Insulation, Radiant and Vapor Barriers - Foaming from the exterior in

Has anybody ever heard of, or have an opinion on, removing siding and spraying foam from the EXTERIOR of the house onto the back of the drywall? It's a retrofit (not new construction) and the cavities have their original 3 1/2" fiberglass batts (30+ years old) which were poorly instaled in the fist place and are now worn out. House construction is simple: 1/2" drywall, 16" OC framing (2 X 4 studs) and T1-11 (plywood) siding. No building paper, additional sheathing, etc. I am residing so removing the T1-11 siding to gain access to the stud cavities from the outside is not out of the question. It would mean removing the existing batts with their paper facing in to the living space. I will probably use HIGH DENSITY foam which one can get as a DIY kit but this implies I will not be filling the whole cavity. Ultimately when I replace teh T1-11 I will add either 3/8" or 3/4" insulated backerboard and vinyl siding. Feedback welcome. - John

I'm curious--Do you have x-ray eyes or have you already done some exploration by removing the T1-11 to tell that the old batt insulation is poorly installed. Also how is it "worn out"?

Personally, I would do the strip off of the old T1-11 and insulation and have a professional company come in and foam the stud cavities.

Depending on the size of your house they may have to come back on several ocassions depending on how much of your house you can expose at one time to the elements.

I've removed and seen behind many panels. The bottoms where air comes in are black and matted. There are also examples of 15" batts in 18" bays and I've seen rodent tunnels through the batt, kind of like a mazz. Didnt know they did that! I've also traced a couple of HVAC ducts that are installed in exterior walls - those walls will be pretty poorly insulated too.

The existing wall insulation may be OK as a system but I've seen enough to want to check the corner studs, odd sized stud cavities surrounding window areas, the HVAC runs and cavities where I know batts are failing. My thought was I'll have to repair the insulation in at least a couple of places even though I may just do a few cavities.

To your point, I wouldnt want to ahve the walls exposed for any period of time so I guess that means just striping the bays, putting the T1-11 back up and having a contractor come over and spray into the cavities from holes.

I get the impression that you are not too excited about a considerable air gap (up to 1.5"?) between the sheathing (t1-11) and foam-against-against-the-drywall. I could be talked into reducing the foam depth (its mainly a vapor / air barrier in this case to replace the paper that was stripped when the original batts were removed) to, say, 1/2" R-7 foam against the drywall and then friction fitting some fresh unfaced batts to fill the other 3" of void.

Thoughts?

- John

Let me see if I have this straight.

You are going to remove the T1-11, then remove and discard the old insulation. After that you are going to replace the T1-11, drill holes in it and inject the foam. Is that right?

First of all, I would not reuse the T1-11. Personally, I think this is the absolute worst siding product ever invented, but again, this is a personal opinion. I would reskin the exterior with OSB or plywood since you are going to reside over it anyway. Taking off the T1-11 will probably result in a lot of damage to it anyway.

With the bays exposed, why wouldn't you foam the entire bay when it was open. This would give a complete vapor barrier and completely fill the bay with insulation. You could tack poly over the open wall until the day the foam contractor shows up. It will take some coordination on your part so you you don't have a plastic wrap house for several days.

Just shooting foam into holes through reinstalled T1-11 may or may not fill the entire stud cavity and won't give you the vapor barrier and R-value you desire if you are going with the DIY kits available. A professional foam job through holes may give better results.

Again, I don't have any idea if you have a ranch or two story, but it sounds like you have access to get all the T1-11off and insulation out so good luck whatever way you go.

Sgt Jerry,

Thanks for taking the time to provide feedback that is clearly based on experience.

You probably wont agree with my rationale but I've decided to go with a spot upgrade option.

- I agree T1-11 is not a real siding product but as an underlayment it's fine. I'm going to cover it up and relegate it to a sheathing function rather than a siding function. It's recentely painted/primed so, IMHO, now is the time to cover it up - before it takes more weather. In my house it is not structural - none of the laps are nailed through - so it if it gets a bit beat up in the removal process that's OK.
- Why then put it back up after I just took it off??? I just think it's a better use of material - theres nothing wrong with the existing plywood and it's already cut to shape, as long as I dont destroy it in removing it.
- Why not foam while the stud bays are open? Because the only option there is commercial as there are no open-cell DIY kits. Although closed cell is avail. in DIY kits it doesnt make sense from a financial point of view, at least in terms of filling the whole cavity, and leaving an air gap to the outside is a risky proposal in my type of house. As you point out, it is a staging issue: I'd have to take off all of the siding, cover the house in plastic and wait for the contractor to come in and fill the bays. Thats not an option for me as I need to do the work in pieces to spread it out.
- Since we talked I updated several more stud bays. As I pointed out earlier I am going after known problem areas. This one was behind the second story bathtub and was a classic: some were part filled with 2" foil backed fg and some were left open. Its original R-11 with no stapling on the VR so I just through out the R-11 and cut in some fresh R-13 that fit nicely. Where the cavities were too shallow due to supports etc I just put in some rigid foam that I cut and foamed it in palce. Putting up the T1-11 again was simple.

Anyway, I could go on but I am nearly boring myself!

Let me know if you really care for more details and, again, in the meantime thanks for the feedback.