wall insulation - bathroom remodel

We are in the process of remodeling our master bath, which will entail some framing changes on the outer wall to modify the location/size of windows. I have started to take off the cedar shake shingles on the outside wall (~130 sqft) and we've gutted the inside down to the studs.

Questions:

1) The way the shingles were installed they used horizontal 1x4's on the studs, then tar paper, then redwood shingles, then the outer cedar shingles. When replacing this, should I use OSB on the studs?

2) If I can afford spray foam insulation, should I finish the outside first, i.e. studs (+ OSB?) + Tyvek + shingles and spray it from the inside? Or should I finish the inside first using a vapor barrier and drywall, and spray the insulation on from the outside?

3) What true alternatives do I have instead of spray foam? There were some fiber glass batts between the studs, but they looked pretty worn down and probably weren't very effective anymore... I'm going to have radiant heat in this bathroom so I'd like the insulation to be effective and long-lasting.

Any advice would be greatly appreciated!

Thanks,
Martin

Welcome to the site. In an eathquake zone I would think solid sheeting would be better, don't know code for CA.
In a wet aera like a bathroom you want the vapour barrier to protect the framing from any moisture that gets there. And foam is also a vapour barrier, and you don't want a gap between them so I think you would foam it from out side.

You can use foam and osb on the outside but your cedar shingles need to breath.That is why the horizontal 1 X4's are on the outside of your studs.

I have been buying the foam in sheet form and cutting it to a close fit and then sealing it in with the spray can foam. To get the big equipment out to do a small job is sometimes cost prohibitive.
The sheets come in all thickness and I cut mine with a reciprocating saw. They cut like butter.

I agree the shingles need to breath. Not sure how much air you need behind them though.

They used the redwood shingles as sheathing, probably it's an older house and you're in CA. They were cheap when the house was built but very expensive now. The 1xs helped as nailers and for them to breath. Much like you would do for a cedar or redwood shake roof.
What you replace it with is predicated on your ability to seamlessly weave the shingles from the existing to the repaired area.
A piece of plywood is cheaper but will it be flush with the old siding once you reshingle the repair? You said the siding itself was cedar, red or white?
White cedar shingles, depending on the exposure, can be nailed or stapled, one right over the other. You just want to match the existing.
I would use plywood to whatever thickness to match the old sheathing, tyvek and shingles. Do the outside first to keep it closed to the weather.
Fiberglass insulation to the R value for your wall cavity depth and wall covering of your choice.
You're not going to get someone out there to spray a small area and as long as the outside wall is weather tight, fiberglass is fine.

Thanks much for all the responses!

It is an older house indeed (built in 1967). We are in CA, but not near an earthquake fault line. (Supposedly. Not sure how much that will be worth when "the big one" changes the coast line - but then I probably won't worry about my siding too much...).

There are two layers of shingles. The inner ones are flat on both sides and look like redwood. The outer ones have a texture on the outside and are painted, but I think they may indeed be white cedar. I'm saving the ones I remove and the previous owners gave me a box of spares so I can replace those that broke during removal.

Thanks for pointing out that the shingles need to breath! Makes sense. I'll try if the transition is noticeable if I add any OSB. But I'll be sure to add the horizontal nailers before putting the shingles back on.

I am waiting for a quote from a spray foam company. I asked for a combined quote for this job (130 sqft outside wall) AND a 500+ sqft underfloor spray foam job in our crawl space. I'll see if they are willing to come out and at what cost.

Otherwise, I think I'll try bud16415's idea with the rigid foam boards plus can foam and use a separate vapor barrier on the inside.

In that case, I'll start cutting the boards to fit snug between the studs from outside (after the plumber is done), seal the seams with can foam, then OSB + Tyvek (or tar paper) + 1x4's + shingles.

Thanks again, everyone!

I believe I read in looking into ridged foam that anything over 1.5 or maybe 2 inches thick is considered a vapor barrier.

I’m doing this right now in my garage workshop. The previous owner left me a mountain of trash to get rid of (treasures to him) in this windfall I inherited was a gigantic pile of used 6” foam that I think was salvaged from some industrial demo job. I have been chopping this up and filling the bays and gluing it in with spray foam. I don’t see any difference than if it was all sprayed in as the end result. The foam is denser than you think and I have been shooting for size for size or slightly under, as it doesn’t want to compress much. Having done a lot of fiberglass in my life there is no comparison in my mind as to improvement in insulating ability the foam has over glass. I can feel the difference with each piece I install.

Bud, they make a caulk for that which is cheaper and cleaner than spray. I would still use a vapour barrier to keep bathroom moisture out of the framing. But the insulation and the barrier should be right next to each other.

Thanks for the tip Neal. I will take a look into the caulk for foam. In my case I’m sometimes filling a tiny crack and sometimes I have an inch to fill so the calk might not do the trick.