“Major thermal bypasses” is building-geek speak for holes in your house – holes through which you are losing an abnormal amount of heat. More formally, they are areas in standard construction where flaws in the building enclosure allow air to escape, bypassing the thermal control layer.

Thermal bypasses: the what and where

The building envelope includes those elements of the building (the floor, walls, and roofs) which separate the comfortable interior from the potentially uncomfortable exterior.

A huge part of an energy audit is inspecting the building envelope for problems. And thankfully, as builders have standardized their building practices, they’ve been considerate enough to standardize how and where they make giant holes in the building envelope. Thanks, guys!

Thermal bypasses and the home buyer’s dilemma

As I mentioned, thermal bypasses are inadvertent holes in the building envelope. If one were to conceive of a perfectly insulated and airtight house (with an appropriate level of fresh ventilation air … no suffocating), it would be very simple. Make it like a box.

Wait, you want to get inside?! OK, we’ll add a door. And you want to see the ocean view you paid an extra $100K for? Well, they have a really lousy R-value compared to walls, but I suppose we can add a few windows. And a cathedral ceiling and recessed lights and a hot tub and a finished room over the garage and a zero-clearance fireplace and a giant hole in the wall for fresh air. (Wait a minute…)

You see where this is heading. Home buyers have wants and needs, and they are most often at cross purposes with the aim of having an easily defined building envelope. And the harder the building envelope is to define, the harder it is to keep the uncomfortable outside away from the comfy interior.

List of thermal bypass locations

When developing their housing certifications, Energy Star put together their list of common thermal bypasses. Thankfully, the good folks at the Department of Energy have collected this info in one place: the Energy Star thermal bypass list:

• Kneewalls like those found in Cape Cod style houses
• The ceilings over porches
• Attic hatches
• Zero-clearance fireplaces (I love these, because the leaks are entirely enclosed and not evident to the naked eye)
• Skylights
• Garage ceiling joists attached directly to the building framing
• Attic stairs built into exterior walls
• Bathtubs and showers built into exterior walls
• Cantilevered floors like those found in Garrison houses.

And the hits just keep on coming. Check out Energy Star’s list of thermal bypasses and take a look around your house. Fixing one of these issues can be a real heat saver and money saver.

Erik North, the owner of Free Energy Maine, is an energy auditor and home performance specialist in Westbrook, Maine. He is also the author of the Energy Auditing Blog.

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