Should building cavities be converted to work as rigid duct work?
The building codes do not permit duct designers and installers to use building cavities as part of rigid duct work. But they are often used as return air pathways. You should know that using building cavities as return air plenums is one of root cause duct leakage. The consequences of duct leakage are well known viz. loss of precious air produced by HVAC unit, increased utility bills, lower comfort levels, and an inefficient HVAC system along with rapid wear and tear. Besides, building cavities, floor joists are also used to make duct ways. Ducts made by framing cavities and floor joists are full of leaks. In all cases the negative pressure in the cavity or joist will pull in outside air through the joints in the construction.
Objects like gypsum boards, sheet metals are nailed with joists or cavities to make rigid pathways of return or supply. In fact there are companies that advertise their insulating sheets that can aid in such practices.
Wall cavities are designed to allow the inside air mingle with outside air. In such a case it is difficult to make the interstitial spaces airtight for return and supply duct pathways. Because the spaces are leaky it will cause pressure imbalance throughout the building envelop. The leaks in the wall cavities will not only pull in external air but also other pollutants that will find their way into your living area. The cavities have types of building materials that can also include woods that do not meet the fire safety criteria as the standard duct materials do. In humid climate moisture will trickle in the cavity space that will encourage mold growth. Besides having an inefficient duct system, your building will lose its quality with time.
Air handler platform, open-floor truss cavities, and dropped ceilings are used as duct ways. Open-floor trusses can invite outside air from any leaky nook and cranny connected to the floor. Air from vented attics and crawlspaces can easily get inside air handler platform being used as duct work.
How to Use Building Cavities as Duct Chases for Supply and Return Pathways
Instead of framing cavities to use them as ducts, you can use cavities to run duct chases. But even for this you have plan the layout from the construction stage of the building. The cavities that will be used to carry the chase should be marked. The duct size should be calculated and specified using ACCA Manual D (ACCA 2009). The cavity space should be made so that it allows the duct to run without any obstruction. The cavities can be used to run insulated, air-sealed metal, flex, or fiberboard ducts. As done in all cases, the duct connections should be sealed with mastic or approved tape. Ducts running through the cavities will be inaccessible as they will get hidden with the placing of a drywall. Therefore their air tightness should be tested with duct blaster test before installing the drywall.
You should avoid using cavities as a rigid duct work. This is because they cannot be sealed and insulated properly. If you are using building cavities as duct work you should install insulation without misalignments, compressions, gaps, or voids. If non-rigid insulation is being used then it should be supported with rigid air barrier to support it and hold it in place. The seams of air barrier should be sealed with caulk.
The US Department of Energy has laid out following specifications in case building cavities will be used as duct way:
• In the unconditioned zones, all supply ducts and return must have an insulation equal to or greater than R-6.
• Duct leakage must not be more than 8 CFM25 per 100 square feet of conditioned area.
• Duct leakage to the exterior must not exceed 4 CFM25 per 100 square feet of conditioned floor area.
• Duct leakage will be calculated by a rater using RESENT-approved testing protocol. This will be done after all components are fitted.