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How to Feed Cable Through Insulated Walls

It's not as hard as you think. Here are a few handy techniques.


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Many installers seem to have a real fear of insulated walls. Fear not.

Drilling up or down and fishing cable through insulated walls is an inconvenience, but nothing more.

Sure, there are certain techniques you can't use through insulation, but there are always work-arounds.

What Are Insulated Walls?


What exactly are insulated walls? They:
  • are mainly wall cavities filled with fiberglass batt insulation
  • were used in nearly all homes starting in the early 1960s, usually in outside walls or in walls that divided "conditioned space" from unconditioned space
  • are almost always associated with applications where in Sheetrock is used as the interior wall covering
Older homes tend to have either no insulation or to have been retrofitted with something like blown-in fiber.

This stuff is usually no problem; it just makes a mess when you make a decent-sized opening in the wall. You will often find it has all settled to the bottom of the wall.

Some newer homes have expanded foam in the walls. This is not impossible to deal with, but difficult.

Where to Drill


Regardless of whether you're drilling up or down in an insulated wall -- from an opening in the wall or into the wall cavity from above (through top plate) or from below (through sole plate) -- the place you want to aim for with the bit is in the corner of the wall cavity.

If you're drilling up from an opening you've made for an outlet or speaker in the Sheetrock:
  1. push the bit through the insulation, using short jabs, at an upward angle, toward either back corner
  2. if there is paper, foil or plastic on your side, just cut through it
  3. when the bit hits the back corner, walk the bit into and up the corner by giving the drill short bursts (forward or reverse, whichever works better)
If you can keep the drill in the corner, you'll have no trouble with the insulation.

If you're drilling down into a basement or crawl space from an outlet opening near the floor, use your drill guide to keep the bit in front of the insulation and aim it into one of the front corners.

If you're drilling into the wall cavity from above or below:
  1. try to make the hole near one of the corners. Sometimes the best corner to use depends on the insulation
  2. if there is no foil or paper backing, it doesn't matter much
  3. if the insulation has a foil or paper backing, then it may have been installed by stapling the paper/foil to the inside of the studs (see here).
This creates a nice open space on either stud near the Sheetrock. Try to make your hole here (use a spring steel "feeler bit" to locate the exact spot).

If the insulation was stapled to the outside of the studs, then there is usually a nice open space in the center of the wall cavity. This makes fishing the cable up/down much easier.





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Article Topics

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About the Author

Grayson Evans
Grayson Evans is founder of The Training Dept. He can be contacted via e-mail at grayson@trainingdept.com.

3 Comments (displayed in order by date/time)

Posted by deasley  on  11/12  at  10:24 AM

Fishing wire in insulated walls is not “inconvenient” it is dangerous! Because of the insulation, you cannot see what is in the wall. There may be electrical wires, water pipes or drain pipes inside the wall. Electricians usually drill their holes in the corners, so walking the bit to the corner is a good way to put your drill bit through an electrical wire. Doing this could be fatal. Occasionally, an electrician will staple wire across the entire top plate or sole plate inside the stud bay to get to the other side of the bay.  Drilling into a sewer pipe will result in a call back too. Use a stud sensor with high voltage detection before drilling inside an insulated wall to prevent getting killed or burning down the client’s house.
The Training Department should certainly know better than to tell people that drilling where and what you cannot see is nothing more than “inconvenient”.

Posted by Grayson Evans  on  11/12  at  02:58 PM

I appreciated deasley’s concern, you should always try to determine where electrical wiring is.  This will vary dramatically depending on the age of the house and type of construction (from knob-and-tube to BX to NM).  I recommend using on of those stud finders with an AC detector in it. 
I disagree about the hazards of drilling where you can’t see.  There is always a risk in retrofit and no one is perfect, you just need to plan accordingly and understand what to do when mistakes happen.  I’ve been doing the retrofit I described in the article for many decades and have NEVER drilled through AC wiring, especially in a corner.  But, I do my best to make sure I know where AC wiring is likely to be installed by observing outlets, switch, and the type of wiring installation done in the structure.

Posted by Craig  on  01/30  at  08:40 PM

Thank you Mr Evans for the excellent article.
Its hard to find good information on the internet these days.

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