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Community and Q&A

Why do we need air barriers?

Lee_roy155 | Posted in General Questions on

I just dont understand the point of an air barrier on the inside of a building? they stop air movement, well isnt that what the insulation is supposed to do? so what is the air barrier doing that the insulation isnt?

Also why vapour barriers? isnt insulation suppose to stop cold air from entering and hot air from escaping? Is it not the case that vapour is only created when hot air enters a cold environment yet if the insulation is working will this not mean that a vapour barrier is pointless?

Apologies for my ignorance I just dont quite understand the elements at play here.

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  1. Expert Member
    Dana Dorsett | | #1

    >"they stop air movement, well isnt that what the insulation is supposed to do? "

    NO, that is NOT what insulation is supposed to do! It's possible to detail some types of insulation as an air barrier (eg: rigid foam board) but with others it's impossible (such as any fiber insulation).

    >"Also why vapour barriers? isnt insulation suppose to stop cold air from entering and hot air from escaping? "

    Neither vapor barriers nor insulation have anything to do with blocking air movement. Vapor barriers are to block moisture transfer via water vapor diffusion. A vapor barrier can sometimes be detailed as an air barrier (usually a good idea, since a small amount of air leak can move a lot more moisture than mere vapor diffusion), but the vapor barrier itself can still be effective at blocking water vapor diffusion even if it's not air tight.

    1. Lee_roy155 | | #4

      thanks for your reply it really helped out

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Lee,

    Most architects and high-performance builders think about controlling four things with a building assembly: water, air, water vapor, and heat.

    Water is an obvious and most important control because if water is getting into our assemblies and staying there, the building will grow mold, rot, and ultimately fail. Water is controlled with roofing, roofing underlayments, gutters, water-resistive barriers, flashings, etc. Home design also has a role in managing water. Roofs, for example can manage water well, or not.

    Air is the next most important control for two reasons. Homes these days are conditioned for most of the year. Our mechanicals are controlling the temperature and humidity levels in our homes. Therefore excessive air infiltration or exfiltration will not allow the mechanicals to perform as designed and the house will become uncomfortable and inefficient. Because we can't rely on a leaky house for fresh air supply, we introduce fresh air and exhaust stale air with ventilation. Also, air can carry a lot of moisture, so if air is getting into our wall assemblies, we have created the potential for moisture accumulation when conditions are right (and air flow through and around insulation degrades its performance). Air barriers are an assembly of different materials used to seal all aspects of the building envelope including gaskets, caulks, spray foam, membranes, sheathing, etc.

    Vapor is probably the trickiest control to understand (at least it is for me), because unlike water and air, which we are simply trying to keep from moving into the building envelop, vapor control is more nuanced. We do want to prevent water vapor from entering walls, but we don't want to trap it in walls. If a home has a poly vapor barrier on the interior for example, the exterior side of the wall needs to be vapor open to allow the wall to dry if it does get wet. Some designers and builders shoot for the most vapor open assemblies as possible. One of the keys here is dew point management so that there is no condensing surface for vapor to condense on inside the assembly. All building materials have a perm rating. Some, like poly sheeting have a very low perm rating and are considered vapor barriers (more accurately, a class I vapor retarders), others have higher perm ratings, like many housewraps, and are considered vapor open.

    Thermal is the final control. This is insulation's job. As I just mentioned it can be used to control the dew point, otherwise thermal control is largely a matter of comfort and efficiency.

    Sometimes a material controls more than one thing, the most common example today is ZIP System Sheathing, which many designers and builders use for both the water resistive barrier and the exterior air barrier. Drywall, is another example. It is a wall covering that can be detailed during installation to be an interior air barrier and when painted with latex paint, can serve as a class III vapor retarder, which offer appropriate interior vapor control in some building assemblies.

    Each of these controls is a hugh topic alone (search the site for much deeper information on all of them). I hope this was somewhat helpful to get you started. Maybe some other members will elaborate.

    1. Jon R | | #3

      > One of the keys here is dew point management so that there is no condensing surface for vapor to condense on inside the assembly

      Except in very mild climates, there will always be some condensation/sorption somewhere and it's rare to build such that some of it doesn't happen in a bad place (like on wood). This is why wetting and drying rates are important.

      Also note that position matters - for all of the barriers.

    2. Lee_roy155 | | #5

      thank you for your response (it was incredibly detailed) and it really helped

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