Building Details in Mild Climates
All,
As I am starting to settle on the design for my 18×20 ADU I’m kind of stuck on figuring out materials/assemblies that will perform well and also be efficient from a labor perspective (I’m doing most all construction myself). This is a simple structure with a mono slope roof and most likely a poured slab.
Most of my trouble comes from my indeciveness I admit but I wish there was some form of prescriptive requirements for people who value high efficiency – for instance I can look up for my area (California Building Climate Zone 4) that my total wall u-factor can’t be over U0.048 and there are easy lookup tables showing I can get there with 24OC studs, R21 cavity insulation and R5 continuous insulation. In a location like San Jose is it economically responsible to go beyond this code minimum, is there any literature that gives an idea at what point adding more insulation is a waste of money (I have seen some analysis based on cost per R and cost per KWH for solar)? I find myself gravitating towards using something like the insulated zip system because it looks to be less labor intensive and should perform well here. But I guess I am getting a bit paralyzed when I start thinking about all the materials that could go into a wall structure instead of just deciding something like osb -> wrb with rainscreen -> hardie siding
While I am trying to be cost conscious I also want to avoid the typical way I see homes (and even my addition a few years ago!) built here with poor detail to building performance since the weather never gets particularly harsh in either direction. And even though this is an ADU it’s more likely we will be using it for ourselves and our parents as the get older so a well performing, healthy structure is important. Hopefully the labor savings combined with a very simple shape will let me spend a bit more on airsealing and insulation, but it’s a matter of having too many choices that I’m stuck trying to pick a sensible assembly to go with. All that to say, is there any advice for high performing assemblies that are not too labor intensive and aren’t overkill for a mild climate?
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Replies
A couple of thoughts:
No matter what, the building should be as airtight as you can make it. A tight building uses less energy, is more comfortable, is less dusty and does a better job of keeping out bugs and rodents.
It's hard to be specific about what level of insulation makes sense because there are so many variables, but one thing to keep in mind is proportionality: it doesn't make sense to super-insulate the walls if there is a hole in the ceiling. The guideline that Joe Lstiburek uses is figure out what R-value your walls will have. Use the same R-value on exterior floors. Put 1.5 times as much in the roof. Put half the exterior walls on below-grade walls, and half that under basement floors. Windows should be 1/8 exterior walls.
Finally, start with what kind of building you want. Then figure out how to build it.
Thanks DC, those are some simple to follow guidelines :). I'm hoping something like the zip panels do a good job of airsealing and really think that may be the way forward for me to keep the labor easier on myself so I don't have to worry about the extra steps of a WRB and continuous insulation.
I built a house in Zone 4 (Virginia) 2 years ago, with similar goals in mind. Part way through the project, I decided to go for Net Zero Ready Certification (ZERH). The rater built an energy model of my house, and it was a real eye-opener. I changed some of the insulation specs on the spot.
So I recommend spending $1000 or so to get an energy model done (and do earlier than I did). You could also do your own energy model with BEOpt, for free, if you have the time and interest.
Hi, Since it is a ADU and will be small = less shear wall space, the better option may be to build a double stud wall. Look at the ZIP limitations on shear wall lengths & requirements in the ESR report.
If you make the wall double stud wall 2x8 wide ( 2-2x4 plates) with cellulose, you can avoid the external foam. This make the detailing easier for the windows & doors. The building would be all plywood shear paneled. The rainscreen could be optional depending on your micro climate in California
I know you want your considerations simplified, but if I were building in CA, I'd be thinking a lot about fire resistance. Maybe your location is urban enough that it's not an issue, but at this point that's hard to be sure of. I think mineral wool boards as the exterior continuous insulation are the best choice in that regard.
In mild climates, there is very little energy saved by insulating more. Even up here in Zone 5, what I was looking at insulation for a garage/studio, ended up installing only R13 walls/roofs. Doubling that up would have saved only $60/year in heating cost with a mini split.
As others have said, focus on air sealing details instead.