How to get air tight around a large bundle of wires (and then some)
New construction, in San Diego, where builders apparently pay little attention to air sealing. I am trying to fix that. We’re at the end of framing and electrical/plumbing roughs, etc., with insulation up next.
The attic is unconditioned, and in one key area we have a 6″ gap between interior walls. Left as is, attic air will spill down between those walls, and through fiberglass batts. In a previous house, this was easily solvable by closing off that open gap in the attic “floor” between the walls and laying batts over it. Not so easy this time. We have at least 30 electrical lines running down between those walls, because one of the walls is where all of our Lutron automated lighting switches go. And we have about 20 A/V, networking, and data lines running there as well.
Ideas:
– close it off as best as possible, run a fat conduit through it for the wires, and try to seal the ends of the conduit as much as possible.
– leave it open, and use rigid foam in the interior walls
– rigid foam the interior wall, then frame another 2×4 wall in front of that existing wall with a lower ceiling, put the switches on that wall, and run the electrical lines one line, one hole” through the now lower top plates so they each can be sealed tightly and I don’t have to worry about trying to seal 20 electrical boxes.
Thanks for any thoughts/ideas!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
The easiest solution, and the way this would be done in a commercial building, would be to stuff the hole with mineral wool as a backer, the cover that with canned foam (commercially we'd use intumescent fire stop caulk or putty pads, but you don't really need that here). It's OK to caulk, foam, or seal (with a putty pad) to the wires.
If the gap isn't too wide across, you might be able to just use putty pads alone (https://www.amazon.com/Rectorseal-66340-6-Inch-7-Inch-Metacaulk/dp/B008A3UA04/ref=sr_1_2_sspa?crid=VFMX05P45AK6&keywords=fire+putty+pad&qid=1696603868&sprefix=fire+putty+pad%2Caps%2C104&sr=8-2-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1). These are a bit like modeling clay, and can be molded around things, but they're also sticky so they stay in place. They are intended to form an air barrier.
Note that it's often desireable to allow A/V cables to be upgraded in the future, especially HDMI cables. I'd run those in conduit. 1.25" conduit is usually big enough for you to run the cable with a connector already attached. You can then seal the ends of the conduit with a putty pad.
Bill
I'm trying to picture what you're describing. You've got two walls -- 2x4, presumably -- and between them is a 6" gap? At at the top, where the ceiling would go, there's nothing covering them?
You say the attic is unconditioned, but how is it insulated? Is the insulation in the floor or the ceiling? The reason I ask is I'm trying to figure out where the boundary of the building envelope is.
My inclination would be to cut a piece of plywood 13" wide, to cover the tops of the walls and the gap. Drill a series of holes in it along the center line, and then rip it in half through the holes. Place it over the gap, align the wires with the hole and then use firestop caulk or foam to seal the holes.
As an aside, this sounds like a fire code violation, you need to have firestopping to prevent fire from spreading inside the walls.
@DContrarian,
Yes, you've pictured it correctly. The attic is not yet insulated, but when it is, it will have batts laying over the the gap between the two walls. Of course air will migrate right through that and down between the walks.
It sounds like a fire code violation to me, too, and it's a very common thing here. We passed our framing inspection (although that doesn't mean it is ok).
While what you suggest (ply, drill holes, cut down the middle so I have one wire per hole) sounds good, but we have so many wires going through one area I don't think it's feasible. I'll mess with that tomorrow and see what's possible. It amazes me the contractors have never seen anyone care about this stuff. But when your temps are 50F-70F about 95% of the time, a whole lot of these things get ignored.
@ Bill, the putty is interesting. I'll add that to my toolbox of solutions.
Using plywood to make a slot, either between the plywood and a top plate or between two pieces of plywood, would be easier to work with than a bunch of individual holes. You just need to close the gap up so it can be easily sealed.
azfcrsf,
It sounds like a clear code violation me. IRC 302.11 requires a separation between each storey and between the upper storey and attic. Codes aside, it's a real safety issue, allowing fire to spread unimpeded to the attic.
Stuffing the cavity with mineral wool and capping that with a fire putty pad should satisfy fire codes, as that creates a fire stop. That may be simpler than trying to cut framing lumber around existing wires to put in wood blocking.
I do agree that there should absolutely not be any completely open vertical cavities in the wall though. Open stud bays act like chimneys in a fire, allowing for rapid spread of fire between floors.
Bill
Bill,
I'm constantly amazed at the stuff that gets described here. Concealed cavities are a routine thing - above fireplaces, bulkheads, built in cabinets - and they get the blocking put in by the framers before any services. So much of the stuff we discuss is trying to retrofit solutions to problems caused by trades not doing the basic tasks they should be performing in their sleep.