Best wall assembly for a Pretty Good House in Zone 7A
I’ve posted a couple times recently as I question everything about our house design. I appreciate everyone’s feedback. My last inquiry was focused on the execution of the Building Science Corporation double-stud wall: https://www.buildingscience.com/documents/enclosures-that-work/high-r-value-wall-assemblies/high-r-value-double-stud-wall-construction
Conversations with builders and insulation contractors (regarding this particular double stud wall) have brought to light challenges that can be overcome, but will be labor intensive. For example:
– Order of assembly, can’t prefab the entire wall and raise as one (taping air-barrier on the exterior of the inside wall)
– Separate dense packing required from both interior and exterior
– Need to “net” the interior and exterior before blowing to allow proper air-movement–fiberboard doesn’t allow enough air to escape
– Need to separate the bays for proper dense-packing
– If we do apply fiberboard first (with air escape holes), we would need to strap down the fiberboard so it doesn’t bulge out
With that, before moving forward, I feel that it is my duty to ask one more time, do I have the right wall system?
I am certain that I don’t have all of the options or pros/cons listed, however, I have distilled this to my front-runners and their main bullet points. I will lay it all out for you.
Cost analysis assumptions: All assemblies have furring for rain screen layer. Cost analysis done by breaking down materials costs for a single 4’x8′ wall section. Does not reflect window detailing, does not include labor, etc. No doubt, the labor involved with these assemblies will vary considerably.
Other notes: Performance is ballpark. Whole wall r-value should probably be considered. Double stud requires service cavity to be insulated, other wall systems include a separate service cavity not necessarily intended to be insulated.
Performance: ~R44 with service cavity insulated
– Lower material cost
– Likely best overall wall performance
– Service cavity must be insulated to achieve r-value
– Potentially higher labor cost
– Order of assembly with OSB on exterior side of interior wall
– Fiberboard will belly out when dense-packing
– Additional steps for insulation contractor (translating to additional cost)
– Modest additional loss of square footage in house
– Slightly increased thickness of foundation to accommodate detailing
Performance: ~R37 (as high as R47 with SC insulated)
– Exterior insulation, keeps sheathing/framing warm
– Requires weather resistant barrier (taped Zip System or sheathing and housewrap) unlike Gutex wall
– Issues with load transfer and compression through material (how to address attaching window eyebrows, porch roofs)
Notes: Image of assembly attached. Could use OSB instead of Intello Plus for interior air barrier. Would use dense-pack cellulose or mineral wool batts instead of Havelock Wool.
Another variation of this wall would be the Bensonwood Open-Built wall (2×8 with less Gutex): https://bensonwood.com/building-systems/panelized-enclosures/#wfb-walls
– Exterior insulation
– Requires NO additional weather resistant barrier or sheathing
– Seals panels via tongue and groove, no need for lining up ends with studs
– Applies in 1 layer at same time as rain screen furring
– Doesn’t compress like mineral wool
– Likely lower labor cost due to eliminating several steps in other walls (no additional sheathing, two layers of exterior insulation, only 1 wall to frame)
– Highest cost
– Inconvenience of obtaining (must order ahead of time)
– Exterior insulation
– Doesn’t compress
– Keeps sheathing and framing warm
– Lowest cost
– Does not breath
– Requires sheathing WRB layer
– Puts air barrier on exterior
– Global warming potential
I greatly appreciate your feedback.