Several of you have probably been following my house remodel and soon I will be blowing cellulose in my addition. The walls will be about r30 to r32. I looked at also dense packing the walls of the rest of the 1988 built house since I suspect the existing 2x4 walls have only r11 batts. It is simple to pop off a row of siding, drill some holes, and dense pack. Well the existing house is covered in 1/2 inch or 3/4 inch Rmax foil faced foam. Looking at the sheathing on the wall in the attic, it doesnt appear to have any OSB or plywood on the house. I had the house resided when I purchased it and the builder took off the wood and applied house wrap and vinyl. I am slightly concerned that could have been a screw up now.

I am concerned about dense packing with only the rmax sheathing and having it blow the sheathing off of the studs. I eventually would like to, one wall at a time, remove the vinyl, add 4 inches of foam, and then put it back on. This is an extensive project for after the addition.

Has anyone ever dense packed with only foam sheathing? Is there a better alternative like a foam pour that would strengthen the wall? Or is my only option the siding removal, wall gutting, and installing OSB??? Thanks.

probably depends on how well the foam is attached & that might vary across the wall/building. I've seen 30 yr old houses like that where the foam has cracked due to house movement, so there is very little strength.

What would you think would be the best way to fix? I am looking at three sides of a 1900 sft house that would need the siding completely removed and put back up...

Do you think that the liquid slow rise foam would do the trick?

A foam pour is less flexible when cured, and far more likely to cause a blow-out during installation. And when it blows out it's a lot of work to rectify the damage, since it glues everything together. (I'm not a big fan of 2lb pours, to say the least.)

A common way of stiffening up the structure from racking forces is to add shear panel reinforcement at the corners. Whether yours really needs that type of treatment (or not) or something else entirely is a bit beyond the scope of a web-forum. But a shear-panel retrofit (if needed) can usually be done without stripping ALL the siding.

Well at least if I have a blow out it will not be on the sheetrock on the inside. Will I guess give me an excuse to strip some siding and add 4 inches of foam!!!

I guess this doesnt make today so bad. I lost my drone in a tree, but know where it is, and can get it tomorrow... having the possibility of having an excuse to redo my house for the better isnt so bad!

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I suppose the worse case is that the dense pack operation applies ~4 PSI over an entire 16"x8' cavity. That's 3 tons of force applied to say 8 nails or 768 pounds to each nail and the surrounding foam. Doesn't sound good although doing a workshop test would be far more accurate. You may have to remove the siding and foam and use wet sprayed instead of dense pack.

If I remove the siding and foam I will be installing OSB and removing the old batts. I will also be putting a bunch of rigid foam on the outside.

I wonder how expensive it is to have all the siding removed and put back....

Well I just confirmed, there is definitly no plywood or OSB between the siding and house. Luckally, there is actual painted wood paneling in the first 3 feet of the first floor which prevents one from simply cutting their way into my house. That being said, and knowing from being in the business, it still seems a lot easier just to break a window. No one is going to slip an LCD TV out of a hole in the wall either on the second floor so our alarm system will still finction.

On a structural note. The sheithing does seem pretty sturdy since not only does it have the original nails holding it on but also house wrap over it and vinyl siding nailed in at every stud. I am still tempted to blow some cellulose in to try when I do the addition. Even lightly filling it couldnt be worse than things are now since I tested the sheithing by looking through a hole in a low voltage housing with no insulation, at all, behind it. When I poked the sheithing with a pin I felt a small amount of cold air come it. This is still September and it is only in the high 50s at night!

Side note, I know the fiberglass batts here are crap. I insulated the crawl space walls last year with fiberglass in R30. Unlike what is in the walls and under the floor it is fully expanded and not compressed. I decided this year to roll that up, insulate the walls with a spray foam kit, and reaplly the fiber. I also decided to rip all of the old R19 out of the crawl space since it will be no longer needed. With the R30 on one side of the crawl space rolled up I completely removed the R19 from half of the house. The R19 batts were compressed heavily by hangers and wiring. It looked like some small rodents, probably chipmunks, had been living in a few places for years, and there was mold spores. After that it got to about 58 deg last night. I felt absolutly no difference in floor temp from the side of the house without insulation. This stuff was utter crap and I could have done better by just stapling platic to the floor joists to prevent air movement.

Even lightly filling it couldnt be worse than things are now

I agree, cellulose with some gaps will be better than no cellulose. And of course air sealing is always helpful.