Tillie Doyle
Joist panning is a technique used to install ductwork for HVAC systems. It involves using the space between floor joists to create a duct run, which is more cost-effective and requires less material than traditional metal ducts. While this method is simple and easy, it is highly discouraged due to the difficulty of preventing air leakage. In addition, the thermal expansion and contraction of the wood can cause sealing materials to fail, leading to leaks. To address this issue, duct mastic, a highly effective sealant, can be applied generously to seal up the ducts. However, it is important to consider local codes and regulations, as some areas may not allow joist panning for HVAC ducts.
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What You'll Learn
Using sheet metal for joist panning
If you're looking to save money and install ductwork for your HVAC system yourself, you might consider using the joist panning method for cold air returns. This method is simple and easy because it uses the space between the floor joists and requires much less material than running metal ducts.
One option for joist panning is to use sheet metal. While this is the most expensive and labour-intensive solution, it is possible to purchase sheets of metal to use for joist panning. To do this, you would screw a piece of sheet metal to the bottom of the joists and attach a duct to it to return the air to the HVAC system.
However, it is important to note that using sheet metal for joist panning can be challenging to seal properly. 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. In addition, leaks can occur due to penetrations for wiring, piping, or ceiling strapping, and inadequate air sealing at connections between the bay and the ducts or between the joists and the sheet metal pan.
If you choose to use sheet metal for joist panning, it is essential to use a generous amount of duct mastic, one of the best sealants for ducts, to seal the junctions between the wood and metal properly.
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Using foil-faced cardboard products
Foil-faced cardboard products, such as Thermopan, are a more user-friendly and cost-effective alternative to sheet metal for HVAC ductwork. They are also a good option if you want to install your ductwork yourself, as they are easier to manipulate and require fewer tools—you will only need a utility knife and a stapler.
However, foil-faced cardboard is not flexible and requires more effort to create an airtight seal. You will need to staple the cardboard to the joists and then seal the seams with reflective tape. Additionally, foil-faced cardboard is susceptible to moisture damage, so it should not be used in humid environments.
If you are looking for a more flexible option, you could consider radiant barrier bubble insulation products, such as EcoFoil. These also come in rolls that are sized to fit between or underneath the floor joists. EcoFoil is also waterproof and mould-resistant, which can be beneficial if your ductwork is located in an attic or crawl space.
When installing any type of ductwork, it is important to consider the potential for air leakage, especially at junctions between different materials, such as wood and metal. Proper sealing is critical to prevent this and to ensure the efficient operation of your HVAC system.
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Using duct mastic to seal ducts
Duct mastic is one of the best sealants for ducts, and it is important to use it generously. It is recommended to use duct mastic that is UL 181-rated, as it is more flexible, adheres better, and lasts longer. Before applying the mastic, ensure that the joint area is clean and free of particles and dust. If the joint is not clean, the mastic will not create a tight seal.
When applying the mastic, use enough to form a continuous coating on the surface of the duct. You can use a brush, trowel, gloved hand, or caulking gun for this. Work the mastic into the joint or crack and press lightly to ensure an even coating. There should be at least a 1/2-inch overlap around any joint, crack, or hole in the duct.
If you are using mastic to seal seams in fiberglass board ductwork, use fiberglass mesh tape for all joints. This is especially important if the area being sealed is a joint that will be under stress. The tape used for sealing the joints should be designed specifically for use with mastic.
When using mastic to seal large gaps, be careful not to get globs of mastic inside the ducts, as this could create an indoor air quality issue if the mastic is not rated zero VOC. Most HVAC professionals use a combination of mastic and tape for sealing ducts.
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Using duct insulation to prevent leaks
Panned joist return ducts are notoriously leaky, and the junctions between wood and metal are challenging to seal. The thermal expansion and contraction of the wood can cause sealing materials to fail. Therefore, it is important to use duct insulation to prevent leaks.
Firstly, you should inspect your ducts for any visible gaps, holes, or disconnected sections. Sealing these leaks before insulating will maximise energy efficiency. You can use duct mastic to seal leaks, which is more reliable than cloth-backed duct tape. Duct mastic is also more durable than any available tape and is generally easier for a DIY installation. However, it will not bridge gaps over 1/4 inch, so these gaps must first be bridged with web-type drywall tape or heat-approved tape.
After sealing any leaks, you can start the process of applying new insulation. By sealing the seams, you reduce the risk of conditioned air escaping and unconditioned air infiltrating the system. Apply a continuous strip of foil tape to all longitudinal seams along straight runs of the duct. Use a paintbrush to apply duct mastic to the joints where an elbow connects to a duct. Check that all connections and seams are thoroughly sealed to prevent air leakage.
Properly insulating your ducts will prevent the loss of conditioned air and improve the overall performance of your HVAC system. When ducts are properly insulated, the heated or cooled air maintains its temperature as it travels through the system, resulting in more consistent temperatures throughout your home. Insulating ducts in unconditioned spaces, such as attics or crawl spaces, is usually very cost-effective.
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Using ductboard or closed-cell foam for basement joist bays
If you're looking to install a return-air duct in a basement joist bay, there are a few options to consider. Firstly, ductboard can be used, with the foil facing inside the duct where airflow will occur. Closed-cell foam, such as Armaflex, is another option. This type of foam does not have a fibrous inside surface, preventing the trapping of dust and mold spores.
Additionally, basement joist bays lined with ductboard or closed-cell foam can help to minimise the introduction of mold spores and unpleasant odours into the supply air. This is in contrast to sheet metal attached to the bottoms of joists, which can lead to mould contamination, especially if the joists already have mould growth.
When installing ductboard or closed-cell foam in basement joist bays, it is essential to follow proper procedures. Sealant or foil tape should be used to ensure that all the joints in the duct are airtight. This helps to maintain the efficiency of the HVAC system and prevents the introduction of unwanted particles into the airflow.
Furthermore, filtration is a crucial component of any air-handling system. It is recommended to install a pleated filter with a minimum depth of 2 inches on the air handler to ensure effective filtration and maintain the quality of the air circulated throughout the house.
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Frequently asked questions
Joist panning is a technique used to install ductwork for HVAC systems. It involves using the space between the floor joists to create a duct run.
Materials such as sheet metal, foil-faced cardboard (e.g. Thermopan), ductboard, closed-cell foam (e.g. Armaflex), and foil-faced insulation board can be used. Sheet metal is the most expensive and labour-intensive option.
You can nail or screw a piece of sheet metal to the bottom of the joists and attach a duct to it. Ensure that all joints are airtight by using sealants or foil tape.
Joist panning is prone to leaks due to inadequate air-sealing, wiring, piping, or ceiling strapping. It is also difficult to clean and can introduce mould spores and odours into the HVAC system if located in a basement with poor air quality.
