From time to time I get questions about thermal bridging, what is it, how does it affect our buildings, how do we manage it and as spray foam professionals, how do we explain it to prospects and customers in the market.
So, let’s talk about that today, what is thermal bridging?
Well, you should be familiar with thermal performance, R-value and how energy or heat transfers through building materials, that’s what R-value is, it is resistance to heat flow.
Thermal bridging describes the phenomena that occurs when you have dissimilar materials with different thermal rates of transfer, so different R-values near each other, and when you have different materials with different R-values next to each other in the same assembly then the rate that the heat transfers through one of those materials is going to be much faster in some cases than the material right next to it.
For example, let’s consider the most common case of thermal bridging. This occurs when we have steel studs, or metal studs, and we insulate the cavity between the metal studs.
The metal transfers heat much faster than the insulation inside the cavity and the result of this is the overall thermal performance of the wall assembly is significantly lower that the cavity R-value because the thermal transfer through the metal studs is so high that it degrades the thermal performance of the overall wall system.
If we put in R-20 inside a metal stud cavity, the thermal performance of the metal studs is so bad that the overall wall assembly is going to perform closer to an R-13, so there is a significant degradation in performance compared to the installed R-value.
Now this is not exact, so don’t quote me on these exact figures, I am just trying to illustrate the situation with this example.
So, how do we overcome thermal bridging?
The answer is continuous insulation.
Continuous insulation overcomes thermal bridging by creating a thermal break and not allowing any structural members, with lower thermal performance – like metal studs, to transfer heat and energy directly through the building envelope.
The building code has started moving in this direction with guidelines for continuous insulation.
For example, on the exterior of a commercial building built with metal studs, or built with a steel frame and CMU, a big way for spray foam to help from a thermal efficiency standpoint is by providing a continuous insulation on the exterior of that assembly.
This method mitigates thermal bridging, it helps reduce the heat transfer through the structural members, so the building doesn’t have thermal bridging occurring.
Another good example of thermal bridging can be seen in roof decks. Now it is not really a failure point, so it is often less noticeable, but if you look at the exterior of a roof, where the roof deck has been insulated between the rafters, after a thin layer of snow you can occasionally see melted lines every sixteen to twenty-four inches on center, where the rafters have allowed heat to pass through the roof assembly and melt the snow above the rafters much faster than the snow above the foam insulated cavity.
The best way to address thermal bridging is to reduce the opportunity for low R-value materials to have direct contact from the inside to the outside of a building envelope assembly; this the purpose behind continuous insulation.
In the roof assembly where you might see melted snow or evaporated dew lines and you can identify roof rafter lines from the exterior, the key to solving this problem is to insulate over the face of the rafters on the inside of the attic, providing continuous insulation and reducing the heat loss through the rafters. I’m glad I could share some thoughts and ideas about thermal bridging with you today, take this information out in the market, share it with your customers and go out and get more business.
