Hello, this is Ono from WALD.
Currently, the mainstream of Japanese home building is “vapor-proofing (stopping)” by using plastic sheets to prevent moisture from entering the walls. On the other hand, you may have heard some companies claim, “Our homes use highly vapor-permeable materials, so the walls breathe and won’t suffer from condensation.”
However, from the professional perspective of Building Physics, both the mainstream “vapor-proofing” and the “simple high vapor permeability” can sometimes harbor risks that damage the home from the inside.
It is not as simple as saying “it’s safe if you stop moisture” or “it’s a good home if the materials are breathable.” Today, I would like to explain the slightly technical but crucial “laws of physics within walls” that we calculate to ensure our homes truly last for generations.
“Vapor-proofing” and “Simple High Permeability” causing congestion inside the walls
Why is there a concern with both mainstream “vapor-proofing” and materials that simply “pass moisture”?
It is because, in both cases, the balance between the “entrance” and “exit” for moisture is often not calculated.
Standard Japanese homes (vapor-proof approach) apply a “vapor barrier sheet” on the indoor side and “structural plywood” on the outdoor side for earthquake resistance.
If moisture happens to enter through a gap in the indoor sheet due to unforeseen circumstances, it becomes trapped by the thick outdoor plywood, making it difficult to escape.
Similarly, even in a home that boasts “high vapor permeability,” the result is the same if the final exit—the outer wall or plywood—does not allow moisture to pass easily.
The moisture entering the wall from the room may cause “congestion,” unable to find an exit. This leads to “interstitial condensation” when the trapped moisture cools down, which is a primary cause of damage to critical pillars and structures.
What matters is not just the material itself—whether it “stops” or “passes” moisture—but the “calculation to ensure any moisture that enters is reliably vented to the outside.”
What is the “sd-value” that quantifies moisture resistance?
Here, I would like to introduce an indicator we use in our design: the “sd-value” (Equivalent Air Layer Thickness).
It sounds technical, but the concept is very simple.
It represents the resistance of a building material to moisture, translated into “how many meters of air thickness” it equals.
For example, an “sd-value of 1m” means the material has the same resistance as 1 meter of air. An “sd-value of 50m” is like a 50-meter thick wall of air. The higher the number, the stronger the guard against moisture.
A typical vapor barrier sheet has an sd-value of over 50m, while the structural plywood used on the outside often has an sd-value of 4 to 10m. Looking at these numbers, you can see how difficult it is for moisture to find an exit once it gets inside the wall.
The core of WALD: Calculating the “Vapor Permeability Gradient”
To solve this problem and keep the inside of the walls dry and safe at all times, we use a vital concept in physics: the “Vapor Permeability Gradient.”
The Vapor Permeability Gradient is the balance of making it “hard for moisture to enter from the inside, while making it overwhelmingly easy for it to escape as it moves toward the outside.”
At WALD, we precisely design this gradient through sd-value calculations:
- [Entrance (Indoor side)] sd-value approx. 2–3m (A moderate brake to limit moisture entry)
- [Exit (Outdoor side)] sd-value approx. 0.1–0.2m (An exit to rapidly vent any entering moisture)
By creating this “slope” from the inside out, we ensure moisture flows out smoothly, much like sliding down a slide.
Designing and building homes based on calculation, not just intuition
Relying solely on intuitive phrases like “it won’t condense because it’s airtight” or “it’s fine because natural materials breathe” can be risky.
We apply the sd-value of every single component to our formulas, ensuring the Vapor Permeability Gradient is correctly established before we even begin the design.
Because of this thoroughly calculated gradient, even if moisture enters the wall due to an unforeseen event, it is instantly discharged, allowing the house to last for decades.
While this has been a bit technical, we demonstrate this “wall cross-section structure” with actual models at our showroom. We invite you to come and see our “calculated slide” for yourself.
