Helpful Analogies for Building Control Layers
It can help to summarize the different control layers of a building if we think about them like layers of clothing we can put on our bodies:
Wearing a sweater is like putting on a thermal control layer. Wool is a good insulator, slowing the flow of heat from our bodies, and the thicker the sweater, the warmer we will be. But when the wind starts to blow, the sweater is not very effective. And if it starts to rain, the sweater does not keep us dry (though it will dry out after a soaking, if the conditions are right).
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Wearing a rain suit is like putting on a weather resistant barrier. A plastic layer can shed the rain and keep us dry, and it will also keep the wind from our skin. But it’s not effective for keeping the heat inside.
The rain suit is also like putting on a vapor barrier. We soon find that moisture from our bodies gets trapped inside the impermeable plastic and starts to condense on the inner surface, causing us to be wet and uncomfortable.
Wearing a Gore-Tex jacket is like putting on a vapor permeable air control layer and WRB.
It will repel most of the rain, and it will keep out the wind, helping to keep us comfortable.