I have an older redwood sided home that has the siding nailed directly onto the framing. There's a layer of felt paper over the framing, but it hardly qualifies as a "moisture barrier" by our modern standards.
Doing an extra careful job of sealing up penetrations and doing an extra careful paint job is the only affordable strategy I can think of to "seal up" the exterior. (Taking off the siding is out of the questions.)
So, assuming that there will be periodic moisture getting into the stud bay and onto the insulation, what would be the best insulation to use? Would spray foam also help to seal up the envelope? And if not and water/moisture got into the stud bay, how would the foam react?
If you define the drywall as your air barrier, then the felt paper is simply a drain plane, not uncommon. Only in the deep south would you need a vapor barrier on the outside, ie ac running 90% of the time. I'm assuming central to northern CA, where you'd have a mixed climate. Although some would like foam, take a look at Roxul. It handles moisture well and is very dense so will reduce any air flow. Just an opinion as I'm about as far away from CA as one can get.
First off felt paper is a great moisture barrier & while me & many others generally use a more modern one, felt is still fine & in fact some might argue a superior one. With that being said if there is any doubts on moisture, there is only 1 product that can or should be used & that is closed cell foam - end of story.
The biggest issue in older homes & why many have lasted so long is because there was no insulation in them to hold water (Sorry Bud, Roxul still holds moisture in) which does not allow the cavities to dry out as quick. If you are considering any other product (or even using CC foam), make sure you take care of any bulk water intrusion points (flashings around doors / windows - incorrectly lapped felt, etc...). The other big issue is the wiring - if it is old romex, aluminum, or lord forbid - knob & tube it should be replaced.
As for paint - make sure it is vapor permeable / not an oil based so that any moisture that may get into the wood can get back out or you are going to be looking at peeling & blistering paint (and an area you have a problem to fix) As for the final 3 questions - I think I answered #1 & 3 above, as for #2 - yes closed cell will help "seal up" the envelope & even strengthen the structure. Drywall as mentioned when done properly can constitute an air-barrier though all climate zones are supposed to have one on the outside & northern climate zones are also required to have an interior one also
From the GBA article below: "We also took pieces [of mineral wool] and submerged them in the pond as a test, because we were so worried about the wall being soggy. The water beaded off. As it turns out, the Roxul will actually help keep the wall dry.”
Mineral wool is good at doing that but as described in the article this install was done on the outside with the "board" product & they used a rain screed detail. That is very different than using a batt system where any bulk water that gets into the cavity can't drain or dry out as quick
I would think you would want to use fiberglass. Calculate and use a good controlled ventilation strategy.
Sean is spot on! However, spfi (spray polyurethane foam insulation) products do not react properly with existing moisture problems. This can change the cellular structure and ruin the product if existing moisture is present at the time of installation. Even if the walls are dry but do get wet from time to time, spfi could cause the structure to rot if the problems are not corrected before hand. This includes open cell and closed cell.
Anyone recommending spfi will (if trained properly) tell you to remove (FIX) the water problem before hand. Considering this sounds like a low budget retro fit with existing issues, Mineral Wool would be your best source and the safest bang for the buck. From what I have read at the Roxul web site mineral wool will not lose it's R value when wet unlike fiberglass and open cell foam. Fiberglass and open cell will regain their R-value when they eventually dry out.
Most importantly, SPF is not a fix all for everything as many treat it! It does have benefits and it has a very costly down fall if the manufacturer's specifications are not followed to their testing lab criteria. All to often salesmen will tell you what you want to hear. SPF is not something you want to mess up and it does require tremendous skill and training to make it right.
Best of Luck!
RetroFoam injection foam is the solution. Use it all the time here in Alabama. Seals all penetrations, has excellent R value and stops air filtration.
Have you considered doing something other than "just" painting? There are several companies making exterior coatings and sealants. One product line I would suggest everyone looking into is CABOT. I have seen it used on homes and decks in the WNC (Western NC) mountains and on the Charleston, SC coast line. It has always been on "upper class' homes (half million dollar and up). But, why shouldn't the rest of us protect our homes to the same level?
The "paints" are a several step process as are the sealants. The results are well worth it time and cost! Afterward, it is as simple as doing the proper maintenance for the product to continue working at its' peek performance.
If one can afford it I would strongly suggest using this or a similar quality product on your home! After all, what dollar amount can you put on your peace of mind. Knowing that the first line of defense for your home is the same used by home owners of seven figure homes!