Best Practice for Service Cavity and Interior Walls
I’m nearly done the structural framing of my double stud wall house. Once it’s dense-pack insulated I will need to start on my service cavities.
For the 2nd floor ceiling service cavity I plan to strap along the bottom of the trusses with 1×3 to hold up my poly vapor barrier (it will have 20″ of cellulose resting on it), then strap across with 1×3 or 1×4 (unless there are better suggestions?). Cavity depth = 1.5″ minus the sag of the poly.
For my exterior wall service cavities I plan to run horizontal 2x4s on edge. Cavity depth 3.5″
Questions:
1. Best practice for attaching interior walls to the ceiling service cavity? Where walls run across the strapping I assume it’s easy to just attach from below with screws where they intersect? Maybe not? How about where they run parallel to the strapping but don’t line up with it? I’d rather not put blocking above the poly, I want to limit penetrations as much as possible.
2. Best practice for exterior wall service cavities? Economical way of attaching the 2x4s to the walls? Dealing with board twist and inconsistent width after drying/shrinking?
3. Best way to attach interior walls to the exterior wall service cavity? I’m concerned about sound transmission between rooms through the service cavity. Should I run the interior walls right to the exterior walls and put the service cavity in afterwards? I do plan to insulate the service cavity with R12 fiberglass.
Thanks for your input!
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The best practice isn't to have service cavities. There's just not that much stuff that needs to go in the walls. Wiring can go in the insulation, plumbing shouldn't be in the exterior walls anyway.
I get this, but part of the plan is to insulate the cavity with R12 fiberglass to add to the insulated wall assembly (takes the 12" double stud from R44 up to R56) and also moves the vapor barrier to a point in the wall where condensation won't take place during the summer months with AC running ( vapor barrier will be several degrees warmer than the interior).
A side benefit is no penetrations in the vapor barrier from electrical and plumbing (we have several places where plumbing vents need to run from sinks mounted on exterior walls). I'm treating the vapor barrier (six mil poly) like the primary air barrier, even though the primary air barrier is teh external Zip sheathing.
If you're set on service cavities (see DC's comment above first), I believe the folks at "Kimchi and Kraut" documented their ceiling service cavity construction at https://kimchiandkraut.net/
Thanks for the link! I remember following that build a while back. They use a steel U bracket to run 2x6 parallel to the bottom truss chords. Another option, for sure.
We regularly integrate service cavities on both the ceiling and outside walls. A typical approach for us is to frame exteriors walls and set roof trusses before any interior partition walls are framed. This creates one wide open space and allows for easy install of air/vapour membrane at the ceiling as well as service cavity strapping. We use 2x3 on edge to create a 2.5" cavity which easily fits low profile recessed lighting etc without the need for additional cross strapping. 4" U2 Universal screws https://u2fasteners.com/product/universal-screw/ are our preferred fastener for the 2x3 strapping and we use Fastenmaster Pro NLB https://www.fastenmaster.com/products/nlb-connector screws to connect our partition walls to the strapping to manage potential truss uplift. For walls that run parallel and in between the 2x3 strapping we cross block with 2x3 on its flat.
This method has worked very well for us and we have had zero complaints from our electricians.
This looks like a great approach - thanks for the tips! Very tidy framing BTW!
I've built our place the same way - no load bearing interior walls, just a wide open space to manage the vapor barrier (poly in our case) and cavity strapping.
Question about the NLB connectors; what do you do for the ceiling to wall drywall transition? Do you float the ceiling drywall at the joint somehow to allow movement without cracking?
Thank you for the kind words. Where we could, we kept our drywall fasteners back from our interior partition walls by 12-16" and let the drywall float.
Sounds like a good approach!
I don't want to hijack your thread but I found this when searching another thread and noticed that both you and the poster RMaglad are building in the Ottawa area. I'm currently in the processes of doing some finishing and mechanical work on a home across the river in Chelsea, QC https://www.greenbuildingadvisor.com/question/blower-door-test-and-air-sealing-for-hvac-sizing and I would love to hear any experiences you all have had on local contractors for the energy audit, HVAC design and ERV installation and commissioning. We are doing all the work remotely so finding reliable contractors with good recommendations has been a bit challenging.
Hey neighbor!
I'm not going to be much help to you as I'm doing all the work myself. I modeled the house with BEopt software (free resource from NREL) to get a rough idea of energy requirements, window shading etc. I made some rough estimates when applying for my building permit, and I'm planning to do the HVAC and ERV as well.
I have not yet arranged a company for a blower door test. I'm planning to do all my air sealing based on pressurizing the envelope with a fan and fogging to identify any leaks, and I probably won't do the actual blower door until the house is more or less complete. Using an anemometer with a variable speed fan and a manometer I can calculate the rough CFM @ 50Pa and get a pretty good idea where my airtightness is at.
Good luck! Taking over a complicated high-performance build must be quite a leap of faith!