You need to know about water perm vapor transmission!
Let’s talk about perms. No, not the hairstyle perms – we’re talking about water perms!
What’s a water perm? A water perm is a measurement of water as it passes through a material over a period of time. The key part is that there needs to be a static vapor pressure difference between the two areas: where the water is coming from, and also, where the water is going.
Wait a minute- what’s ‘vapor pressure’? It’s a combination of relative temperature and humidity. If the vapor pressure in one area is greater than the vapor pressure in an adjoining area, the water vapor (water molecules that are air-borne) will want to move from the higher pressure area to the lower pressure area. Think about when it’s early August here in St. Louis and the temperatures are flirting with triple digits. The humidity is probably pretty high as well, maybe around 90 percent. Your A/C is running nonstop and that means cool air is keeping your comfortable… but it’s a fight.
The cool air from the A/C invades your office and it’s dry because of no humidity. Outside is hot and humid, but inside is cool and dry. The heat outside is putting pressure on your building’s energy envelope as the humidity in the suffocating air outside is pushing against your insulation barrier to come inside. The opposite is true in early January when it’s in the 20s and we’re bracing for another city-stopping ice storm. Your furnace is blasting hot air through your building and it’s causing the moisture in the air to rise in contrast with the cold outside.
What does this have to do with a perm of water? One of the ways we measure a building’s energy efficiency is by measuring how much water vapor seeps through an insulation type over a period of days, weeks, and months. This is often a minuscule amount but it lets us know where we may need to apply more insulation. You can’t always detect air in insulation applications, but you can often tell the effects that moisture is having on a building’s energy load.