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Are Wall Dew Point Calculations for the Summer in Climate Zone 5 Critical?

user-413071 | Posted in Energy Efficiency and Durability on

I am wrestling with new house build decision in Climate Zone 5 regarding the wall design. I am a fan of the REMOTE Wall concept (CCHRC), i.e. external insulation outside of the sheathing of a stick frame wall. I am also a fan of mineral (rock) wool and prefer it over foam board as long as it will work and also avoid moisture and mold problems in the wall assembly. I have studied the literature and have seen support on within GBA for the idea of substituting mineral wool boards for foam boards in the REMOTE Wall concept. I understand the goal of keeping the sheathing above the dew point temperature and the way in which a 2/3rd ratio of external/internal insulation R value accomplishes that goal. I have made the calculations myself of what that R ratio should be, what thickness of rockwool should be used on both the inside and outside, and what the sheath temp is compared to the dew point temp.  These calculations suggest that even though rockwool does not provide an impervious layer like foam board, it should nonetheless keep the sheathing drip it is done correctly. This seem to me to imply that foam board not absolutely necessary Рat least for winter conditions.

However, I have always seen these calculations made for northern climates with respect to the heating season when we want to assure that the moisture from inside air is not condensing on the sheathing which is cold from the outside weather conditions. I have not come across (in the same document) a corresponding calculation also made of sheathing temp during hot summer conditions when humidity from the outside could reach the sheathing through a permeable insulation like rockwool and condense on the sheathing Рthe backside of which is cooled by the internal air conditioning. If one calculates an R ratio that would keep the sheathing above dew point for this summer condition, it would be the inverse of the R ratio for the winter condition. But if the installed R ratio is chosen for the winter condition, it begs the question of whether I am going to have a problem in the summer, especially as the climate keeps getting warmer.

What if one argues that because rockwool can breath, any moisture that condenses on the outside surface of the sheathing on the few summer days when the dew point is high (and the R ratio is that designed for winter), is not a long term problem because the sheathing will dry out to the outside, especially f there is an air gap between the external cladding and the rockwool. A corollary to this argument may be that drying to the outside is easier than drying to the inside. Is this a legitimate/valid argument & rationale? The wall can only have one R ratio; if the winter condition takes precedence in designing the wall, is it possible for the wall to failure because of the R ratio is improper for summer conditions? If this conflict between winter and summer design conditions is both unavoidable and is likely lead to failure, is using foam board the only solution because of its imperviousness? If that is the case, is a SIP wall – because it is just foam between two OSBs, an even easier solution? I have other reasons to prefer a stick frame wall, but the hangup right now is whether this wall design, whatever it is, must use foam board – one way or another. By the way, I am aware of the double wall concept with blown cellulose but I choose not to go there. So my choices are a REMOTE Wall with either foam board or rockwool board, or a SIP wall, that will work for all seasons.

Thank you in advance for anyone who can advise me. The GBA site is an invaluable resource, especially for difficult and complicated questions.

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  1. user-413071 | | #1

    I was doing some more research tonight on dew point for my local area. It appears that I may have misunderstood the calculation of dew point temp itself and how it relates to ambient temperature and humidity. rather than trying to calculate dew point, I searched directly on historical dew point for my location rather than trying to calculate dew point. It appears that the historical dew point is not as high as I was getting by plugging temperature and "humidity" into a dew point calculator. It the dew point is actually as low as the historical dew point data suggests during the summer (between 60 and 75 degrees), then the likelihood of frequent condensation on the sheathing in the wall during the summer is much lower than I was fearing. Any comments are still welcomed.

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