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What is Thermal Bridging? A RoofTec Guide

Ryan Cash

UAMCC Associate Member
Why is the algae on my home growing in a weird pattern?
Why does it look like I can see the studs on the outside of the house?
What is stud shadowing?

And most importantly, why does any of this matter??

Before doing a deep dive into this, let’s define a few terms

Thermal Bridge: an area of the building enclosure- whether it be the walls, roof, or foundation- that has significantly higher heat flow than intended.

R-Value: a measure of resistance to heat flow through a given thickness of material. The higher the R-value, the more thermal resistance the material has and therefore the better the insulating properties.

Dew Point: The temperature to which air must be cooled to become saturated with water vapor. When cooled below the dew point, moisture capacity is reduced and airborne water vapor will condense to form liquid water known as dew. When this occurs via contact with a colder surface, dew will form on that surface.

Now...let's get into it:

Exterior walls and roofs of homes all contain insulation to increase the building’s thermal performance. The purpose of the insulation is to help keep the outside temperature outside and the inside temperature inside by limiting how much heat escapes to the exterior of the home. This insulation is typically measured by it’s R-value. The higher the R-Value, the better it is at insulating. The insulation on the exterior walls of a typical home will fall somewhere between R13 and R23.

However, something can occur on exterior walls that is not often discussed: Thermal Bridging. Thermal bridging usually occurs when interior heat is more easily transferred to the outside of the home through the wall assembly. The framing of the home (wood studs, joists, etc.) is much more conductive than the insulation inside the stud bays which contain the insulation. Wood studs actually only have an R-value of R1 per inch. So a 2x6 framing stud would only have an R-value of R6. Therefore more heat escapes through the frame of the wall (Studs) than through the insulation. Research suggests that insulation effectiveness can be reduced by as much as 40% with standard framed walls.

Because of this, the warm interior air will transfer to the building exterior faster through the wood framing than through insulation within the stud cavities. You can see this here in this Thermal image of the outside of a home:

Thermal 1.png

In this photo, you can clearly see the shadows of the framing studs. These sections of the wall are warmer than the rest of the façade.

So, getting closer to the point…pun intended.

The dew point is the temperature to which air must be cooled to become saturated with water vapor. In simple terms, hotter air can hold more water than cold air. So if you have warm humid air and then cool it down, the water vapor condenses and leaves behind dew (water droplets). The dew point is the temperature that the air can no longer hold the water. If you drop below it, condensation occurs.

This is why cars, grass, and other exterior surfaces are covered in dew in the morning. The humid warm air comes in contact with the cool surface, condenses and leaves behind water droplets. This is the same reason that the outside of a glass of ice water will become covered in condensation. The cold surface draws water out of the surrounding, warmer air.

Now let’s look at how this may be an important factor when discussing thermal bridging.. Under the right conditions, the exterior wall temperature at the areas of thermal bridging can be above the Dew Point temperature (no condensation occurs), and the temperature at stud cavities that contain insulation can be below the Dew Point temperature (condensation occurs).

So overnight, when the temperature drops, the wall at the stud cavities (insulated) can form condensation while the areas with the actual studs does not since the interior of the home is heating up the studs.

Alternatively, on some colder nights, the entire wall can drop below the Dew Point, allowing condensation to form on the entire surface. The condensation typically evaporates as the sun rises and temperatures pick back up. However, after the sun rises and the temperature begins to increase, condensation will first evaporate at the warmer areas of the wall along the studs.

All this equates to stud cavities being wet sooner and longer than the section of the wall with the actual studs.

So now on to the point of all this: Why does any of this matter?

Overtime, particulate (Dirt, dust, debris, organic spores) will bond to exterior walls. The moisture on the surface of a wall will actually attract this particulate and increase the amount of pickup that occurs.

If nightly condensation occurred evenly across the entire wall surface, then dirt pickup would generally occur uniformly. However, since some areas of walls may condensate quicker due to thermal bridging, the particulate pick up will sometimes show up in patterns that correspond to the underlying framing.

With more particulate on the walls, bacterial growth has more to feed on, which will likely result in algae staining that mimics the interior studs of the walls. This will often lead to growth like in the picture below:

Thermal Bridging.jpg

You can see how this mimics the thermal imaging photo taken as well:​

Thermal 1.png

So, the next logical question is: What am I supposed to do with this information?

First, being more knowledgeable is better. Knowing not only what a stain is, but why it is occurring is hugely important if you want to be able to present yourself as an expert in this industry. Customers are much more likely to pay a high ticket price for someone who is educated and can explain to them the entire process.

This shows that you’re not just going and spraying bleach on someone’s house and calling it good.

So take the time to educate the customer. Explain to them what the stain is, where it’s coming from, why it showed up and how you are going to remedy it.

Secondly, use this information to talk to them about potential remedies. This doesn’t just apply to thermal bridging either. Maybe there’s a downspout that feeds onto a lower roof and algae growth continues to show up. A quick fix would be to extend the downspout down the roof so that it direct feeds into a lower gutter.

The same applies here. While thermal bridging is not easily (or cheaply) fixed, you might suggest to them to look into continuous exterior insulation if they ever plan to replace the siding on their home. Additionally, if you see only one small area or square section of siding that doesn’t show signs of algae growth while the rest of the home is covered, that may imply that there is a problem In that stud cavity with the insulation. Customers will almost always appreciate the expert knowledge that you can offer them to better understand their home.

Having a general understanding of thermodynamics and structural framing will go a long way to boosting your credibility on a job site. The key here is simply never stop learning. Be curious. When you see a particular stain or growth pattern, don’t just ask yourself what it is. But also ask why!

The cleaning process for this particular home ended up being fairly straight forward.

We treated the home with a 1.5% mix of Sodium Hypochlorite with some added WashTec surfactant.

The solution was applied, from the ground, with the RoofTec XCS500 at roughly 600 PSI.

The solution was then thoroughly rinsed and left this 30 year old home looking new again!

thermal bridging after.jpg