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HJ_AJS

Effects of dew point in fully grouted insulated wall

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In a northern climate, for an integrally insulated, fully grouted, single wythe wall, would the location of the dew point throughout the year be a factor in keeping the building leak-free?  The air barrier requirement would be covered by the fully grouting, but would this assembly also need a vapor barrier?

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This is a great question HJ – one that causes a lot of confusion and many (often conflicting) opinions. No, this assembly would not need a vapor retarder. In fact, I’m not a big fan of vapor retarders in single wythe masonry construction in general as they tend to cause two real problems for every pseudo-concern they intend to solve. (There are some exceptions – such as indoor pools in northern climates – which shouldn’t automatically trigger the inclusion of a vapor retarder, but rather an assessment of the vapor permeance of the wall assembly being used.)

Dew point is another common point of confusion. For most of the U.S. where we heat during the winter (and the vapor drive is generally to the exterior) and cool during the summer (reversing the vapor drive to the interior), the vapor retarder will be on the wrong side of the wall for some portion of the year. (And the last thing any assembly deserves is a vapor retarder on both sides of the wall as the wall would never be allowed to dry.)

Far more moisture can get into (or through) a wall assembly by air leakage rather than vapor diffusion. Hence, starting with a solid grouted assembly a great start – but good performance also relies on good design and construction practices. The units and mortar should contain an integral water repellent; openings (and if present) copings should be flashed; mortar joints tooled to a concave profile; and if appropriate, consider a post-applied sealant. More detailed discussion is provided in NCMA TEK 19-2B.

TEK 19-02B.pdf

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