Using a Stud Finder on Walls With Exterior Polyiso Foam
This is likely an esoteric question, but are there any stud finders that work through 1-1/2″ thick polyiso foam?
Basically, I have such foam glued to the exterior framing (2×6 studs, 24″ O.C.) and am going to attach 1×3 furring strips as rain screen standoffs. The strips are screwed through the foam and directly into the 2x6s with some pretty beefy screws.
I do know within an inch or slightly less where all of the 2×6 framing studs are, but I’d like to know PRECISELY where they are so my rain screen screws hit as close to dead center as possible. The edges of the 2x6s aren’t as forgiving to beefy screws as they are to nails. That is, if a nail is on the outer third of a two-by, then it’ll likely hold just fine. A thick screw in the same outer third is likely to blow completely out and just spin (in my experience)… hence my wanting to know the center as close as possible.
I cannot see the studs at all since they are covered up by OSB on the inside. I can see where I nailed the OSB and that’s honestly what is giving me the accuracy I referenced above. But as I also said, that can only get me so far because I don’t know how close I was to hitting center when nailing on the sheets.
So what I’d like is a stud finder that would accurately locate the 2×6 framing members directly through the foam.
Any thoughts?
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Replies
Kurt,
Can you not locate the studs by transferring your framing layout to the foam? If you know the stud spacing, and which end you started from, it should give you a very accurate reference to what lies beneath.
Using something like this between your cordless drill and the screws will help keep them straight. https://www.busybeetools.com/products/drill-mate-accu-milescraft.html?gclid=EAIaIQobChMIgdiYoOq18AIVRQPnCh1NFQuiEAQYCCABEgL7BfD_BwE
In theory, yes, I could just transfer the layout. I even have a Sketchup model of the entire framing which was intended to be incredibly precise. In reality... well, I was inexplicably careless during the framing stage (a carelessness that has haunted me in every stage since) and now I can't trust the model to the precision I want it to be at. At least, it's only going to give me the precision close to what I can get from the interior wall OSB nailing pattern.
Honestly, one idea if I can't find an accurate stud detector is to just use a thin nail (or similar) to probe all around each stud to find the extents and then just seal up the holes after. That would be super accurate... but certainly not as quick by far as a mythical accurate detector.
BTW, you know you are asking a truly esoteric question when you do a Google search on the topic just 20 minutes after asking the question and find that it's already the number one hit. Heh.
Oh, and thanks for the drill guide link -- that's a great idea and I'm definitely going to do that!
Another gadget that might be useful: https://www.amazon.com/Rafter-finder-Stud-Center-line-Locator/dp/B07DWFB3XC
Intended for solar installation where you want roof mounting bolts to hit the center of rafters and may not have access to the interior to measure. If you can drill a hole "close" to the side, then thread this in and twist til the probe hits the side of the stud, and the exterior portion will indicate the centerline. You could probably make your own version with some bent wire and wood, too. You'd still have a hole to patch, but only one hole instead of a bunch of probing around with a nail.
Bosch has a tool that uses ultrawideband radar to allegedly "look" thru up to 6 inches of material: https://www.boschtools.com/us/en/boschtools-ocs/stud-finders-detection-devices-d-tect-150-29179-p/
It's pretty expensive and the reviews seem mixed, but it's the only one I've heard of that might work thru thick foam. Might see if you can get a used one for this job and re-sell later
Take a piece of clear poly. Tack it up tight on the inside and draw the stud locations with a Sharpie. Tack it up on the outside, being careful to keep the right orientation, and use it as a map.
If you can find the studs accurately on the inside, I would transfer one to the outside using a long pilot bit (you can get long 1/8" drill bits, and similarly small sizes, for doing things like this -- this is a common wire fishing technique to find things before drilling the "big" hole). Find all the studs on the inside, and carefully mark all the centers. Measure from one site, noting the center location referenced to one corner (the one you accurately located and transferred to the exterior with the pilot bit), and record all this on paper.
Now go outside, measure from your reference corner, and mark all the center locations. This is an accurate way to do it, and something I've done before. This way you only need to transfer ONE stud location to the exterior, all the rest measure from that reference point. This avoids drilling or poking lots of extra holes in the foam.
BTW, I described a simple jig you can make with some scrap 2x4 to help run the screws (and the pilot bit) in straight. If you can't find it, I can repost the drawing. Malcolm still hasn't forgiven me for that simple jig, he had been hoping to buy a new tool :-)
Bill
Im not sure where you are geographically, but might you be able to pick up the studs with an infrared camera if you heated the interior with space heaters?
The way I've done this is drive a very long screw on the top and bottom from the inside all the way to the outside beside a stud. Do this at 10' or so increment, the rest of the studs should be close enough to hit. Once you get one stud, you can usually find the rest. If you don't have your house wrap on yet, go for commercial Tyvek which has a convenient 8x8 grid printed on it that really helps with this type of work.
So many great replies!
@Jonny_H - Bosch and DeWalt both have the radar-style scanners, but even then, I haven't seen one way or another if they'd work through foil-faced foam. And for how much money they cost, I'd expect PERFECT results every time... which doesn't appear to be the case? But yeah, that might be the only option that could even have a chance of working. On the flip side, that Chiptoolz locator you linked to is ingenious! Simple but effective. I'm definitely getting one of those.
@DCContrarian - I love the idea of using poly as a see-through "map"! Regardless if I use it or not in this case, I can think of a bunch of cases where this would match or beat using a story pole. Sweet.
@Zephyr7 - That's a great technique - the "through" hole would also be self-sealing on one side by the very screws I'm trying to locate. Nice. And I did find your drawing of the drill guide jig - very easy to make and use. That said, I've been trying to think of a justification for buying one of those portable drill guides for awhile and now that Malcom has given me just the right reason, I am certainly not going to pass up that opportunity!
Kurt,
Not much help for your situation, but a decade or so ago i worked with a guy who 'True Framed".
He picked one corner of the house and took all his layouts from there. Every stud, both exterior and interior started with reference to the same p0int, so you could look through the house before the sheathing was on and line them all up. That meant you could use a tape measure to locate any of the studs, even once the drywall was in. Too OCD for me to continue doing, but it sure as a treat to work with that framing.
That's like having a machinist do your framing -- they "pick a zero" somewhere on the part, then always work off of that same zero reference point. This is actually a good technique to learn, since a lot of things magically work out when you do this: stuff stays square, centers stay aligned, etc... It does take a little more work in the beginning, but it can save you time later. I usually try to do things this way whenever I can.
My current house is very much not built like this, however. The framers apparently randomly changed stud spacing to get stuff to fit into whatever wall they were working with. I get to find exciting things sometimes when I open walls too -- in my home office, when I replaced all the drywall (less work than stripping wall paper), I found an old window opening framed out inside one of the exterior walls that was covered up. My OCD didn't like thinking about the thermal bridging of the unneeded header, so I re-framed that section of the wall before closing things up.
Bill
@Kyle R - I do actually have a FLIR infrared camera, but it wouldn't work at all in this case. My exterior is foil-lined foam and the low emissivity of the foil massively messes with the infrared detector. That does give me the idea to bring that camera out during the heat of the day and see just how much is visible from the inside...
@Akos - Ah, putting one "through" indicator on both the top and bottom would very accurately locate the entire length of the stud... and if one of the long screws was on the right side and one on the left, then splitting the difference would give a one-time precise center, too. Cool.
If you put a screw all the way through, it's very important to keep it straight. Sometimes the advantage to a hole over a screw is that you can use a piece of wire as a sort of feeler gauge to make sure your hole is spaced off the stud correctly on both sides.
BTW, another trick I've ocassaionally used to gain some room to work is to cut a hole on the interior the size of a single gang electrical box. This gives you hole of roughly 2 x 3.5 inches or so to work through. After doing whatever I need to do, I use a plastic low-voltage ring and a blank cover plate to "patch" the hole. This doesn't give you an air seal though, so be careful how/where you use this trick.
Bill