Well, asjmama, you're going to have to work fast, unless DH is on a business trip...

Looks like you're going to need a backer of some sort. I would suggest a couple of pieces of wooden lath cut about 2"-3" longer that the width of he hole. Apply liberal amounts of construction adhesive to the outside inch or so, then work them into the hole (punching through the remaining paper) so they create a backstop for your patch.
Squeeze tight against the back of the drywall, and clamp, if possible.

While that is curing, cut the piece of scrap drywall to fit the hole as good as possible. When you feel the lath is firmly enough adhered to the existing drywall (probably an hour) GENTLY adhere your patch to the two lath slats with more construction adhesive (don't press too hard or you'll break the bond to the old drywall).

Once this little patch is in place, you can try your setting type compound to fill the crack and create a solid surface.

To tell you the truth, I don't understand why your original application didn't work. It's possible the drywall had some underlying issues (e.g., moisture from that window) that weakened it on the back side. If that's the case, you may have to expand the hole till you get to where the back paper is obviously solid. You need something solid to attach the backer lath to.

Best of luck!

My uncle who is a civil engineer said he'll come over Sunday and take a look - suspects moisture too. Of course this didn't show up until after the baseboard, window stool and casing was installed. But if there's a problem with window flashing (penetration or condensation?), wouldn't the whole wall from stool to floor be wet and not just this one spot halfway between?

The wall faces south and the problem first showed up this July when the house was 2 yrs old and I had repainted under the windows b/c of scrapes/gouges from putting up baseboard, taking out separate stools. And I still have a little more touchup to do since putting in the single stool in the bay and the casings. Might be more than a little touchup now...

Come to think of it, June was rain practically every day - more like April. Could have gotten wet...

That's the thing about most moisture issues, ajsmama, they can be difficult to see (especially when in the wall) and when you do see something it's like the tip of an iceberg.

That particular area is one of the most notorious for leakage issues from the window - underneath one or both corners where some flashing failure may be occurring. It may not be a lot of leakage, but over time it can be enough to cause damage.

Have a feel inside the hole (might as well break through that paper) to see how solid the rest of the back of the drywall is, or if the insulation seems wet (if it has rained recently). I guess I'd suggest waiting for your uncle to have a look before you cover it up - just tell DH how lucky it was you discovered the problem...

Ask the person who puts the casing trim and apron on the window to patch it.

Or follow the instructions on any one of a dozen videos on the internet.

Here is a link that might be useful: How to patch a small hole in drywall

This is not a big deal to patch. But dealing with moisture, if that's an issue, is another matter.

But why do you need to do this so urgently before DH gets home? Do we need to worry about you or something?

No need to worry about me - DH is pretty easy-going, I was just embarrassed that the little cosmetic problem turned into a big deep hole in the past 2 days while he was at work (I must have too much time on my hands LOL). He *did* ask yesterday "Why is there a hole in our wall?" even though I told him this weekend that I was going to try to fix that spot and it might require some cutting (possibly even replacing drywall stud to stud under the window - 18' x 27" piece). I told him I was going to fill it with several layers of setting compound today - oops! Ended up looking worse...

My uncle is going to come take a look to rule out any moisture problems. If he finds anything wrong with flashing we'll call the builder since I think our "2-10" warranty should cover this sort of thing (modular home). If not we'll patch either trying a small drywall patch inserted into hole or cutting it out stud to stud. Not the first place in this house we've had to (have someone) do that - though that was due to bad framing/taping and was before we got baseboard in.

I *could* give my cousin (trim carpenter) a call to see what he thinks, but I don't know how good he is at patching drywall - gotta be better than me! He saw this when he put the stool in, but had no idea what would have caused it. But if uncle (on other side of family) doesn't show, I can call cousin for help tracking down the cause. Question would be if he has time to look at this/fix it. Builder probably won't come fix it - after more than 2 years. So keep your fingers crossed, this could turn into a big drawn-out thing if moisture/warranty issue.

Just buy some drywall repair clips. I've used them and they do a good job. You'll have to spackle & sand the cut edges but it will be much easier than cutting stud to stud.

I always keep some clips and fiberglass tape in my tool box!

Here is a link that might be useful: repair kits

Thanks - I'm wondering if I should cut the backing if there's no water damage?

I'll have to look for drywall clips, or maybe I'll use plywood (since hole is so small, maybe just paint stir sticks?). Try a combo of techinques, either clips or a wood backer to screw a patch into, then either try the "flap" method from Fine Homebuilding *or* just try "no tape" method diy_guy just posted - anything to avoid having to cover mesh tape!

I'll let you know what we find - we had a lot of rain yesterday, the backing paper seems dry to me, we'll see if uncle wants to cut it out to look at insulation. I think it's moving *now* just b/c it's just thin (1/8" thick?). It appears that original movement was just b/c the gypsum was damaged or had a void. When I first cut an X in the paper to do exploratory surgery, and stuck a chopstick in, it moved around, came out covered with dust and a few chunks fell out. I didn't *shove* the chopstick in, there was no resistance. So something crumbled/crushed the gypsum in that area w/o damaging the paper.

If I can't get it patched invisibly, I may just hang artwork under the 3 windows, or make "paneling" with foam core and molding until we enventually (10 years from now?) put in the bookcases/window seat I originally had planned for the area.

Thanks everyone.

It's only 2" in diameter. Put mesh tape over it and cover it with compound.

My uncle says it doesn't look like water penetration, but since it's going to rain tomorrow I'll wait to see before I fix it.

Now, he said don't cut through and remove the back of the sheet that's there b/c the vapor barrier is right on the other side? I always thought vapor barrier was over insulation and sheathing. Maybe he was talking about the paper on the drywall being "treated" and not the Tyvek? Do they make special drywall for exterior walls?

Also, I never sealed under the windows after the new stool was put in. I pulled out the builder's fiberglas and used low-expansion foam around the windows before we put the finished casing up. I was wondering if I could just fill the hole with the same stuff, since it's toolable (is that a word?) and then either mud over it (don't fill all the way), or tape and mud. The DAP stuff is paintable but it would likely be a different sheen/texture than the surrounding wall so I wouldn't just use that - it would have to be skimmed with mud. But I was thinking it would be easier to apply as a backer material to fill the 1/2" deep hole than the setting compound, and I'm not comfortable with just taping and mudding over the hole with no insulation or backer under (not that the original gypsum is a great insulator, but it's better than air). I'm afraid my patch would sink like the original paper did if I leave a void behind it.

You're wasting a lot of time and energy trying to fill this hole with various materials that aren't intended for such big holes. It will never be right unless you use a proper sheetrock patch.

First, get rid of all the patching stuff you've put into the hole. Then get a thin piece of wood that's a little narrower than the hole from top to bottom, and is about 2" wider than the hole. Put a sheet rock screw into the center of the wood so you have a "handle" to hold onto. Insert the wood into the hole then drive in a screw through the sheet rock and into each end of the wood backer piece. If you do this right, the piece of wood will be tight against the back of the sheetrock. Now remove the screw that you used to hold onto the piece of wood. Get a scrap of sheetrock and cut a patch that fits into the hole with as little gapping as possible. It will simplify the later steps if you sand away the front edges of the paper face so nothing rough is sticking up. Hold the scrap in place and drive a screw through its center to fasten it to the wood backer. Then get yourself some paper tape and some joint compound and patch it up. When you've applied the joint compound, and it's dried, apply more joint compound to get everything flat. When you're satisfied, gently sand it and paint.

I haven't wasted much time - just the time to put in the setting compound, then dig it out, and post here. I haven't touched the hole since Friday, it looks the same as in the last pic. It's raining today, we'll see if it gets wet.

I am trying to figure the best way to patch this. What kudzu said is what I was leaning toward - think a paint stir stick would work?

The only thing I'm wondering is if I should try to wedge it into the hole w/o cutting out the back paper, or if I should cut through that against my uncle's recommendation. Or maybe leave it and skip the wood backer, just cut the "plug" of sheetrock, mud the back and edges of it b4 I insert it so the existing paper backing will act as the backer board. Leave the paper on the patch a little bigger than the hole to act as "tape" as in this video from Fine Homebuilding? Thanks

Here is a link that might be useful: Fine Homebuilding patch drywall

You're making a mountain out of a mole hill. If it were a larger hole, the wood backing method that others have menntioned works great. You're only talking a small blemish here. Mesh tape it, work joint compound into it followed by a couple of coats feathering the last around it, lightly sand, primer, paint.

A paint stirrer might work...I usually use something that's 1/4"-1/2" thick, though. In addition, if you don't cut out what's in the hole it will keep the repair from being flat: you'll have a lump in the wall. I don't know why your uncle thinks the vapor barrier would be right against the sheet rock...it should be on the outside of the wall. In addition, if you cut out the paper, you can actually look inside the wall to confirm there is no water damage.

I'm still of the opinion that a 2" hole needs a decent patch. You can use the technique where you have a sheetrock plug with the paper around the edges. I just prefer to have something solid backing up my repairs so I'm sure I won't have to redo it because of movement or bumping of the spot in the future. I'm a firm believer in taking the time to do something right once, rather than poorly several times.

That's why I'm taking the time to ask about proper way to do it. This is a pretty visible spot in my house and I want to do it right. If uncle was wrong about vapor barrier I'll cut the whole thing out and use some sort of backer, was thinking a stir stick was the right size but can see if we have anything thicker (but narrow) that might work, or buy the clips. Still need a "plug" whether or not I use screws and/or tape, I really don't want to have to cut it stud to stud for such a small hole, but don't want one of the kids to bust through tape and mud if I don't fill the void.

Builder fixed 3 spots under BR windows (and one spot in ceiling at bottom of stairs) replacing stud to stud when we had cracked seams keep coming back, but that was before baseboard was up (and he replaced the entire depth, no problem with vapor barrier that I know of, you could just see insulation while in progress). Those problems were due to bad framing (high/low studs) and in one case duct tape on back of seam instead of drywall tape on house side (modular house)!

I don't know why your uncle thinks the vapor barrier would be right against the sheet rock...it should be on the outside of the wall.

Infiltration barriers, i.e., Tyvek, are on the outside or cold side of the wall. Infiltration barriers stop the wind but are moisture permeable, allowing any moisture that migrates from inside of the house from being trapped inside the wall. Vapor barriers, i.e., Visqueen, are between the studs & drywall, on the warm side of the wall. What vapor barriers do is to prevent warm moist interior air from moving thru the insulation and condensing when they hit the cold side of the wall, rendering the insulation ineffective & worse--moisture problems inside wall cavities.