Thermal Bypasses Can Thwart the Best HVAC Systems


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It seems so simple. There’s outside, and there’s inside. The walls, ceilings, and floors that separate the cold outside from the warm inside should be the only surfaces that rob heat from a home. Interior walls have conditioned space on both sides, so they shouldn’t be a problem, right?

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Wrong! I’ve seen and worked on several houses that lose heat through their interior walls, including the one I wrote a while back in my article on hidden attic air leaks. They’re usually older houses that have been modified from the original, but I’ve seen these problems in new houses, too.

In such houses, the problem results from the top of the walls being open to the attic. You can go into the attic and look down into the interior walls and see the drywall. That means that cold attic air gets down into those cavities.

With cold air inside the wall and warm air on each side in the house, heat moves through the drywall, which has a low R-value, and warms up the air in the cavity. The warm air then rises into the attic (a process called the stack effect). As that warmed air leaves the interior wall, cold attic air moves into the cavity to take its place, resulting in more heat loss. This process continues as long as the attic stays cold and the house is warm.

To fix the problem, you have to stop air from moving into or out of the interior wall cavities. One way to do this is to cover the opening with a rigid material and use caulk or spray foam to seal the edges.

Another way is to stuff a piece of fiberglass insulation into the gap at the top of the wall and spray foam over it. It’s important to note that the fiberglass by itself is not sufficient because fiberglass does not stop air movement. (That’s why it’s used in filters.)

Another problem I see in attics is insulation in the wrong place. A lot of old houses with high ceilings have had lower ceilings put in below the original ceiling. In that case, insulation on top of the original ceiling fails to keep the house warm. Again, it’s a matter of understanding where the attic air is going. These houses have two ceilings: the original ceiling and the current ceiling, which is lower.

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The insulation is almost always on top of the original ceiling because that’s the ceiling you see from the attic. In the course of modifying the houses, however, the workers opened many pathways for air to move from the attic to the space between the original ceiling and the current ceiling.

The result is that the air under the insulation (between the two ceilings) is at (or close to) the same temperature as the attic air. That cold air is separated from the house only by a layer of drywall. As that air warms up from heat moving through the drywall, it moves through the openings into the attic and is replaced by more cold air. The insulation is useless because it’s in the wrong place!

In another article I wrote about a house that a friend of mine was financing with an Energy Efficient Mortgage, and it had both of the problems I described here – cold walls and useless insulation.

When you have building enclosure problems like the ones described here, you can’t solve them fully with the HVAC system. Even the best designed and installed system in the world can’t stop your customers from calling in comfort complaints when the walls, floors, or ceilings are close to—or below—the freezing point. That’s one reason the best HVAC contractors understand that home performance contracting is the way to go.

Allison Bailes, III, PhD
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Posted In: Building Performance, Residential Buildings

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