Ella Walter
Panned joists are used as return ducts in HVAC systems. This is done by attaching sheet metal or foil-faced cardboard to the bottom of the floor joists. While this method is simple and cost-effective, it is highly discouraged because it is almost always leaky. Leakages can be due to penetrations for wiring, piping, or ceiling strapping, or inadequate air-sealing at connections between the bay and the ducts. This can cause several issues, including the introduction of pollutants, fire safety hazards, and increased energy costs.
| Characteristics | Values |
|---|---|
| What is joist panning? | A method of installing ductwork for an HVAC system that uses the space between floor joists. |
| Why use joist panning? | It is a simple, easy, and cost-effective method that requires less material than running metal ducts. |
| Materials used for joist panning | Sheet metal, foil-faced cardboard (e.g. Thermopan), bubble insulation (e.g. EcoFoil), ductboard, closed-cell foam (e.g. Armaflex), gypsum board, OSB. |
| Concerns with joist panning | Leaky bays due to penetrations for wiring, piping, or inadequate air-sealing; mould and allergen issues; fire safety concerns. |
| Solutions to concerns | Generous use of duct mastic as a sealant; use of ductboard, closed-cell foam, or foil-faced cardboard to prevent mould and allergens; ensuring compliance with fire safety codes. |
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What You'll Learn
Why panned floor joists are not recommended
Panned floor joists are still commonly used as return ducts in HVAC systems. However, there are several reasons why this practice is not recommended:
Leakage
Panned floor joists can cause significant air leakage because negative pressure in the cavity will draw air from the outside into the cavity through the construction joints of the rim area at the end of the joist cavity. This can result in higher utility bills, reduced comfort levels, and decreased efficiency of the HVAC system.
Inadequate Air-Sealing
The junctions between wood and metal in panned floor joists are difficult to seal properly. The thermal expansion and contraction of the wood can cause sealing materials to fail over time, leading to increased leakage. Even with the use of duct mastic, one of the best sealants available, it can be challenging to achieve a complete seal.
Indoor Air Quality Concerns
The leakage associated with panned floor joists can introduce mould spores, bad odours, and pollutants from unknown sources into the indoor air. This can be particularly problematic if the panned-bay returns are located in basement ceilings, as basements are often damp and prone to mould growth.
Fire Safety
Building materials used in panned floor joists, such as wood products, may not meet the same flame- and smoke-spread criteria as approved duct materials. This could potentially pose a fire safety risk.
Cleaning and Maintenance
Panned-bay returns can be challenging or even impossible to clean thoroughly due to obstacles such as pipes, wires, and cross-bracing. This can be especially concerning for individuals with allergies or respiratory issues.
In summary, while panned floor joists are still used, there are several significant drawbacks that should be carefully considered before employing this practice. Proper planning and the use of approved duct materials and sealing techniques are essential to mitigate these issues.
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Using sheet metal for joist panning
Panned joists are commonly used as return ducts in HVAC systems. However, this practice is discouraged because it can cause leakage due to the negative pressure in the cavity, which draws outside air into the cavity. To create a panned joist, a solid panning sheet material, such as sheet metal, is attached to the bottom of a floor joist to create a return-air pathway.
To install sheet metal for joist panning, you can purchase sections of round metal ducts or use sheets of metal. If using sheets of metal, attach them securely to the bottom of the floor joists, ensuring all joints are airtight. You can use sealant or foil tape to seal the joints.
While sheet metal can be an effective material for joist panning, it is important to consider the potential challenges of achieving an airtight seal and the higher costs compared to other materials.
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Using foil-faced cardboard for joist panning
Panned floor joists are still commonly used as return ducts in HVAC systems. However, this practice is highly discouraged because air leakage can be difficult to prevent. Joist panning can be done by attaching a solid panning sheet material to the bottom of a floor joist to create a return-air pathway.
If you must install a return-air duct in a basement joist bay, you can use foil-faced cardboard products such as Thermopan. These products are more user-friendly than sheet metal but require some effort to create a sealed return. They are also not flexible and can be damaged by excess moisture. To install foil-faced cardboard for joist panning:
- Cut the foil-faced cardboard to the appropriate size to fit between or underneath the floor joists.
- Securely attach the foil-faced cardboard to the bottom of the joists using staples, steel staples, sheet metal screws, or roofing nails. Ensure that the print side is facing out or down.
- Tape the seams with reflective tape, such as aluminium tape or UL181 aluminium tape, to create an airtight seal.
- If necessary, cut and bend the foil-faced cardboard to fit around any obstacles, such as pipes or wires.
- Ensure that all gaps and penetrations for wiring, piping, or ceiling strapping are properly sealed to prevent air leakage.
It is important to note that while foil-faced cardboard can be used for joist panning, it may not be the best option if there is excess moisture present or if a more flexible material is required. Other alternatives to consider for joist panning include composite materials, such as Reflectix® RAP (Corrugated Sheet) Return Air Duct Panning, which is rated Class 1/Class A and has a reflective surface on both sides.
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Using bubble insulation for joist panning
Panned joists are commonly used as return ducts in HVAC systems. However, this practice is highly discouraged because air leakage is very difficult, if not impossible, to prevent. If you must use the joist panning method for your HVAC system, you can use bubble insulation for joist panning.
Bubble insulation, such as EcoFoil, is an excellent solution for joist panning as it is less expensive and more flexible than sheet metal. It comes in pre-cut widths that make it perfect for installing between your floor joists. The flexibility of the bubble insulation also makes it ideal for installation, and when you wrap the seams using reflective tape, it becomes completely airtight.
To install bubble insulation for joist panning, you need to set up the material between or beneath the floor joists and then cover the closures. If you use EcoFoil, you need to staple it precisely to the beams. EcoFoil bubble insulation is also waterproof and resists the growth of mould and fungi. It reflects up to 97% of radiant heat back to its source, making it an excellent choice for unconditioned spaces such as attics and crawl spaces.
While bubble insulation is a more affordable and effective option than sheet metal, it is important to consider the potential drawbacks of using any type of joist panning. Joist panning can be leaky, and these leaks can be due to penetrations for wiring, piping, or ceiling strapping. In addition, the junctions between wood and metal are difficult to seal, and the thermal expansion and contraction of the wood can cause sealing materials to fail. As a result, using joist panning can lead to increased energy loss, higher utility bills, and decreased comfort levels.
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Sealing the seams
If you are using sheet metal, you will need to seal the seams with duct mastic. Duct mastic is one of the best sealants for ducts and should be applied generously. It is important to ensure that the mastic is applied to all necessary areas.
In some cases, leaks may be due to penetrations for wiring, piping, or ceiling strapping. It is important to seal these areas properly to prevent air leakage. One way to do this is by using foil tape or sealant to make all the joints in the duct airtight.
Additionally, it is important to note that the junctions between wood and metal can be difficult to seal. The thermal expansion and contraction of the wood can cause sealing materials to fail. As a result, it is recommended to use a sealant that can withstand these movements, such as duct mastic.
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Frequently asked questions
Joist panning is a technique used to install duct work for an HVAC system. It uses the area between the floor joists and requires less material than running metal ducts.
Metal sheets, foil-faced cardboard products, and bubble insulation are all materials that can be used for joist panning.
Joist panning is a simple and cost-effective method for installing ductwork. It can also help improve the efficiency of an HVAC system.
Yes, one of the main drawbacks of joist panning is that it can be leaky, especially at the junctions between wood and metal. This can lead to issues with mould, allergens, and energy loss.
