Understanding Air Barriers and Vapor Controls: Why and Where to Place Them - HypoAir (2023)

If you're trying to build your own home, or just renovating part of it, you probably want to get it right the first time. Here are some inspirations for good planningarbeitszone.com:

"By not preparing, you are preparing to fail." ― Benjamin Franklin, Founding Father of the United States

“Every minute invested in planning saves 10 minutes in execution; that gives you a 1,000% energy return!” ― Brian Tracy, author and motivational speaker

"Give me six hours to chop down a tree and I'll spend the first four sharpening my axe." ― Abraham Lincoln, former President of the United States

And truth. One of my personal skills is being able to paint walls well and having fun doing it. I know planning works because the best walls I've ever painted were the ones that I spent 75% of my time preparing and 25% of my time painting. Forward-looking planning makes the work much easier and more successful.

Building something to last is the same thing: the investment of time and money is worth it if the house works. Layout and design seem to be top priorities today, but function must come first. It's like picking the color of the room and buying that color before any structural decisions are made or executed. Does the roof work? Do the walls serve their purpose? Does the base work? Let's build and cover these bones so that they protect all the beauty of the form and the inner workings. (comes out of my soapbox now).

Here is an article worth reading:Build the perfect wallby Joe Lstiburek. Mister. Lstiburek argues that there are four barriers that must be built to protect the home from the elements, and here they are.arranged according to importance but not in the order of installation.

  1. a rainproof layer
  2. an airflow layer
  3. a vapor barrier layer
  4. a thermal control layer

He also argues that all of these layers must be on the outside of the structure (the bones, be it wood, steel, or concrete) because the structure is where the money is! Aside from the rainproof layer and maybe a vapor barrier, many builders don't realize this. Install the insulation on the outside of the structure? It's not enough done, but it's brilliant. Here is a simplified diagram (bauwissenschaft.com):

Here are some key points to understand why these layers of control are necessary and why they have these priorities.

Rain control:

  • Water does a lot of damage.
  • Ultraviolet light also does great damage.

air control:

  • To control the indoor climate and the health of the building and its occupants, you need to control the air. Controlling the air means sealing it off with controlled penetrations.
  • Air can carry (transport) a lot of water: see the first point above about water doing a lot of damage.

Steam Control:

  • Water vapor travels in two ways: air transport (see air control above) and vapor diffusion.
  • In order to control the indoor climate, you in turn need to control the flow of steam in and out of it.

Thermal control:

  • Thermal control prevents condensation when temperatures and dew points differ inside and outside the home.
  • A final word: comfort!

Precipitation control and thermal control are relatively easy to understand; Finally, all 4 season homes now have siding and insulation. It took a while to understand the limitations and the relationship between air barriers and vapor retarders.It is helpful to know that air barriers are actually rated for their ability to retain vapor, so air barriers are also used as vapor retarders in modern buildings.(The term "vapour barrier" used to be in vogue, but it's actually more accurate to use "vapour retarders"). There are two terms that refer to a material's ability to retain vapor: permeability and permeability. To understand them, it helps to know something about water vapor.

Water vapor moves through a building in two ways: diffusion through the building materials themselves and air transport.Here is a diagram illustrating the two:


Water vapor has its own pressure that contributes to the total air pressure.The vapor pressure differential between the two sides of a building envelope is the driving force behind diffusive vapor transfer.(buildingenclosureonline.com)

Water vapor absorbs heat differently than air with which it is mixed. If you think of sunlight entering through a window and heating a room, the sun will heat the air and water vapor in the room, but at different rates. Heat added to the air directly raises its temperature (so-called sensible heat). Heat added to the water vapor increases its temperature more slowly (also sensible heat), increases the vapor pressure and as the ambient temperature increases, the capacity of the air to hold more water vapor also increases; thus causes the evaporation of water from the environment (e.g. from our skin) without changing its temperature (so-called latent heat). Heat that can be felt causes a temperature increase, but latent heat causes a change of state without a change in temperature. (Latent heat is the heat applied to melt ice or boil water; an ice cube does not change temperature as it melts, and water in a boiling pot does not change temperature as it boils.) The vapor pressure seeks the equilibrium, i.e. an area of ​​high vapor pressure tries to diffuse into an area of ​​low pressure. It does this by "diffusing" through the wall itself. This step is the reason for installing a vapor barrier.

In terms of air transport of water vapor, hot air naturally contains more moisture than cooler air, so you want to prevent hot, humid air from entering in summer and hot air from escaping in winter (causing condensation at the inlet/outlet). . The air barrier is responsible for stopping this movement.

permeabilitydescribes the water vapor permeability (through diffusion). It is the rate over one hour through a square foot of a material of a specified thickness at a specified vapor pressure, expressed in perms (grams/hour·ft2·inch of mercury). (buildingenclosureonline.com) Being water vapor, permeability is also a description of how vapor retarders slow latent heat transfer.

permeabilityin building materials, permeability is per unit thickness - or inch perm, which is useful when comparing different thicknesses of insulation.

With the permeance standard we can compare the vapor permeability of different building materials. The less permeable a building material is, the greater its resistance to water vapor transmission. A vapor barrier is basically any building material that has very low permeability (high water vapor resistance). (buildingenclosureonline.com) Here is a table with some common building materials and their permeances:

Below is a diagram of The Perfect Residential Wall. The steam arrows denote the desired moisture flow because "we want the set to dry inside the control layers and dry outside the control layers". (Joe Lstiburek, Civil Engineering) You never want to put material between 2 vapor barriers. In fact, steam is definitely retained there in that sandwich, leading to condensation and mold!


Note that this wall has two layers of insulation, which is good as the insulation doesn't trap moisture (although it's probably wise not to insulate without kraft paper inside, more on that below). The outer rigid insulation prevents heat transfer to the wooden structure, and the inner insulation provides greater comfort.Article The perfect wallalso describes a "smart wall" that combines all three: air, vapor and heat control layers with an outer layer of closed-cell, high-density foam insulation.

Vapor retarders have come a long way in just a few decades. In the early to late 1900's, felt/tar paper was typically the only thing that went over the top of the liner and behind the liner. There was no drainage behind the siding (and most medium sized houses still don't have this) and no air barrier. Sometimes the arrangement worked well to protect the structure, sometimes not, but without an air barrier, only moderate thermal control could be achieved inside. Additionally, the kraft paper backing on the fiberglass insulation is a vapor permeable barrier (see table above).

Polyethylene was introduced as a vapor barrier in the 1950s (approx.onstructioncanada.net) and had disastrous effects in many homes because despite being a Class 1 (0.1 Perm or less) vapor retardant, correct placement of this layer was critical to avoiding condensation problems. This brings us to our final point, where is the best place for the air barrier and vapor barrier?

There is a lot of confusion in the construction industry about where to place these layers. For a long time, installers were instructed to run the kraft paper liner of fiberglass insulation “on the hot side.” They were also instructed (to this day) to install it “facing them”, i.e. towards the inside of the building. What about buildings in the south that are cooler on the inside for most of the year and only a few months cooler on the outside? You can't seasonally rearrange insulation after the wall has been caulked (!). The good thing is that kraft paper is semi-impermeable and, as it turns out, is one of the first "smart" vapor barriers, meaning it has variable permeability: low permeability in colder, drier climates and higher permeability in drier and drier climates. humid climate. (greenbuildingadvisor.com) Newer vapor retarders do this to give buildings some flexibility in these fluctuating climates. Some brands are Intello Plus, Pro Clima DB+ and MemBrain (haha). Each of these products is marketed as both an air barrier and a vapor barrier, so by installing one outside the structure (wood, metal or concrete) you protect the structure by slowing down the diffusion of vapor through it and climate control inside the building maximize , sealing air leakage.Since airtightness takes precedence over vapor barrier, it is crucial that the product is installed correctly and completely to give airtightness priority.For example, on a busy construction site, various companies and change orders can drill many unauthorized holes in a properly installed air barrier. Therefore there must be signs to protect and rules to permit penetrations.

Since YouTube and Pinterest are the inspiration for many home improvement projects, if you are a DIYer doing a home renovation and even using a contractor, go back to the beginning of this article for more planning inspiration. Reading and researching air barrier and vapor control layer products and installation methods takes extra time, but getting them right is just as important to your family's long-term health and your home's longevity. Some recommended sites (in no particular order) are GreenBuildingAdvisor.com, BuildingScience.com, EnergyVanguard.com, Inspectapedia.com, BuildingEnclosureOnline.com, and more. Good planning is more than half of good building!

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