How to keep house framing relatively simple, inexpensive and better than code?
I have a house design that I hope to build that is approximately 900 square feet, consisting of a story and a half. Second story has short knee walls and a ceiling that follows the roof line until 8′ at which point it will be flat. The south side of the house has a shed dormer to gain headroom and windows.
I would like to keep this pretty simple. I want a “pretty good house” but not a Passivehaus or a LEED house (I would rather put that money into the actual house). I plan to use 24″ on center advanced framing, and want to do balloon framing, making continuous studs for the first floor and short second story wall. I am considering using 2×8″ for studs. I plan to used dense pack cellulose, with drywall on the interior and plywood or zip board sheathing on the exterior (probably not OSB). Probably using Tyvek or a similar better product.
For environmental reasons, I DO NOT like foam products and really don’t want to use them other than to fill RO gaps with spray foams for doors and windows. I would like to avoid exterior insulation. The house will likely have an HRV and minisplits for heating, in addition to solar gain from the south facing windows. The house is in Colorado, zone 5.
I just want to keep this relatively simply, KISS and not expensive. I think a truly greener house is smaller and economically scalable, so average people can afford them.
http://www.thesmallhousecatalog.com/afewsmallwords/cottonwood-cottage
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
2x8 framing 24" o.c. isn't necessarily the right way to go, and would barely meet IRC 2015 code-min for zone 6, though it would has a small margin for zone 5.
With a simple footprint you can do just fine with 2x4 16" o.c. with a double studwall approach set to whatever thickness you need to achieve the thermal performance you're looking for, and it can be done with about the same board-feet of lumber as 2x8 24" o.c. framing. The inon-structural set of studs can be built like any partition-wall- it's nothing special.
ZIP board is a brand of OSB, but it's still OSB.
Kneewalled half-storys with mini-attics are a PITA to air seal properly. A simple shed roof on a 2-story is simpler to frame, easier to air seal, and the roof can be easily vented without heat leaks & thermal bypasses.
I was originally thinking of a double stud wall, but then started looking at the benefits of a balloon framed wall. Stronger, simpler and a lot less wood. I could potentially do a 2x6 balloon frame with a 2x4 interior wall, but it would depend on what the engineer mandates. There is a pretty fast diminishing return on insulation once you get over R30. I know beauty is in the eye of the beholder, but I think shed roofs are ugly, and would force the whole massing of the house to much larger to get usable head room. I would also have to put the high side on the south making the roof pitch north which would preclude the PV system.
Curt,
For your design, the balloon framing is essential. (Otherwise, the second-floor kneewalls will hinge at the bottom plates.)
Lots of people build double-stud walls with 2x6 load-bearing walls (in your case, these would be the balloon-framed walls) and 2x4 walls that aren't load bearing.
The alternative is a 2x6 wall with continuous exterior mineral wool insulation.
These are the best solutions. If you are resistant to these approaches, you can build your home any way you want, as long as your house meets local code requirements.
For more information on this issue, see How to Design a Wall.
-- Martin Holladay
Thank you Martin. The double stud approach does seem like an elegant solution, with the exterior being the structural load bearing balloon framing. I love the idea of wiring a double stud wall as well. If I have to use exterior insulation to protect the sheathing, I like the mineral wool option since it still avoids foam and would allow the wall to dry to the outside, with a rain screen.
You once talked about a Venn diagram with hippies on one side and engineers on the other. I am perhaps in that middle overlap, but leaning toward the hippie side. I am involved with housing affordability and seeing housing become more out of the reach of more people. If a house has to be super complicated and expensive in order to be "green" It isn't very scalable, and may be good for bragging rights at a cocktail party, but I have to question it's sustainability.
Curt,
While the balloon framing may in practical terms help resist the lateral loads exerted by the roof, as I understand your codes the only approved ways of dealing with them are:
1) Rafter-ties located in the bottom 1/3rd of the span.
or
2) A load-bearing ridge.
Just something to keep in mind as you continue to plan.