Walls with rigid foam sheathing and a vapor barrier on the roof
user-6122944
| Posted in Energy Efficiency and Durability on
With the exterior foam with vapor barrier on the outside of the house, and interior vapor barrier on the roof, how does the moisture flow from ceiling to the outside of the house? how do the 2 vapor barriers connect to evacuate moisture?
Any resources on what the detail should be to prevent moisture problems in that case?
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Replies
Generally, your envelope is not designed to "evacuate moisture". What you want the envelope to do is:
- Keep water out.
- Let water out if it gets in.
- BSC article: https://buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers
- GBA article: https://www.greenbuildingadvisor.com/blogs/dept/musings/vapor-retarders-and-vapor-barriers
Typically, flashing and air barriers are more critical than vapor barriers. Also, typically, you want to be thinking about vapor retarders not complete vapor barriers.
When you take a hot shower and your bathroom fills with moisture that needs to be removed, you want it to be removed by your bath fan, not by moving through your envelope, so it seems like you might be thinking about the goals of a vapor retarder the wrong way with your question about how the envelope "evacuates moisture"
Does that help at all?
Tatiana,
Brendan is right. Here's how I explained the situation in one of my articles ("Worries About Trapping Moisture"): "Water doesn’t need to escape from your house. Although it’s true that indoor air is warm and humid during the winter, while outdoor air is cold and dry, that doesn’t mean that indoor moisture needs to “escape” from your house. It’s perfectly OK if the indoor moisture stays where it is without “escaping.” Some old-time carpenters look at walls that include a layer of rigid foam and exclaim, “That won’t work! If you put in a layer of foam, the moisture will have nowhere to go!” Moisture in your home isn’t like your retired parents in Michigan on New Year’s Day, itching to go to Florida. The moisture can stay right where it is, all winter long."
Tatiana,
You mentioned that you have an "interior vapor barrier on the roof." What type of vapor barrier are you talking about? Where is it located?
In cold climates, some builders install a polyethylene vapor barrier between the drywall ceiling and the ceiling joists. In most cases, the polyethylene vapor barrier doesn't cause any problems, but it isn't required by code. Most building codes only require a vapor retarder (a less stringent layer than a vapor barrier) in this location. The requirement for a vapor retarder can usually be met with vapor retarder paint.
Tatiana,
Q. "How do the 2 vapor barriers connect to evacuate moisture?"
A. Vapor barriers have nothing to do with the need to evacuate moisture. Vapor barriers are installed when we want to stop the transfer of moisture (for example, under a slab on grade).
You may be thinking about air barriers, not vapor barriers. Air barriers should be continuous -- not interrupted.
we are building a cottage with exterior foam sheathing. my husband is experienced with inner vapor envelope ( hope i use the right terms, he is not near to correct me) and this is the first time he is building with exterior vapor barrier. the roof will have a poly vapor barrier on the interior side of insulation and while he is considering ways to create an envelope, i was wondering if moisture needs to travel. thanks to your responses, i understand that's not the main concern. the air barrier then- how is it achieved in construction with exterior and interior barriers meeting?
Tatania,
Here is a link to an article that provides introductory information on air barriers: Questions and Answers About Air Barriers.
From your questions, I'm guessing that you would also benefit from reading these two articles:
Calculating the Minimum Thickness of Rigid Foam Sheathing
How to Install Rigid Foam Sheathing
Tatiana,
You also should search the site for articles on building a "pretty good house." Here is one: https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/pretty-good-house
I think this is the type of home you are interested in building.