Replies

1. 
   user-2310254 | May 03, 2018 09:31pm | #1

   Ian,

   Here is a good article that may answer some of your questions. https://www.greenbuildingadvisor.com/articles/dept/musings/how-design-wall

2. GBA Editor
   Martin Holladay | May 04, 2018 04:41am | #2

   Ian,
   Your question is too general to answer easily.

   It usually makes sense to follow local practice (unless local practice violates building science principles).

   If SIP construction is common in New Zealand, there is nothing wrong with SIP construction. The key issues with SIPs are: (a) It's essential to pay attention to airtightness, and (b) It's a good idea to have a rainscreen gap between the exterior OSB and the siding.

   In the U.S., frame construction is more common than SIPs. If insulation is installed between the studs, thermal bridging through the studs lowers the R-value of the wall, so many builders install a continuous layer of insulation (rigid foam or mineral wool) on the exterior side of the wall sheathing.

   All of this said, your climate is mild, and in a mild climate, walls don't necessarily need a high R-value.

   If you build a SIP wall, the entire SIP assembly becomes your air barrier (if the SIP seams and penetrations are properly sealed). Best practice is to seal the seams with canned spray foam as well as interior tape.

   If you build a frame wall, most builders establish the air barrier at the sheathing level by taping the seams of the sheathing with high quality tape. If you use fluffy insulation between the studs (fiberglass, cellulose, or mineral wool), it's also a good idea to have an interior air barrier (the gypsum drywall) as well.

3. 
   IanVosper | May 04, 2018 05:57pm | #3

   Martin,
   Thank you for taking the time to respond. I appreciate the initial question was somewhat vague but I didn't want to go into too much detail and end up with an epic!
   I have read your book and many of your blogs but still remain somewhat confused by the optimum order of layers to protect from condensation damage within walls in a mild but permanently humid climate like NZ.
   Deciding between SIP and stick/fluff construction is one problem; NZ is increasingly adopting SIP and one of the newer versions I am considering is a magnesium oxide paneled SIP called MagRoc which at the onset sounds a whole lot better than OSB or even ply. I would also have a cladding and rain screen gap. However, I haven't been able to get vapor permeability data on MagRoc SIPs so it just adds to the unknowns.
   Back to the original question; with such a SIP construction providing the air barrier, how should I address the vapor? What should be used (permeability range) and where should it go in the layers of wall?
   The same fundamental confusion arises over the relative locations of air barrier and vapor permeable membranes on a stick & fluff wall in that climate...
   Other details: The house would be well insulated (R number to be determined) as the location gets some pretty cold winter winds. It will have mini-splits for conditioning. We may install a HRV (funds permitting).
   Thanks for your patience!

4. GBA Editor
   Martin Holladay | May 05, 2018 05:43am | #4

   Ian,
   You didn't mention air sealing. Especially in your climate -- windy and mild -- but really, for any home, airtightness matters more than R-value. Pay attention to airtightness during construction, and verify performance with a blower door test.

   The vapor permeance of the magnesium oxide facing of the SIPs isn't very important. The rigid foam -- presumably EPS, but possibly polyiso -- has a low permeance, and there aren't any wall components that need to dry, besides the siding (which gets soaked by wind-driven rain). Your rainscreen gap handles that issue.

   With fluffy insulation in a framed wall, builders traditionally worried about the migration of interior vapor toward the cold sheathing during the winter. Most of this migration occurred as a consequence of air leakage, so paying attention to airtightness matters. Some -- not much -- of this migration happens as diffusion, so an interior vapor retarder can make sense in a cold climate. Your climate isn't very cold, though, so I wouldn't worry. Ordinary latex paint should be adequate in your climate to limit outward vapor diffusion -- but if you're worried, you could purchase vapor retarder paint if it is available.

   On the exterior side of your wall sheathing -- whether you are talking about a frame wall or a SIP wall -- you'll be installing a water-resistive barrier (WRB). These days, that usually means plastic housewrap. A WRB should be vapor-permeable, to allow any moisture that might have reached the sheathing level to evaporate outward.