I own a two story, 3200 Sq ft house in Los Angeles. The exterior is stucco and the walls are lathe and plaster. A year ago we had some water damage after a rain storm. When the contractor demo'd the lathe and plaster for repair of the wall, I was shocked to discover there was no insulation between the inner and the outer walls. I want to lower my heating/AC bills, and make the house more livable. What is the best route to go (Cost v. Benefit) to insulate my walls? What are my considerations? (Based on what I saw during the demo I do not believe there is any kind of "wrap" on the exterior of the house).
It IS possible to insulate with a "wrap" on the outside, but then you need to extend all the window frames: a huge undertaking, and really hard to seal properly.
Your practical options are blown in insulation from the outside or from the inside. Then you can choose from fiberglass, cellulose (most common) or maybe maybe foam.
From the outside is less disruptive, but introduces more chances for leaks. From the inside works OK. Two holes are drilled per wall cavity. You'll want to use a contractor who uses a thermal gun, to ensure all the cavities are filled.
As mentioned here: Alternatives to conventional weep screed, for stucco to the ground homes? the stucco and interior plaster of that era is just reaching maximum strength, and it's rock rock rock rock hard. Drilling from the outside is a serious chore.
You also have a SEVERE danager. Existing leaks in your stucco are no big deal, they dry out. With blown in cellulose in particular you now have wet compost in your walls. Paying a professional energy consultant is needed to give a better answer and to really evaluate your house, habits and opportunities for energy savings and comfort upgrades.
Two options really. Or maybe 3.
1) Sprayed in insulation either from inside or outside the home. Outside is easiest and repair is simple. You'll hardly notice it was done. Did this for most of the old house with a product they called airfoam. It's not really recommended though because it goes in wet and has to dry out. As it dries it shrinks. Better to use a product that goes in dry. Advantages: Quick to install. little mess. Cost effective. Disadvantages: Breaks the last layer of defense against water intrusion - the building wrap, tar paper. Some go in wet and must dry out. They shrink as they dry.
2) Remove the wall linings on the inside and add in insulation and then reline. This is far more expensive but could be done. I did it myself in my old house and it made quite a difference to the room that I did it in. But it was heaps of work.
Disadvantage: Cost, need to redecorate. Time consuming. Advantage: Doesn't break the wall lining/builders paper.
3) Re-clad & insulate. A good option if you need to re-clad anyway.
Disadvantage: Extremely expensive.
Don't forget, much heat is also lost through windows. Here in NZ, homes are being upgraded to double glazing which is standard in many countries. Also, a set of good quality heavy thermal drapes helps.
Also see the answer to a similar question here. I'm cautious about spray foam now. Because it prevents breathing and drying out. And it sounds like you have no builders paper/wrap beneath the stucco.