Thursday, July 11, 2019

Thermal Bridging in Foundations and Footers

Exposed concrete foundations are notorious for glowing yellow on thermal imaging. All that concrete acts as a highway for heat to leave the building, and should be given as much attention as windows, balconies, and the rest of the building envelope.

Let us join the chorus of builders in emphasizing that it’s not just about heat loss.

When you have an uninsulated basement wall or slab, yes, you’ll certainly see lower interior temperatures. But the direct result of a lower interior temperature is not just a cooler space, it’s also an environment where condensation is likely to form. And no one wants a damp, dank, musty environment at the base of their building.

Recap: What is thermal bridging?

Thermal bridging (also called a cold bridge, heat bridge, or thermal bypass) is the movement of heat through a material that’s more conductive than the air around it. Thermal bridging can account for heat loss of up to 30%, so you can imagine the importance of addressing this when constructing your foundation! (Brush up on Thermal Bridging 101 here.)

Which materials act as heat highways, you ask? Steel, concrete, and wood (core construction materials) are prime offenders – and can’t really be avoided when building. But you CAN take into account some design considerations that will minimize thermal bridging, if not stop it in its tracks.

We’ve covered how windows are a prime offender for thermal bridging in your home, and we addressed some design considerations for decks, cantilevers, and balconies. Now, let’s drill down into foundations and footers. Challenges for foundations

If you’ve ever had a basement, you may have noticed that any musty smell becomes stronger in the summer months. That’s because the warmer, more humid air is coming in contact with cooler surfaces that are below the dewpoint of the air inside (more on dewpoint in a minute). Particularly guilty are rim joists, as they tend to run colder.

In a two-story home, basements can account for 10-30% of the home’s annual heat loss in winter – more for a single-story building. Let’s take a look at what’s happening where.

Heat loss
All the cement, studs, and supports that go into shoring up a foundation are perfectly primed for thermal bridging if you don’t take the necessary steps to insulate and construct appropriately.

You might think, “heat loss, no biggie for a basement.” But it’s not just about reducing energy bills and keeping temps comfortable. As we mentioned, because thermal bridging also moves condensation and wetness along those pathways, enter the potential for expensive damage due to moisture and mold.

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