Thermal Performance of Walls - Additional Information

Following up on the last post regarding thermal performance of different wall assemblies. That post looked at a code minimum wall vs ICF because those are two common wall assemblies we see.

A quick reminder, I am sharing this 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.

This post will look at some other wall assemblies that are being proposed and will soon become code minimum.

The above two images are a typical 2x6 stud wall with cavity insulation WITH continuous R5 rigid insulation outboard of the wall framing and an insulated slab on grade with exterior insulation at the edge of the slab. This is the 2024 IECC (international energy conservation code) minimum for climate zone 4C (there are a couple of other wall assemblies (R30 cavity insulation, R13 cavity insulation with R10 continuous outboard, and R0 cavity with R20 continuous outboard are the other options for 4C) . The 2024 IECC is not yet adopted in Oregon but it likely will be in the coming years. 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 in the green band, near the yellow band 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 cavity insulation is fiberglass batts).

The one benefit this wall has to the current code minimum wall is that it pushes the point of freezing in the wall further outboard.

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?

So, in summary this updated code minimum wall will have essentially all of the same problems the current code minimum wall has. Is there a better wood framed wall?

The above two images are a typical 2x6 stud wall with cavity insulation WITH continuous R13 rigid insulation outboard of the wall framing and an insulated slab on grade with exterior insulation at the edge of the slab.

To spare repeating myself, this wall assembly has essentially all of the same problems we saw with the other two wood frame wall assemblies.

Is there a way to make a wood frame wall assembly work as well as an ICF wall? Some wood frame wall assemblies can and have been shown to work, but they are significantly more complex than an ICF wall and typically also more expensive. AND wood is always more prone to water damage than concrete.

I will cover other topics that make wood frame wall assemblies less prone to problems, such as actively controlling the humidity inside the entire home in future posts. Again, wood frame walls can be made to function but if you have the option to build better, why not? Code minimum wood frame wall assemblies are less expensive upfront than ICF walls but are more costly over the long term. If upfront costs are the main, or only, concern, then we can look at ways of making wood frame perform and be durable. I recommend slightly reducing the home’s size and switching to ICF, which is by far the best assembly right now.

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.