Ducts in Conditioned Space: Installing HVAC for Comfort and Savings Spray-foam insulation changes how the HVAC duct work should be hung.

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The ductwork is hung in the conditioned attic, which is insulated with sprayed foam. Investing in Energy Savings "By insulating the attic and knee walls, we were able to downsize the mechanical unit by half a ton," Black says. "We were right on the edge between 2-1/2 and 3 tons, but because there were so many windows, we opted for 3 tons. We also saved about $120 on the cost of duct board by using 1" instead of 1-1/2" because the ducts were in conditioned space. We put these savings toward the cost of the Icynene insulation, which is more expensive. "To spray Icynene along the attic roofline cost about $5,000 for this 2,000-square- foot home. But this money will be more than made up over time, because insulating the attic roofline and installing the ductwork in conditioned space will reduce total energy cost by 20 to 30 percent. It will provide added protection against water infiltration from hurricanes as well. "Spraying the roofline also gives you some additional useable storage in the attic where the temperature is only 10 degrees above air-conditioned temperatures in the living areas. This helps solve a storage problem, since the high water table in coastal Florida prevents us from installing basements." Improving Durability and Comfort "Because we insulated below the roof deck, we didn't use vented soffits," Black continues. "After the last two years of hurricanes in Florida, we found out that a lot of wind-driven rain was getting in through the soffit areas. It also keeps the hot, humid air in the summer from coming in contact with the ductwork or other cool parts of the building, which can cause condensation and then mold. Instead of having attic temperatures in the 130-degree range, the attic temperature will be in the 80-90 degree range. "In addition, by installing the insulation in the roof early on, we're also keeping the temperature more stable inside while we're working, which makes a big difference in Florida." Simple Steps for Success "What we are doing is not rocket science," Black says. "People everywhere are doing this every day using easily available materials. Your standard mechanical contractor shouldn't have any problems adapting. He can basically do things just as he always has. "Because overspray from the spray foam insulation tends to get everywhere, all the roof penetrations, electrical boxes, and any penetrations for exhaust fans need to be covered and sealed," Black says. "A lot of times, people put a plastering and maybe a wire behind a roof penetration. The problem is that it gets buried in the foam insulation and then we have to dig around for the wire. Other times, somebody has forgotten to seal up a box. As the general contractor, I need to remember this and warn the subs. After the first time, you learn really fast." "Hanging duct work is also different," he says. "In certain situations, you might nail a hanger to the top chord of a truss, but here you can't because it's going to be covered with foam insulation. But that's the advantage of Icynene. We had to reconfigure how we were going to hang the duct and take it off of web members of the roof trusses. For example, from the bottom chord of the truss, we built a support of two uprights made from scrap 2x4, and hung the black strap to support the duct work from these two upright pieces. "When you put the A/C in conditioned space in a tightly sealed building, you also have to think about introducing make-up air to compensate for the air expelled by exhaust fans. That air should be introduced from outside through a small duct on the return side. That way it is filtered and either cooled or heated before it gets pumped through the house. In our case, since it's a small house, the make-up air duct is 4". It's a totally passive system, so when the HVAC is on, it will pull air into the house. You want to keep a small positive pressure inside the building to keep the pollutants and outside air out. Experts recommend about 4 Pascals of positive pressure in the building at all times." PREVIOUS  1 | 2 | 3  NEXT