Hello all. I've been doing some research over the last year, and I've learned most of the basic principles to build an efficient home. I'm planning on keeping it simple, and just thought I might hear some advice from some of you with more knowledge and experience before I start. Let me be clear, I know I'm not doing as much as I should... for time sake, money, lack of local options. The plan is to build a 1500-1600 sq/ft ranch house, over an insulated unfinished basement (plan to finish it later). It will be South facing, in a dry Southwest Kansas climate. I am doing some pricing at www.wardcraft.com which builds very nice homes. I know they might not use the BEST options... but still pretty efficient. They use Johns Mansville Spider Fiber® blown fiberglass insulation in 2x6 walls for R23, or optional R26. They caulk all exterior sheathing seams, and tape all house wrap seams, and seal bottom plate of wall to the floor. There is an option for a 12" raised truss for extra roof insulation. You can read more here http://wardcraft.com/Standard-Specs/index.shtm and here http://wardcraft.com/Green-Options/index.shtm I was going to customize the windows to 10-12% of sq/ft, and do tile or concrete floor for some passive heating. Budget depending, I would like to have it prewired for solar, and add some panels if if possible. I know this is far from a Passivhaus, but would this simple option make for a house with pretty efficient plan? Any feedback, or changes would be appreciated. Thanks.

With 2x6 16" o.c. framing the framing fraction of all framing runs about 25% of the total surface area. When you account for the thermal bridging of the framing, as well as the R-value of the gypsum/sheathing/siding the "whole-wall-R" with R23 cavity-fill is about R15.2.

With R26 cavity fill it's about R16.0.

Don't spend a whole lot of money for the R26, since it's buying you only about R0.8 (a little less, actually.) There's a rationale for going to 1.8lbs density with Spider, but in a house that is also being air-sealed during construction there is very little to be gained by taking it up to 2.2lbs.

If you added an inch of rigid foil-faced polyiso outside the sheathing under the siding you'd be at about R21. If you added 1.5" you'd be at R25 whole-wall, which is about where you want to be in SW KS. See table 2, p10 in this document:

http://www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones

SW KS is on the cool edge of zone 4 or the warm edge of zone 5, so figure out which zone you're actually in, and use the right zone-row as a rough guide for planning. Note, those are all "whole assembly" numbers, not center-cavity.


http://www.greenbuildingadvisor.com/sites/default/files/images/H-T%20Zones%20with%20Cities%20ABC+.jpg

Where time-is-money it's sometimes easiest to do the foundation with insulated concrete forms, and set the stud plates such that your sheathing foam is even with or slightly proud of the exterior foam of the ICF. The gap is then easily sealed with expanding foam, for a perfect thermal break at the foundation/upper house interface. With ICF you also have the advantage that your under-slab foam can butt up to then interior EPS of the ICF, for a perfect thermal break at the footing. Under the slab use 2" of EPS (R8) rather than 1.5" of XPS (R7.5) under the slab- it's cheaper/better, since in 50 years or so the slow loss of the blowing agent for the XPS reduces it's R-value to less than what 2" of EPS would be. The blowing agents used in XPS is a serious greenhouse gas too (~1400x CO2), not so much for EPS (7x CO2). It's the same polymer, different process. At any specified thickness and density they'll perform identically to one another in 100 years, even if the early-years average gives a 25% edge to XPS.

Insulate the attic with cellulose, not fiberglass. Cellulose is far more air-retardent and opaque to infra-red in comparison to blown-fiberglass. To get to R55-60 takes about 18", initial blow at about 1.5 lbs density (it'll settle an inch or more over the first decade or so, falling to it's rated R from an initially somewhat higher number.) That adds a bit over 2lbs per square foot of dead load, and if you have a big span that may push the load-limits of the truss chords, but that can be solved by adding 1-2 trusses and tighter truss spacing. It will also push the limits of half-inch gypsum with trusses on 24"o.c. spacing, but 1x furring screwed or ring-shank nailed to the underside of the truss chords at 16" o.c. or tighter spacing can deal with that aspect.

Definitely go with the raised heel trusses; no question. The biggest problem with "energy efficient" modulars is joining the halves together; this is where you'll find the majority of air leakage and sloppy work. The "set" is typically done not by the shop crew but by an outside sub who is interested in getting it done and getting out of there. First, ask the manufacturer to replace the crummy "gaskets" at the marriage wall with EDPM rubber gaskets. you may have to buy them (conservationtechnologies.com) but they will, if dealt with correctly at the set, work well. Next, do a blower door test PRIOR to sheetrocking the ceiling at the marriage wall. A tight, well built house can be compromised by a hole in the middle. You will probably have to crawl into the attic, move/remove insulation and make sure that joint is well sealed and air tight. Then you'll have to replace the attic scuttle with a good one. you can find instructions or buy one online.