New guide on vented attics with buried ducts

Jan 17, 2017 1:35:49 PM Energy Efficiency, moisture By Jordan Doria
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Back in December we wrote about a change in the 2018 IECC that laid out a new, prescriptive path for the use of buried ducts in standard vented attics. This practice offers the potential to save significant energy over typical vented attic designs, and can even rival the savings of an unvented attic done with ccSPF under the roof deck, at much lower cost. However, like much of the building code, actually understanding and applying the language is not always easy. To help, we contracted with Home Innovation Research Labs (HIRL) to create a “TechSpec” that lays out how you undertake the practice in a simple, straightforward manner. In addition, HIRL also provides both construction and energy costs of different attic design approaches, including variants of buried ducts. The document speaks for itself, but since our first communication on this new practice we have heard feedback on some areas of confusion that need to get cleared up. Here are the answers to 5 common questions about buried ducts.

  1. “Are you really burying ducts in hot/humid climates? Won’t condensation form?”

Yes, you can absolutely bury ducts in these climates. Evolving building science showed how to do this safely. This evolution is what precipitated the need for code language in the first place, to make clear that the practice is now proven, when done the right way. The TechSpec has all the details, but here is the cliff notes version of what the research has demonstrated: higher duct insulation requirements, specifically R-13 (climate zones 1A-3A) with a vapor retarder, proper levels of insulation above and around the ductwork and very tight ducts prevent condensation from forming. Years of research has led DOE, Building Sciences Corporation, Home Innovation Research Labs and others to conclude that this practice works when all the requirements are met.

  1. “Are you saying buried ducts in vented attics are more energy efficient than ducts in an unvented attic with ccSPF?”

Yes and No. Anyone making highly generalized statements around energy savings should be view with skepticism. All energy saving statements should come with numerous qualifiers. We are saying that buried ducts in vented attics CAN rival the energy savings of unvented (conditioned) attics, but there is a great deal of variability. This variability is due not only to home size, location and roof pitch but also to the variety of buried duct approaches one can employ. Really, “buried ducts” refers to a suite of options. The TechSpec is again a good reference, as it lays out these options, as well as the costs and projected energy savings for a two model homes in different climates. This segues nicely to another question…

  1. “What exactly does it mean to ‘bury ducts’? How many approaches are there?”

The TechSpec models a number of different design approaches, however from a code standpoint there are really three categories of approaches one can consider:

  • General buried ducts. This element of the code lays out general criteria all buried duct designs must follow.
  • Deeply buried ducts. This code language lays out requirements for deeply buried ducts. It also spells out how to calculate the duct R-value when modeling energy code compliance.
  • Buried ducts in conditioned space. This final piece lays out the requirements for buried ducts to be considered “in conditioned space” for energy modeling purposes. As you might expect, the requirements for this are more stringent than for just general buried ducts.
  1. “This is all well and good but I am not operating under the 2018 IECC, so why do I care?”

The IRC provides a process under section R104.11 “alternative materials, design and methods of construction and equipment” by which you can get approval for practices and products not explicitly approved under the code in your jurisdiction. The fact that the language is in the 2018 IECC should help secure the approval. In fact, the TechSpec itself can help, as a tool to both educate the code official on the practice and a way to help the building inspector ensure the requirements are met.

  1. “R-13 duct insulation is not a standard product, so how do I meet this requirement?”

We don’t know what manufacturers will offer in the future. In the short term, there are a few options for builders who need to meet the R-13 duct insulation requirement in CZs 1A-3A. One is to do “concentric flex ducts”, basically sliding an R-6 flex duct through an R-8 flex duct to create a single duct that meets the R-13 requirement. Another would be R-5 duct wrap over R-8 flex duct. The TechSpec speaks to both of these methods. A slightly different approach that works is doing “encapsulated ducts”. This refers to using ccSPF in conjunction with insulated ducts. For example, 1.5” of ccSPF around an R-4 flex duct would get you to the R-13 needed. This approach has been done in many of the test homes which helped prove out this approach. These are just a few examples, and code does not limit choices so long as the R-13 level is reached (the requirement is only R-13 in CZs 1A-3A, it is R-8 everywhere else).

We will continue to provide education on this evolving practice as it becomes available. We believe the energy efficiency benefits, combined with the cost savings, make it an attractive practice for builders around the country.



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