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Wire before insulation or visa versa?

Posted by spafrica2003 (My Page) on
Fri, Jan 21, 11 at 10:37

I'm adding a room into some unfinished space in my basement. I've already got the walls framed and I'm ready to move on to the next step... I'm just not sure what it is. Should I lay in the fiberglass batting insulation first and then run the wires over the top of it, or should I run the electric and then stuff the insulation either behind or over the top of the wires?


Follow-Up Postings:

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RE: Wire before insulation or visa versa?

With respect, you missed the first step by using fiber glass instead of extruded or expanded or polyiso foam boards or spray polyurethane insulation. You're setting yourself up for mould and mildew problems.

As for the wiring, it normally goes first.

Here is a link that might be useful: Building Science on Basements


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RE: Wire before insulation or visa versa?

You never want to run the wires over the top of the insulation since this would mean that they are close to the backside of the sheetrock and vulnerable to being pierced by a sheetrock screw/nail, or by anything later attached to the wall (picture, toilet paper holder, towel rack, etc.). Wires in stud cavities are generally required to be run through holes that are centered in the studs for just that reason.


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RE: Wire before insulation or visa versa?

"Wires in stud cavities are generally required to be run through holes that are centered in the studs for just that reason."

The requirement is for wires in bored holes in studs to be far enough back to not be hit by drywall screws.
Any hole closer than 1.25 inches to the face of the stud requires a metal plate to protect the wire.

The plates are available by the box and are inexpensive.
They even have prongs formed in each end to attach them to the stud face, and are as wide as the stud.


While it is possible to hit a wire between the studs, it is actually pretty hard to do since it can move slightly.


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RE: Wire before insulation or visa versa?

Run your wire.

Then when you install the fiberglass batts, install the batt starting at the top of the stud cavity, tucking it in so FG contacts all sides of the stud bay.

When you get down to where the wire is, split the batt thickness by peeling it into two at the end. Place half the insulation thickness behind the wire (between the wire and the exterior wall sheathing) and the other half in front of the wire (between the wire and the eventual drywall).


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RE: Wire before insulation or visa versa?

ae2ga-
Yes...why would you want the headache of trying to do drilling and wiring after the foam was in place, unless it was a remodel situation and you were rewiring?


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RE: Wire before insulation or visa versa?, part 2

brickeyee-
Thanks for clarifying my less-than-precise language about where the wires need to be run.

I also recognize that wires have some movement, but I visualized the OP running the wire from the stud hole to the face of the insulation, across, and then back in to the next hole, etc. In that situation, with the wire pressed to the back of the sheetrock by the insulation, I could see the wire getting hit by a sheetrock screw that missed the stud or by a drill when locating holes for a fixture. Perhaps not highly likely, but, if something can go wrong....


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RE: Wire before insulation or visa versa?

or by a drill when locating holes for a fixture.

That's why electrical wires are typically located midway or lower in the wall. Most people don't hang pics three feet from the floor.


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RE: Wire before insulation or visa versa?

"Everything inside of the walls (electrical, plumbing, etc.)..."

"So now I know the order of operations begins with electrical first. "

Not if you are doing plumbing.

It is a lot harder to run pipes around wires than wires around pipes.

The HVAC guys have the largest items that are very hard to route.
They usually get first crack.
The plumbers typically come next.
After the plumbing rough in the wiring is still easy.
Insulation next (unless you are spraying foam).


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RE: Wire before insulation or visa versa?

Okay then - HVAC, plumbing, wiring? And i will definitely be using spray foam insulation- so when is that?


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RE: Wire before insulation or visa versa?

worthy-
I know where wires "typically" are run. I also know that people put in things like stair railings, toilet paper holders, and coat hooks for kids at lower levels. And I just finished a kitchen remodel where I ran into five cables running horizontally in a wall at eye level. The point is, when doing something that penetrates sheetrock at any level, you can never be sure what is behind it.


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RE: Wire before insulation or visa versa?

Thanks Brickeyee;

Your comment had me go look at the document I cobbled up and I had rough elec before pipe or hvac.

Bill


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RE: Wire before insulation or visa versa?

Spray foam depends on how thick it is going to be applied.

If the stud cavity will be filled completely just about everything, including wiring, needs to be in place.

If a less than cavity filling layer is to be applied (good article in the recent Fine Homebuilding) then the spray foam may go in first).

A shallower layer of spray foam for air sealing, with other insulation on top to get the desired R-value, costs a lot less in many cases than just filling the cavities with foam and provides a much better infiltration stop than just fiberglass or even wet applied cellulose.


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RE: Wire before insulation or visa versa?

"If I were to have that done, would it be necessary to add furring strips to later attach the drywall, or would there be enough space?"

The idea is to fit the same R-value in the available space while saving some money on the foam.

"Would the method of sealing with an inch or so of foam and then other insulation (would you recommend batts or spray cellulose?) provide a comparable R-value as completely filling the cavity with foam?"

It is going to depend on how much of each material is needed.
It is the first decent article that has discussed the dew point placement by controlling the different types of insulation R-values, and that the concept may not save money in every location (if you need a lot of foam for higher R-value there may be fill problems in the cavity).

I have seen a couple of 'super insulated' houses with fiberglass that developed problems when the dew point ended up inside the fiberglass, even around Northern Virginia and we are not known for very harsh winters.

One of the owners saved on his heating bills, but the repair costs likely ate up most of the savings (and more).

I would note that the pictures in the FHB article show wiring done before a thin enough layer of foam that the order could have been changed.


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RE: Wire before insulation or visa versa?

Thanks - I'm going to the bookstore to buy the current issue this evening.


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