Thermal Bridging in Foundations and Footers

Thermal Bridging in footers and foundations

This post is part of a series on thermal bridging.

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.

Moisture

Let’s do a little refresher on dew point.

When a thermal bridge whisks that heat out, you get cooler interior surfaces (good if you wanted a root cellar, not so good if you’re hoping for a cozy place to hang out). And those cooler interior surfaces invite moisture – the root cause of mold, mildew, and decay. Why? It’s all about dew point, the temperature at which the vapor in the air begins to condense.

To control moisture, it’s more or less a matter of properly insulating your foundation and installing vapor barriers. We’ll get to that in a minute.

Foundation-to-wall transitions

One particular area to keep in mind is where the foundation meets the rest of the house – a particularly problematic area for thermal performance. Foundations of course are still considered part of your building envelope, so wherever the concrete (slab, foundation, or foundation wall) meets the exterior wall will need extra attention because any insulation at those points is generally non-continuous.

Challenges for footers

Footers are typically poured concrete with rebar reinforcement and set just underground. Footers support the foundation and help prevent settling. You may also find yourself constructing a footer for a deck, pergola, wall, or garage.

While heat loss through footers can be reasonably low, if you’re going for Passive House, you’ll still need to address it.

Mitigation strategies

When it comes to thermal bridging, two things are critical to consider when constructing your foundation or footer: Insulation (interior, exterior, midlayer) and permeability (vapor and air barriers). Keep this in mind: Your goal is to keep the interior warmer than the exterior in the winter. However, condensation can also happen in summer when the air can hold more moisture.

Insulation

Foundations, basements, and crawl spaces can be insulated internally, externally, between layers of concrete, or both internally and externally. While exterior insulation is the most effective, interior insulation is more common – but it comes with more moisture problems.

Interior or exterior, you may want to check which materials are recommended for your climate zone, as your insulation needs will depend on temperature and humidity ranges in your region.

Exterior insulation

Let’s cover exterior insulation first.

In a perfect world, the best solution is to wrap the exterior of your entire building envelope with rigid insulation, including the foundation.

For below-grade foundations, select an insulation material that can withstand conditions underground, which means it should hold up to moisture, freezing, and thawing. If your insulation will be in direct contact with the soil, extruded polystyrene (XPS) holds up well and will retain most of its original R-value. However, XPS has some reasonably concerning environmental drawbacks (global warming!). Higher-density, rigid mineral wool board is a more environmentally considerate choice.

Heat loss tends to be greatest at the corners of a building (where more material is in contact with the soil which absorbs more heat away from the walls). So, it helps to overlap the insulation at corners – including recesses for doors and windows.

Bonus: Insulation installed outside the foundation wall, as a wing or straight down vertically, can help prevent or absorb some of the effects of frost heaving (for those of you blissfully in warmer climates, frost heaving is the upward swelling of the soil as it freezes). Contact us if you would like more information on this.

Interior insulation

Interior insulation is designed to protect the interior air in the basement or crawlspace from contact with cold surfaces (the concrete and framing).

At one point or another, you’ve probably stuffed insulation into framing cavities and called it good. It’s easy, it’s cheap… but you’re not stopping thermal bridging through the studs and cold hard concrete floor.

Several options for insulating your stud cavities:

  • ROCKWOOL or fiberglass batts. While a touch more costly, ROCKWOOL is our top choice for several reasons (which we’ll likely explore in a blog post down the road).
  • Open and closed-cell spray foam insulation. Closed-cell spray foam rings in with a high R-value rating. Note that these spray foams act as a vapor barrier at around 2″ thickness. The downside: This creates an impermeable layer which you’ll want to avoid if you have an impermeable layer on the exterior, too. Open-cell spray foam diffuses moisture and can completely fill cavities. The downside: This creates a lot of waste when trimmed. Also, note also the environmental impacts of urethane foams as well as the duration of the air-sealing of urethane. These are likely for another discussion.
  • Flash and batt or flash and fill is a hybrid approach to insulation that combines closed-cell spray foam insulation (to create an air seal) with fiberglass batt (usually, or blown fiberglass, blown cellulose, or sprayed cellulose) insulation.

Whatever you use, aim for an insulation layer with a permeance rating that allows for drying – this will lower the risk of moisture accumulation. (Rule of thumb: The greater the permeance, the greater the drying capability.)

Above-grade foundation insulation

While you’re probably used to seeing exposed concrete meeting the siding, you can imagine this is an undesirable scenario when it comes to thermal bridging.

But the laws of physics are on your side. Simply installing an R-10 insulation from siding to footing can cut heat loss by about 70% (for a heated basement). Applying a protective “board” or coating over the foundation insulation above grade will help protect it from damage due to the trades, sunlight, pests, and … weed whackers. There also are boards and coatings that can be colored and textured so as to add to the building’s visual appeal.

Notes for footers and foundation walls

While many kinds of rigid foam insulation have a good compressive strength higher than most soils, you may still want to consult with a structural engineer to verify the likelihood of “creep” – slow compression of the insulation under the footings. One option that is getting more recognition is foam glass. It can be costly but has great compressive strength.

Insulation on the interiors of any stem walls and a horizontal layer of continuous rigid foam or mineral wool under the slab can help address thermal bridging. A lesser-known option for insulating under the slab is perlite, a naturally occuring, expanded volcanic rock that has similar properties to glass. Contact us to learn more about perlite below slabs.

Retrofitting

If you’re retrofitting a building, focus on insulating the top half or top third and save yourself some digging. Warm air rises!

Vapor & air barriers

Make sure you get a vapor barrier beneath the slab to prevent moisture from rising up through the concrete. A sheet polyethylene will work well for this purpose, as well as a capillary break between the footing and the perimeter of the foundation wall. Then you can use tapes or sealant to seal the basement wall to the slab.

Be very cognizant of the ramifications of using  interior vapor barriers as you’ll want to allow drying. The general idea here is to keep the moisture out (obviously). We talk more about vapor barriers and breathing in this post.

Wrapping it up

Hopefully, this post has helped give you some insight into how to properly address thermal bridging as you construct your foundation or building footers. Now that you know what to look for, you won’t be able to un-see all the uninsulated concrete poking up around your neighborhood!

Serious about energy efficiency and want to get thermal bridging right on your next project? Talk to us.

Want to learn more about the impact of thermal bridging? Start with this post: What Are Construction Thermal Bridges in Buildings?

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