Thermal Performance of Walls
/Hi everyone,
I am sharing some thermal modeling we have had performed to compare different wall assemblies and in particular where condensation is predicted to occur. These models were done for a few projects in climate zone 4C, so a somewhat mild climate that sees typical temperature ranges from the 20’s overnight in the winter to 100 (ish) on peak summer days.
Of particular note, these models appear to be very accurate based on older homes we have remodeled for clients over the years and where we have found mold in wall systems. I will cover mold, indoor air quality and proper flashing in later posts. It is worth noting that the majority of the mold we have found in wall assemblies during remodeling is at least partially due to improper flashing and not controlling the relative moisture inside homes. But, the typical wall assembly is flawed from the start as you will see from the below thermal models.
The above two images are a typical 2x6 stud wall with cavity insulation and an insulated slab on grade with exterior insulation at the edge of the slab. This is currently the code minimum wall assembly in 2024 in Oregon. This model is shown with a 23 degree outside temperature and a 72 degree inside temperature. Condensation (depending on the relative humidity) is expected to occur at the green/yellow interface which is on the interior third of the cavity insulation. That is a major problem, how will it dry out? Along with other problems, for example wet insulation loses it’s insulation ability (reduced r-value) and will have a tendency to sag over time (typical insulation in a code minimum wall is fiberglass batts).
The next point I often hear is, “but no water will get into the wall.” But this is not true, was every electrical penetration sealed perfectly? Was every HVAC and plumbing penetration sealed perfectly? On a code minimum wall I can guarantee that the answer is no. If air can get into the wall then water WILL get into the wall as well, carried by the air. So what is a better wall assembly?
The above two images are a typical ICF (insulated concrete form) wall and an insulated slab on grade with exterior insulation at the edge of the slab. This model is shown with a 13 degree outside temperature and a 72 degree inside temperature. Why did we model the ICF wall at a colder outside temperature? We know ICF will perform significantly better than a typical stud frame wall so we wanted to check colder climates at the same time. Condensation (depending on the relative humidity) is expected to occur in the green band, towards the yellow side which is in the solid concrete core of the wall system.
A well placed concrete wall will allow nearly zero air through it. If no air is able to pass into the concrete where the condensation is predicted to occur then also no water will get to that point in the wall assembly. AND even if a very small amount of water did get into the concrete core where it would condense there is no concern because concrete does not mind water.
We have been starting to switch entirely to ICF walls because the thermal performance of the wall is significantly better than a traditional wall (I will cover thermal performance in a separate post), an ICF wall is significantly more durable (will last much longer) and an ICF wall will help maintain much better indoor air quality.
The above modeling is just a few snapshots of what we do for clients and projects to ensure that the entire project will perform as expected. We also model the different components in wall and roof assemblies, for example the framing around the openings in an ICF wall and the roof/wall interface. In a future post I will go through some of our modeling we have done for different options to frame out window and door openings in ICF.
If you have any questions please let me know. We also consult on projects in many different capacities, from reviewing details to providing owner’s representative services and can help ensure your project achieves long term success.