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I'm looking at installing thin canless LED lights in our soon to be finished garage.

Our garage roof is truss roof and I would like to insulate it before drywalling. So my question has to do with the logistics of the wiring of these lights and insulating the ceiling before drywall.

My initial thought is to just install in the following order:

  1. install the rough wiring into the junction boxes and let them just hang down (below ceiling level) in roughly the right area.
  2. Then install (staple) the insulation in the spaces between the trusses and feed the wiring through/around the insulation to get the lights in the right spot.
  3. Then install the drywall, pushing the junction boxes in between the kraft paper and the ceiling drywall.
  4. cut the holes for the lights
  5. fish out the junction box and pigtail and connect the light and snap into place

Does this make sense or would you do things differently?

Do these junction boxes need to be attached to the roof trusses or can they just lay somewhere (between the insulation and the drywall)? Most of the videos I've seen are people installing them in finished ceilings and they just lay the junction box up on the drywall.

The main difference I have is that i'm planning to have insulation right above the drywall so I'm trying to figure out how to install it easily.

thanks,

Edit: Just added a pic of the open truss system.

enter image description here

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  • One important note is rafter ties are not rated to carry a ceiling. It is likely you need to add additional timbers to support the drywall. Apr 15 at 17:46
  • Disclaimer I'm British, but it's definitely against US code to have cables effectively in direct contact with insulation for any notable length - they need to go inside conduit. This is true even for low load circuits such as LED lighting, as it might be LED lighting now but something else/ higher load in the future. Generally easier to have your cabling running broadly above the insulation and then take the shortest route punching through it. Modern building practices would suggest a warm roof instead though. Apr 15 at 17:56
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    @LukeBriggs - I know you're British, but you have any code citations for what you're claiming? I don't think there is any prohibition to have cables in contact with insulation. It's important to use the proper cable (with the right thermal rating, which NM-B (the most commonly used residential cable) has). Some cities in the US (Chicago & NYC, most notably) require conduit, but it's not because of insulation.
    – FreeMan
    Apr 15 at 18:14
  • @FreeMan That one was from a retrofit in AZ where they were required to put cabling through conduit as the insulation made it a fire hazard. By the sound of it, probably one of those things that varies by state. It's certainly the law here in the UK (i.e. all regions here) as well. Apr 15 at 18:25
  • Luke, my garage and thus the bottom (2x4) chord of the trusses are only 11 feet across. I'm in CA with no snow load, so wouldn't it be safe to assume that my trusses can handle drywall and insulation? Apr 15 at 18:43

2 Answers 2

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On a recent episode of This Old House, I saw Heath Eastman the resident electrician do something different.

He laid a long run of cable with plenty of slack all through the ceiling joists where he knew roughly he wanted to place his lights.

Then, AFTER the drywall was up, he went to the locations where he wanted to place the lights, and drilled the 4-6" hole in the drywall where he wanted and reached up to the cable, pulled it down through the hole, cut the cable and spliced in the junction box, pushed the junction box through the hole and then attached the LED puck with the LV cable, clipped into the ceiling and then went down the line to drill the next hole in the drywall.

In this way, he got the benefit of both speed AND the ability to accurately place the holes for the lights after the drywall was installed.

I looked up a bit more detail about this and determined that, as far as I know, it's code-compliant. I don't think that it would have been a segment on This Old House if it wasn't code compliant.

So if I were installing these lights again (I just did a whole house renovation where I installed about 16 of them) I would do it the way Mr Eastman did it, not the "old fashioned" way of wiring all the junction boxes before the drywall is put up.

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  • that's kind of what I am thinking except that there's the added factor of the insulation which makes it hard to just reach up into the hole and grab the wires which will be running on top of the insulation. Either way, (wiring up junction box before or just leaving wire ends in the right spot) I think I need to make sure the wires for a given light come down to the general area where the light should be and that I don't cut the wire with the hole saw when I drill the hole. Apr 16 at 0:32
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Assuming use of a somewhat common rock fibre insulation, here is the manufacturer guidance on how wiring interacts with insulation:

To prevent possible overheating, electrical cables, transformers and the housings to light-fittings which protrude into the loft space should not be fully covered by insulation. If electrical cables have sufficient slack in them, they should also be raised above the insulation. Transformers should be lifted clear of the insulation. If cables cannot be raised above the insulation, a ‘derating’ of 50% should be assumed. It is important to ensure that the current-carrying capacity of any cables to be covered is sufficient for the application – this is particularly important for heavy-duty cables serving high power items such as showers, cookers, dense arrays of lights, etc.

This guidance has been codified into regulations at least here in the UK and in some states in the US. Codes then go on to propose running cable in conduit to avoid the quite substantial derating. However, even if it isn't in your local code, it is of course always advised to follow the manufacturers guidance as much as possible.

So, in order to make sure your wiring is good to go, it would be best to run the wiring just above the insulation, and then punch down through it to each light fitting.

As a side note, modern building practices would suggest using a warm roof design instead where your insulation is up at the roof deck level - this would of course completely eliminate the need to have holes through your insulation and vapour control for lighting as both of those layers end up instead at the roof deck. The roof design in the question here is known as a cold roof, with insulation at the ceiling level:

Cold vs warm roof

Image source: https://www.pinterest.co.uk/pin/661536632734114935/

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  • thanks for the answer. The garage already has gable vents on the ends, so unless I want to change it and add soffet and ridge vents, insulating the ceiling is the easiest option. Apr 15 at 20:48
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    @Chris_engagingdata don’t worry so much about the insulation. As long as the light junction boxes are IC rated (and they almost certainly are) then they can be in direct contact with the insulation Apr 15 at 21:17

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