Detail Feedback
Hello GBA Community,
We’re designing a single family home in a mixed dry climate (zone 3). We’re inspired by Pretty Good House and trying to find that sweet spot between low carbon, high performance and affordability, though of course there are compromises. We’ve finished up our floor plan and have not yet applied for a permit, so it seems like a good time to consider details.
To that end we’re looking to incorporate some of the ideas found on this site, double stud, service cavity, sheathing as air barrier, etc. The attached details have been adapted from details found on this site…not looking to reinvent any wheels.
I think there could be some criticism that this is overkill for our climate, but it’s hot here, getting hotter. I currently live in a super insulated house and it’s great…I don’t want to go back to code minimum, even if our code is pretty good here now.
We’re in a dry climate (average rainfall 18″), so I think we can get away with no rain screen. Maybe plywood rather than OSB for sheathing is wise.
Other than general feedback, I’m interested in specific feedback on:
1) Termite barrier: I’ve read the GBA articles on the subject, while valuable, didn’t arrive at a ‘do this’ conclusion from them.
2) Size of service cavity truss…my thinking is this would be a non-load bearing truss.
While I am an energy nerd and work tangentially in the building industry (home energy audits, HERS rating, various other services), I am *not* a builder, so there are likely some dumb errors in my details, please point them out. If more information is needed, let me know. Hopefully the details are readable enough for feedback.
Thanks everybody!
Edit: Dense pack cellulose in the walls, loos fill cellulose in the attic. Walls are ~12″ thick.
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Replies
Kent,
The details look good. With a double wall it won't matter how wide the stem-walls end up being, and hanging the ceiling trusses avoids having to insulate their perimeter, which is often a weakness with similar service cavities.
Something to ask your engineer is whether they would like a 2"x4" plate around the perimeter of attic sub-floor to deal with truss movement.
Thank you, Malcolm.
Kent,
I realize I didn't directly answer your two questions.
1. Sorry, I know next to nothing about termites.
2. Like all service cavities it's best to size them once you now exactly what will be there and where. If as I recall you are building a two storey house it may make more sense to run all the ducting in between the two floors, and reserve the service cavity for just the electrical.
No problem...I take what I can get. :)
It's one storey. There's going to be a ducted mini split, a separately ducted ERV and fire sprinklers that need to be accommodated. Maybe it's time to do some 'back of the napkin' duct design to figure out what I'll need.
If the exterior OSB is going all the way up to the underside of your insulation baffle that's good. If the OSB stops at the bottom of the soffit, your baffle needs to wrap around and come down so that it blocks air movement for the extreme outside edge of the insulation layer.
The service cavity probably needs to be either small (to fit electricals), or big enough for an air duct. If you have a mechanical plan already, the duct layout will show you if you'll need to fit ductwork up there.
If you're main concern is future electricals, I recommend leaving access hatches near any outside corners of interior walls, so that you have a way to get up there to fish wires. I've used a wire fishing tool called a "gopher pole" for long runs like this, and it needs some room to maneuver. The Labor Saving Devices (a company that makes wire fishing tools) "Grabbit" would work very well for runs within it's reach (12 feet or so). You'll need a rigid fishing tool to work in a truss, flexible things like fish tapes and even fish sticks will just jam on every truss's lower chord -- your fishing tool needs to be stiff enough to be able to ride over the tops of those lower chords. If you don't have access hatches, you'll have to cut access holes every time you have to get up there.
BTW, I generally think leaving service cavities like this aren't worth the effort. If you have a few specific places you need access (such as behind a wall-mounted TV, or behind an equipment rack for an A/V room), I would run conduit and use that for future cabling changes. I've also selectively run conduit into some electrical boxes if I think I might need to make future wiring changes. I like to do that between basements and multigang boxes that handle switches for outdoor lights, since that allows changes to the outdoor lighting easily by just running an extra few wires into a conduit run.
Bill
Thank you, Bill.
I was thinking the OSB would go all the wall up the wall, cut so that I can tape the transition to the plywood 'lid' above the service cavity. I was thinking the plywood was needed to act as a dam for the insulation.
I don't have a mech plan yet but it sounds like the time is right.
The service cavity will be open above the mechanical room, which may provide some room to maneuver. I'll keep in mind that access hatches might come in handy in corners.
I'm sympathetic to the idea that spaces like this might not be worth it. I've worked in a few crawl spaces and wondered mid job whether it might be better if they didn't exist. But without a service cavity, I think air sealing would be more difficult, and it allows the duct work to be in conditioned space.
You need that insulation dam yes, but it's also an air barrier to prevent wind washing. It doesn't need to be detaield airtight though.
Concealing duct work is a good reason to leave that space open, but keep in mind you can also use soffits. It depends on what you want as a finished appearance, since the open cavity housing the ductwork will allow for a cleaner finished appearance in the room without the "boxed out" look of a soffit. If you want to allow for future wiring in there too, be sure that the ductwork doesn't completely block off access to any areas you might want to run wire in, and if it does by necassity, be sure to include an access hatch on the "other side" of the duct work so that you can work from access location to access location and avoid the need to blind fish the entire length of the run (which is challenging even with lots of experience).
Note that any areas you really think you might be making wiring changes could use conduit in the service cavity too, which will greatly simplify those future changes. Plan ahead and make your future work easier! :-)
Bill
It looks like you have about 5" of ventilation clearance over the insulation. You could get by with trusses having a shorter heel, bringing the overhangs closer to the window heads and also saving a bit on wall sheathing and cladding.
Good catch, that is excessive. I'll change that.
For me it looks like the 2x6 is the structual wall, then you hang the ceiling truss on the 2x4 which might need to reduce the spacing to accomodate that.
Would it make sense to rearrange that? 2x6 inside as the structual wall? etc. - so many options..
Or - shorten the 2x4 and put the ceiling truss on that top plate.
Yes, the outer 2x6 wall is the structural wall.
I was talking to another builder and he was worried about the 2x4 wall bearing on the slab edge. His solution was to make some connection between the service cavity/plywood lid and the roof trusses so that the cavity was at least in part hanging from the roof trusses.
I’d lay a race way thru the truss, use scrap 3/8 or 1/2 ply, OSB whatever is handy.
Buddy of mine built a mini potato gun and uses it to shoot a chunk of ‘spud with a messenger line attached. That only works though if you have access to where the potato is going to land (foam balls bounce, potato usually stops where it lands).
That potato gun is hilarious but probably works pretty well. For the OP, you might consider just strapping the ceilings for services and having taller ceilings in your public spaces. With good mechanical layout you might be able to keep the ducts within a core area around the mechanical room and along a spine through the middle of the house. This can allow for some more impressive public spaces and saves $$ in not doing the truss thing everywhere. Bedrooms, bathrooms, etc can have dropped ceilings with trusses if necessary. We kept our mechanicals to the core and just dropped most of the flat ceilings with 2x4 cross strapping and spacers for electric. The cross strapping and spacers gave us a 3" cavity to work with and that's plenty for wiring. Where necessary we dropped the ceilings a foot, but just with joists resting on interior walls, not trusses. This gave us 9' ceilings in the main areas and 8' in the others for some design flexibility. The truss cavities would probably be wonderful to work in but at somewhat considerable expense and you're paying to condition a lot of space that you can't see and don't use. Just a thought.
If you do take this approach make sure you know where the return duct goes. It's big and can't make tight turns.
Lots of ways to do these things and and maybe too many cooks in the kitchen, but a couple of thoughts…
*I’d definitely go plywood instead of osb. Stucco is a reservoir cladding and osb doesn’t handle moisture. Good window flashing will be critical. If you can find that rare mason, lime based stucco is more resilient than Portland.
* with your insulation levels and good air sealing, your heating and cooling loads should be quite low—in the neighborhood of what can be served by a single mini split? If so you can condition with a single head and use the mechanical ventilation to even out temps between individual rooms. This in turn would mean you could lose the larger hp ducts and also reduce your service cavity to 2x4 or 2x6.
I’m in a much different climate, but here’s how we do it: run your sheathing over the top plate and 6” or so into the room, then set your trusses, screw and tape sheathing to the bottom of the trusses for your air seal, then strap 2x to the bottom of the truss to give you a mech chase and hang sheetrock. That way all your pipes and wires and ducts are inside a simple continuous envelope.
Good luck!