Posted By bkcasa on 06 Jun 2011 01:52 PM
Wow. So much info to take in. I want to build a very tight well insulated house. I built the home we are in now, 2x6 and Hardi-Plank, it seems pretty well insulated. We had fiberglass sprayed in. Now I am considering a double wall with hardi-plank siding and trim.
Will I be correctin building it with 2x6 exterior and 4x8 OSB sheer around entire home. 2x4 inside walls.I was thinking of 15 inchwall thickness. We plan on building in Washington State, near the Columbia or on the Peninsula.
I want to do alot of he work myself. I will have to pay to have foundation and slab in. We can do electric and some plumbing, cabinets, doors etc. I will look for help to frame it in and get gong on siding. Our living space wil be about 1200 square foot.
So will I need 1x2 to give me space between siding and OSB?
Insulation ? Spray fiberglass vs spray in foam or combo for best bang for the buck?
I plan on sealing all joints in the OSB. I have used Tyvek on top of OSB. Mu building experience is mostly in Arizona, warm and dry.
Rather than picking a wall thickness, design the thing for it's U-values and let the modeling determine how to achieve that reasonably economically.
Spray foam is an impossibly expensive way to go high-R. It's primary benefit is the ability to get a good air-seal, but there are other methods than a full-fill of spray foam. Defining an all-6-sides of the cube air barrier using caulks, OSB, gypsum, edge & seam-sealed with the appropriate materials (including some spray foam) can be reliable. Test the air-barrier a soon as you have it all shelled in, before blowing any fiberglass or cellulose
Type-I EPS roofing insulation is a fraction of the cost per unit R, and for only a bit more than EPS fiber-faced polyiso roofing insulation gives you about R6/inch at PNW temperatures. But it's still several times more expensive than cellulose. If you buy recycled/reclaimed roofing iso it's typically 25-35% the cost of virgin stock, approaching cellulose pricing, and would allow a thinner wall at the same U-values than with EPS, fiberglass or cellulose. Eg: 6" of iso + a 2x6 cellulose studwall yields about R60 (whole wall, thermal bridging included) if the iso is on the exterior and extends down to or below the foundation sill. Add a couple inches for siding, rainscreen and gypsum and you're talking 13.5" from exterior paint to interior paint. But you probably don't need quite that much R in the PNW to hit PassiveHouse levels.
With spray fiberglass you'll have to dense-pack it to perform well, and it'll be more expensive per unit R than dense-packed cellulose, but you can do a slightly thinner wall with it. (R4/inch vs R3.6/inch). The performance difference in a simple studwall is negliglble, since the bridging of the framing reduces the whole-wall R to well under the center-cavity R, but in double-wall construction it can mean a wall done with 9" of new-school fiberglass performs about as well as 10" of cellulose. You're paying a bit for the extra inch though.
Yes, you'd need some furring between the siding & OSB to form the
rainscreen. Fiber-cement siding is a moisture-reservoir making the
quality of the rainscreen even more critical.
Rather than going full-on PassiveHouse, in the milder parts of the PNW there's an economic argument for going R30-40-ish whole wall/R65 attic, and heating the whole shebang with a ductless mini-split, since you'll get better than 3.0 for a coefficient of performance out of a fairly cheap but high efficiency heating system. Even a tight 1200' 2x4 house with R13 batts (~R11 walls) is easily heated with a mini-split in that climate if you have an open floor plan. (I'm getting my brother in Port Orchard off the propane tank and onto a mini-split in his open-floor plan home, hopefully before October.) Building ~ R30 walls with no thermal bridging at the foundation, and R20 ICF (or R12 slab on grade) with an R60+ attic you could heat the place with even the very smallest mini-splits. It wouldn't run a high duty cycle, and could end up using similar amounts of electricity as a resistance-heated PassiveHouse at the COPs it would operate at there. The cost of the extra insulation to get the heat load low enough to use fewer kwh than the mini-split may cost considerably more than the mini-split. (Of course you'll still have to replace the mini-split every 15-25 years, whereas the insulation could last a century or more.)
A 2x6 cellulose insulated studwall with 3" of exterior iso comes in at ~R34 (whole wall), and is relatively easy to build, more than 2x the whole-wall R of a 2x6 fiber or opencell foam studwall with no exterior foam (R14). A 2x6 wall with 3" of exterior EPS comes in at ~R26, still almost twice the whole-wall R of a traditional 2x6. These are decent performance walls for Zone-4 climates, but you'd probably need R45-50 and some very careful design work to hit true PassiveHouse performance. No matter what, once you're air-tight and over R20+ whole-wall , the window performance and thermal bridging of structural elements starts to swamp the heat-loss picture, so even if not going the whole way on R-values, DO pay close attention to all of those details.