Replies

1. 
   creativedestruction | Jun 23, 2020 03:38pm | #1

   Assuming your sheathing will be OSB or plywood, either will act as a suitable vapor control, but more importantly they can be a great air barrier if joints are properly taped. Sheathing air barriers mitigate vapor drive concerns from either direction if you have sufficient exterior insulation -- go with at least 20% of your total R value outside the sheathing for your climate. Mineral wool would give you the "flow through" assembly you're after.

2. GBA Editor
   Brian Pontolilo | Jun 23, 2020 03:43pm | #2

   Hi Peter.

   If you have 3 inches of rigid EPS (or equivalent R-value) on the exterior of your walls in your area, I think you will be keeping the sheathing warm enough for condensation control and shouldn't need a class I or class II interior vapor retarder, which by the way, are used to prevent outward vapor drive in the heating season. I suggest you read this article which will help you get the details for a wall with continuous exterior insulation in your climate right: Walls that Work.

   It seems like people jump to variable-perm membranes when they are making unconventional decisions, and perhaps this is fine. But if you are making this decision based on your expected indoor RH, I'd suggest another approach.

   Planning to have 60%-70% indoor relative humidity is planning to have indoor RH that is too high. I'd suggest you instead plan to do some dehumidification. I asked Kristof Irwin just this question last week during A Deep Dive into Dehumidification. Indoor RH levels north of 60% are risky for building durability and occupant health, in his opinion.

   A qualified HVAC designer should be able to help you design an efficient system that will keep you comfortable and keep RH in check. Of course air tightness is imperative for an efficient system to work well, so I recommend you make an air sealing plan and test it with a blower door before closing up your walls.

1. 
   Peter_Francis | Jun 23, 2020 04:00pm | #3

   Hi Brian;

   Thank you so much for your answer.

   This is rather an unconventional small building itself. It is like a small vacation hut. So, let's imagine will not control de humidity at all. Just install a regular mini-split without RH setpoint.

   Is there any occasion where faced batt insulation is the most suitable option? What´s the purpose of that material in the residential sector? (sorry for my existential questions).

   Also, regarding vapour variable materials, imagine that I have a non dehumidified kitchen (except for natural ventilation (winter is problematic because I don't want my windows open)) with periodic elevated vapour production (I love pasta and soups). Drywall and faced batts. Should I be concerned that this vapour will difusse through the drywall, open my vapour variable material (because of the hight HR) and condense on a cold surface? Imagine there is no exterior insulation. Can variable vapour materials be problematic in these scenarios?

3. GBA Editor
   Brian Pontolilo | Jun 23, 2020 04:29pm | #4

   Hi Peter.

   I guess it depends on how you will use this building. I don't have experience with buildings that are not occupied full-time, but I see many people post on here that if you are only conditioning the building occasionally, you don't have to worry too much about these issues because much of the time, when the building is unoccupied, you essentially have the same conditions inside and out and therefore no vapor drive, etc.

   If you do not have adequate continuous exterior insulation and will be heating the space for lengths of time in the winter, a class II or variable-perm warm-side vapor retarder is a good idea. Faced fiberglass batts, installed well can provide R-value and vapor control. If you will have fibrous insulation in the walls though, a warm side air barrier is also a really good idea--that can be your vapor retarder membrane or your drywall, if they are detailed for air sealing. Sealing all six side of the stud bays is a good idea with fiberglass insulation.

   Again, if you are going to be occupying the building for any lengths of time, an exhaust fan in the kitchen (and in the bath) are a very good idea. These do not have to be expensive to buy or install but will go along way if you are cooking or showering in the hut.

   I hope this is helpful and hope that some folks with experience with occasionally-occupied buildings will share their experience.

4. Expert Member
   Dana Dorsett | Jun 23, 2020 05:53pm | #5

   >"I will not have moisture control to expect for natural ventilation. Let´s assume an interior RH of 60%-70%."

   Then you're stuck with mold & moisture risk no matter how the assembly is built. At 68-70F room temperatures 70% RH would support rapid mold growth around cooler areas such as window trim, floors, etc. even indoors.

   Marine zone 4 is actually pretty DRY, reputation for rain notwithstanding. It's likely that you are totall overthinking this- it's actually an EASY climate to design for in most respects. Set up a 10mm rainscreen behind the siding, give it a modest amount of ventilation and forget about it. The outdoor dew point temperature averages in zone 4 marine are pretty reasonable compared to other parts of North America, averaging under 50F pretty much year-round. In a tight house that's kept at 50F or warmer it won't take much mechanical dehumidification to keep mold issues at bay. Even with modest amounts of solar gain and 10cfm of mechanical ventilation unoccupied "vacation hut" would be fine. Bump up the ventilation rates as needed when occupied.