Who can describe to me what Thermal Bridging is?
Parts of this article have been provided by Havelock Wool (Insulation)
3 reasons you need to know about Thermal Bridging.
1. NCARB will ABSOLUTELY ask you a few questions about Thermal Bridging.
2. In practice the IBC (IECC actually) has minimum requirements for insulation.
3. You are studying and it is important to understand R-Value.
What is R-Value?
R-Value is the measurement of Resistance to heat flow THROUGH a material (assembly) over time. Higher R-Value = greater resistance to heat flow = better insulating.
(I remembered this as: R value is for Resistance - R=R)
[R Value is the inverse of U-Factor. U Factor is the HEAT FLOW COEFFICIENT of a material. and represents the measure of how much heat a material conducts. A material has a U factor. You use that U factor to determine the R Value - or the material's resistance to heat flow.]
* In reality assuming that your wall assembly will realize the R-Value of your insulation ignores a crucial variable in building science… thermal bridging.
What is thermal bridging?
Thermal bridging is when a particularly conductive (or poorly insulated) material allows heat flow across a thermal barrier. Physics dictates that hot wants to go to cold and a thermal bridge is a perfect invitation. Think studs within an insulated wall. The insulation on either side of a stud will do its job of limiting heat flow but the wooden stud, which has a much lower R-Value, will not. These bridges can become superhighways of heat loss.
How does this work in a building?
Let’s continue with the stud example. The typical R-Value of a 2×4 wooden stud is 4.4 while the insulation on either side is much higher – typically greater than R13. As discussed, the result of this bridge is heat transfer and a significant loss in the wall’s effective R-Value.
In a wood stud wall with R20 insulation, thermal bridging can reduce the effective R-value to as low as R15. An even worse thermal bridge is a metal stud that can cut effective R-Value by as much as 80%.
Along with studs, other typical building materials that create thermal bridges are metal fasteners, plates, headers (structure), curtain walls and windows. Typically you can observe these all-around a building – roofs, walls, fenestrations for example. Anywhere metal (strapping) crosses the line of insulation. Window headers are packed out wood without insulation. Curtain walls (windows) are metal window systems inside and out, and without thermal gapping, provide a complete bridge from outside to in.
NCARB will ask you to identify areas of thermal bridging on building sections and detail drawings.
The Impacts of Thermal Bridging
A building loses heat through thermal bridges either into or out of the enclosed space. The efficacy of the climate control system decreases (and the building uses more energy!) and it’s just not as comfortable. Lose-Lose. Further, thermal bridges can create cold spots within your walls where condensation can easily form. This moisture can produce a whole host of issues to the wall cavity…. mold and mildew or rot, to name a few. Excessive moisture also challenges the structural integrity of your wall.
How do we prevent or limit thermal bridging?
Here are some ways to effectively address this issue:
Exterior insulation: Maybe a better term would be “out-sulation”. The premise is simple. By wrapping the exterior of your home in insulation you can prevent (or dramatically reduce) the common thermal bridging via wall studs.
Advanced house framing or Optimum Value Engineering: The goal of this building technique is to reduce lumber used in framing which reduces thermal bridging. Lots of strategies are incorporated here including spacing wall studs up to 24 inches on center, eliminating headers in non-load-bearing walls, designing on two-foot modules to make the best use of common sheet sizes, and reducing waste.
“Alternative” Wall Assemblies: Structural Insulated Panels (SIPs) are a common alternative to stick-built construction and can help limit thermal bridging. These panels have an insulated foam core between two rigid board sheathing materials which when installed properly can provide a continuous air and vapor barrier.
HYPERFINE Architecture (By Ben Norkin) has a PPD-PDD 10-week study program available here.
(Archizam) Ben and I used this study program to help us pass our PPD and PDD exams. It covers many topics, provides in depth explanation from many different sources, and generally makes you think deeper and really work to UNDERSTAND what you are learning. I bring this up on this topic because I give full credit to Hyperfine for making me learn about R-Value and U-Factor - WEEK 1 - ASSIGNMENT 4