Alexei Volkov
Wall cavities are essential for maintaining good airflow throughout a building. Return air ducts can be installed into wall cavities to help with this. However, this is not always necessary, especially if the ducts are only being used sparingly during construction. In addition, while drywall can act as a return duct, it is important to ensure that the entire return path is sealed. If installing supply ducts within the walls, it is also crucial to verify that the duct can output the required airflow. If installing return ducts using the floor or ceiling structure, it is recommended to seal both the exterior and interior of all return boxes to prevent air leakage. Furthermore, while plastic boxes are generally not allowed in air-handling spaces, it is possible to install a junction box in a cold air return using plenum-rated materials such as a metal junction box.
| Characteristics | Values |
|---|---|
| Necessity of return air ducts in wall cavities | Not necessary, especially if they are only being used a little during construction |
| Necessity of lining a return air cavity | Not necessary unless there is above-average humidity |
| Materials to line a return air cavity | Sheet metal |
| Sealing a return air cavity | Sufficient to compress drywall and studs to create a channel in the wall; caulk the corners if there are gaps |
| Installation of a junction box in a cold air return | Possible as long as "plenum-rated" materials are used; plastic boxes may not be used in air handling spaces |
| Using a cavity as a return-air pathway | Not a fire hazard in itself, but will encourage a fire to spread throughout the building; may cause mold growth or the deterioration of building materials in humid climates |
| Installation of supply ducts within walls | Double-wall assemblies are required to allow for proper insulation and duct size |
| Installation of return ducts using the floor or ceiling structure | ENERGY STAR recommends sealing both the exterior and the interior of all return boxes to prevent air leakage |
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What You'll Learn
- Fire safety: Return air cavities can increase fire risk by providing oxygen and a path for flames to spread
- Mould and structural issues: Cavities can cause mould and structural damage due to trapped debris, water, and humidity
- Installation: Drywall can act as a return duct, but proper sealing is essential to prevent air leakage
- Building regulations: Using cavities as return ducts is common but may not comply with updated building codes
- Materials: Metal is preferred over plastic due to fire safety concerns and potential fumes from burning plastic
Fire safety: Return air cavities can increase fire risk by providing oxygen and a path for flames to spread
Return air cavities are commonly used as supply or return ducts for HVAC systems. While this practice is not a fire hazard in itself, it can increase the risk of fire spreading throughout a building. This is because the cavities often create a connection between inside and outside air, providing a path for flames to move from one compartment to another.
A recent survey of homes in the UK found that most buildings were not constructed with sufficient and up-to-standard fire protection. In particular, timber-frame properties were found to have improperly fitted or missing cavity barriers. Cavity barriers are blocks of fire-resistant material used to close off cavities and prevent the spread of fire, smoke, and hot gases. They are designed to be highly sensitive to extreme temperatures and will expand when subjected to fire, sealing off any spaces.
The use of return air cavities as ducts can also lead to depressurization, causing outside air to be sucked into the cavity. This can result in the spread of pollutants, mould, and humidity throughout the building, potentially causing health issues for occupants. It is recommended that return air ducts be sealed to prevent air leakage and the associated issues.
To address the fire safety concerns associated with return air cavities, it is crucial to install effective cavity barriers. These should be installed vertically at every party wall and horizontally at every floor to facilitate fire compartmentation. By dividing and compartmentalizing hidden cavities, the spread of fire from one area of the building to another can be prevented.
Additionally, it is important to note that the use of plastic in air handling spaces should be avoided due to the potential for toxic smoke to be released if it catches fire. Instead, plenum-rated materials such as metal junction boxes and metallic wiring methods should be used. Proper sealing of all seams, gaps, and holes in the air barrier with caulk or foam is also essential to minimize the risk of fire spread.
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Mould and structural issues: Cavities can cause mould and structural damage due to trapped debris, water, and humidity
Return air cavities can cause mould and structural damage due to trapped debris, water, and humidity.
Firstly, in terms of mould, the humidity caused by cavities can encourage mould growth. In humid climates, a cavity space used as a return-air pathway will pull humid air into the cavity space, possibly leading to mould.
Secondly, structural damage can occur due to the deterioration of building materials caused by humidity. This can also be caused by fire, as the use of cavities as return or supply ducts is not a fire hazard itself, but it will encourage a fire to spread throughout the building.
Furthermore, structural issues can arise from the installation process of return air cavities. For example, if the seams, gaps, and holes of the air barrier are not properly sealed with caulk or foam, it can lead to air leakage. Additionally, the use of plastic in air handling spaces is not recommended due to the potential for toxic smoke if it catches fire.
Finally, the design of the cavity itself can impact the structural integrity of the building. For instance, the placement of cooling channels in mould cavities is crucial. If they are too widely spaced, there will be wide temperature fluctuations, and if they are too close together or too near the cavity surface, the mould becomes structurally weak.
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Installation: Drywall can act as a return duct, but proper sealing is essential to prevent air leakage
Drywall can indeed act as a return duct, but it is important to ensure proper sealing to prevent air leakage. This can be achieved by sealing all seams, gaps, and holes with caulk or foam. It is worth noting that while drywall can be used, it is not the only option for creating a return duct. Metal ducting is a popular alternative, although it is not necessary.
When creating a return duct, it is essential to consider the location of the duct and coordinate with the relevant contractors. This is to ensure proper spacing for installation. If installing supply ducts within the walls, it is crucial to verify that the duct can provide the required airflow. Typically, only double-wall assemblies will have enough depth to accommodate proper insulation and duct size.
It is also worth noting that using cavity spaces as return-air pathways can present some issues. One concern is fire safety. While building cavities are not inherently fire hazards, they can encourage the spread of fire throughout a building. This is because the running fan can draw air and potentially flames into the wall cavity, igniting the framing. Additionally, in humid climates, cavity spaces used as return-air pathways can pull in humid air, promoting the growth of mould or the deterioration of building materials.
To address these issues, it is recommended to seal both the exterior and interior of all return boxes to prevent air leakage. This can be done using tape, mastic duct sealant, or other approved methods. It is also important to ensure that any debris or waste is removed from the cavity to maintain good airflow and prevent contamination of the air being circulated.
Overall, while drywall can act as a return duct, proper sealing and coordination with contractors are crucial to ensure effective and safe airflow throughout the structure.
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Building regulations: Using cavities as return ducts is common but may not comply with updated building codes
Using building cavities as return ducts is a common practice. However, this may not comply with updated building codes, which restrict the use of cavity spaces as supply ducts.
According to the U.S. Department of Energy's ENERGY STAR program, there are specific requirements for insulation and duct leakage if building cavities are used as supply and return ducts. For instance, supply ducts in an unconditioned attic must have insulation equal to or greater than R-8, while all other supply ducts and return ducts in unconditioned spaces must have insulation equal to or greater than R-6.
Additionally, the ENERGY STAR program mandates that all ducts in exterior walls must be within the air barrier and the thermal boundary. It is crucial for the framer and HVAC contractor to collaborate on the location of the return duct to ensure proper spacing for installation.
Another important consideration is fire safety. Building materials such as wood products do not meet the same flame- and smoke-spread criteria as approved duct materials. While using cavities as return ducts is not a fire hazard in itself, it can encourage the spread of fire throughout the building.
Furthermore, using cavities as return ducts can lead to issues with humidity and mould growth, particularly in humid climates. Cavity spaces used as return-air pathways can pull humid air into the cavity, potentially resulting in mould growth or the deterioration of building materials.
To address these concerns, it is recommended to use recognised and acceptable duct materials for all HVAC airways. Approved duct materials include galvanised steel, aluminium, fibreglass duct board, and flexible duct. It is also essential to ensure proper sealing and insulation of the ducts to prevent duct leakage and energy loss.
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Materials: Metal is preferred over plastic due to fire safety concerns and potential fumes from burning plastic
When it comes to materials for return air cavities, metal is generally preferred over plastic due to fire safety concerns and the potential release of toxic fumes from burning plastic.
Metal, such as galvanized steel, aluminum, or sheet metal, is often the material of choice for return air cavities, also known as ductwork or ducting. This is because metal meets the stringent flame- and smoke-spread criteria required for duct materials. In the event of a fire, metal ductwork can help prevent the rapid spread of flames and smoke throughout the building.
Plastic, on the other hand, is typically discouraged due to its combustible nature. While some plastics, like polypropylene, can be made more fire-resistant through the use of flame retardant additives, they still pose environmental and health risks. The additives can complicate recycling, impact the material's recyclability, and have negative effects on ecosystems and living organisms.
Additionally, burning plastic can release toxic fumes, further exacerbating the hazards of a fire. This is a critical consideration for the safety of building occupants and emergency responders.
Furthermore, metal ductwork is preferred because it can be properly sealed and insulated. According to the U.S. Department of Energy, all supply and return duct connections should be sealed with mastic or approved tape. Proper sealing helps ensure airtightness and prevents air leakage, which can impact energy efficiency and indoor air quality.
While plastic may be more cost-effective and easier to work with, metal is the preferred material for return air cavities due to its fire-resistant properties, ability to be properly sealed and insulated, and lack of toxic fume emissions during a fire.
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Frequently asked questions
No, you don't need to install a return air duct into a wall cavity, especially if you're only using them a little during construction. However, wall cavities are necessary for maintaining good airflow throughout a structure. Return air ducts can help keep good airflow through wall cavities, preventing structural issues in the long run.
Using a wall cavity as a return air pathway can cause fire safety issues. Although the cavity itself is not a fire hazard, it can provide oxygen for a fire to spread. In addition, in humid climates, a cavity space used as a return-air pathway can pull humid air into the cavity space, possibly encouraging mould growth or the deterioration of building materials.
No, plastic boxes may not be used in air handling spaces. The issue with installing plastic is that the PVC blue boxes and romex cable produce nasty smoke if they catch fire, and you don't want that in the air handling system.
Drywall itself can act as a return duct. You can also use a metal duct or sheet metal. If you are installing a junction box in the cold air return, you must use "plenum-rated" materials such as a metal junction box, metallic wiring method, or conduit and caulk the openings.
