The 3 Levels of the Building Enclosure Hierarchy
When you approach the subject of building science, especially as it regards heat, air, and moisture flows through the building enclosure, it's easy to get confused. There's a lot to learn: blower door testing, insulation grading, R-value, permeance, radiant barriers, combustion safety, solar heat gain coefficients... That's why we break things down into simpler pieces. In science, that process has led to all kinds of discoveries (atoms, nuclei, quarks, organs, cells, bacteria...).
In building science, the first big breakdown I find helps people grasp the basic ideas is seeing the house as the mechanical systems and the building enclosure. I heard an analogy on the radio once that helps make sense of this: Your HVAC system is the faucet; the building enclosure is the cup. The mechanical systems are very important, of course, but they depend on the quality of the building enclosure. And that's where another important classification clarifies things.
The building enclosure is the boundary between conditioned space and various types of unconditioned space (outdoors, garage, attic...). It's a key concept in building science and in understanding the house as a system. This is Building Science 101. But there's a hierarchy here. We should be thinking of this in three levels:
The enclosure includes floors, walls, and ceilings. Each of those components is typically made up of assemblies. And assemblies are made of materials.
Why it matters
"OK, that's all well and good," you're thinking about now. "But what does this mean to me?"
Here's why it's important to understand the distinction between these three levels of the building enclosure. Products have ratings. Sometimes it's the material that's rated. Sometimes it's an assembly. And sometimes shady salespeople try to confuse prospects by blurring the lines between what was rated.
Let's take the case of foil-faced bubble wrap. It's a product made up of a couple of materials - plastic and aluminum foil. But when used in the building enclosure, it's only one part of an assembly.
By itself, foil-faced bubble wrap has an R-value of about one, almost entirely due to the air trapped in the plastic bubbles. Yet go to a trade show and you'll see sellers of these products claiming much higher numbers. At the Southeast Building Conference in 2010 I asked a foil-faced bubble wrap seller what the R-value of his product was and he said R-15.4.
No! That's not for the bubble wrap itself. That's for bubble wrap in a particular assembly. Martin Holladay covered this topic really well in his article, Stay Away from Foil-Faced Bubble Wrap. See especially the section titled, "Blurring the line between product R-values and assembly R-values."
Of course, R-value isn't the only place you need to understand the distinction between material, assembly, and enclosure. Here are a few others.
- Windows - U-value and solar heat gain ratings apply to the whole assembly, not just the glass
- Blower door tests - Done for a whole enclosure
- Permeance - Individual product ratings must be combined with the other materials in an assembly to find the net drying capacity of an assembly
When evaluating the performance of a building, you'll be better prepared to make the right call if you understand these three levels.
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