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Still need clarity whether or not permeance matters with exterior foam

joenorm | Posted in General Questions on

Climate zone 4C. 

Trying to decide on the right exterior foam for a new build. I had written off foil faced polyiso because it doesn’t breath at all compared to EPS but in my last post Dana suggested using it. 

This is a confusing topic, everyone seems to have their own idea of what is most important.

One article says there is a very specific R-value per climate zone, another article says it really doesn’t matter much, and everyone has an opinion somewhere in between.

There is a lot of talk about vapor permeance on this site. 475 building supply has basically based its entire sales operation on very expensive ways to remain vapor permeable.

So why is it ever OK to to wrap a building in foil?

Thanks for fielding my all my questions, very much appreciated.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Joe.

    You'll hear a lot of anecdotes from builder and remodelers who have done this or have done that and have never had a problem. And a lot of it is true. If you want to build your house based on those anecdotes and the inspector is okay with it, that's your prerogative. If you want to build your house based on current building science, then permeance matters in the design of your building envelope.

    By now you know that the reason to use less permeable materials is to keep water vapor from entering walls, where it could condense and cause problems. The reason to use more permeable materials is to allow water that has already accumulated in a wall to dry to one side of the wall or the other. You can design your wall a lot of different ways, but as I said in response to another one of your questions on this topic, you can also use the guidelines written in the International Residential Code.

    Again, look at section R702.7 on vapor retarders. In your area, if you don't meet certain conditions (basically adequate continuous exterior insulation or vented siding), then you would install a class I or class II vapor retarder on the inside of the walls (poly or kraft faced batts, for example). If you do meet those conditions, then you could use a class III vapor retarder (drywall with latex paint).

    So, to answer your question, it is okay to wrap a building in the adequate R-value of vapor impermeable, foil faced rigid foam because when you do that, the sheathing stays warm enough and doesn't present a condensing surface and you also omit the class I or class II interior vapor retarder and your wall now has lots of inward drying potential should it get wet.

    If you don't want to follow the IRC on this, and want to design an alternative assembly, read this article: Building Science Corp. on Understanding Vapor Barriers

  2. Expert Member
    Michael Maines | | #2

    Joe, for better or worse, designing and building a house is complicated, and the more you increase energy efficiency measures, the more sensitive the building is to mistakes. 475 has built their business on vapor-open assemblies, and there is a lot to be said for the resilience of that approach. Walls that can dry in two directions, to the interior and to the exterior, are more forgiving of less-than-perfect conditions, and they allow the use of natural materials.

    On the other hand, if certain conditions are met, you can safely build a house wrapped in foil-faced insulation. The difference is that the wall can only dry to the interior. But if all of the required conditions are not met, you could develop a moisture problem.

  3. Jon_R | | #3

    +1 on what Michael said.

    Be conservative. A very conservative approach is design your walls to be code compliant (of course), have some exterior insulation to warm the sheathing (in appropriate climates), have a rainscreen, conform to these Building Science recommendations and provide some ability to breath in both directions (no < .1 perm vapor barriers anywhere). In almost all cases, it is easy and affordable to do ALL of these.

    An opposite extreme would be to build something barely code compliant and go against the other recommendations. I wouldn't do this, but it will probably work. For even more unnecessary risk, use standard latex paint as a vapor retarder (see page 62 here).

  4. Aedi | | #4

    I think the simplest explanation is that it is okay -- even desirable -- to have one vapor barrier. Any moisture behind the vapor barrier can dry to the interior, and any moisture in front of the barrier can dry to the exterior. The trouble arises when you have multiple vapor barriers, as you can end up inadvertently trapping moisture in your assembly, leaving it no place to dry to.

    When you combine impermeable exterior foam with interior polyethylene (or similar product), you have two vapor barriers. Any water that gets between the polyethylene and the foam has nowhere to dry, and can quickly lead to damage. And it is best to assume that water will eventually make its way there, no matter what you do. There are two solutions to this problem: to omit the polyethylene and use a thick impermeable foam, or keep the polyethylene and use a permeable foam.

    When omitting the polyethylene, foam thickness plays a role to prevent condensation. Since there is no interior vapor barrier, vapor can travel freely to the face of the foam. A thin foam will get cold in the winter, and water vapor will condense on the inside face. In limited amounts, this is fine, the water can simply dry to the interior. But if the foam is too cold, the condensation can outpace the wall drying capacity, and lead to moisture damage. Thicker foam prevents this. The colder it gets, the thicker the foam needs to be.

    On the opposite end, you can elect to use a vapor barrier to limit the vapor reaching the foam. In that case, you should ensure the foam remains permeable, so that any condensation (or other moisture sources) that do occur can dry to the exterior. An inch or two of EPS is pretty permeable, and so can be used with an interior vapor barrier without issue.

    Both approaches have their pros and cons. I favor the impermeable foam approach, simply because polyiso is a better product than EPS, easily available reclaimed where live, and the cold climate makes thicker foam desirable. Milder climates have more flexibility.

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