Design Charrette: General Building Envelope Questions for Mixed-Humid Climate
I’m in need of some focused input on the overall insulation strategy for a home I’m designing, and rather than taking hours upon hours searching through the archives and reading what now seems to be seemingly countless (and often divergent) opinions on these matters, I’m hoping through a quick online charrette a general consensus could be found to help me and my client understand the best route to achieving a fairly high-performance home for a mixed-humid climate (only!) in the Blue Ridge Mountains (meaning we don’t get quite as high humidity levels as lower elevations, but can get some fairly low temperature wintertime cold spells).
Some background, assumptions, parameters, and a couple specific questions to guide the discussion:
I’ve already learned quite a bit over the years from this website, but as I suggested above, there is often such divergent opinion on even some of the fundamentals that it can leave one’s head spinning. Thus my desire for “focused input”.
Assume that the builder is very competent, but not necessarily fully knowledgable about all the intricacies of green building design and detailing, such that you have to design to the builder, rather than trying to fully move the builder into a world he’s not yet fully familiar with. Besides, I’m still learning, too. I know this is a separate discussion that goes to “Pretty Good Building” concepts, so please set that aside—not to ignore it, just not to spend time on it here. In my area of the country it is still rare to find a builder who is even willing to move in this direction, without, I might add, using that move as an opportunity to up-charge so significantly, thus putting out of reach better building practices for the average homeowner, who tends to be the type of clientele I serve. For example, I’m lucky to get a builder to use crinkled house wrap for a little bit better drainage plane. I will spec it, but once I’m out of the picture, the builder says to the owner it’s “x” amount more and the owner chooses standard house wrap. In the case of this particular home, however, the builder will generally move in the right direction, and the homeowner does want to get it right and is willing to pay a bit (not to say exorbitantly) more to do so.
The shell of this one-story home is as follows: Floor: radiant slab-on-grade (also set aside that discussion of whether or not it’s worth it. The client wants it. Period.) Walls: brick (primarily, with small areas of cedar lap over furring for drainage plane) over air space over house wrap over 1/2″ sheathing (yes, probably OSB) over 11 1/4″ double stud walls. Roof: all ventilated cold roof with deep enough trusses and shed roof rafters of either 16″ TJIs or trusses for 15″ max. thick insulation. Windows: uPVC German-style, u-factor 0.27 or so, possibly with triple-glazing for 0.17 or so u-factor (willing to hear input on worthwhileness of triple glazing in mixed-humid. Most of the discussions at GBA seem to deal with cold climates. Also, the additional cost is a major factor on whether the client goes to triple).
Assume tight construction and GSHP for heating (again, set aside that discussion of whether or not it’s worth it, but in this case if you can provide a compelling argument for an alternative, please do so), a Zehnder ERV system for ventilation, no air conditioning, and possibly a standalone dehumidifier if found to be warranted.)
The goal is not net zero or PH standards, though if we get close without exorbitant (there’s that word again) cost, we’d all be happy.
Question: in the mixed-humid/mountain climate with the wall shell as proposed is there any particular insulation to be avoided or recommended to keep the sheathing from getting wet—and at what r-value? Or is that really more a cold climate concern, as opposed to here where drying potential is in both directions?
At what level of insulation r-values do we reach the point of diminishing returns? Like I said about divergent opinions, I’ve seen where both “more is always better” and “‘x’ is good enough” propounded.
Finally, I’ve not mentioned the size of the home, but does that really matter with regard to the questions I’m seeking answered?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part