UPDATED on August 13, 2017 with new concluding paragraphs
Millions of Americans live in states where residential HVAC contractors routinely install ductwork in unconditioned attics. In many cases, these attics also contain a variety of appliances, including air handlers, furnaces, or water heaters.
Because the disadvantages of this arrangement are fairly well known, I’ll mention them only briefly:
- During the summer, attic temperatures often exceed outdoor temperatures.
- Attic ducts almost always have thinner insulation than ceilings, in spite of the fact that the delta-T (that is, the temperature difference) between the air in the ducts and the air in the attic is even greater than the delta-T between the inside of the home and the exterior.
- Most duct seams leak; as a result, supply ducts lose conditioned air to the attic, while return ducts suck in attic air — air which is hot in summer and cold in winter — and bring it to the air handler.
- If access to the attic is through a hatch, servicing any HVAC equipment in the attic is awkward at best.
The bottom line: running ducts through an attic saves money for the builder, but costs the homeowners dearly in increased energy costs.
Ductwork belongs inside a home’s thermal envelope
Ideally, HVAC appliances and ductwork should be located inside a home’s conditioned envelope. In the northern half of the country, appliances and ductwork are routinely located in basements or crawl spaces. If your house has a slab foundation, HVAC appliances can be located in an equipment room and ductwork can be located in interior soffits.
Another solution is to move the insulation from the attic floor to the sloped roof, thereby creating a conditioned attic.
Assuming you want to create a conditioned attic — either during new construction or as a retrofit project — how…
Get building science and energy efficiency advice, plus special offers, in your inbox.