Thermal Bridging in Foundations and Footers

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?

Thermal Bridging and Decks, Cantilevers, and Balconies

This post is part of a series on thermal bridging.

Have you been charged with designing an energy-efficient deck or balcony? Or maybe you’re looking at blueprints with cantilevers that has your gut telling you something’s not quite right. Perhaps you’re a homeowner itching to start a project, but the term “thermal bridging” stopped you in your tracks. Whatever you’re working on, we hope this entry helps clarify what thermal bridging is, why you can’t afford to ignore it, and how to address it appropriately when constructing a deck, balcony, or cantilever.

Recap: What is thermal bridging?

Thermal bridging, quite simply, is the movement of heat through a material that’s more conductive than the air around it. Anytime a thermal-conductive material like steel, concrete, or wood penetrates the building envelope, it creates a highway for heat to exit (or enter) the building. Don’t think it’s worth taking seriously? Consider that this can account for heat loss at a rate of up to 30%! (Want to learn more? Read our 101 here.)

The inefficiencies created by thermal bridging not only reflect poor design, but can result in high energy bills and discomfort for the homeowner. Worse, because these materials move condensation and wetness along with temperature differences, thermal bridging introduces the potential for expensive damage due to moisture and mold. Just think about the havoc persistent moisture within your walls could wreck!

We’ve already covered how windows are a prime offender for thermal bridging in your home. Now, let’s talk about what you need to take into consideration when building a deck or balcony and incorporating cantilevers into your design.

Thermal bridging in action

Thermal bridging is most pronounced with materials like steel (you’re probably thinking of beams and support, but fastening items are guilty too), although wood will also transfer heat. Basically, if you’re designing or constructing any element that projects from or enters the building, you need to pay attention to our points below in order to get energy efficiency right.

A cantilevered steel deck or balcony might as well be a case study for how thermal bridging works. Or, for that matter, a concrete slab (just look at multi-level apartment buildings). Decks and cantilevered design elements project out from their origins within the building, breaking through the building envelope and really quite efficiently conducting heat from (or into) the building.

Imagine these elements as giant radiator fins and you’ll start to get the picture of exactly how thermal bridging works!

However, with the right strategy and materials, you needn’t drop the trending cantilever look from your design toolbox. Stay with us as we get into some solutions and techniques to help you address this problem elegantly and efficiently.

Risks associated with thermal bridging

Heat leaks

Decks, balconies, cantilevered bump-outs, and concrete slabs are notorious for leaking heat. In the winter, you might notice that the interior floor near a deck feels colder to your feet – that’s poor design helping heat escape through the structure of your home.

Keep in mind this is not just about staying cozy: All of this wasted energy costs a homeowner money and impacts the environment.

Moisture issues

Let’s head back to grade school for a second. Remember what happens when warm air hits a cooler surface? You guessed (or Googled) it: Condensation. Now picture those beautiful decks, cantilevers, and balconies. Not only are they exposed to the elements, but when that warm summertime air travels into the AC-cooled building envelope thanks to thermal bridging, not only is condensation bound to happen, but you could soon have a serious mold problem on your hands. It’s not just a wintertime issue.

Working with a pre-built structure? Here’s how to tell if there’s a moisture problem: If you’re lucky enough not to find mold, you’ll see darkened areas where the moisture has attracted dirt.

One last thing to keep in mind here: having a vapor-open approach to the envelope assemblies is important. If moisture condenses with the envelope assemblies, it has to be able to migrate out of the assemblies or you will have mold issues.


Prevention & mitigation strategies

Ultimately, thermal bridging solutions are all about mitigating heat transfer – but let’s not forget air leaks. Use the following tips and tricks to help you “break the bridge.”

Good design

No surprise, preventing thermal bridging starts with good design. And the best way to get good design from the get-go? Tell your architects and structural engineers to work together and think “energy smart” first. (This may sound easier than it is!)

Ideally, good design doesn’t compromise the building envelope – which means you should try to construct your deck or balcony separately from the building and secure via load-bearing brackets to the walls or support independently. Even better, on its own foundation.

For decks:

So best-case scenario for a deck is an independently constructed structure on its own foundation. Otherwise, anywhere that a deck is attached to or penetrates the structure of the building, anticipate thermal bridging to occur. And if heat transfer is happening, you better believe moisture transfer is happening too.

You need a plan to keep air out of your walls, since air (or more precisely, the vapor in the air) is the primary culprit for moisture condensing within the wall assembly. You might consider installing enough continuous rigid exterior insulation so that the dewpoint occurs outside the envelope assemblies. Using a vapor-open approach will help ensure that if your walls do get wet, they can dry.

For balconies:

You might try supporting the outer corners of the balcony with steel rods or cables attached higher up the building – which can add visual appeal to your exterior design. Or, you might support the balcony with wooden brackets affixed to the building’s exterior. You may also be able to support the corners on independent posts, like you would for a deck.

If none of this possible or the design is already set in stone, using the right materials, insulating them, and creating an air barrier can significantly reduce or even eliminate thermal bridging issues. Read on for more about these strategies.

Structural thermal breaks

A thermal break is a material used to block the path of heat transfer. Incorporating structural thermal breaks (like specialized plates, pads, or foam) between a balcony and floor slab can reduce heat transfer by up to 75%. Bonus: This also improves condensation control.

You can purchase manufactured thermally broken balcony connectors from manufacturers such as Schock and Halfen.   

Air sealing & taping

Air sealing is an important step to making sure your building envelope is airtight. Don’t make the mistake of thinking you’ve covered your bases simply by insulating. Air can move around insulation, bringing higher/lower temperatures – and moisture. Since any penetration through the structure breaks the building envelope and creates the potential for airflow, you’ll want to make sure you have a plan for air sealing.

In fact, you should start the air sealing process even before you add any insulation by using a weather barrier system that is also an air barrier. Arguably, adding an interior air barrier can be a belt-and-suspenders approach. Learn about interior air barriers here.  

The Holy Grail here is an airtight, vapor-open building envelope. (There’s even a certification for that. Learn about Passive House.)

Insulation

Exterior insulation is often recommended and should be considered part of that building envelope you want to avoid penetrating. You’ll especially want to insulate around the highly conductive steel studs and structural framing. Here’s one exterior insulation we trust to do the job well.

Continuous rigid exterior insulation is used for wrapping the building structure. Ideally, you are using enough insulation to move the dewpoint out of the structural wall assembly and into the exterior rigid insulation. “Continuous” is the key word here – you want to install it without breaks. If you need to cut out the rigid insulation around penetrations (which would obviously short-circuit your attempts at preventing thermal bridging), taping and spray foam will be your friends.

Double stud construction: With double stud construction, the exterior studs will act as the structure, while the inner studs are used for chases and insulation with a gap between the two. Insulating the gap between the studs provides the thermal break. Decks and balconies can then be bolted to the structural exterior studs. Other cantilevered details like bump-outs are not recommended without an air barrier and a continuous rigid exterior insulation layer.

An educated crew

Ultimately, a well-informed crew (from architects to structural engineers to construction teams) will know what to look for and the steps they can employ to minimize thermal bridging while making the assemblies airtight.

Wrapping it up

We hope this post has helped underscore the importance of addressing thermal bridging and given you some strategies to make your design work. The good news: With these strategies and the right materials, you can significantly lower or eliminate the risk of thermal bridging in your deck, balcony, or cantilever design.

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?