Thermal Performance of Walls - Final Thoughts
/We have walked through different thermal models for different wall types, but so far, we have mostly focused on the condensation potential of the different walls. First, I will provide a few final thoughts on how to help wood frame walls with condensation potential. Then, I will go through other considerations, like heating/cooling, associated with the different walls.
So, how can we mitigate the condensation issue in wood-frame walls? I will only touch on the two broad methods I see commonly used. The first method is to put a significant amount of the wall insulation outside of the wall cavity, ideally in the form of continuous insulation. What does that mean? It depends (doesn’t it always??) on the climate, but it can easily be 60% or more of the wall insulation value placed outside of the wall cavity; this also requires special attention to providing a well-detailed vapor barrier. The second method, typically employed along with the first, is actively managing the humidity inside the home. Actively managing the humidity inside the house moves the point in the wall cavity where condensation will occur. For example, with an air temperature of 70 degrees and a relative humidity of 65%, the dewpoint temperature is 57.7 degrees (that means that if the air comes into contact with a surface lower than 57.7 degrees, it will start to condense). But reducing the relative humidity to 45% moves the dewpoint temperature to 47.7 degrees! Why is moving the dewpoint to a lower temperature good? It moves the place in the wall where condensation will occur, and the goal in a wood frame wall is to move the condensation location out of the wall cavity (ideally to outside of a rain screen where there is excellent drying potential for the assembly). It should be noted that simply controlling humidity is a great help in all buildings and can make otherwise deficient wall assemblies function for much longer.
One thing to note is that humidity in a home should be actively controlled regardless of the type of building system used.
How do wood frame walls compare to ICF walls for heating and cooling loads? A “mass wall,” is a wall assembly like ICF; the code gives us a good indication of how much better it will perform than a traditional wood frame wall. Let's look at the 2024 IECC (International Energy Conservation Code). For climate zone marine 4, we see that a wood frame wall needs R30 cavity insulation or R20 cavity and R5ci (ci means continuous insulation) or R13 and R10ci, or R0 cavity and R20ci. Alternatively, a mass wall requires R13 (if half or more of the insulation is on the exterior of the mass wall, like ICF) or R17 if more than half of the insulation is on the interior of the mass wall (I have never seen a mass wall constructed in that way). So, if we look at the wood frame wall requirement of R0 cavity and R20ci vs the typical mass wall of R13, we can see that the code recognizes that the mass wall performs approximately 43% better than the wood frame assembly. THEN, we look at the typical R-value of an ICF wall, which is R22+ (most manufacturers are in the R22 to R24 range, but it can be increased); we can see that an ICF wall is over two times the performance of a wood frame wall. OR, put another way, to get comparable performance from a wood frame wall, it would need to have a continuous insulation value of R55 to perform similarly to the R24 ICF wall. Many studies show that ICF performs even better than what the code recognizes compared to wood frame. What does that translate to? Well, that 43% performance difference translates really well into heating and cooling costs, and many studies show that ICF uses 40% to 50% less energy (with some showing significantly more).
Another critical component of the wall assembly is how airtight it is. A wood frame wall assembly requires special care for the entire wall area to ensure that every gap/joint/penetration is sealed, whereas an ICF wall simplifies the air sealing (the concrete core is essentially airtight). However, wood frame walls and ICF walls can both be relatively airtight if good detailing is followed, especially around all openings/penetrations. By having an airtight assembly, we can control the air in the home (great for having clean air inside the house), potential wall condensation (only of concern for wood frame walls), and minimize heating/cooling costs. ICF makes it much easier to achieve an airtight wall, but again, it is possible to achieve this with a wood frame wall.
One essential item I will not touch on in this post because it needs a dedicated post is windows and doors. They are a critical component in the wall assembly, but an entire post is required to cover the nuances adequately. Similarly, I will not be covering ERVs or HRVs and the need to bring in fresh air in this post, but I will cover that topic in a future post.
In summary, code minimum wood frame wall assemblies are less expensive upfront than ICF walls but are significantly more costly over the long term. I recommend slightly reducing the home’s size and switching to ICF, which is by far the best assembly right now. It is also worth noting that ICF is typically less expensive than a high-performance wood frame wall.
For information, we build ICF homes and wood-frame homes. Ultimately, it is the client’s choice, and it is possible to build either one well. I recommend ICF because it is far superior, but we build using both.
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.