different climates areas, heating/cooling, duct location).
Even more importantly, we are going to check the insulation R-values of flex ducts, duct boards, duct wraps, duct liners, and other duct insulation materials. This will help you choose the correct materials to insulate your ducts with.
Before we check the 3 charts interpreting the ‘Table 6.8.2 Minimum Duct Insulation R-Value’ by ASHRAE, we need to familiarize ourselves with the metrics in that table:
Table of Contents
3 Factors That Determine The R-Value For Duct Insulation
ASHRAE duct insulation table uses 3 factors, based on which they specify the required duct insulation R-values. These are:
- HVAC purpose – Heating, Cooling, or Heating + Cooling. It also matters if your supply ducts and return ducts are used only for heating (R-value up to R-12), only for cooling (R-value up to R-8) or for heating and cooling; that’s why you will see 3 duct insulation R-value charts below, one for each purpose.
- Climate Zones. Where you live is important. The further north you go (Example: Chicago, Canada), the higher R-value heating duct insulation you will need. There are 8 climate zones in the USA, based on temperature and humidity levels, and for each climate zone, you have to use a specific R-value. You can use this simple chart to check in which climate zone you live:
- Duct Location. ASHRAE specified duct R-value insulation requirements based on where the supply and return ducts are located. In short, they differentiate between 3 locations: Exterior Ducts, Unconditioned Space and Buried Ducts, and Indirectly Conditioned Space. Exterior ducts include the attic ducts (installed above insulated ceilings), and ceiling ducts in garages and crawl spaces. Indirectly conditioned ducts include air plenums as well.
With all of this in mind, you can check each chart (for heating and cooling, for heating only, for cooling only) to see what insulation R-value to use on ducts in climate zones 0, 1, 2, 3, 4, 5, 6, 7, and 8 and in all 3 different locations:
R-Values For Heating And Cooling (Supply Ducts And Return Ducts)
| Climate Zone: | Exterior Ducts: | Unconditioned Space Ducts, Buried Ducts: | Indirectly Conditioned Space Ducts: |
| Climate Zone 0 | R-8 | R-6 | R-1.9 |
| Climate Zone 1 | R-8 | R-6 | R-1.9 |
| Climate Zone 2 | R-8 | R-6 | R-1.9 |
| Climate Zone 3 | R-8 | R-6 | R-1.9 |
| Climate Zone 4 | R-8 | R-6 | R-1.9 |
| Climate Zone 5 | R-12 | R-6 | R-1.9 |
| Climate Zone 6 | R-12 | R-6 | R-1.9 |
| Climate Zone 7 | R-12 | R-6 | R-1.9 |
| Climate Zone 8 | R-12 | R-6 | R-1.9 |
Most homeowners use supply and return ducts for both heating and cooling. In this case, you will have to use R-8 duct insulation on the exterior ducts and R-6 on unconditioned ducts in warmer climates (up to climate zone 4).
In colder climates (climate zone 5 to 8), you have to use R-12 duct insulation on exterior ducts. Be aware that this also applies to ducts in the attic, garage, and crawl spaces.
Let’s look at what R-insulation value you need if you only use ductwork for heating:
R-Values For Heating Only (Supply Ducts And Return Ducts)
| Climate Zone: | Exterior Ducts: | Unconditioned Space Ducts, Buried Ducts: | Indirectly Conditioned Space Ducts: |
| Climate Zone 0 | No Insulation | No Insulation | No Insulation |
| Climate Zone 1 | No Insulation | No Insulation | No Insulation |
| Climate Zone 2 | R-6 | R-6 | R-1.9 |
| Climate Zone 3 | R-6 | R-6 | R-1.9 |
| Climate Zone 4 | R-6 | R-6 | R-1.9 |
| Climate Zone 5 | R-12 | R-6 | R-1.9 |
| Climate Zone 6 | R-12 | R-6 | R-1.9 |
| Climate Zone 7 | R-12 | R-6 | R-1.9 |
| Climate Zone 8 | R-12 | R-6 | R-1.9 |
As you can see, if you live in a very hot zone – climate zone 0 (we don’t have that in the USA) or climate zone 1 – you don’t need duct insulation when it comes to heating. If, however, you live in the northern part of the US (climate zones 5, 6, 7, or 8), you will require R-12 insulation on exterior ducts.
In general, most ducts for heating should be at least R-6 insulated.
R-Values For Cooling Only (Supply Ducts And Return Ducts)
| Climate Zone: | Exterior Ducts: | Unconditioned Space Ducts, Buried Ducts: | Indirectly Conditioned Space Ducts: |
| Climate Zone 0 | R-8 | R-6 | R-1.9 |
| Climate Zone 1 | R-8 | R-6 | R-1.9 |
| Climate Zone 2 | R-8 | R-6 | R-1.9 |
| Climate Zone 3 | R-8 | R-6 | R-1.9 |
| Climate Zone 4 | R-8 | R-6 | R-1.9 |
| Climate Zone 5 | R-8 | R-6 | R-1.9 |
| Climate Zone 6 | R-8 | R-6 | R-1.9 |
| Climate Zone 7 | R-1.9 | R-1.9 | R-1.9 |
| Climate Zone 8 | R-1.9 | R-1.9 | R-1.9 |
Cooling is a different story altogether. As you can see, you will require the least insulation (R-1.9) in very cold climates (climate zone 7 and 8).
In normal and warmer climates, you will require:
- R-8 insulation on exterior ducts.
- R-6 insulation in buried ducts and unconditioned space ducts.
- R-1.9 insulation on indirectly conditioned space ducts.
Using these 3 charts, you can adequately determine when is insulation required on air ducts and what R-values you need. Let’s check some R-values for some of the materials used to insulate air ducts:
R-Values Of Duct Insulation Materials
To insulate ducts, we use different kinds of insulations, including duct boards, wraps, lines, and so on.
In order to get that duct insulation R-value up to R-6, R-8, or even R-12, you will have to have an idea of how much R insulation values these materials add to the duct insulation. Here are the most common duct insulation materials and their respective R-values:
- Duct liner R-value is about R-4 per inch of thickness. That means that a 1-inch duct liner has an R-value of R-4, a 1.5-inch duct liner has an R-value of R-6, and a 2-inch duct liner has an R-value of R-8. For R-12 flex duct insulation, for example, you would need 3-inch duct liners. These liners are the most common way to insulate ducts due to ease of installation, high R-value per inch, and affordable cost.
- Duct wrap R-value is about R-4.2 per inch of thickness. Duct liners and duct wraps are quite comparable.
- Duct board R-value is about R-4.35 per inch of thickness. Typically you would use 2-inch duct boards with R-8.7 insulation value; that is what most air ducts need.
It’s hard to talk about which is the best duct insulation. Duct liners, wraps, and boards are all quite good and there are no substantial differences between R-values of these duct insulation materials.
Hopefully, this gives you sufficient information on if and when is insulation required on air ducts and what should the duct R-value be, as well as which types of duct insulation materials you can choose from.
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