Are dew point calculations for all wall assembly’s important?
nlbailey
| Posted in General Questions on
I am planning on doing an R-30 wall assembly with faced fiberglass batts. I have landed on this after a previous post as it is simple to do with 2×8 studs and doesn’t require exterior insulation to achieve this r-value (I will be using vinyl siding and have ants so am hesitant to use foam board). I have read about others using this assembly and similar wall assembly’s such as bonfiglioli and double stud walls.
It seems with this level of insulation, according to dew point calculations, condensation would happen in the middle of the stud bay. Won’t this cause issues? Or, since there isn’t any exterior insulation and there is an interior vapor retarder, will the wall will be able to dry to the outside without having any issues? Since I know others have used this assembly and the bonfiglioli assembly / double stud walls have been used without issue too I’m thinking I don’t really need to worry about it.
So, is this important or something that doesn’t need to be worried about in this situation or am I misunderstanding ?
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Replies
nlbailey,
You don't need to worry. It isn't so much where the dew point is that matters, it's where that moist air meets it's first condensing surface of interest which is below the dew point. https://buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion
The short version is that as long as the sheathing has a drying path that allows it to dry faster than it is getting wet, the wall is safe. Dew point calculation make sense for complex or novel walls. We know how most of the common ones (like the one you are proposing) work, so they aren't necessary for those.
Thank you for the link and information.
So, is there a specific perm level for the sheathing that should be adhered to for ensuring enough drying potential? My understanding is that plywood has a perm of ~10 and OSB is ~2 (including sheathing like zip). I assume plywood would be a safer choice but is OSB then considered a bad choice or will it be just fine?
Also another question. Since the dew point relates to the first condensing surface below the dew point, does that mean in a double or staggered stud wall the interior face of the exterior studs is at risk of having moisture condense on it?
nlbailey,
Plywood is a lot safer. It acts as a variable-perm vapour-retarder, which can be useful if the wall gets wet.
Yes, in broad terms the thicker the wall the colder the sheathing will be, and therefore more vulnerable to moisture accumulation. However, absent other flaws these walls don't typically experience problems: https://www.greenbuildingadvisor.com/article/the-mythical-threat-to-double-stud-walls
The family of walls with permeable cavity insulation of various widths - from 2"x4" to double stud - do pretty well as long as they have a good air-barrier, and good ability to dry to the outside - preferably including a rain-screen cavity.