What’s the most efficient way to build an R-52 wall in Climate Zone 5?
KrisCurrie
| Posted in Energy Efficiency and Durability on
Looking to build a passive-inspired net-zero home, 2,264 square feet, two stories.
What’s the most cost-effective or beneficial way to get that higher R-value? Double stud – blow in, 2×8 fiberglass batt with 2-2″ polyiso, ICF, etc.?
Also, what are the risks with WRB (roll on) on sheathing, 1/2″ plywood, 2′ on center framing? Is there a concern for movement, cracks, or air leaks, as there will be no poly on the inside?
Thanks,
Kris
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
You don't really need R52 to hit PassiveHouse levels in zone 5. Are you looking for R50-ish whole-wall R (U-0.020 walls ) or is that a center-cavity number? You should be able to hit Net Zero fairly easily with R30-ish whole wall R (U-0.033) in zone 5. Download a copy of BA-1005 and take a peek at Table 2. Read the whole first chapter.
http://buildingscience.com/documents/bareports/ba-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones/view
2x8 studs are a waste of lumber, since the studs thermally bridge the insulating layers. A 2x6 studwall 24" o.c. has the same structural strength and about the same board feet of lumber as a 2x4 16" o.c. wall. Insulate with R23 rock wool batts and sheath with half-inch gypsum interior, half-inch OSB /plywood sheathing (Huber ZIP-R is good, since it's a pre-coated WRB, easy to redundantly air seal), and put 3" of exterior polyiso on the exterior held in place with 2x4 furring through-screwed to the studs with 6" pancake head timber screws 24" o.c. hanging the siding of your choice on the furring. That comes in right around R30 whole-wall (or a bit more), and is a fairly straightforward thing to build. Tape the seams of the polyiso for good measure. If the polyiso is foil faced with shiny foil at the furring established gap it adds about R1 to the whole wall number compared to fiber faced goods.
In zone 5 as long as you have at least 25% of the center-cavity R value on the exterior of the sheathing, you don't need or want an inteiror side vapor barrier. With 3" polyiso derated to R15-R17 for mid-winter performance, and R23 rock wool in the cavities you'd be looking at R38-R40 center cavity you'd have about 40% of the total R on the exterior, which means you have HUGE dew point margin at the sheathing.
Detail the structural sheathing as the primary air barrier by caulking it to the framing inside every stud bay, and caulking between any doubled up headers & plates, and under the bottom plate, etc.
If you build the foundation with a standard 2.5" +2.5" ICF you could place the 2x6 stud plate 1/2-3/4" back from the edge of the concrete, and the 3" wall foam would slightly overhang the 2.5" ICF foam which would eliminate the need for flashing there,with a continuous thermal break over the band joists and sill plate, with enough room on the ICF foam to apply a cementicious EIFS finish such as Quikrete Foam Coat on the above-grade section.
Put 2" of EPS (Type-II or Type-IX) for an honest R8.4 under the slab, butted right up to the interior ICF foam. That floats the slab with a full thermal break from both the subsoil and the wall footing.
If you are really going for R50 whole-wall numbers it takes more careful analysis, and you're probably looking at a double studwall all-cellulose/no-foam solution- a fairly fat one, about 15-16" thick, with a lot of details to attend to around band joists and window/door framing. Read the discussions on different high-R stackups here:
http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/high-r_value_walls_case_study_2011.pdf
In the R-30 to R-40 range, there are several options that can be competitive depending on local materials and labor costs and skills, experience, and preferences of individual builders. But when you say R-52, I think you are solidly in double-stud wall territory. Everything else needs more of the expensive stuff as you go from R-40 to R-50. But with double stud, you only need more cheap fluffy insulation.
I'm not sure you need to go to R-52 to get to net zero in climate zone 5, but that wasn't your question.
As far as risks with the combinations of materials, I would be concerned about whether the roll-on WRB has high enough vapor permeance to allow your cold sheathing to dry sufficiently. I'd at least add a smart membrane on the interior, and I'd also consider locating the sheathing closer to the middle of the wall so it's warmer.
My other concern would be that cellulose that deep is more difficult to install, and that dense packing is more critical, because you can't count on friction to prevent settling. So I'd want to be sure you had a skilled cellulose dense pack installer. The mid-wall sheathing actually helps that some too, although 3.5 inches into a 15" deep wall still leaves one very deep cavity.
I am pretty much with Dana here. I think 2x6 walls (24"oc) with air sealing, dense pack cellulose, quality innie, tripane windows- 2 layers of reclaimed polyiso or EPS foam totaling 3 inches (allows you to use 6" screws to attach firring strips) is an efficient way to fly. Use drain wrap under the foam. Your weather resistant barrier should be integrated with the sheathing and window flanges. I would be less concerned with liquid applied membranes than with attention to flashing details, overlapping the foam board, quality house wrap job, and well done winds trim that drains water away. I used 2 1/4 high density EPS under my basement slab. Well worth it.
Kris,
First, net-zero just means, typically, that you send as much energy back to the electrical grid as you took from it, over the course of, say, any given year. It really doesn't have anything to do with energy efficiency. You could get to net zero with a tent with resistance heaters if you had enough sun and grid connected PV. In the big picture, you have to determine what capacity you will have to send energy back to the grid and coordinate that with how much energy you will need to use. As an example, I retrofitted to achieve net-zero and beyond with a small roof area to hold PV, but there would not have been enough roof area if I had not first dramatically reduced my electricity usage. See my guest blog here on GBA under "Green Homes." I'm in climate zone 4, almost 5, and have been living comfortably with about R35, but I really thought I would end up with more than that, and wouldn't mind more than that. If my location was deeper into zone 5, I have no doubt I would want R-45 or more walls
I'm not sure what you mean by "passive inspired" but if I were building a Passive House, I would invest in the Passive House Planning Package (PHPP) software, or whatever it's called these days.
As others have told you, double wall and cellulose might be your best bet for R-50. But, from my experience, I might consider trying 3 layers of 2 inch polyiso over 2X6 stud walls, for about R-45 or so, cold weather performance. My understanding is that the packed cellulose takes expensive equipment and special skills. But it was pretty easy for me to attach 5 inches of rigid foam over studs using Fasten Master screws. I don't believe it would be any harder to run screws through 6 inches of exterior foam.
Kris,
I agree with Dana that an R-52 wall is unnecessary in your climate. In general, it makes sense to stop adding R-value to a wall when the cost of the additional R-value is more than the cost of a PV array able to generate the energy that would be saved by the additional R-value.
Builders have discovered that the two most cost-effective ways to build a high-R wall are:
1. A double-stud wall insulated with dense-packed cellulose. If you go this route, make sure to include (a) a ventilated rainscreen gap between the back of the siding and the WRB, and (b) a layer of MemBrain (or a similar smart vapor retarder) on the interior side of the wall.
2. A 2x6 wall with one or more layers of rigid foam on the exterior side of the wall sheathing. If you go this route, you can lower the total cost of the wall by buying used (reclaimed) rigid foam. This type of wall usually has vertical furring strips between the rigid foam and the siding; these furring strips create a ventilated rainscreen gap.
Thanks Dana, Charlie, Rick & Martin.
I may have used an improper reading on zones, Canada zone B on related graph GBA zone 6-7 like Rick mentioned(thanks for the contact at Accurate Dorwin)
I am about 6 hrs north of Maine. One recent build here went with double stud 24" deep-r-60-wall- blow in cellulose- 2x4 service wall, talking with my insulation guys they cant guarantee the 3 psi it needs to reduced the settling, also the sqft I would loose on 24" walls. Plus labour and material? I'm not convinced this is the best way. I like the idea of air and vapour barrier on the ext. to minimize air leakage, Single wall stud for framing labour, window sill depths at 12"(rain,ice,snow problems reduced) but like Dana stated drying the sheathing may be difficult in my climate avg.yr- temp 10C we see +30C summer and -25c winter?
Currently pricing these options to see what is best! Thanks again for all your input. Kris
When it's cold out the sheathing doesn't have to be dry, since the biological activity is much reduced. But if built with rainscreened siding it'll stay pretty dry anyway.
Six hours north of Maine headed toward the maritimes is much more temperate than 6 hours from Maine headed somewhat inland. It could be a climate zone 6, or it could be climate zone 7 (or even 8).
How many annual heating degree-days do you expect in that location, on average?
http://www.atlasroofing.com/media/images/original/1910.jpg
http://ftp2.cits.rncan.gc.ca/pub/geott/atlas/archives/english/5thedition/environment/climate/mcr4033_browse.jpg
http://www.weatherdatadepot.com/
http://www.degreedays.net/
Dana, sorry I missed this, heating degree days 6700-7900 .Wood siding will be the cladding
Kris,
With your announcement of that range of HDD, I'm liking your proposed R50 walls even better than I did before.
Harold Orr described superinsulation this way. HDD divided by 180 for the wall R-value and HDD divided by 120 for ceiling R-value.