If you are building a new home or undergoing extensive remodeling, it might be tempting to tuck your new high-efficiency furnace into the vented attic to save living space. While this is not an uncommon practice, our modern understanding of building science suggests this can be a major liability for energy efficiency, comfort, and indoor air quality of your home.

Installing a furnace in an unconditioned, vented attic creates a series of "penalties" that can quickly negate the benefits of a high-efficiency system. Here are the primary issues associated with this setup, especially in cold environments like Montana.

1. Energy Losses

When you place a furnace and its ductwork in a vented attic, you are essentially putting your heating system "outside".

• Thermal Exposure: The air handler and ducts are exposed to extremes of cold and heat, which increases the energy required to condition your home by approximately 10%.
• Leaky Ducts: Duct air leakage in these spaces commonly exceeds 20% of conditioned air flow. In a cold environment, this means you are paying to heat your vented attic rather than your bedrooms.
• Pressure Imbalances: Because ductwork and air handlers typically leak, they can create negative pressure inside the house, pulling cold, unconditioned air in through other cracks and gaps in the building envelope.

2. Ice Dams & Mold

One of the most destructive issues in cold climates is the formation of ice dams. When a furnace or leaky supply ducts are located in a vented attic, they release heat into the space. This warm air escapes into the attic and warms the roof deck. When snow on the roof melts due to this escaped heat and then refreezes at the colder eaves, it creates an ice dam that can back up under shingles and cause significant water damage.

Condensation: Humid indoor air that escapes into the attic from the house or through leaky HVAC components can condense on the cold roof sheathing, leading to mold growth.

4. Condensate Freezing

Because these units produce liquid condensate as part of the high-efficiency heating process, exposing the unit to the extremes of cold found in a vented attic increases the risk of system failures and damage to the adjacent structure if those lines are not properly protected from freezing.

Strategies for a Better Setup

If you absolutely must keep your furnace in a vented attic, you can minimize the damage by following these steps:

• Seal and Insulate: Use "copious amounts of mastic" to seal every duct joint and insulate the air handler and ducts as much as possible.
• Keep it Low: Install the furnace and ducts as low in the attic as possible (ideally on the attic floor) to keep them closer to the thermal boundary of the house.
• Consider an Unvented Attic: The best long-term solution is often to move the "thermal boundary" to the roofline by creating a conditioned, unvented attic. This brings the furnace "inside" the house's envelope, dramatically improving efficiency and disaster resistance.