With our choice on using a metal structure, including steel studs, and with the stringent Passive House requirements, the selection of using continuous exterior insulation was an obvious one.
Steel studs are excellent thermal conductors, and they act as thermal bridges by conducting the heat (energy) from the inside to the outside during the cold months and move the heat in the opposite direction in the hot months. Using the insulation in the wall cavity, as typically done for wood construction, is heftily penalized by the presence of these thermal bridges, as heat will find the path of least resistance and will use the steel studs to efficiently transfer heat. If doing so, the overall thermal performance of the wall will be so poor that it will not be able to meet any building standards. Many approaches have been developed to address this poor thermal performance of cavity insulation with steel studs, and a good article describing these solutions is Steel Stud Walls: Breaking the Thermal Bridge from homeenergy.org. However, even if all of these solutions are implemented together, the thermal performance of the wall is still not to the standards of the Passive House requirements.
The solution? Place the insulation on the outside. This is not just a good solution for steel studs wall, but an optimal solution in general. A good article by Joseph (Joe) Lstiburek, titles “The Perfect Wall” can be found here on the buildingscience.com website. By the way, I have read many of the articles written by Joe, and watched many of his videos on YouTube (I recommend it to everyone who wants to learn about building science and good design practices).
A typical wall section is shown in the picture. Note that the insulation in the stud wall cavity is for sound, not for thermal, and we are still evaluating if it will be necessary. This wall reflects “The Perfect Wall” as outlined by Joe in his article. There are still design details to be addressed: a critical one is the clips that support the cement board panels, as they are also a source of potential thermal bridge, and needs to be properly designed to meet the Passive House requirements. This will be addressed on the next article ….
