Thanks to GBA members' answers I received to an earlier question ( www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durab... ), I have decided to abandon local building practices. I am aiming to build a well-insulated & airtight 600 square foot house for my family. I would like to come close to the insulation R-values that Alex Wilson recommended in this blog post: http://www.greenbuildingadvisor.com/blogs/dept/energy-solutions/how-much... Those numbers are R-4 windows (0.25 U-factor), R-30 walls, and an R-60 roof. I would love to read your opinions on my building envelope design. The house I plan to build can be seen here: http://www.tumbleweedhouses.com/products/whidbey#ad-image-0
Double-stud 2x4 walls set 3” apart allow for 10” of dense-packed cellulose with the exterior wall bearing the load. This should give us an R-37. I am hoping the wall is thin enough to avoid too many window framing complications. The interior will be drywall using the Airtight Drywall Approach without a vapor barrier. The exterior would be sheathed with plywood or OSB and covered with housewrap. This is followed by a rain screen and siding. The vapor permeable wall would be able to dry to both the interior and exterior.
The roof for the house plan is not overly complex but there is a gabled dormer on the front and a shed roof over a room in the back which would prevent a straight run from the soffits to the ridge for a properly vented roof as mentioned here: http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulat.... I was thinking of doing an unvented roof using 2x10 rafters with bays insulated by dense-packed cellulose. Below the rafters, attaching airtight drywall to keep out most of the moisture-laden air. Directly above the sheathing, adding 2 layers of 2” XPS with staggered and taped seams, followed by metal roofing. This would give an R-54 which is not quite up to the R-60 that I wanted but I am hoping that it is enough. The 4” of XPS should prevent moisture accumulation in the roof sheathing. See http://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minim...
Foundation & Floor:
As far as I know, most residential foundations in Japan have crawl spaces. GBA mentions that crawlspace foundations have the same insulation and moisture control issues of basements without the usable space so I would like to avoid them. The building site is sloped so a slab foundation doesn't seem like a good option. This leaves a pier foundation. One of my future neighbors' house is on piers and they suffered from very cold floors. Later, they built a perimeter wall to keep the wind from passing beneath their feet. This may be more an issue with many Japanese builders' lack of understanding regarding insulation and air sealing than the pier foundation itself and I am willing to give it a try. There is a passive house in VT that is on piers so I know that a comfortable floor on a pier foundation can be had. See: http://vermontpassive.com/2011/08/insulating-floor-passivhaus-piers My plan is to get R-36 by insulating 8” joists with dense-packed cellulose and placing 2” of XPS sandwiched between OSB or plywood sheathing beneath the joists. Would it be wiser to instead use 2x10 joists for R-44? I will also caulk, gasket, and tape as if I owned stock in the air sealant industry.
I know there is a lot here for one post. Thank you for taking the time to read it and I appreciate any friendly advice.