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Clewless1 | Nov 25, 2010 10:28am | #1. Calculating the temperature at each layer of a wall is relatively easy to do. Most would assume that the RH inside would tend to drift outside ... increasing RH as it does. Making that assumption, the dewpoint would be easy to determine. But as Dan says ... the vapor transmission rates are difficult at ...
Rigid foam must be thick enough to prevent condensation on the roof sheathing. When combining exterior foam insulation with fiberglass, cellulose, or mineral-wool roof insulation, the foam layer must be thick enough to keep the sheathing temperature above the dew point. Otherwise, water vapor can condense on the back side of the sheathing ...
Answer: It depends on the temperature differential at any given point in time. Could be half way into the wall cavity, or 1/3rd. And it varies depending on real R values at any point in the wall. Adding exterior insulation simply moves the dew point more to the exterior. Adding most anything to the exterior moves the dew point further out.
Closed-cell foam is so dense that it is difficult to trim. To avoid having to trim closed-cell foam in a 2×4 wall, the installer will usually stop at a maximum depth of about 3 in., instead of the full 3-1⁄2 in. depth of the stud cavity. This leaves the typically bumpy surface of cured foam and about a 1⁄2-in. gap to the back of the drywall.
If you take some warm moist air at, say, 70 degrees and 40% relative humidity, lowering the temperature will raise the relative humidity. When the temperature gets to about 45 degrees, the relative humidity reaches 100%, meaning that moisture in the air will condense into a fog, and collect on any solid surface nearby.
The 2012 IRC has a handy way to make sure the sheathing stays above the dew point most of the year: table R402.1.1, which shows the minimum R-value of cavity and exterior insulation. For Rhode Island (zone 5), it’s 13 + 5. In other words, minimum R-13 cavity insulation with minimum R-5 exterior insulation.
The R-value of dense-packed cellulose (the most common insulation used in double-stud walls) is about 3.7 or 3.8 per inch, so a 12-in. double-stud wall has an R-value of about 45. Both the design of the building and your performance goals affect the optimum thickness of your walls. The walls might be 16 in. thick for a Passive House in Vermont ...
Enter flash and batt. This hybrid system relies on a thin flash coat of foam sprayed against the inside of the sheathing, with the remainder of the framing cavity filled with fiberglass. Flash and batt costs as much as $2 per sq. ft. less than meeting R-value requirements with foam alone. Like any element of building, however, there’s more to ...
Use a dehumidifier to keep the humidity below dew point. Stop breathing, sweating, cooking and washing to keep water vapor out of your home. Leave your home empty during cold weather. If you keep your indoor temperature at 70°f and your relative humidity 65% then dew point is 57°f. If you change your relative humidity to 45% then dew point is ...
Vapor drive is the least significant cause of moisture problems in a house, but it’s still really important. When water vapor passes through an assembly and comes in contact with a surface that has a temperature below the dew point (the temperature at which water vapor condenses), then it becomes a wood-rotting, mold-feeding liquid.