What is Vapour Diffusion
Water and moisture accumulation can be potentially dangerous when it comes to construction, and dampness can wreak havoc on basements and the insides of walls, causing structural damage or even mold build-up. There are several ways that water and moisture can get into a structure, and the more common causes are things we can all recognize, like a leaky roof or poorly sealed window. Today we’re going to talk about a less commonly understood method of water transfer: vapour diffusion.
Vapour diffusion is the movement of water molecules through a porous, vapour-permeable material. Wood and drywall are examples of porous surfaces and, therefore, susceptible to vapour. Wood and drywall absorb or soak up moisture if they are left in a damp environment and release moisture via evaporation if left in a dry place.
Vapour Diffusion: The Basics
Let’s say you have a damp bathroom situated next to your living room. The bathroom, generally, is a high humidity room in a house. Moisture would be absorbed on the damp side of the bathroom drywall and pass through to the drier living room side via evaporation. This is vapour diffusion, in this case moving from a high moisture content side to a low moisture content side.
Moisture Moves From Warm to Cold
Vapour diffusion does not always move from the higher moisture side to the lower moisture side, and rather, the motion or drive of vapour diffusion should be defined as moving from the warm side to the cold side. This has important implications for exterior wall insulation, and improper use of vapour-permeable materials can lead to issues discussed above, including water build-up causing mold and structural damage.
Vapour diffusion is well-understood in the construction world, but changing energy-code practices have increased insulation levels, particularly in colder climates. Improved thermal efficiency has several benefits, including cost savings associated with greener energy practices. However, the various approaches to insulation construction will perform differently when it comes to vapour diffusion and moisture control
Vapour Drive in Cold Climates
In colder climates, the flow of moisture typically moves from the indoors to the outdoors, from warm to cold. This is known as an outward vapor drive. Moisture will always try and move from warm to cold, but materials can be used to control an outward vapor drive and are graded according to how vapor-permeable they are. Some materials are considered vapor barriers in that they are nearly or completely impermeable.
The use of vapor barriers, or highly impermeable materials, on the interior side of a home can help prevent vapor drive from moving through the wall. The practice of placing impermeable materials on the interior side is considered to be effective for cold climates specifically, where outward vapor drive takes place.
When Condensation Build-Up Can Factor In
Sticking with the outward vapour drive model, if a vapour barrier was placed on the cold side during wall construction, then condensation would start to build up in the wall. The misplacement of a vapour barrier on the cold side of a wall can have potentially damaging effects.
Condensation occurs at the dewpoint temperature when air is holding the absolute maximum amount of water vapour that it possibly. Once the dewpoint temperature is reached, the vapour condenses and becomes liquid.
Consider our wall with a vapour barrier on the cold side. The temperature of the barrier would be colder than the temperature within the wall itself. When the cold side barrier is colder than the dew point temperature of the warm side, the materials on the cold side provide greater resistance to vapour diffusion than the materials on the warm side.
Vapour Diffusion Promotes Drying
While vapour diffusion is often thought of in strictly negative terms, it also provides a chance for the wall to “breathe” and for moisture to dry. Remember that vapor always moves from warm to cold. Builders can use this process as a benefit and design walls in such a way that there is the maximum amount of drying potential taking place.
Split insulated are one way that builders can control outward vapour drive and promote drying. Split insulated walls use an interior, warm wall vapour barrier as well as exterior insulation to help with the diffusion of moisture. By splitting the wall, the exterior or cold side stays closer to the interior temperature of the home and reduces the possibility of condensation.
Smart Builds Promote Vapour Control
If you’re interested in protecting your home from condensation and moisture while choosing environmentally friendly energy practices, call Lucas at GenCorp Homes Inc. We keep up to date on all the latest trends and techniques for eco-friendly, reliable home building.
